Wavefield imaging of the upper mantle transition zone across by slappypappy116

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									                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Josh Stachnik1 and Aaron Ferris2                                                                                                          WESTON

                                                                                                                             Wavefield imaging of the upper mantle transition zone across USArray                                                                                                                                                                                                                                                                                                                    1
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    2
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      Univ. of Wyoming, Dept. Geology and Geophysics, Laramie WY
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 GEOPHYSICAL
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      Weston Geophysical, Lexington MA 02420



                  1. Motivation                                                                                                                                                                                                           3. Composite azimuth swaths and corresponding τ (p) curves                                                                                                                                                                                                                                            4. General observations of the wavefield
                                                                                                                                55˚

• The USArray network (Figure 1) provides high-quality record-                                                                                                                                                                   • Example processing of a magnitude 6.2 Queen Charlotte earthquake recorded by USArray on 09 JAN 2008.                                                                                                                                                                                     • Clear P410 and P660 primary and secondary arrivals are seen between 11◦ - 26◦.
  ings of seismic wavefields as they propagate across the entire                                                                                                                                                                  • Wavefield images from three separate back azimuths are shown: 115◦, 130◦, 140◦.                                                                                                                                                                                                           • τ (p) curve inflection points are consistent with velocity discontinuities near 410 km and 660 km depth.
  western United States.                                                                                                        50˚
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            • Variations in the back-azimuth swaths (e.g. P410 cross-over distance and gradient changes in τ (p)) reflect
• For earthquakes at regional to far-regional distances, record sec-                                                                                                                                                                                                                                                                                                                                                                                                                                                          differences in upper mantle structure.
  tions can be constructed that illuminate the triplicate arrivals                                                              45˚
                                                                                                                                                                                                                                        (a)                                                   (b)                                                                                  Distance (deg)
                                                                                                                                                                                                                                                                                                                                                                                                                                       (c)                                      slowness (s/deg)

  from the upper mantles discontinuities at 410 km and 660 km.                                                                                                                                                                                                                                                                                        5                    10                   15                      20                          8                  10                   12                 14



• We apply wavefield continuation methods (e.g. Walck and Clay-                                                                  40˚
                                                                                                                                                                                                                                                                                                                                                                                                                                               0                                           P410                                                 5. Downward continuation (depth migration) of τ (p) curves for velocity profile
  ton 1984) to derive upper mantle profiles for azimuth swaths                                                                                                                                                                                                                                                                                    0


  across the USArray network.




                                                                                                                                                                                                                                                                                               Reduced−travel time (s) (slowness=11.2 (s/deg))
                                                                                                                                                                                                                                           50˚
                                                                                                                                35˚
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           Velocity (km/s)                                    Velocity (km/s)                                                                                  Velocity (km/s)

• While this work is preliminary, our goal is to further address                                                                                                                                                                                                                                                                                                                                                                              50
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       8   10                12
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          8   10                12
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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            8                  10                12


  questions concerning transition zone variations, particularly                                                                 30˚
                                                                                                                                                                                                                                                 5



                                                                                                                                                                                                                                                                                                                                                                                                                 P410
  finescale velocity structure above the 410-km discontinuity and                                                                                                                                                                                                                                                                                                                                                                                                                                                                                200                                                200                                                                         200




                                                                                                                                                                                                                                                                                                                                                                                                                                       τ(s)
                                                                                                                                                                                                                                                                10

  its relation to water flux.




                                                                                                                                                                                                                                                                                 15
                                                                                                                                25˚
                                                                                                                                  −135˚         −130˚      −125˚   −120˚   −115˚      −110˚      −105˚       −100˚        −95˚                                                                                                                                                                                                                                                                                                                  400                                                400                                                                         400
                                                                                                                                                                                                                                                                                                                                                                                                                                              100
• In this poster, we present wavefield continuation results for a                                                                                                                                                                           40˚

                                                                                                                                Figure 1: Reference map of western North America
  single Queen Charlotte Island earthquake recorded at a distance                                                               showing the extent of the transportable component of the
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                600                                                600                                                                         600




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   depth (km)




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      depth (km)




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              depth (km)
  of 5◦−26◦ across the western US.                                                                                              USarray network.
                                                                                                                                                                                                                                                     15

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                800                                                800                                                                         800

                                                                                                                                                                                                                                                                                                                                                 50
                                                                                                                                                                                                                                                                     20                                                                                                                                                                       150
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                1000                                               1000                                                                        1000


                  2. Wavefield continuation method to determine earth structure                                                                                                                                                             30˚
                                                                                                                                                                                                                                                     −130˚           −120˚            −110˚                                                           1/09/2008 M6.2 depth: 10.0 km QUEEN CHARLOTTE ISLANDS REGION                                  1/09/2008 M6.2 depth: 10.0 km QUEEN CHARLOTTE ISLANDS REGION                                1200                                               1200                                                                        1200



                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                1400                                               1400                                                                        1400
                                                                                                                                                                                                                                  Figure 4: Event to station azimuth of 115◦, (a) Reference map showing stations within 1.5◦ of projection line (red triangles) used to construct record section and the region corresponding to the
• Triplicate phases (i.e. pre-critical, critical, and post-critical re-                                                                                                                                                           ray bottoming depths between 410 km and 660 km depth (purple circles). Contour lines indicate distance in degrees away from epicenter. (b) Time-reduced record section. Red corresponds to
  flections/refractions) are generated when a wavefield bottoms                                                                                                                                                                     negative amplitude arrivals and blue corresponds to positive amplitude arrivals. (c). Slant-stack of record section showing the resulting τ − p curve.
  near a seismic discontinuity (Figure 2 and 3a.)                                                                              200                                                                             410
                                                                                                                                                                                                                     km
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              Figure 7: Azimuth = 115 ◦                           Figure 8: Azimuth = 130 ◦                                    Figure 9: Azimuth = 140 ◦
                                                                                                                                400
• The depth and impedance contrast of the discontinuity affects                                                                                                                                               660
                                                                                                                                                                                                                     km
                                                                                                                                                                                                                                        (a)                                                   (b)                                                                                  Distance (deg)
                                                                                                                                                                                                                                                                                                                                                                                                                                       (c)                                      slowness (s/deg)
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            • The above figures show τ (p) curves migrated in IASPEI91 P-wave velocity model (red line).
                                                                                                                                 600                                                                                                                                                                                                                  5              10               15               20                    25                     8                  10                   12                 14
  the amplitude, slowness and arrival time of the triplicate phases.                                                                 800                           10˚         15˚         20˚
                                                                                                                                                          5˚                                           25˚
                                                                                                                                                                                                                                                                                                                                                                                                                                               0                                           P410                               – To be implemented:
• Several different techniques have been developed to deduce                                                                               0˚                                                                        30˚

  earth structure from triplicate arrivals. In this study, we apply a                                                                                                                                                                                                                                                                            0                                                                                                                                                                              ∗ Update starting velocity model according to migrated image.
  wavefield continuation method. This method entails transform-                                                                  Figure 2: Upper mantle raypaths for far-regional events                                                                                                                                                                                                                                                                                                                                         ∗ Iterate process until migrated image matches input velocity model.




                                                                                                                                                                                                                                                                                               Reduced−travel time (s) (slowness=11.2 (s/deg))
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     Velocity (km/s)
                                                                                                                                                                                                                                           50˚
                                                                                                                                (10◦-28◦), highlighting the bottoming rays at the 410 km                                                                                                                                                                                                                                                                           P660
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               6        7        8         9           10   11        12

  ing composite record sections from the distance-time domain                                                                   and 660 km discontinuity. Paths are for the IASPEI91                                                                                                                                                                                                                                                          50
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           0



  (∆ − T ; Figure 3a) to the slowness-tau domain (p − τ ; Figure                                                                reference model.                                                                                                 5
                                                                                                                                                                                                                                                                                                                                                                                                                    P660
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               100



  3b) and finally to the velocity-depth domain (v − z; Figure 3c).                                                                                                                                                                                                                                                                                                                          P410                                                                                                                             • Alternative method - Damped least squares inversion:                                                                             200




                                                                                                                                                                                                                                                                                                                                                                                                                                       τ(s)
                                                                                                                                                                           Velocity (km/s)
                                                                                                                                                                                                                                                                10
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              – Digitize τ (p) curve along highest amplitude.                                                                                  300




                                                                                                                                                                                                                                                                                 15
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               400
                                                                                                                                                                   10      9          8            7                 6                                                                                                                                                                                                                        100                                                                             – Invert for model layer thicknesses for a given velocity model.




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 Depth (km)
                                                                                                                                                                                                                                           40˚
                                                                                                                                                          0                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    500
                                                                                   0
                                                                                                                                                                                                         Crust
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                ∗ Damped least squares inversion using data and model stan-
                  30                                                                                                   Crust                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   600


                                                           F
                                                                                                                                                                                                                                                     15
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  dard errors.                                                                                                                 700
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       Az=115°
                                                                                                                                                         200                                                                                                                                                                                     50                                                                                                                                                                           – Recalculate ray path turning point for updated velocity profile.                                                                                        Az=130°
  T − 11.2∆ (s)




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       Az=140°
                  40
                                                                    τ(s; T−p×∆)




                                                                                                                                            Depth (km)




                                                                                                                                                                                                                                                                     20                                                                                                                                                                       150                                                                                                                                                                                                              800
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       IASPEI
                                A                          D
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              – Iterate process until residual between input and predicted                                                                     900
                                                                                                                                                                                                                                           30˚
                                                                                  50             410 km
                  50        C
                                                                                                                                                         400                   410 km                                                                −130˚           −120˚            −110˚                                                           1/09/2008 M6.2 depth: 10.0 km QUEEN CHARLOTTE ISLANDS REGION                                  1/09/2008 M6.2 depth: 10.0 km QUEEN CHARLOTTE ISLANDS REGION                models is minimized.                                                                                                          1000

                                            B      660km
                                410km                                                                                                                                                              ∆ range
                                                                                                                   ∆ range
                                                                                                                    5° − 28°                                                                        5° − 28°                      Figure 5: Event to station azimuth of 130◦ , (a) Reference map (b) Time-reduced record section. (c). Slant-stack of record section showing the resulting τ − p curve.                                                                                                                                                                                                                              Figure 10: Velocity profiles after 4 iterations of direct
                  60                  E                                                                                                                  600
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     inversion method.
                                                                                                                                                                        660 km
                                                                                            660 km
                                                                                  100
                  70
                       10       15         20       25         30                       8   10   12      14       16    18       20                            8    10      12        14         16          18          20             (a)                                                   (b)                                                                                  Distance (deg)
                                                                                                                                                                                                                                                                                                                                                                                                                                       (c)                                      slowness (s/deg)
                                                                                                                                                                                                                                                                                                                                                      5               10                15                  20                    25                8                  10                   12                 14                               6. Preliminary results and future work
                                          ∆(deg)                                                      p (s/deg)                                                                  p (s/deg)
                                                                                                                                                                                                                                                                                                                                                                                                                                               0                                           P410
                                    Distance−time                                                       slowness−tau                                                                 velocity−depth
                                                                                                                                                                                                                                                                                                                                                 0

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            • One-dimensional velocity profiles derived from downward continuation of teleseismic wavefield show az-



                                                                                                                                                                                                                                                                                               Reduced−travel time (s) (slowness=11.2 (s/deg))
                                                                                                                                                                                                                                           50˚
 Figure 3: (a) Reduced-travel-time curves for a distance range of 10◦−30◦ (IASPEI91 reference model). The region of 410 km triplications are                                                                                                                                                                                                                                                                                                                                                                                  imuthal dependence using an earthquake from the Queen Charlotte region.
 highlighted. (b) The corresponding p − τ curve, where τ is the intercept-time. (c) The velocity-depth profile that results from migrating the p − τ curve.                                                                                                                                                                                                                                                                                                         P660
                                                                                                                                                                                                                                                                                                                                                                                                                                              50
                                                                                                                                                                                                                                                 5
                                                                                                                                                                                                                                                                                                                                                                                                                    P660
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            • Sampling points of these triplicated phases represent the nature of the transition zone beneath the Juan de
                                                                                                                                                                                                                                                                                                                                                                                         P410                                                                                                                                 Fuca plate.




                                                                                                                                                                                                                                                                                                                                                                                                                                       τ(s)
• Processing Methods                                                                                                                                                                                                                                            10

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            • Future Work
                                                                                                                                                                                                                                                                                 15




  – Azimuth swath record sections are created using a 2-D Gaussian operator similar to Neal and Pavlis                                                                                                                                     40˚
                                                                                                                                                                                                                                                                                                                                                                                                                                              100
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              – Process events from Mexico to sample the transition zone where significant water flux to the upper mantle
    (1999).                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     may exist.
  – No static corrections are applied.                                                                                                                                                                                                               15

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              – Use western U.S. P-wave velocity model (e.g. Burdick et al. 2008) for static corrections.
  – Seismograms are bandpass filtered at 0.01-1.5Hz.                                                                                                                                                                                                                  20
                                                                                                                                                                                                                                                                                                                                                 50
                                                                                                                                                                                                                                                                                                                                                                                                                                              150                                                                             – Process S wave triplications for shear velocity profiles.
  – Wavefield record sections are then transformed into slowness and intercept time using a slant stack oper-                                                                                                                               30˚
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              – Extend processing to 2-D where appropriate.
    ator over a 8 s/deg to 15 s/deg slowness range.                                                                                                                                                                                                  −130˚           −120˚            −110˚                                                           1/09/2008 M6.2 depth: 10.0 km QUEEN CHARLOTTE ISLANDS REGION                                  1/09/2008 M6.2 depth: 10.0 km QUEEN CHARLOTTE ISLANDS REGION


  – The slant-stack wavefield (τ − p image) is then downward continued (depth migrated) to a velocity depth                                                                                                                                                                                                                                                                                                                                                                                                                  References
                                                                                                                                                                                                                                                                             ◦
    profile. This is an iterative process seeking convergence between the input model and migrated wavefield.                                                                                                                       Figure 6: Event to station azimuth of 140 , (a) Reference map (b) Time-reduced record section. (c). Slant-stack of record section showing the resulting τ − p curve.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            Burdick, S., et al., Upper mantle hetergeneity beneath North America from travel time tomography with global and USArray Transportable Array data,
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              Seismological Research Letters, 79(3), 384–392, doi:10.1785/gssrl.79.3.384, 2008.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            Neal, S. L., and G. L. Pavlis, Imaging P-to-S conversions with multichannel receiver functions, Geophys. Res. Lett., 26(16), 2581–2584, 1999.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            Walck, M. C., and R. W. Clayton, Analysis of upper mantle structure using wave field continuation of P waves, Bulletin of the Seismological Society of
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              America, 74(5), 1703–1719, 1984.

								
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