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					Subduction tectonics:
Earthquake cycle and
long-term deformation
                            Charles DeMets
                           Dept. of Geology &
                              Geophysics
                           Univ. of Wisconsin-
                                 Madison
                        Acknowledgments to
                        Francisco Correa-Mora
                        and Stuart Schmitt, who
                        developed some of the
                        graphics below as advisees
                        of C. DeMets at UW-
                        Madison.
               Presentation goal and outline

Goal: Develop useful spatial and temporal frameworks
   for students to understand short-term and long-
   term deformation related to subduction

Outline:
•   2-D & 3-D spatial models for shallow subduction
•   Describe tectonic processes in subduction zones
•   Characterize subduction earthquake cycle
•   Categorize types of long-term deformation
Note to participants: The majority of this presentation focuses on short-term processes
      that influence subduction zone tectonics (earthquake cycle). The latter third or so
      deals with long-term deformation, but uses GPS measurements (short-term once
      again!) to reveal one example of long-term upper plate deformation.
                  Conceptual model – 2D




Major components of the “system”
1. Upper plate (free to move in any direction - shown as fixed above)
2. Subducting plate – moves downward into mantle
3. Subduction interface – contact zone between upper and subducting
   plates. Frictional “surface” along which thrust earthquakes occur.
   Interface is subdivided into free-slip, seismogenic, and transitional zones.
   These are differentiated by their respective frictional properties.
         3-D spatial model – layer properties
                       1. Upper layers – accumulate and release strain elastically
                       2. Lower layers – respond to stress “jump” via protracted
                          viscous flow.
Subduction interface
       2D model – Earthquake cycle processes – Seismogenic zone

          Zone of interseismic locking




1. Seismogenic zone is locked by friction during long inter-earthquake periods (decades
   to centuries) – no motion between upper and subducting plates. Crust accumulates this
   “deficit” in slip elastically. Unrelieved slip accumulates at rates of mm to tens of mm per
   year.
2. Earthquake eventually ruptures seismogenic zone and recovers most or all of the
   interseismic slip deficit – meters of slip occur in just seconds to tens of seconds
          2D model (continued) – After an earthquake

                    Triggered by earthquake




Earthquakes “trigger” three transient processes in the crust and mantle.
•   Fault afterslip – a logarithmically decaying dynamic frictional response DOWNDIP from the
    seismogenic zone – manifested at surface as log-decay elastic response. Can equal 50-100% of the
    coseismic elastic deformation! Requires weeks to years to decay away.
•   Viscoelastic flow – coseismic stress jump in the viscoelastic lower crust and upper mantle triggers
    protracted FLOW in these regions, measured at surface as exponentially-decaying deformation.
    Continues for decades after large events.
•   Poroelastic deformation – coseismic volumetric changes in crust alters pore volumes and forces
    fluid flow, which in turn causes measurable surface deformation. Weeks to months.
Spatio-temporal model for
subduction earthquake cycle

Upper diagram shows movement of a
hypothetical GPS site through the seismic
cycle.

 - Interseismic – superposition of steady elastic
strain accumulation across locked seismogenic
zone due to steady plate convergence and
occasional short-duration aseismic strain release
across frictionally-transitional zone downdip from
locked region. Free slip areas contribute nothing
to surface deformation

- Coseismic – Rapid opposite-direction release
of accumulated elastic strain with slip dominantly
along seismogenic zone

 - Postseismic – Superposition of triggered
afterslip in transitional region and viscoelastic flow
in mantle wedge and lower crust and poroelastic
response in fluid-bearing regions of crust. Decays
through time back to steady strain accumulation.
     Continuous GPS along          Conceptual model
      Mexican Pacific coast
 illustrates different phases of
        the seismic cycle

Oaxaca CGPS




                                        Jalisco CGPS
Dense GPS network
samples seismic cycle
deformation in
southern Mexico
The next 5 slides show
our imaging of the
spatial relationship
between interseismic
locking and transient
strain beneath Mexico
from Cocos plate
subduction.
Ph.D. research of F. Correa-
Mora advised by C. DeMets
Shallow regions of most subduction interfaces are characterized by
occurrence of large shallow-dipping thrust earthquakes that define the
seismogenic zone.
3-D modeling of a
subduction zone
permits different
material properties to
be assigned to different
layers and zones, e.g.
oceanic crust is
“stiffer” than
continental crust and
hence has a diminished
elastic response. Here,
a dense 3-D mesh
simulates the geometry
of the Middle America
subduction zone in the
study area of southern
Mexico.
Continuous and annual measurements of ~30 bedrock geodetic pins in
the region with GPS are used to establish their motions through time.
In the following two slides, I show results from inverting these GPS
motions to estimate the location and magnitude of frictional locking along
the subduction interface.


continuous site example      „01-‟07 velocity field
Left – (A) location
and magnitude of
TRANSIENT slip in
2004. White dashed
lines indicate areas
of earthquake
rupture in 1968 and
1978 mega-thrust
earthquakes
(defines
seismogenic zone).
(B) Location and
magnitude of
INTER-SEISMIC
frictional LOCKING.
Note that LOCKING
occurs across
seismogenic AND
downdip zones
(C) Slip magnitude
and location during
2006 transient
event. Note
similarity to 2004
result!
Inversion of GPS site motions during a transient slip event in 2004
       to define location and magnitude of the transient slip
                  from Brudzinski et al. (2006) GJI

Note that slip is DEEP – well downdip from the seismogenic zone.
              Reinforces results from previous slide.
     Synoposis of seismic cycle (short-term)
                  deformation

1.   Period of steady interseismic strain accumulation that
     ends with major thrust earthquake also includes
     occasional aseismic releases of elastic strain that has
     accumulated downdip from seismogenic zone.
2.   Earthquakes trigger three post-seismic responses.
     Separating the three decaying processes (afterslip,
     viscoelastic flow, and poroelastic deformation) from
     each others is a “challenging” modeling exercise and is
     frequently non-unique.
     What about long-term deformation ?

Thus far, we have focused largely on elastic and
  thus recoverable deformation, which leaves little
  or no long-term permanent record. But UPPER
  plates clearly deform in a permanent manner
  (faulting, folding, uplift, subsidence) inboard from
  subduction zones. Geologists are more
  frequently interested in the long-term
  deformation record, as I imagine many of you
  may be…..
Let‟s quickly review the three end-member types of upper-plate
    deformation and their causes….
1. Upper plate shortening (mountain building) - Possible causes: Rapid
   trenchward motion of upper plate, overrides subducting plate,
   associated with shallow subduction, deformation far inboard from
   trench. Possible other cause – collision of seamount, oceanic plateau,
   or continental fragment traveling on subducting plate with the trench.
2. Upper plate extension (back-arc spreading) – Possible cause: Upper
   plate motion AWAY from trench induces upper plate extension.
   Possibly trench roll-back, but any lateral motion of a subducting slab
   must PUSH a great deal of mantle out of its path. Is roll-back possible?
3. Coast-parallel sliver transport – Possible cause: Oblique subduction
   and partitioning of obliquity into trench-normal subduction and trench-
   parallel upper-plate shear. Other cause – coastwise lateral escape
   from collision zone between a buoyant subducting feature with
   trench/upper plate.

   Let’s focus on the third of these, which is
   relevant to parts of the Middle America
   trench…..
Tectonic setting
of Nicaragua/El
Salvador segment of
Middle America
subduction zone.
- oblique subduction
 - strain partitioning
yields sinistral trench-
parallel forearc shear
 - Basin and Range-like
extension in Honduras,
Guatemala, El Salvador
 - strike-slip tectonics
along Motagua-Polochic
faults (CA-NA plate
boundary)
         2000-2005 GPS velocity field – CA plate fixed.
(1) cent/eastern Hond/Nic sites are on CA interior. (2) forearc slip obvious, (3) E-W
                                 stretching obvious
 (Proprietary results from ongoing M.S. research by D. Alvarado and M. Rodriguez – UW-
                                         Madison)
  Dramatic difference in
onshore character of GPS
   velocity fields along
shallow-dipping Mexican
 segment of the MAT and
 steeply-dipping Central
     American MAT.




               Oaxaca




                           El Salvador
   Oaxaca segment                    Prediction of fully coupled elastic model
   moderate to strong
    frictional coupling
           (~75%)
 inferred from measured
        site motions
    High EQ hazard
                            50% coupling



Trench-normal
GPS site motions
                                               El Salvador/Honduras

   Salvador segment
  weak or zero frictional
  coupling inferred from
   measured GPS site
         motions

   Low EQ hazard ?
Nicaraguan GPS sites also move parallel to coast – no inland
   component of motion - indicates weak coupling across
   subduction interface
Absence of subduction “overprint” on onshore velocities affords
     clear view of upper plate LONG-TERM deformation!
   Conclusions: Long-term deformation

• Deformation of upper plate is dictated in part by upper
  plate motion relative to the trench. The subducting plate
  and hence trench cannot migrate laterally with ease and
  thus stays in place no matter how upper plate moves.
• Deformation also depends on geometry of trench
  (bends), buoyant oceanic or continental fragments that
  are being subducted (or not), and RELATIVE direction of
  upper and subducting plate convergence (obliquity).

       THE END: (A GPS appendix follows, but is provided for GPS
       novices)
  2-slide GPS appendix for skeptics – proof of
                  technique
If one or more of you are unfamiliar with the application of
    GPS geodesy to crustal deformation research, the
    following two slides show results from processing 24-
    hour continuous GPS station data to high precision in
    order to measure changes through time in the absolute
    coordinates and height of the fixed GPS monument.
Graphic 1 shows that the daily site coordinates show
    random scatter superimposed on linear motion (well
    behaved).
Graphic 2 illustrates the remarkable velocity pattern defined
    by numerous GPS sites, representing a powerful proof of
    the concept that GPS can be used to map plate motions
    and other forms of crustal deformation
                       Using GPS tracking to
                           monitor plate
                            movements




Technical background
                       GPS Site motions

                       - Raw GPS processing done at UW-
                       Madison
                       - Continuous GPS station motions
                       relative to ~mantle-fixed reference
                       frame (above)
Technical background   - Motion around and toward best pole
                       of rotation (left)

				
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posted:2/28/2010
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