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					Earthquakes in the Ocean:
 Where, Why, and What Happens?
         Prof. Julia Morgan
         Dr. Meghan Miller
     Department of Earth Science
           Rice University

              As prepared for
 ESCI 323 - Earth Structure & Deformation
 And Sally Ride Festival, Houston (10/25/06)
                   An aside:

           Dale Sawyer’s
    Discovering Plate Boundaries
              Exercise
    (http://terra.rice.edu/plateboundary)
•   Designed for students to observe & classify
•   Appropriate for broad range of ages/exper.
•   Designed for 3 days, w/ variations
•   Excellent framework for MARGINS & similar
Seismology Map – Earthquake Locations and Depths
Volcanology Map – Recent Volcanic Activity
Geochronology Map – Seafloor Age
Geography Map – Topography and Bathymetry
Plate Boundary Map
Where do all the earthquakes occur??




                    (Press et al., Understanding Earth, 4th Ed.)
       Kurile Earthquake
        Nov. 15, 2006




Juli
Tectonic Setting
   - Kuriles
• Subduction of Pacific
  Plate Plate beneath
  Okhotsk/N. America.
• Conv. rate: ~ 9 cm/yr.
• Sakhalin Island &
  Japan are rifted cont.

Political Setting
• Russia “stole” from
  Japan after WW II
  Kurile
Earthquake
 Nov. 15,
   2006
 Kurile
Tsunami -
Nov. 15,
  2006
 Convergent Boundaries: Three Types
In the Oceans
                                                               Ocean-ocean
                                                            Sumatra earthquake!
                                                               M ~9.3 (2004)

                                                             Ocean-continent
                                                              Chile earthquake!
                                                               M ~9.5 (1960)


                                                           Continent-continent
                                                           South Asian earthquake!
            (Press et al., Understanding Earth, 4th Ed.)        M ~7.5 (2005)
                        Chile - 5/22/1960
(Source: Sawyer, Discovering Plate Boundaries)




M 9.5 Earthquake (Largest worldwide)
• Rupture zone ~1000 mi long.
• Generated a tsunami that spread across Pacific.
• Run-up: 25 m in Chile; 10.7 in Hilo; 1.7 in CA.
• ~ 6000 people died, most from tsunami.
• $3.5 Billion property damage
                      Alaska - 3/27/1964
(Source: Sawyer, Discovering Plate Boundaries)




  M 9.2 Earthquake (3rd or 4th largest worldwide)
• 750 mi long rupture zone.
• Large landslide and tsunami across Pacific.
• Ground displacements 25 ft x 3 ft.
• 122 people died worldwide, most from tsunami.
• Property damage $500 million.
Alaska Tsunami




         (Press et al., Understanding
               Earth, 4th Ed.)
           Southeast Asia - 12/26/2004
(Source: Sawyer, Discovering Plate Boundaries)




  M 9.3 Earthquake (2nd largest worldwide)
  • 1000 km long fault rupture.
  • Generated a tsunami that went around the world.
  • No warning.
  • More than 200,000 people died.
  • Untold property damage….
  Sumatra Earthquake




                                Sumatra

Epicenter
                            Indian
     Fault zone rupture      Plate
      ~ 1000 km in length
                               (Source: USGS)
             What is a tsunami &
             How does it form?
• A sudden impulse pushes the water, forming a train of
  waves that spreads outward from the source.
• Tsunami front propagates at ~450 mi/hr (~600 km/hr).




                              QuickTime™ and a
                       Sore nson Video decompressor
                       are neede d to se e this picture.




              (Source: Prof. Miho Aoki, U. Alaska Fairbanks)
 Tsunami Generation & Propagation
                            (http://staff.aist.go.jp/kenji.satake/animation.gif)




                                               QuickTime™ and a
                                               GIF decompressor
                                         are neede d to see this picture.




Sumatra Tsunami
   (12/26/04)
- red: rise in sea level
- blue: fall in sea level
       What Happens Nearshore?
• As the water column shallows, wave height grows.
• Tsunami run-ups can be MUCH larger than original
  wave.
                            (Press et al., Understanding
                                  Earth, 4th Ed.)
What are the Consequences?
       (Source: Prof. Miho Aoki, U. Alaska Fairbanks)




                  QuickTime™ and a
           Sore nson Video decompressor
           are need ed to see this picture.




• Coastal damage and destruction.
Can This Happen in
   N. America?

      Yes!!
      Oregon-Washington - 1/26/1700
(Source: Sawyer, Discovering Plate Boundaries)




  M ~9 Cascadia Earthquake (7th largest?)
  • No historic record in North America.
  • Recorded in Japan ~9 hrs later.
  • Tsunami deposits found along Oregon coast.
  • Recurrence interval is 300-500 yrs!
January 1700 Cascadia Tsunami




                           QuickTime™ and a
                           GIF decompressor
                     are neede d to see this picture.




  (Source: K. Satake, http://www.pgc.nrcan.gc.ca/press/index_e.php)
Where do Great Earthquakes occur?

                   Rocks              Sediments

                               Tsunamigenic
                                   Slip



                                       Aseismic
                         Coseismic       Slip
                           Slip
           Seismogenic
               Zone

           • Plate boundary mega-thrust

Why do Great Earthquakes occur?
Where can we
 study great
earthquakes?
Nankai Trough
• Subduction of the
  Philippine Sea Plate
  beneath Eurasia.
• Convergence rate:
  ~4 cm/yr.
• Thick clastic-rich
  sediment section
  (hemipelagites and
  turbidites).
                     Earthquake
                     Recurrence
               • Nankai margin is subject
                 to repeated large (M ~ 8)
                 earthquakes.
               • Evidence for tectonic
                 segmentation of the
                 margin.
               • Earthquake recurrence
                 ~180 yrs.

                 -> There will be future
                 earthquakes!!
(Ando, 1975)
               Co-seismic Slip Zone
(Bangs et al., 2004)




                                      Up-dip
                                       limit
Recent Ocean Drilling & Surveys
              Toe of Muroto Transect
NANKAI PRISM         PROTO-THRUST               NANKAI TROUGH
                         ZONE
 Frontal thrusts      Proto-          Deformation
                      thrusts            front                     1 km

                   Trench Fill
                    turbidites




                                               Proto-decollement
        Decollement
                                 Ocean Crust
                         Prism Architecture




  Seismogenic Zone                      (J.C. Moore and Zone
                           Coseismic Zone         Aseismic
                                        Saffer, 2001)
  (after G.F. Moore et al., 2001)        Up-dip
                                          limit
• Décollement partitions the incoming section:
  - Accreted section vs. underthrust section.
• Décollement steps down near the updip limit.
             Décollement Amplitude




                                            (Bangs et al., 2004)

• Downdip decrease in décollement amplitude.
• Suggests dewatering of underthrust section.
                       Pore Fluid Pressures




    Seismogenic Zone         Coseismic Zone

  (after G.F. Moore et al., 2001)             Updip limit
                                                            (J.C. Moore and Saffer, 2001)


• Modeled pore fluid pressures are highest near up-dip
  limit.
   Seismic-Aseismic Transition
     -> Onset of Earthquakes
• Coincident with onset of coseismic sliding:
   –   Out of sequence thrust.
   –   Step down in décollement.
   –   Reduced décollement amplitude.
   –   Increased pore fluid pressures.
• Why? Something changes along décollement:
   – Frictional behavior: stable vs. unstable?
   – Loss of strength in underthrust sediments?
   – Something completely different?

  Future fault zone drilling may tell us….

				
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