Earthquakes in the Ocean: Where, Why, and What Happens?

1. 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)

2. 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

3. Seismology Map – Earthquake Locations and Depths

4. Volcanology Map – Recent Volcanic Activity

5. Geochronology Map – Seafloor Age

6. Geography Map – Topography and Bathymetry

7. Plate Boundary Map

8. Where do all the earthquakes occur?? (Press et al., Understanding Earth, 4th Ed.)

9. Kurile EarthquakeNov. 15, 2006 Juli

10. 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

11. Kurile EarthquakeNov. 15, 2006

12. Kurile Tsunami - Nov. 15, 2006

13. 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! M ~7.5 (2005) (Press et al., Understanding Earth, 4th Ed.)

14. 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

15. 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.

16. Alaska Tsunami (Press et al., Understanding Earth, 4th Ed.)

17. 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….

18. Sumatra Earthquake Sumatra Epicenter Indian Plate Fault zone rupture ~ 1000 km in length (Source: USGS)

19. 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). (Source: Prof. Miho Aoki, U. Alaska Fairbanks)

20. Tsunami Generation & Propagation (http://staff.aist.go.jp/kenji.satake/animation.gif) • Sumatra Tsunami (12/26/04) • red: rise in sea level • blue: fall in sea level

21. 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.)

22. What are the Consequences? (Source: Prof. Miho Aoki, U. Alaska Fairbanks) • Coastal damage and destruction.

23. Can This Happen in N. America?Yes!!

24. 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!

25. January 1700 Cascadia Tsunami (Source: K. Satake, http://www.pgc.nrcan.gc.ca/press/index_e.php)

26. Where do Great Earthquakes occur? Rocks Sediments Tsunamigenic Slip Aseismic Slip Coseismic Slip Seismogenic Zone • Plate boundary mega-thrust Why do Great Earthquakes occur?

27. 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).

28. 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)

29. Co-seismic Slip Zone (Bangs et al., 2004) Up-dip limit

30. Recent Ocean Drilling & Surveys

31. Toe of Muroto Transect NANKAI PRISM PROTO-THRUST ZONE NANKAI TROUGH Frontal thrusts Proto- thrusts Deformation front 1 km Depth (m) Trench Fill turbidites Shikoku Basin Strata hemipelagic sediments Proto-decollement Decollement Ocean Crust

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

33. Décollement Amplitude • Downdip decrease in décollement amplitude. • Suggests dewatering of underthrust section. (Bangs et al., 2004)

34. Pore Fluid Pressures Seismogenic Zone Coseismic Zone Updip limit (after G.F. Moore et al., 2001) (J.C. Moore and Saffer, 2001) • Modeled pore fluid pressures are highest near up-dip limit.

35. 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….