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

As prepared for ESCI 323 - Earth Structure & DeformationAnd Sally Ride Festival, Houston (10/25/06)

Prof. Julia MorganDr. Meghan Miller

Department of Earth ScienceRice University

Dale Sawyer’s Discovering Plate Boundaries

Exercise(http://terra.rice.edu/plateboundary)

An aside:

• 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 EarthquakeNov. 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

Ocean-oceanOcean-ocean

Sumatra earthquake!M ~9.3 (2004)

Ocean-continentOcean-continent

Chile earthquake!M ~9.5 (1960)

Continent-continent Continent-continent

South Asian earthquake!M ~7.5 (2005)

In the Oceans

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

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

Chile - 5/22/1960Chile - 5/22/1960(Source: Sawyer, Discovering Plate Boundaries)

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

Alaska - 3/27/1964Alaska - 3/27/1964(Source: Sawyer, Discovering Plate Boundaries)

Alaska TsunamiAlaska Tsunami

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

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

Southeast Asia - 12/26/2004Southeast Asia - 12/26/2004(Source: Sawyer, Discovering Plate Boundaries)

Sumatra EarthquakeSumatra Earthquake

Fault zone ruptureFault zone rupture~ 1000 km in length~ 1000 km in length

EpicenterEpicenter

Sumatra

Indian Plate

(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 aSorenson Video decompressorare needed to see this picture.

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

Tsunami Generation & PropagationTsunami Generation & Propagation

Sumatra Tsunami (12/26/04)

- red: rise in sea level- blue: fall in sea level

QuickTime™ and aGIF decompressor

are needed to see this picture.

(http://staff.aist.go.jp/kenji.satake/animation.gif)

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?

• Coastal damage and destruction.

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

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

Can This Happen in Can This Happen in N. America?N. America?

Yes!!Yes!!

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

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

January 1700 Cascadia Tsunami

QuickTime™ and aGIF decompressor

are needed to see this picture.

(Source: K. Satake, http://www.pgc.nrcan.gc.ca/press/index_e.php)

Where do Great Earthquakes occur?

Seismogenic Zone

Coseismic Slip

Aseismic Slip

Tsunamigenic Slip

• Plate boundary mega-thrust

SedimentsRocks

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-diplimit

Recent Ocean Drilling & Surveys

Toe of Muroto Transect

Decollement

1 km

Proto-decollement

Deformationfront

Proto-thrusts

Frontal thrusts

NANKAI TROUGHNANKAI PRISM PROTO-THRUSTZONE

Dep

th (

m)

Trench Fillturbidites

Shikoku Basin Strata

hemipelagic sediments

Ocean Crust

(J.C. Moore and Saffer, 2001)

Prism Architecture

• Décollement partitions the incoming section: - Accreted section vs. underthrust section.

• Décollement steps down near the updip limit.

Seismogenic Zone Coseismic Zone Aseismic Zone

(after G.F. Moore et al., 2001) Up-diplimit

Décollement Amplitude

(Bangs et al., 2004)

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

(J.C. Moore and Saffer, 2001)

Pore Fluid Pressures

• Modeled pore fluid pressures are highest near up-dip limit.

(after G.F. Moore et al., 2001)

Seismogenic Zone Coseismic Zone

Updip 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….