Download Source: Sawyer, Discovering Plate Boundaries

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
(Press et al., Understanding Earth, 4th Ed.)
South Asian earthquake!
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
Fault zone rupture
~ 1000 km in length
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 a
Sorenson Video decompressor
are needed to see 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 needed 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
Sorenson Video decompressor
are needed 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 needed 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
Seismogenic
Zone
Coseismic
Slip
Aseismic
Slip
• 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
Frontal thrusts
PROTO-THRUST
ZONE
Protothrusts
NANKAI TROUGH
Deformation
front
Trench Fill
turbidites
Decollement
Proto-decollement
Ocean Crust
1 km
Prism Architecture
Seismogenic Zone
and Zone
Coseismic Zone(J.C. Moore
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….