Sendai Earthquake
NE Japan
March 11, 2011
Some explanatory slides
Bob Stern, Dave Scholl, others
updated March 14 2011
• Earth has 11 large plates and many more smaller ones.
• Plates are 100-200 km thick and up to many thousands of km
across.
• Plates are moving at speeds of ~1 - 10 cm/yr.
Diagram summarizing principal
features of plate tectonics
Subduction Zone
Note that places where plates come together (convergent
plate boundaries) are places where the plate sinks deep into
Earth’s mantle; Isaacs et al. 1968
Tectonics is the study of how
Earth’s outer rind deforms
and affects humans.
The circum-Pacific is the
most tectonically active
region on the planet.
Japan
USA
“Circum-Pacific Ring
of Fire” should be
“Circum-Pacific Ring
of Earthquakes
Pacific plate is by far the fastest plate, moving towards Subduction
Zones in the western Pacific, including the one beneath NE Japan
Thanks to T. Becker, USC
Earthquakes define plate boundaries
Japan
Pacific Ocean
Magnitude > 4, 1961-1967; Barazangi & Dorman, 1969;
0-70 km deep = shallow; 70-300 km deep = intermediate; >300 km deep = deep. Spreading
ridge EQ are always shallow and focal mechanisms indicate extension; similar EQ for
continental rifts. Transform EQ are shallow and generally have strike-slip mechanisms, but
sometimes show compression or extension. All intermediate and deep EQs are associated
with subduction zones, like that beneath NE Japan.. Deepest EQ are ~670 km. JapaSendai
earthquake was shallow, which is one of the reasons that it was so damaging.
Plate Tectonic setting of Japan
Japan lies above two subduction
zones; one beneath NE Japan due to
subduction of Pacific plate (green lines
show depth), the other beneath SW
Japan subducting the Philippine Sea
plate (dashed lines show depth).
Modified from Kimura and Stern 2008
Seismic Tomography and earthquakes show the
Subduction Zone beneath NE Japan
Honshu
Sea of Japan
Pacific
Ocean
0
Moho
100
Colors show
seismic velocity
differences;
sinking Pacific
plate is cold and
has relatively fast
seismic velocities
(blue color).
Open circles =
earthquake foci
-4%
0%
Relative P-wave velocity
5%
Zhao et al., 1994
NE Japan Trench
Eurasian Plate
(Japan)
Seismic reflection imaging
Pacific Plate
Tsuru et al. JGR
Seismogenic Zone
The biggest earthquakes occur in subduction zones at depths of ~20-50km (12-30
miles)
The Seismogenic Zone is especially dangerous because of the kind of
earthquakes and because it is near the surface, beneath coastal areas.
Sendai earthquake was caused by massive slip on a very shallow part
of the subduction zone interface. This part of the subduction zone
generates the most violent earthquakes (magnitudes 8 and 9) and is
called “the Seismogenic Zone”.
Washington Post
Concept of earthquake epicenter is misleading in a quake this big; in fact a very large
region ruptured, outlined by the dashed box.
NY Times
Visualization of main shock
http://seismology.harvard.edu/research_japan.html
Map is ~300 miles N-S. Rupture of shallow subduction interface takes 2.5
minutes, moves from N to S
Earthquake (red beach ball) with respect to the NSF-supported EarthScope USArray
Transportable Array (390 stations, orange region in central USA).
* *
Earthquake
energy
release
*
*
The duration of the event is
inferred to be about 150 seconds.
Normalized peak amplitude as a function of time, showing four significant
episodes of high-frequency energy release at about 50, 80, 95, and 120
seconds.
Harvard seismology
Sendai earthquake “focal plane solution”
1/03/11 5:46:23
Epicenter: 38.32°N
142.37°E
MW 9.0
Fault plane orientation:
Strike=193° Dip=14° W
Upper plate was thrust up
relative to upper plate
From USGS
1&4
2&3
1) Plate convergence is continuous but the two plates are locked across the
plate interface. This causes uplift and compression of the overlying plate
margin (forearc).
2) Strain builds up until it exceeds the strength of the fault; the locked zone
breaks and a great earthquake occurs.
3) During the rupture, the built-up strain is released. This lets the upper plate
relax. Subsidence & horizontal extension occurs in regions that were uplifted &
compressed previously.
4) Cycle starts over
modified from http://gsc.nrcan.gc.ca/geodyn/eqcycle_e.php
MODIS images
of Sendai region.
Note extensive
coastal area that
subsided below
sealevel.
Upward movement of the seafloor above the rupture pushed water and made a
tsunami around Pacific.
NY Times
Lots of aftershocks as
the plate boundary
settles down to a new
equilibrium. Interesting
earthquakes east of
trench (yellow line).
Concern is now that next
rupture will occur N or S
of region that slipped on
March 11.
British Geological Survey
“Roughly 80 % of all great megathrust earthquakes (Mw>8.5) occur along subduction
zones where the downgoing plate has thick sediments (>1-2 km thick in the trench).
Subduction of such a thick sediment layer smooths out the interface between the two
plates. This favors the long, lateral continuation of rupturing that is characteristic of
giant megathrust events (Ruff, 1989).
From Dave Scholl: But the Sendai
Mw8.9 ruptured along a sector of the
Japan Trench that is not thickly
sedimented. In this way it's like the
northern Chile and also Kamchatka
subduction zones, two other poorlysedimented subduction zones that
have had similar ruptures and
earthquakes (Mw in the high 8s to
9). A common characteristic of all
three locales is that the rupturing
occurs opposite a bathymetrically
smooth subducting seafloor. This
also appears to be the circumstance
for the sector of the Japan Trench
that broke. The Sendai event ripped
mainly to the south and appears to
have terminated at the
underthrusting Kashima seamount
chain. Termination of rupturing at a
subducting ridge of seamount chain
is also typical of megathrust events.