Transcript Tsunami - Department of Earth and Planetary Sciences

DECEMBER 2004 INDIAN OCEAN EARTHQUAKE
AND TSUNAMI
BASIC CONCEPTS: RIGID PLATES
Earth's outer shell made up of ~15 major rigid plates ~ 100 km thick
Plates move relative to each other at speeds of a few cm/ yr (about
the speed at which fingernails grow)
Plates are rigid in the sense that little (ideally no) deformation occurs
within them,
Most (ideally all) deformation occurs at their boundaries, giving rise to
earthquakes, mountain building, volcanism, and other spectacular
phenomena.
Style of boundary and intraplate deformation depends on direction &
rate of motion, together with thermo-mechanical structure
BASIC
CONCEPTS:
THERMAL
EVOLUTION OF
OCEANIC
LITHOSPHERE
Stein &
Wysession
2003
Warm mantle material upwells at spreading centers and then cools
Because rock strength decreases with temperature, cooling material
forms strong plates of lithosphere
Cooling oceanic lithosphere moves away from the ridges, eventually
reaches subduction zones and descends in downgoing slabs back into
the mantle, reheating as it goes
Lithosphere is cold outer boundary layer of thermal convection system
involving mantle and core that removes heat from Earth's interior,
controlling its evolution
INDIAN PLATE MOVES NORTH
COLLIDING WITH EURASIA
Gordon & Stein, 1992
COMPLEX
PLATE
BOUNDARY
ZONE IN
SOUTHEAST
ASIA
Northward motion of
India deforms all of
the region
Many small plates
(microplates) and
blocks
Molnar & Tapponier, 1977
India subducts
beneath Burma
microplate
at about 50 mm/yr
Earthquakes occur
at plate interface
along the Sumatra
arc (Sunda trench)
These are
spectacular &
destructive results of
many years of
accumulated motion
INTERSEISMIC:
India subducts beneath
Burma microplate
at about 50 mm/yr
(precise rate hard to infer
given complex geometry)
Fault interface is locked
EARTHQUAKE
(COSEISMIC):
Fault interface slips,
overriding plate
rebounds, releasing
accumulated motion
HOW OFTEN:
Stein & Wysession, 2003
Fault slipped ~ 10 m = 10000 mm / 50 mm/yr
10000 mm / 50 mm/yr = 200 yr
Longer if some slip is aseismic
Faults aren’t exactly periodic for reasons we don’t understand
MODELING
SEISMOGRAMS
shows how slip varied
on fault plane
Maximum slip area
~400 km long
Maximum slip ~ 20 m
Stein & Wysession
TWO VIEWS OF THE PART OF THE SUMATRA
SUBDUCTION ZONE THAT SLIPPED
ERI
C. Ji
Seismogram analysis shows
most slip in southern 400 km
Aftershocks show slip
extended almost 1200 km
Earthquakes rupture a patch
along fault's surface.
Generally speaking, the
larger the rupture patch, the
larger the earthquake
magnitude.
Initial estimates from the
aftershock distribution show
the magnitude 9.3 SumatraAndaman Islands Earthquake
ruptured a patch of fault
roughly the size of California
For comparison, a magnitude
5 earthquake would rupture
a patch roughly the size of
New York City's Central Park.
NORMAL MODES
(ULTRA-LONG
PERIOD WAVES)
SHOW SEISMIC
MOMENT 3 TIMES
THAT INFERRED
FROM SURFACE
WAVES
IMPLIES SLIP ON
AREA 3 TIMES
LARGER
Entire 1200-km
long aftershock
zone likely slipped
0S2
YIELDS SEISMIC
MOMENT Mo =
1 x 1030 dyn-cm
2.5 TIMES BIGGER THAN
INFERRED FROM 300-s
SURFACE WAVES
CORRESPONDING MOMENT
MAGNITUDE Mw IS 9.3,
COMPARED TO 9.0 FROM
SURFACE WAVES
Comparison of fault areas,
moments, magnitudes,
amount of slip shows this was
a gigantic earthquake
“the big one”
IF ENTIRE ZONE
SLIPPED, STRAIN
BUILT UP HAS
BEEN RELEASED,
LEAVING LITTLE
DANGER OF
COMPARABLE
TSUNAMI
Risk of local tsunami
from large aftershocks
or oceanwide tsunami
from boundary
segments to south
remains
EARTHQUAKE MAGNITUDE 9.3
Stein & Wysession after IRIS
One of the largest earthquakes since seismometer invented
~ 1900
SUCH GREAT
EARTHQUAKES
ARE RARE
Stein & Wysession, 2003
SOME MAJOR DAMAGE DONE BY EARTHQUAKE SHAKING ITSELF,
BUT STRONG GROUND MOTION DECAYS RAPIDLY WITH DISTANCE
0.2 g
Stein & Wysession, 2003
DAMAGE DEPENDS ON BUILDING TYPE
RESISTANT CONSTRUCTION REDUCES EARTHQUAKE RISKS
0.2 g
Damage
onset for
modern
buildings
“Earthquakes don't kill people; buildings kill people."
Coburn &
Spence 1992
TSUNAMI - water wave generated by earthquake
NY Times
TSUNAMI GENERATED ALONG FAULT, WHERE SEA
FLOOR DISPLACED, AND SPREADS OUTWARD
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Hyndeman and Wang, 1993
Red - up motion, blue down
http://staff.aist.go.jp/kenji.satake/animation.gif
TSUNAMI SPEED IN
DEEP WATER of
depth d
c = (gd)1/2
g = 9.8 m/s2 d = 4000 m
c = 200 m/s = 720 km/hr
= 450 m/hr
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Tsunami generated
along fault, where sea
floor displaced, and
spreads outward
Reached Sri Lanka in 2
hrs, India in 2-3
http://staff.aist.go.jp/kenji.satake/animation.gif
WAVE PATH GIVEN BY SNELL’S LAW
Going from material with speed v1 to speed v2
Angle of incidence I changes by
sin i1 / v1 = sin i2 / v2
SLOW
FAST
Stein & Wysession
Tsunami wave bends as water depth & thus speed changes
TRACE RAY
PATHS USING
SNELL’S LAW
RAYS BEND
AS WATER
DEPTH
CHANGES
FIND WHEN
WAVES
ARRIVE AT
DIFFERENT
PLACES
DENSITY OF
WAVES
SHOWS
FOCUSING &
DEFOCUSING
1 hour
Woods & Okal, 1987
NOAA
IN DEEP OCEAN tsunami has long wavelength, travels fast,
small amplitude - doesn’t affect ships
AS IT APPROACHES SHORE, it slows. Since energy is
conserved, amplitude builds up - very damaging
TSUNAMI WARNING
Deep ocean buoys can measure
wave heights, verify tsunami and
reduce false alarms
Because seismic waves travel much
faster (km/s) than tsunamis, rapid
analysis of seismograms can identify
earthquakes likely to cause major
tsunamis and predict when waves will
arrive
HOWEVER, HARD TO PREDICT EARTHQUAKES
recurrence is highly variable
Sieh et al., 1989
Extend earthquake history
with geologic records paleoseismology
M>7 mean 132 yr s 105 yr
Estimated probability in 30 yrs 7-51%
EARTHQUAKE RECURRENCE
AT SUBDUCTION ZONES IS
COM PLICATED
In many subduction zones, thrust
earthquakes have patterns in
space and time. Large
earthquakes occurred in the
Nankai trough area of Japan
approximately every 125 years
since 1498 with similar fault areas
In some cases entire region
seems to have slipped at once; in
others slip was divided into
several events over a few years.
Repeatability suggests that a
segment that has not slipped for
some time is a gap due for an
earthquake, but it’s hard to use
this concept well because of
variability
GAP?
NOTHING YET
Ando, 1975
EARTHQUAKE PREDICTION?
Because little is known about the fundamental physics of faulting, many
attempts to predict earthquakes searched for precursors, observable behavior
that precedes earthquakes. To date, search has proved generally unsuccessful
In one hypothesis, all earthquakes start off as tiny earthquakes, which happen
frequently, but only a few cascade via random failure process into large
earthquakes
This hypothesis draws on ideas from nonlinear dynamics or chaos theory, in
which small perturbations can grow to have unpredictable large consequences.
These ideas were posed in terms of the possibility that the flap of a butterfly's
wings in Brazil might set off a tornado in Texas, or in general that minuscule
disturbances do not affect the overall frequency of storms but can modify when
they occur
If so, there is nothing special about those tiny earthquakes that happen to
grow into large ones, the interval between large earthquakes is highly variable
and no observable precursors should occur before them. Thus earthquake
prediction is either impossible or nearly so.
“It’s hard to predict earthquakes, especially before they happen”
PLATE TECTONICS IS
DESTRUCTIVE TO HUMAN
SOCIETY
Mt Saint Helens
1980 eruption
USGS
1989
Loma Prieta
earthquake
BUT PLATE TECTONICS
IS ALSO CRUCIAL FOR
HUMAN LIFE
Plate boundary volcanism produces
atmospheric gases (carbon dioxide
CO2 ; water H2O) needed to support life
and keep planet warm enough for life
("greenhouse" )
May explain how life evolved on earth
(at midocean ridge hot springs)
Plate tectonics raises continents above
sea level
Plate tectonics produces mineral
resources including fossil fuels
Press & Siever
“CIVILIZATION EXISTS
BY GEOLOGICAL
CONSENT”
The same geologic processes
that make our planet
habitable also make it
dangerous