2. 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: RIGID PLATES
3. BASIC
CONCEPTS:
THERMAL
EVOLUTION
OF OCEANIC
LITHOSPHERE
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
Stein &
Wysession
2003
4. Gordon & Stein, 1992
INDIAN PLATE MOVES NORTH
COLLIDING WITH EURASIA
6. 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
7. 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:
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
Stein & Wysession, 2003
9. TWO VIEWS OF THE PART OF THE
SUMATRA SUBDUCTION ZONE THAT
SLIPPED
Seismogram analysis shows
most slip in southern 400 km
Aftershocks show slip
extended almost 1200 km
C. Ji
ERI
10. 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 Sumatra-
Andaman 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.
11. 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
12. 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”
13. 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
16. SOME MAJOR DAMAGE DONE BY EARTHQUAKE SHAKING
ITSELF, BUT STRONG GROUND MOTION DECAYS RAPIDLY
WITH DISTANCE
0.2 g
Stein & Wysession, 2003
17. 0.2 g
Damage
onset for
modern
buildings
DAMAGE DEPENDS ON BUILDING TYPE
RESISTANT CONSTRUCTION REDUCES EARTHQUAKE RISKS
“Earthquakes don't kill people; buildings kill people."
Coburn &
Spence 1992
19. TSUNAMI GENERATED ALONG FAULT, WHERE
SEA FLOOR DISPLACED, AND SPREADS
OUTWARD
Red - up motion, blue downHyndeman and Wang, 1993
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
20. 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
Tsunami generated along fault, where sea floor displaced, and
spreads outward
Reached Sri Lanka in 2 hrs, India in 2-3
21. 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
Tsunami wave bends as water depth & thus speed changes
Stein & Wysession
24. 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
25. 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
TSUNAMI
WARNING
Deep ocean buoys can measure
wave heights, verify tsunami and
reduce false alarms
26. HOWEVER, HARD TO PREDICT EARTHQUAKES
recurrence is highly variable
M>7 mean 132 yr σ 105 yr
Estimated probability in 30 yrs 7-51%
Sieh et al., 1989
Extend earthquake history
with geologic records
-paleoseismology
27. 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
GAP?
NOTHING YET Ando, 1975
28. 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”
30. 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
BUT PLATE
TECTONICS IS ALSO
CRUCIAL FOR HUMAN
LIFE
Press & Siever
1906 SF – 4 m of slip on 450-km long fault 3 x 10**16 Joules of elastic energy – equivalent to a 7 Megaton bomb (Hiroshima was 0.012 Mt)
1960 Chile – 21 m of slip on a 800 km long fault 10**19 J of elastic energy (more than a 2000 Mt bomb – larger than all nuclear bombs ever exploded – largest was a Soviet atmospheric test of 58 Mt)
