Transcript 35 mm/yr - Northwestern University

GEOLOGY 324 TECTONOPHYSICS:
EARTHQUAKES & TECTONICS
Seth Stein, Northwestern University
INTEGRATE
COMPLEMENTARY
TECHNIQUES TO
STUDY
LITHOSPHERIC
DEFORMATION
Each have strengths
& weaknesses
Important to
understand what can
& can’t do
Jointly give valuable
insight
Introduction
Earthquakes: fundamental concepts & focal mechanisms
Earthquakes: waveform modeling, moment tensors & source
parameters
Tectonic geodesy
Earthquake recurrence & hazards
Plate tectonics, relative plate motions
Absolute plate motions
Spreading centers, Subduction zones & driving forces
Plate boundary zones & changes in plate motions
Plate interiors
Faulting & deformation in the lithosphere
Class notes:
http://www.earth.northwestern.edu/people/seth/324
Most material from
Stein, S. and M. Wysession, Introduction to Seismology, Earthquakes, and
Earth Structure, Blackwell Publishing, 2003.
Studying the lithosphere involves integrating plate tectonics,
seismology, geodesy, geology, rock mechanics, thermal studies,
modeling and much more
No clear dividing lines between subfields
“When we try to pick out anything by itself, we find it hitched to
everything else in the universe.”
John Muir
“Half of what we will teach you in the next few years is wrong. The
problem is we don’t know which half”
Medical school dean to incoming students
Locations map
plate boundary
zones & regions
of intraplate
deformation even
in underwater or
remote areas
Focal
mechanisms
show strain field
Slip & seismic
history show
deformation rate
Depths constrain
thermomechanical
structure of
lithosphere
EARTHQUAKES & TECTONICS
36 mm/yr
NORTH
AMERICA
PACIFIC
San Andreas Fault, Carrizo Plain
PLATE KINEMATICS, directions and
rates of plate motions
Can observe directly
Primary constraint on lithospheric
processes
PLATE DYNAMICS, forces
causing plate motions
Harder to observe directly
Observe indirect effects (seismic
velocity, gravity, etc)
Studied via models
Closely tied to mantle dynamics
Kinematics primary constraint on
models
EARTHQUAKES & SOCIETY
In general, the most destructive earthquakes occur
where large populations live near plate
boundaries. The highest property losses occur in
developed nations where more property is at risk,
whereas fatalities are highest in developing
nations.
Estimates are that the 1990 Northern Iran shock
killed 40,000 people, and that the 1988 Spitak
(Armenia) earthquake killed 25,000. Even in
Japan, where modern construction practices
reduce earthquake damage, the 1995 Kobe
earthquake caused more than 5,000 deaths and
$100 billion of damage. On average during the
past century earthquakes have caused about
11,500 deaths per year.
The earthquake risk in the United States is much
less than in many other countries because large
earthquakes are relatively rare in most of the U.S.
and because of earthquake-resistant construction
Hazard is the intrinsic natural occurrence of
earthquakes and the resulting ground motion and
other effects.
Risk is the danger the hazard poses to life and
property.
Although the hazard is an unavoidable geological
fact, risk is affected by human actions.
Areas of high hazard can have low risk because
few people live there, and areas of modest hazard
can have high risk due to large populations and
poor construction.
Earthquake risks can be reduced by human
actions, whereas hazards cannot
Bam, Iran earthquake: M 6.5 30,000 deaths
San Simeon, Ca earthquake: M6.5 2 deaths
Earthquakes don’t kill people (generally, tsunami
exception), buildings kill people
NATURAL DISASTERS:
HAZARDS
AND RISKS
Earthquake locations map narrow plate boundaries, broad plate
boundary zones & regions of intraplate deformation even in
underwater or remote areas
DIFFUSE BOUNDARY
ZONES
INTRAPLATE
NARROW
BOUNDARIES
Stein & Wysession, 2003 5.1-4
BASIC
CONCEPTS:
KINEMATICS
CONTROL
BOUNDARY
NATURE
S&W
5.1-4
Direction of relative motion between plates at a point on their boundary determines
the nature of the boundary.
At spreading centers both plates move
away from boundary
Arabia
4 mm/yr
At subduction zones subducting plate
moves toward boundary
Sinai
At transforms, relative plate motion
parallel to boundary
Real boundaries often combine
aspects (transpression, transtension)
Transtension - Dead Sea transform
NOMENCLATURE:
Boundaries are described either as
- midocean-ridges and trenches, emphasizing morphology
- or as divergent (spreading centers) and convergent (subduction zones),
emphasizing kinematics
Latter nomenclature is more precise
because there are
- elevated features in ocean basins
that are not spreading ridges
- spreading centers like the
East African rift within continents
-continental convergent zones like
the Himalaya may not have active
subduction
- etc
EULER VECTOR
Relative motion between two rigid plates on the spherical earth can be
described as a rotation about an Euler pole
At a point r along the boundary
between two plates, with latitude
 and longitude , the linear
velocity of plate j with respect to
plate i , v ji , is given by the
vector cross product
Linear velocity
v ji = j i x r
r
r is the position vector to the
point on the boundary
j i is the angular velocity vector
or Euler vector described by its
magnitude (rotation rate) |j i |
and pole (surface position) (, )
Stein & Wysession, 2003
Direction of relative motion is a small circle
about the Euler pole
First plate ( j) moves counterclockwise ( right
handed sense) about pole with respect to
second plate (i).
21
2 wrt 1
Boundary segments with relative motion
parallel to the boundary are transforms, small
circles about the pole
Segments with relative motion away from the
boundary are spreading centers
Segments with relative motion toward
boundary are subduction zones
12
1 wrt 2
Magnitude (rate) of relative motion increases
with distance from pole because
|v ji | = |j i | | r | sin  , where  is the angle
between pole and site
All points on a boundary have the same
angular velocity, but the magnitude of linear
velocity varies from zero at the pole to a
maximum 90º away.
Stein & Wysession, 2003
BOUNDARY TYPE
CHANGES WITH
ORIENTATION
CONVERGENCE ALEUTIAN TRENCH
54 mm/yr
PACIFIC NORTH AMERICA
STRIKE SLIP SAN ANDREAS
PACIFIC wrt
NORTH
AMERICA
pole
EXTENSION GULF OF CALIFORNIA
Stein & Wysession, 2003 5.2-3
SAN ANDREAS FAULT NEAR SAN
FRANCISCO
Type example of transform on land
1989 LOMA PRIETA, CALIFORNIA EARTHQUAKE
MAGNITUDE 7.1 ON THE SAN ANDREAS
Davidson et al
1989 LOMA PRIETA,
CALIFORNIA
EARTHQUAKE
The two level Nimitz
freeway collapsed
along
a 1.5 km section in
Oakland, crushing cars
Freeway had been
scheduled for retrofit to
improve earthquake
resistance
1989 LOMA PRIETA,
CALIFORNIA EARTHQUAKE
Houses collapsed in the
Marina district of San
Francisco
Shaking amplified by low
velocity landfill
Stein & Wysession 2003 2.4-10 (USGS)
TRENCH-NORMAL
CONVERGENCE ALEUTIAN TRENCH
54 mm/yr
1964 ALASKA
EARTHQUAKE
Ms 8.4 Mw 9.1
Pacific subduction
beneath North America
PACIFIC
NORTH AMERICA
~ 7 m of slip on 500x300 km2
of Aleutian Trench
Second or third largest
earthquake recorded to date
~ 130 deaths
Catalyzed idea that great
thrust fault earthquakes
result from slip on
subduction zone plate
interface
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
1971 Ms 6.6 SAN
FERNANDO
EARTHQUAKE
1.4 m slip on 20x14
km2 fault
Thrust faulting from
compression across
Los Angeles Basin
Fault had not been
previously recognized
65 deaths, in part due
to structural failure
Prompted
improvements in
building code &
hazard mapping
Los Angeles Basin
Thrust earthquakes
indicate shortening
1994 Northridge
Ms 6.7
AFTTERSHOCKS
Caused some of the highest ground accelerations
ever recorded. It illustrates that even a moderate
magnitude earthquake can do considerable
damage in a populated area. Although the loss of
life (58 deaths) was small due to earthquakeresistant construction the $20B damage makes it
the most costly earthquake to date in the U.S.
S&W 4.5-9
ELASTIC REBOUND OR SEISMIC CYCLE MODEL
Materials at distance on
opposite sides of the
fault move relative to
each other, but friction
on the fault "locks" it
and prevents slip
Eventually strain
accumulated is more
than the rocks on the
fault can withstand, and
the fault slips in
earthquake
Earthquake reflects
regional deformation
S&W 4.1-3
ELASTIC REBOUND OR SEISMIC CYCLE MODEL
Earthquakes are most dramatic part of a seismic cycle occuring on segments of
the plate boundary over 100s to 1000s of years.
During interseismic stage, most of the cycle, steady motion occurs away from
fault but fault is "locked", though some aseismic creep can occur on it.
Immediately prior to rupture is a preseismic stage, that can be associated with
small earthquakes (foreshocks) or other possible precursory effects.
Earthquake itself is coseismic phase, during which rapid motion on fault
generates seismic waves. During these few seconds, meters of slip on fault
"catch up" with the few mm/yr of motion that occurred over 100s of years away
from fault.
Finally, postseismic phase occurs after earthquake, and aftershocks and
transient afterslip occur for a period of years before fault settles into its steady
interseismic behavior again.
1906 SAN FRANCISCO
EARTHQUAKE (magnitude 7.8)
~ 4 m of slip on 450 km of San Andreas
~2500 deaths, ~28,000 buildings
destroyed (most by fire)
Catalyzed ideas about relation of
earthquakes & surface faults
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
S&W 4.1-2
Boore, 1977
SEISMIC CYCLE AND PLATE MOTION
Over time, slip in earthquakes adds up
and reflects the plate motion
Offset fence showing 3.5 m of leftlateral strike-slip motion along San
Andreas fault in 1906 San Francisco
earthquake
~ 35 mm/yr motion between Pacific and
North American plates along San
Andreas shown by offset streams &
GPS
Expect earthquakes on average every
~ (3.5 m )/ (35 mm/yr) =100 years
Turns out more like 200 yrs because
not all motion is on the San Andreas
Moreover, it’s irregular rather than
periodic
EARTHQUAKE RECURRENCE IS HIGHLY VARIABLE
Reasons are unclear: randomness, stress effects of other earthquakes on
nearby faults…
Extend earthquake history
with paleoseismology
Sieh et al., 1989
M>7
mean 132 yr
s 105 yr
S&W 1.2-15
CHALLENGES OF STUDYING EARTHQUAKE CYCLE
Cycle lasts hundreds of years, so don’t have observations of it in any one place
Combine observations from different places in hope of gaining complete view
Unclear how good that view is and how well models represent its complexity.
Research integrates various techniques:
Most faults are identified from earthquakes on them: seismology is primary tool
to study the motion during earthquakes and infer long term motion
Also
- Historical records of earthquakes
- Field studies of location, geometry, and history of faults
- Geodetic measurements of deformation before, during, and after earthquakes
- Laboratory results on rock fracture
GEODETIC DATA GIVE INSIGHT INTO DEFORMATION BEYOND THAT
SHOWN SEISMOLOGICALLY
Study aseismic processes
Study seismic cycle before, after, and in between earthquakes, whereas we
can only study the seismic waves once an earthquake occurs
SAR image of Hayward fault
(red line), part of San Andreas
fault system, in the Berkeley
(east San Francisco Bay) area.
Color changes from orange
to blue show about 2 cm of
gradual movement.
This movement is called
aseismic creep because the
fault moved slowly without
generating an earthquake
ELASTIC
REBOUND
MODEL OF
STRIKE-SLIP
FAULT AT A
PLATE
BOUNDARY
Large
earthquakes
release all strain
accumulated on
locked fault
between
earthquakes
Coseismic and
interseismic
motion sum to
plate motion
Interseismic
strain
accumulates near
fault
Stein & Wysession, 2003 4.5-12
ELASTIC
REBOUND
MODEL OF
STRIKE-SLIP
FAULT AT A
PLATE
BOUNDARY
Fault parallel interseismic motion on fault with far field slip rate D,
locked to depth W, as function of cross-fault distance y
s(y) = D/2 + (D / π) tan -1 (y/W)
Width of strain accumulation zone comparable to locking depth
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
FAR FIELD SLIP RATE
~ 35 mm/yr
S&W 4.5-13
D
Z.-K. Shen
~ 50 mm/yr
plate motion
spread over
~ 1000 km
PACIFIC-NORTH AMERICA PLATE BOUNDARY
ZONE: PLATE MOTION & ELASTIC STRAIN
Broad
PBZ
~ 35 mm/yr
elastic strain
accumulation
from locked San
Andreas in
region
~ 100 km wide
Elastic
strain
Locked strain
will be released
in earthquakes
Since last
earthquake in
1857 ~ 5 m slip
accumulated
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Stein & Sella 2002
EARTHQUAKE CYCLE
INTERSEISMIC:
SUMATRA TRENCH
BURMA
INDIA
India subducts beneath
Burma at about 20 mm/yr
Fault interface is locked
Tsunami generated
EARTHQUAKE
(COSEISMIC):
Fault interface slips,
overriding plate
rebounds, releasing
accumulated motion and
generating tsunami
Stein & Wysession, 2003 4.5-14
HOW OFTEN:
Fault slipped ~ 10 m --> 10000 mm / 20 mm/yr = 500 yr
Longer if some slip is aseismic
Faults aren’t exactly periodic, likely because chaotic nature of
rupture controls when large earthquakes occur
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
SEISMIC WAVES
COMPRESSIONAL
(P)
AND SHEAR (S)
WAVES
P waves
longitudinal waves
S waves transverse
waves
P waves travel
faster
S waves from
earthquake
generally larger
Stein & Wysession, 2003
Accuracy (truth) depends primarily on
velocity model
Precision (formal uncertainty) depends
primarily on network geometry (close
stations & eq within network help)
Locations can be accurate but
imprecise or precise but inaccurate
(line up nicely but displaced from fault)
Epicenters (surface positions) better
determined than depths or hypocenters
(3D positions) because seismometers
only onQuickTime™
surface and a
TIFF (LZW) decompressor
are needed to see this picture.
EARTHQUAKE LOCATION
Least squares fit to travel times
IMPROVE EARTHQUAKE LOCATION
Precision can be improved by relative
location methods like Joint Epicenter
Determination (JED) or master event
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Dewey, 1987
Or via better velocity
model, including methods
that simultaneously
improve velocity model
(double-difference
tomography)
IMPROVE EARTHQUAKE LOCATION
Precision can be improved by relative
location methods like Joint Epicenter
Determination (JED) or master event
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Dewey, 1987
Or via better velocity
model, including methods
that simultaneously
improve velocity model
(double-difference
tomography)