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Nov 16, 2013 (5 years and 5 months ago)

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GEOLOGY 415/515:

EARTHQUAKE GEOLOGY

C. Rubin,
Central Washington 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: magnitude, earthquake cycle

Tectonic geodesy

Strike
-
slip faults

Normal faults

Subduction zones (Megathrust earthquakes)

Thrust/Reverse faults

Plate interiors

Earthquake recurrence & hazards

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

EARTHQUAKES & TECTONICS

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
thermo
-
mechanical
structure of
lithosphere

PACIFIC

NORTH
AMERICA

San Andreas Fault, Carrizo Plain

36 mm/yr

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)

Models are key

Closely tied to mantle
dynamics

Kinematics primary constraint
on models

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

EARTHQUAKES & SOCIETY

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

INTRAPLATE

NARROW
BOUNDARIES

DIFFUSE BOUNDARY

ZONES

BASIC
CONCEPTS:

KINEMATICS
CONTROL
BOUNDARY
NATURE

Direction of relative motion between plates at a point on their boundary
determines the nature of the boundary.

-

both plates
move away from boundary

Subduction zones

-

subducting
plate moves toward boundary

Transform faults

-

relative plate
motion parallel to boundary

Real boundaries often combine
aspects (transpression,
transtension)

Transtension
-

Arabia

Sinai

4 mm/yr

Boundaries are described either as

-

midocean
-
ridges and trenches
-

emphasizing morphology

-

or as divergent (spreading centers) and

-
convergent (subduction zones)
-

emphasizing kinematics

NOMENCLATURE:

Latter nomenclature is more precise
because there are

-

elevated features in ocean basins that

-

East African rift within continents

-
continental convergent zones like the
Himalaya may not have active
subduction

BOUNDARY TYPE
CHANGES WITH
ORIENTATION

PACIFIC
-

NORTH
AMERICA

PACIFIC wrt

NORTH

AMERICA

pole

CONVERGENCE
-

ALEUTIAN TRENCH

54 mm/yr

EXTENSION
-

GULF OF CALIFORNIA

STRIKE SLIP
-

SAN ANDREAS

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

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

EARTHQUAKE

M
s

8.4 M
w

9.1

Pacific subduction
beneath North America

~ 7 m of slip on 500x300 km
2

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

TRENCH
-
NORMAL

CONVERGENCE
-

ALEUTIAN TRENCH

54 mm/yr

PACIFIC

NORTH AMERICA

1971 M
s

6.6 SAN
FERNANDO EARTHQUAKE

1.4 m slip on 20x14 km
2

fault

Thrust faulting from
compression across Los
Angeles Basin

previously recognized

65 deaths, in part due to
structural failure

Prompted improvements in
building code & hazard
mapping

Caused some of the highest ground
accelerations ever recorded. Even a moderate
magnitude earthquake can cause
considerable damage in a populated area.

Although the loss of life (58 deaths) was small
due to earthquake
-
resistant construction the
\$20B damage makes it the most costly
earthquake to date in the U.S.

Los Angeles Basin

Thrust earthquakes
indicate shortening

1994 Northridge
Ms 6.7

AFTTERSHOCKS

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

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
pre
-
seismic 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

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

ELASTIC REBOUND OR SEISMIC CYCLE MODEL

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)

earthquakes & surface faults

Boore, 1977

Over time, slip in earthquakes adds up
and reflects the plate motion

Offset fence showing 3.5 m of left
-
lateral 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

SEISMIC CYCLE AND PLATE MOTION

EARTHQUAKE RECURRENCE IS HIGHLY VARIABLE

Reasons are unclear: randomness, stress effects of other
earthquakes on nearby faults…

M>7 mean 132 yr s 105 yr

Sieh et al., 1989

Extend earthquake history
with paleoseismology

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

Plus

-

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

SAR image of Hayward fault
(red line), part of San Andreas
fault system, in the Berkeley
(east San Francisco Bay)
area. Color changes from

This movement is called
aseismic creep
because the
fault moved slowly without
generating an earthquake

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

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 should
equal relative
plate motion

Interseismic strain
accumulates
near fault

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

FAR FIELD SLIP RATE D
~ 35 mm/yr

Z.
-
K. Shen

PACIFIC
-
NORTH AMERICA PLATE BOUNDARY
ZONE: PLATE MOTION & ELASTIC STRAIN

~ 50 mm/yr
plate motion
~ 1000 km

~ 35 mm/yr
elastic strain
accumulation
from locked
San Andreas
in region
~ 100 km wide

Locked strain
will be
released in
earthquakes

Since last
earthquake in
1857 ~ 5 m slip
accumulated

Elastic
strain

PBZ

EARTHQUAKE CYCLE

INTERSEISMIC:

India subducts beneath
mm/yr

Fault interface is locked

EARTHQUAKE
(COSEISMIC):

Fault interface slips,
overriding plate
rebounds, releasing
accumulated motion
and generating tsunami

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

INDIA

BURMA

Tsunami generated

SUMATRA TRENCH

TSUNAMI GENERATED ALONG FAULT, WHERE SEA FLOOR

http://staff.aist.go.jp/kenji.satake/animation.gif

Red
-

up motion, blue down

Hyndeman and Wang, 1993

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

EARTHQUAKE LOCATION

Least squares fit to travel times

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
on surface

IMPROVE EARTHQUAKE LOCATION

Precision can be improved by relative
location methods like Joint Epicenter
Determination (JED) or master event

Or via better velocity
model, including methods
that simultaneously
improve velocity model
(double
-
difference
tomography)

Dewey, 1987

IMPROVE EARTHQUAKE LOCATION

Precision can be improved by relative
location methods like Joint Epicenter
Determination (JED) or master event

Or via better velocity
model, including methods
that simultaneously
improve velocity model
(double
-
difference
tomography)

Dewey, 1987