GEOLOGY 324 TECTONOPHYSICS: EARTHQUAKES & TECTONICS Seth Stein, 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

EARTHQUAKES & TECTONICSLocations 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 motionsCan observe directly

Primary constraint on lithospheric processes

PLATE DYNAMICS, forces causing plate motions

Harder to observe directlyObserve indirect effects (seismic

velocity, gravity, etc)Studied via models

Closely tied to mantle dynamicsKinematics primary constraint on

models

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

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 deathsSan 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

Stein & Wysession, 2003 5.1-4

BASIC CONCEPTS:

KINEMATICS CONTROL

BOUNDARY NATURE

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

At subduction zones subducting plate moves toward boundary

At transforms, relative plate motion parallel to boundary

Real boundaries often combine aspects (transpression, transtension)

Transtension - Dead Sea transform

Arabia

Sinai

4 mm/yr

S&W 5.1-4

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 are not spreading ridges

- spreading centers like theEast African rift within continents

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

- etc

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

v ji = j i x 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) (, )

EULER VECTOR

Relative motion between two rigid plates on the spherical earth can be described as a rotation about an Euler pole

Linear velocity

r

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).

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

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.

21

2 wrt 1

12

1 wrt 2


BOUNDARY TYPE CHANGES WITH ORIENTATION

PACIFIC - NORTH AMERICA

PACIFIC wrt NORTH

AMERICApole

CONVERGENCE - ALEUTIAN TRENCH

54 mm/yr

EXTENSION -GULF OF CALIFORNIA

STRIKE SLIP - SAN ANDREAS


SAN ANDREAS FAULT NEAR SAN FRANCISCO

Type example of transform on land

1989 LOMA PRIETA, CALIFORNIA EARTHQUAKEMAGNITUDE 7.1 ON THE SAN ANDREAS

Davidson et alDavidson et al

1989 LOMA PRIETA, CALIFORNIA

EARTHQUAKE

The two level Nimitz freeway collapsed

alonga 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)

1964 ALASKA EARTHQUAKE

Ms 8.4 Mw 9.1

Pacific subduction beneath 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

TRENCH-NORMALCONVERGENCE - ALEUTIAN TRENCH

54 mm/yr

PACIFIC NORTH AMERICA

QuickTime™ and aYUV420 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

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 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

S&W 4.5-9

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

S&W 4.1-3

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.

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)

Catalyzed ideas about relation of earthquakes & surface faults

QuickTime™ and aTIFF (LZW) decompressor


Boore, 1977

S&W 4.1-2

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 VARIABLEReasons are unclear: randomness, stress effects of other earthquakes on

nearby faults…

M>7 mean 132 yr 105 yr

Sieh et al., 1989Extend earthquake history with paleoseismology

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

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

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 faultbetween earthquakes

Coseismic and interseismic motion sum to 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. ShenS&W 4.5-13

PACIFIC-NORTH AMERICA PLATE BOUNDARY ZONE: PLATE MOTION & ELASTIC STRAIN~ 50 mm/yr

plate motion spread over ~ 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

Broad PBZ



Stein & Sella 2002

EARTHQUAKE CYCLE

INTERSEISMIC:

India subducts beneath Burma at about 20 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 yrLonger 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 DISPLACED, AND SPREADS OUTWARD

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

Red - up motion, blue downHyndeman 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

QuickTime™ and aTIFF (Uncompressed) decompressor


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

GEOLOGY 324 TECTONOPHYSICS: EARTHQUAKES & TECTONICS Seth Stein, Northwestern University

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