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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

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Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

Introduction DEFORMATION

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

Class notes: DEFORMATION

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.


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

Studying the lithosphere involves integrating plate tectonics, seismology, geodesy, geology, rock mechanics, thermal studies, modeling and much moreNo 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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

EARTHQUAKES & TECTONICS tectonics, seismology, geodesy, geology, rock mechanics, thermal studies, modeling and much more

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

NORTH AMERICA

36 mm/yr

PACIFIC

San Andreas Fault, Carrizo Plain


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

PLATE KINEMATICS, directions and rates of plate motions tectonics, seismology, geodesy, geology, rock mechanics, thermal studies, modeling and much more

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

EARTHQUAKES & SOCIETY tectonics, seismology, geodesy, geology, rock mechanics, thermal studies, modeling and much more

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

BASIC CONCEPTS: plate boundary zones & regions of intraplate deformation even in underwater or remote areas

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

At subduction zones subducting plate moves toward boundary

At transforms, relative plate motion parallel to boundary

Real boundaries often combine aspects (transpression, transtension)

Arabia

4 mm/yr

Sinai

Transtension - Dead Sea transform


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

NOMENCLATURE: plate boundary zones & regions of intraplate deformation even in underwater or remote areas

  • 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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

EULER VECTOR plate boundary zones & regions of intraplate deformation even in underwater or remote areas

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

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)(, )

Linear velocity

r

Stein & Wysession, 2003


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

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

Stein & Wysession, 2003


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

BOUNDARY TYPE CHANGES WITH ORIENTATION Euler pole

PACIFIC - NORTH AMERICA

CONVERGENCE -

ALEUTIAN TRENCH

54 mm/yr

PACIFIC wrt

NORTH

AMERICA

pole

STRIKE SLIP -

SAN ANDREAS

EXTENSION -

GULF OF CALIFORNIA

Stein & Wysession, 2003 5.2-3


San andreas fault near san francisco type example of transform on land

SAN ANDREAS FAULT NEAR SAN FRANCISCO Euler pole

Type example of transform on land


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

1989 LOMA PRIETA, CALIFORNIA EARTHQUAKE Euler pole

MAGNITUDE 7.1 ON THE SAN ANDREAS

Davidson et al


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

1989 LOMA PRIETA, CALIFORNIA EARTHQUAKE Euler pole

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

1989 LOMA PRIETA, CALIFORNIA EARTHQUAKE Euler pole

Houses collapsed in the Marina district of San Francisco

Shaking amplified by low velocity landfill

Stein & Wysession 2003 2.4-10 (USGS)


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

TRENCH-NORMAL Euler pole

CONVERGENCE -

ALEUTIAN TRENCH

54 mm/yr

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

NORTH AMERICA

PACIFIC


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

1971 M Euler pole s 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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

Los Angeles Basin Euler pole

Thrust earthquakes indicate shortening

1994 Northridge Ms 6.7

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.

AFTTERSHOCKS

S&W 4.5-9


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

ELASTIC REBOUND OR SEISMIC CYCLE MODEL Euler pole

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

ELASTIC REBOUND OR SEISMIC CYCLE MODEL Euler pole

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.


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

1906 SAN FRANCISCO EARTHQUAKE (magnitude 7.8) Euler pole

~ 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

Boore, 1977

S&W 4.1-2


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

SEISMIC CYCLE AND PLATE MOTION Euler pole

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

EARTHQUAKE RECURRENCE IS HIGHLY VARIABLE Euler pole

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

Sieh et al., 1989

Extend earthquake history with paleoseismology

M>7 mean 132 yr s 105 yr

S&W 1.2-15


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

  • CHALLENGES OF STUDYING EARTHQUAKE CYCLE Euler pole

  • 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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

FAR FIELD SLIP RATE D ~ 35 mm/yr BOUNDARY

S&W 4.5-13

Z.-K. Shen


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

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

Broad PBZ

Elastic strain

Stein & Sella 2002


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

EARTHQUAKE CYCLE ELASTIC STRAIN

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

SUMATRA TRENCH

BURMA

INDIA

Tsunami generated

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

TSUNAMI GENERATED ALONG FAULT, WHERE SEA FLOOR DISPLACED, AND SPREADS OUTWARD

Hyndeman and Wang, 1993

Red - up motion, blue down

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

SEISMIC WAVES AND SPREADS OUTWARD

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

EARTHQUAKE LOCATION AND SPREADS OUTWARD

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

IMPROVE EARTHQUAKE LOCATION AND SPREADS OUTWARD

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

Dewey, 1987

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


Geology 324 tectonophysics earthquakes tectonics seth stein northwestern university

IMPROVE EARTHQUAKE LOCATION AND SPREADS OUTWARD

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

Dewey, 1987

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