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

2. INTEGRATE COMPLEMENTARY TECHNIQUES TO STUDY LITHOSPHERIC DEFORMATION
Each have strengths & weaknesses
Important to understand what can & can’t do
Jointly give valuable insight

3. 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

4. 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.

5. Medical school dean to incoming students
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

6. 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
36 mm/yr
NORTH AMERICA
PACIFIC
San Andreas Fault, Carrizo Plain

7. 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

8. 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 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

9. NATURAL DISASTERS: HAZARDS
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 ,000 deaths
San Simeon, Ca earthquake: M deaths
Earthquakes don’t kill people (generally, tsunami exception), buildings kill people
NATURAL DISASTERS: HAZARDS
AND RISKS

10. 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,

11. KINEMATICS CONTROL BOUNDARY NATURE
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
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

12. Boundaries are described either as
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

13. 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
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

14. 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

15. BOUNDARY TYPE CHANGES WITH ORIENTATION PACIFIC - NORTH AMERICA
CONVERGENCE -
ALEUTIAN TRENCH
54 mm/yr
PACIFIC wrt
NORTH
AMERICA
pole
STRIKE SLIP -
SAN ANDREAS
EXTENSION -
GULF OF CALIFORNIA
Stein & Wysession,

16. SAN ANDREAS FAULT NEAR SAN FRANCISCO Type example of transform on land

17. 1989 LOMA PRIETA, CALIFORNIA EARTHQUAKE
MAGNITUDE 7.1 ON THE SAN ANDREAS
Davidson et al

18. 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

19. Houses collapsed in the Marina district of San Francisco
1989 LOMA PRIETA, CALIFORNIA EARTHQUAKE
Houses collapsed in the Marina district of San Francisco
Shaking amplified by low velocity landfill
Stein & Wysession (USGS)

20. Pacific subduction beneath North America
TRENCH-NORMAL
CONVERGENCE -
ALEUTIAN TRENCH
54 mm/yr
1964 ALASKA EARTHQUAKE
Ms 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

21. 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

22. Thrust earthquakes indicate shortening
Los Angeles Basin
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

23. 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

24. 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.

25. 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
Boore, 1977
S&W 4.1-2

26. SEISMIC CYCLE AND PLATE MOTION
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

27. 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

28. 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

29. 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

30. 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,

31. 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

32. FAR FIELD SLIP RATE D ~ 35 mm/yr
S&W
Z.-K. Shen

33. 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 ~ 5 m slip accumulated
Broad PBZ
Elastic strain
Stein & Sella 2002

34. EARTHQUAKE (COSEISMIC):
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
SUMATRA TRENCH
INDIA
BURMA
Tsunami generated
Stein & Wysession,
HOW OFTEN:
Fault slipped ~ 10 m --> 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

35. Red - up motion, blue down
TSUNAMI GENERATED ALONG FAULT, WHERE SEA FLOOR DISPLACED, AND SPREADS OUTWARD
Hyndeman and Wang, 1993
Red - up motion, blue down

36. P waves longitudinal waves
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

37. 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

38. IMPROVE EARTHQUAKE LOCATION
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)

39. IMPROVE EARTHQUAKE LOCATION
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)