C. Rubin, Central Washington University 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

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

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

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

EARTHQUAKES & SOCIETY 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

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 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

KINEMATICS CONTROL BOUNDARY NATURE BASIC CONCEPTS: KINEMATICS CONTROL BOUNDARY NATURE Direction of relative motion between plates at a point on their boundary determines the nature of the boundary. Spreading centers - 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) Arabia 4 mm/yr Sinai 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

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

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

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

Pacific subduction beneath North America TRENCH-NORMAL 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

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

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. 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. AFTTERSHOCKS

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 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 its steady interseismic behavior again.

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

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

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

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

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

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 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 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

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 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

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

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

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 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 Dewey, 1987 Or via better velocity model, including methods that simultaneously improve velocity model (double-difference tomography)