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Strike slip faults Strike slip faults Earthquakes on strike slip faults San Andreas fault Fault segments & slip patches Southern California earthquakes San Diego, central California, Mojave desert Transform (sliding) Boundaries • fault zones between plates • neither destruction nor construction occurs (usually) • relative motions are parallel to faults (usually) • small regions may be collisional or extensional • transform boundaries connect two diverging boundaries, two converging, or one of each diverging and converging • earthquakes are generally shallow • transform faults that cease to be plate boundaries are called fracture zones. NORTH AMERICA 80 MA & 20MA - PRESENT DEVELOPMENT OF SAN ANDREAS FAULT Triple Junctions • there are 16 possible arrangements of triple junctions • see Kearey &Vine (1990), page 100 • some are always stable, some are always unstable and cannot form • some are unstable, but will decay to a stable configuration • an example of the effect of changing triple junctions can be seen in California Oligocene Farallon Plate Mendocino FZ Pacific Plate Murray FZ NA • Farallon plate separated from Pacific plate • subduction of Farallon beneath NA was faster than supply of new ocean crust • as a result, the ridge system moved east, toward NA. Triple Junctions 28 Ma F NA Pacific Plate F 28 Ma F FFT RTF Pacific Plate F NA • momentarily, a quadruple point existed, FRTT • quadruple point decayed to two triple points almost immediately (FFT, RTF) • both of these are stable triple junction configurations (see K&V, p.101) for the velocity space solutions • Mendocino triple junction migrated north along NA boundary • Murray triple junction migrated south to the ridge-transform contact. Triple Junctions 18 Ma F Sanas Ft. re And Pacific Plate F NA • by 18 Ma, the trench had overridden the entire Mendocino-Murray ridge section • both triple junctions now revert to RTF configurations • the situation is stable • it has survived for 18 Ma. TRANSFORM FAULT EARTHQUAKES SAN ANDREAS FAULT Cape Mendocino to Gulf of California Coast line in blue. San Andreas fault heavy dark line.Very complex system of faults especially in southern California. TRANSFORM EARTHQUAKES LOCKED SECTIONS 1857 Fort Tejon and 1906 San Francisco Two sections of the fault locked for long periods of time. Failed in 1857 and 1906 creating two major ruptures 1857 Fort Tejon 360 km long 1906 San Francisco 430 km long. Two very large earthquakes TRANSFORM EARTHQUAKES 1906 SAN FRANCISCO Locked section moved Average 6m offset along 430 km of fault. 60 seconds of shaking. Magnitude 7.8 earthquake Most damage due to fire and disease. Important for developing elastic rebound theory for earthquakes TRANSFORM EARTHQUAKES 1989 LOMA PRIETA During World Series Bend of San Andreas, where two faults split off from main fault, ruptured. Earthquake did not rupture the ground. Moved 1.9 m horizontally and 1.3 m vertically. Magnitude 6.9 Extensive damage in San Francisco. TRANSFORM EARTHQUAKES LOMA PRIETA 1989 Damage in Marina District San Francisco Water saturated sediments were shaken became a slurry and flowed. Foundations collapsed especially soft buildings. Bottom floors used for garaging cars collapsed. TRANSFORM EARTHQUAKES 1989 LOMA PRIETA Interstate 880 collapse Portion of the elevated 880 built on soft mud. The soft mud resonated with the earthquake waves and the mud liquified. The hard rock had much smaller accelerations TRANSFORM EARTHQUAKES 1989 LOMA PRIETA Interstate 880 collapse The bridge had roughly the same frequency as the amplified surface waves and shook severely. The support columns failed at the joints. There were 20 #18 steel bars in the columns but they were discontinuous at the joints. With repeated shaking the joints failed and the bridge collapsed. HOW FAULTS WORK Elastic rebound theory Active fault with road as reference. Elastic deformation occurs either side of the fault but because of friction at the fault, no movement Elastic deformation exceeded strength of rock - fault ruptures and two side snap away from each other. Current models add propagation of fault break. Like a ripple on a carpet. PALEOSEISMOLOGY Tectonic Geomorphology - examples General topography along the central San Andreas Sag ponds - down dropped areas along the fault. Sediments deposited along fault preserve prehistoric record of faulting. PALEOSEISMOLOGY Sag ponds Soda Lake Road at the southern end of Carrizo Plain National Monument. PALEOSEISMOLOGY Sag ponds San Andreas Lake and Crystal Springs reservoir from the air, looking southeast. PALEOSEISMOLOGY Fault scarp San Andreas fault from the air PALEOSEISMOLOGY Fault scarp Offset of rows in plowed field A right-lateral fault trace crosses a plowed field during the Imperial Valley earthquake. The agricultural industry suffered heavy losses from damage to canals, irrigation ditches, and subsurface drain tiles disturbed by the movement along the Imperial fault. PALEOSEISMOLOGY Plastic deformation Left-Lateral Strike-Slip Faults in the cultivated field west of El Progresso, Guatemala, February 4, 1976. The thick, saturated, unconsolidated deposits have yielded by plastic deformation rather than rupture along the left- lateral strike-slip fault. This EQ resulted in the deaths of 23,000 people and $1.1 billion dollars in property damage. PALEOSEISMOLOGY Tectonic geomorphology - summary Scarp Beheaded stream Shutter ridges Offset streams Sag ponds Linear ridges Mole tracks Water gaps Wind gaps THRUST FAULT EARTHQUAKES BEND IN SAN ANDREAS Complex plate pattern south of bend Left stepping right lateral fault creates compression at the bend Leads to many little plates south of the bend. Compression creates the Transverse Ranges and the San Gabriel Mts. Thrust faults under Los Angeles THRUST FAULT EARTHQUAKES BEND IN SAN ANDREAS SAN ANDREAS FAULT BLIND THRUST FAULTS San Fernando 1971, Northridge 1994 SEISMIC GAPS Cross section of seismicity, San Andreas 1969 to 1989 Dense cluster in central creeping section of the fault. 1989 Loma Prieto earthquake and after shocks filled in a gap Gap exists south of San Francisco. Probability of EQ magnitude along SAF Prediction of magnitude and probability of occurring before 2030. Forecasts based on historic records, dated trench wall offsets and GPS The Parkfield Experiment is a comprehensive, long-term earthquake research project on the San Andreas fault. The experiment's purpose is to better understand the physics of earthquakes what actually happens on the fault and in the surrounding region before, during and after an earthquake Prediction by USGS in April, 1985: "A 90% probability of an earthquake with Magnitude 5.5 to 6.0 occurring sometime between 1985 and 1993." The San Andreas fault defines an approximately 1300 km portion of the boundary between the Pacific and North American plates. Along its length, the fault undergoes horizontal strike-slip motion that accommodates most of the relative motion between the plates. To the north, a complex of transform faults and spreading centers accommodates the motion of the Gorda and Juan de Fuca plates. To the south, a similar complex of spreading centers and transform faults accommodate the displacement in the Gulf of California. Foreshocks and Aftershocks The size of the aftershocks depends on the size of the initial large earthquake Press et al., 2004 Two Big Questions: *What causes earthquakes to start in specific locations? *What causes earthquakes to stop in specific locations? Three Big Possible Answers: *Fault Zone Rheology Friction, Deformation Styles *Fault Zone Stress Conditions Tectonics, Stress Triggers/Shadows, Pore-pressure *Fault Zone Geometry Fault Continuities and Discontinuities March 2006 WGCEP Workshop The San Andreas fault in central California. A "creeping" section (green) separates locked stretches north of San Juan Bautista and South of Cholame. The Parkfield section (red) is a transition zone between the creeping and southern locked section. Stippled area marks the surface rupture of the 1857 EQ Most of the northern section of the fault is also currently locked, with no detectable movement and few earthquakes since 1906. Between these locked sections, the San Andreas fault creeps (slips aseismically). From San Juan Bautista to Parkfield, the creeping section produces numerous small (mostly M=5 and smaller) earthquakes but no large ones. Between Parkfield and Gold Hill defines a transition zone on the SAF between the creeping and locked behavior of the fault Waveforms recorded on regional seismographs are strikingly similar for the 1922, 1934 and 1966 earthquakes, These earthquakes may have involved repeated rupture of the same area on the fault. Recordings of the east-west component of motion from the 1922 earthquake (shown in black) and the 1934 and 1966 events at Parkfield (shown in red) are strikingly similar, suggesting virtually identical ruptures. Suggest that there may be some predictability in the occurrence of earthquakes, at least at Parkfield?? Do earthquakes occur completely randomly, or do they have a pattern that tends to repeat? If they're random, there's no hope for earthquake prediction. Regular repetitions of the same rupture event, (characteristic earthquake) may be occurring at Parkfield. This repetition was part of the basis for developing the Parkfield experiment. Vertical component seismograms from clustered micro-earthquakes on the San Andreas fault at Parkfield. Three types of events (numbers on left) are identified on the basis of subtle differences in the waveform. Adding to the sense of repetition, similar-size foreshocks occurred 17 minutes before both the 1934 and 1966 Parkfield earthquakes. The seismicity at Parkfield: Since 1857, six similar, M~6 earthquakes have occurred on the San Andreas fault near Parkfield with apparent regularity -- one approximately every 22 years. Little is known about the first three shocks Available data suggest that all six earthquakes may have been "characteristic” The Eqs occurred with some regularity (mean repetition time of about 22 year) and may have repeatedly ruptured the same area on the fault. 2004 Parkfield Earthquake Are these six earthquakes "characteristic" with a mean repetition time of about 22 year? September 28, 2004 Parkfield Earthquake Ruptured the same segment of the fault that broke in 1966. 8 km depth. Northwest rupture primarily along the San Andreas fault. Strong shaking lasted for about 10 seconds. This earthquake is the seventh in a series of repeating earthquakes on this stretch of the fault. M>2 aftershocks following the September 28, 2004 M6.0 earthquake The previous events were in 1857, 1881, 1901, 1922, 1934, and 1966. Lu Dongao/Xinhua Parkfield recurrence σ1 represents the failure stress of the fault. Most characteristic earthquakes occur at al; the 1934 shock occurred at a2. A constant loading rate of 2.8 cm per year and a coseismic slip of 60 cm for the Parkfield earthquake sequences in 1881, 1901, 1922,1934, and 1966 are assumed. Lu Dongao/Xinhua b) Series of earthquake sequences at Parkfield since 1850. The line represents the linear regression of the time of the sequence obtained without the 1934 sequence, The anticipated time of the seventh Parkfield sequence for the regression is January 1988. c) Shocks of ML greater than 4 since 1930 have tended to occur when the stress exceeded σ2. Parkfield (1) Will the strain release during the next earthquake be approximately the inverse, both in amount and distribution, of the strain accumulation since the 1966 shock? The answer is crucial to the basic assumptions underlying earthquake recurrence models, such as the timepredictable and Parkfield recurrence models, which are the foundation of longterm prediction efforts. (2) Are there changes in the details of the deformation field that might permit a refined estimate of the time of the next earthquake? The answer to this question will have a major impact on efforts toward medium- and short-term prediction. Predicting earthquakes requires an understanding of the underlying physics, which calls for novel multidisciplinary approaches at a level never yet undertaken. We still have very limited precise quantitative measurements of the many parameters involved. Video camera looking along San Andreas fault at Parkfield, CA. The physical phenomena underlying earthquakes are much more intricate and interwoven and we do not have a fundamental equation for the crustal organization. Two Big Questions: *Where will earthquakes start? * Where will earthquakes stop? Possible Answers: Both initiation and termination are likely caused by *Fault Zone Rheology Friction, Deformation Styles *Fault Zone Stress Conditions Tectonics, Stress Triggers/Shadows, Pore-pressure state *Fault Zone Geometry Fault Continuities and Discontinuities) It is probably best to assume that earthquakes can start anywhere (on faults). Large earthquakes can be stopped by 1) Encountering large creeping sections of a fault 2) A Stress shadow from a recent large earthquake on the same fault 3) Big changes in fault geometry. (e.g., inter-fault distances of >5 km) Earthquake Cycle and Geodetic observations Modeled as elastic lithosphere overlying a viscoelastic asthenosphere. Displacement on a single fault that slip periodically in large earthquakes. The fault that cuts through entire elastic lithosphere. The lower part of the fault creeps at continuously at a slip rate equal to plate rate. Flow rate constant in asthenosphere. The stresses on the fault are generated by far field load (tectonic motions) and time-dependent loading of the lithosphere due to viscoelastic flow in the asthenosphere. Earthquake Cycle and Geodetic observations Stage 1 Interseismic deformation. Far field deformation present on the Earth's surface at about a fault depth away from the locked zone Earthquake Cycle and Geodetic observations Stage 2 Coseismic deformation, showing discrete slip along fault, note far field base line has elastically recovered along a horizontal datum Earthquake Cycle and Geodetic observations Stage 3 Possible post seismic transient response - viscoelastic relaxation post earthquake. Sometimes called “afterslip”. Fault plane between two crustal blocks Rupture (slip) along fault plane causes earthquake Stress builds up and strain accumulates Elastic rebound after earthquake The Physics of Friction Why friction? Because slip on faults is resisted by frictional forces. • Earthquake cycles, • Earthquake depth distribution, • Earthquake nucleation, • The mechanics of aftershocks, Question: Given that all objects shown below are of equal mass and identical shape, in which case the frictional force is greater? Question: Who sketched this figure? Da Vinci law and the paradox Leonardo Da Vinci (1452-1519) showed that the friction force is independent of the geometrical area of contact. The paradox: Intuitively one would expect the friction force to scale proportionally to the contact area. Amontons’ laws (1699) - verified by Coulomb later in 1781 Amontons' first law: The frictional force is independent of the geometrical contact area. Amontons' second law: Friction, FS, is proportional to the normal force, FN: FS = µFN € Bowden and Tabor (1950, 1964) A way out of Da Vinci’s paradox has been suggested by Bowden and Tabor, who distinguished between the real contact area and the geometric contact area. The real contact area is only a small fraction of the geometrical contact area. Figure from: Scholz (1990) FN = pAr , where p is the penetration hardness. € FS = sAr , where s is the shear strength. Thus: € FS p µ≡ = . FN s Since both p and s are material constants, so is µ. € This explains Da Vinci and Amontons’ laws. Byerlee’s law For σ N < 200MPa : µ = 0.85 For σ N > 200MPa : µ = 0.60 € Byerlee (1978) Static versus kinetic friction The force required to start the motion of one object relative to another is greater than the force required to keep that object in motion. µstatic µdynamic € € µstatic > µdynamic Ohnaka (2003) Velocity stepping - Dieterich Dieterich and Kilgore, 1994 • A sudden increase in the piston's velocity gives rise to a sudden increase in the friction, and vice versa. • The return of friction to steady-state occurs over a characteristic sliding distance. • Steady-state friction is velocity dependent. Slide-hold-slide - Dieterich Dieterich and Kilgore, 1994 Static (or peak) friction increases with hold time. Slide-hold-slide - Dieterich • The increase in static friction is proportional to the logarithm of the hold duration. Dieterich (1972) Monitoring the real contact area during slip - Dieterich and Kilgore Change in true contact area with hold time Dieterich and Kilgore (1994) • The dimensions of existing contacts are increasing. • New contacts are formed. Dieterich and Kilgore (1994) • The real contact area, and thus also the static friction increase proportionally to the logarithm of hold time. Summary of experimental results • Static friction increases with the logarithm of hold time. • True contact area increases with the logarithm of hold time. • True contact area increases proportionally to the normal load. • A sudden increase in the piston's velocity gives rise to a sudden increase in the friction, and vice versa. • The return of friction to steady-state occurs over a characteristic sliding distance. • Steady-state friction is velocity dependent. • The coefficient of friction response to changes in the normal stresses is partly instantaneous (linear elastic), and partly delayed (linear followed by non-linear). The constitutive law of Dieterich and Ruina * ⎞ ⎛ ⎛ ⎞ τ V θ V ∗ = µ = µ + Aln⎜ * ⎟ + Bln⎜ ⎟ ⎝ V ⎠ σ ⎝ DC ⎠ and dθ θV αθ dσ /dt = 1− − , dt DC B σ € were: • V and θ are sliding speed and contact state, respectively. • A, B and α are non-dimensional empirical parameters. • Dc is a characteristic sliding distance. • The * stands for a reference value. The set of constitutive equations is non-linear. Simultaneous solution of non-linear set of equations may be obtained numerically (but not analytically). Yet, analytical expressions may be derived for some special cases. • The change in sliding speed, ΔV, due to a stress step of Δτ: ( ) ΔV = exp Δτ Aσ . • Steady-state friction: * ⎞ ⎛ ⎛ ⎞ V θ V µss€= µ* + (A − B)ln⎜ ss* ⎟ = µ* + (B − A)ln⎜ ss ⎟ . ⎝ V ⎠ ⎝ Dc ⎠ • Static friction following hold-time, Δthold: µstatic ∝ (B − A)ln(θ 0 + Δt hold ) . €