Living on an Active Earth Perspectives on Earthquake Science (2003) / Chapter Skim
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5. Earthquake Physics and Fault-System Science
5. Earthquake Physics and Fault-System Science
Pages 256-349
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From page 256... ...
Applied earthquake science seeks to predict seismic hazards by forecasting earthquakes and their site-specific effects. Research on the first problem began with attempts to place earthquake occurrence in a global framework, and it contributed to the discovery of plate tectonics; research on the second was driven by the needs of earthquake engineering, and it led to the development of seismic hazard analysis.
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From page 257... ...
The dynamics of the continental lithosphere involves not only the sudden fault slips that cause earthquakes, but also the folding of sedimentary layers near the surface and the ductile motions of the hotter rocks in the lower crust and upper mantle. Moreover, because earthquake source regions are inaccessible and opaque, the state of the lithosphere at seismogenic depths simply cannot be observed by any direct means, despite the conceptual and technological breakthroughs described in Chapter 4.
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From page 258... ...
Individual events are also complex in the disordered propagation of their rupture fronts and the heterogeneous distributions of residual stress that they leave in their wake. At the smallest scales, earthquake initiation appears to be complex, with a slowly evolving nucleation zone preceding a rapid dynamic breakout that sometimes cascades into a big rupture.
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From page 259... ...
Earthquake faults, or arrays of coupled faults, seem to be natural candidates for this kind of behavior; such systems are constantly being driven by tectonic forces toward slipping thresholds (8~. If the thermodynamic analogy were valid, then the fluctuations the slipping events would be self-similar and scale invariant, and their sizes would obey power-law distributions.
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From page 260... ...
The more conventional and perhaps obvious assumption is that the heterogeneity of fault zones their geometric disorder and strong variations of lithological properties plays the dominant role. It appears that earthquake faults, when modeled in any detail, have relevant length and time scales that invalidate simple scaling assumptions.
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From page 261... ...
Most of the seismic energy is released in the large events; thus, it seems reasonable to suppose that the system suffers most of its memory loss during those events as well. If this supposition were correct, earthquake prediction on a time scale of months or years intermediate-term prediction of the sort described in Section 2.6 would, in principle, be possible.
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From page 262... ...
how theological properties of the fault-zone material interact with rupture propagation and fault-zone heterogeneity to control earthquake history and event complexity, and (2) to what extent scientists can use this knowledge to predict, if not individual earthquakes, then at least the probabilities of seismic hazards and the engineering consequences of likely seismic events.
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From page 263... ...
It is possible that many features of this small-scale behavior are imprinted in important ways on the subsequent large-scale events, but it is also possible that only one or two parameters pertaining to nucleation perhaps the location and initial stress drop (plus the surrounding stress and strain fields, of course) have to be specified in order to predict accurately what happens next.
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From page 264... ...
There are strongly different conceptions of fault systems, all of which may have merit for some purposes (18~. Faults can be modeled as smooth Euclidean surfaces of displacement discontinuity in an otherwise continuous medium; fault systems can be represented as fractal arrays of surfaces; fault segments can be regarded as merely the deforming borders between blocks of a large-scale granular material transmitting stress in a force-chain mode.
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From page 265... ...
This diffuse nature is clearly related to the greater thickness and quartz-rich composition of the continental crust, as described in Section 2.4. The structure of continental fault zones is thought to be complicated by variations in frictional behavior with depth, changes in wear mechanisms, and a brittle-ductile transition (Figure 4.30)
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From page 266... ...
These interactions have created a zone of deformation a thousand kilometers wide that extends from the continental coastline to the Rocky Mountains. The "master fault" of this plate-boundary zone is the strikeslip San Andreas system, but other types of faults participate in the deformation, from extension in the Basin and Range to contraction in the Transverse Ranges.
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From page 267... ...
One approach to evaluating repeatability and periodicity of earthquakes employs seismic data from smaller earthquakes. Along the creeping portion of the San Andreas fault in central California, M 4 to M 5 earthquakes have been frequent enough to enable studies of their similarity.
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From page 268... ...
Seismicity and Scaling Earthquake scaling laws, and the circumstances under which they break down, furnish insights on fault interactions that carry important ramifications for seismic hazard analysis and earthquake prediction. For instance, the seismicity of individual faults does not follow the GutenbergRichter relation (28)
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From page 269... ...
following large events along the San Andreas fault system (32) , the 1999 Izmit earthquake in Turkey (33)
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From page 270... ...
This approach has resulted in a statedependent model for earthquake rates that provides quantitative explanations for observed aftershock rates in response to a stress change, the Omori decay law, and various other features of aftershocks (Box 5.1~. Aftershocks can also be generated by dynamic stresses during the passage of seismic waves.
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From page 272... ...
shock triggering by seismic waves are poorly understood but may in volve fluid-rock interactions or triggering of local deformations that pro duce permanent stress changes after the waves have passed through a region. Foreshocks Foreshocks are generally thought to arise by one of two mechanisms.
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From page 273... ...
and following the M 6.9 earthquake that struck Kobe, Japan, in 1995 (41~. Accelerating Seismicity and Intermediate-Tenn Prediction A central issue for earthquake prediction is the degree to which the seismicity clustering can be used to monitor the stress changes leading to large earthquakes.
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From page 274... ...
Are mature faults such as the San Andreas weak?
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From page 275... ...
· What special processes occur at borders or transition regions between creeping zones, whether localized on faults or distributed, and fault zones that are locked between seismic events? Do lineations of microseismicity provide evidence for processes along such borders?
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From page 276... ...
Nevertheless, over the same time, close geological investigations of exhumed fault zones (53) have strengthened the viewpoint that much of the observed complexity of damage zone and secondary fault structures bordering large-slip faults could indeed be a relatively inactive relic of evolution and that, with ongoing slip accumulation, faults become more like Euclidean surfaces (54~.
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From page 277... ...
Friction of Fault Materials Experimentally determined constitutive laws, such as those presented in Box 4.4, have been validated for slip rates between about 10-~° and 10-3 meter per second. As such, they cover the range from plate rates to rates at which incipient dynamic instabilities are well under way, so they probably provide an appropriate description of frictional processes during earthquake nucleation and postseismic response.
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From page 278... ...
Earthquake Mechanics in Real Fault Zones It may be conjectured that different physical mechanisms prevail at contacts during the most violent seismic instabilities, when average slip rates reach 1 meter per second and maximum slip rates near the rupture front might be as great as 102 meters per second. In that range, the dynamics of rapid stress fluctuations from sliding on a rough surface, openings of the rupture surfaces, microcracking, and fluidization of finely comminuted fault materials may result in a different velocity dependence, possibly with a dramatic weakening.
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From page 279... ...
Thus, one option is that faults slide during large earthquake slips at stresses on the order of 100 megapascals. This is, however, in conflict with the well-known lack of a sharply peaked heat outflow over the San Andreas fault (see Section 2.5~.
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From page 280... ...
Yet another possibility is that dynamic weakening may be responsible for the low-stress observations along the San Andreas fault. Sources could include severe thermal weakening, including melt formation, in rapid, large slips, as above, or the formation of gouge structures that accommodate slip by rolling with little frictional dissipation (79~.
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From page 281... ...
Can damage during large earthquake ruptures explain the discrepancy between the small values of the critical slip distance found in the laboratory (less than 100 microns) and the large values inferred from the fracture energies of earthquakes and assumptions about the drop from peak strength for slip initiation to dynamic friction strength (5 to 50 millimeters if the strength drop is 100 megapascals, but an order of magnitude higher for 10 megapascals)
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From page 282... ...
waves at some distance from the fault. Better knowledge of the physics of rupture propagation and frictional sliding on faults is therefore critical to understanding and predicting earthquake ground motion.
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From page 283... ...
A second line of evidence comes from rupture dimensions of the smallest earthquakes, which place an upper bound on the size of the nucleation zone since slip over an area less than LC must be stable. Microearthquakes on the San Andreas fault recorded on the downhole instruments of the deep Cajon Pass borehole have source dimensions of about 10 meters (85~.
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From page 284... ...
Lines show the slip speeds at suc
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From page 285... ...
SOURCE: l.H. Dieterich, Earthquake nucleation on faults with rate- and state-dependent strength, Tectonophysics, 211, 115-134, 1992.
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From page 286... ...
The character of the onset of microearthquakes suggests that very small events also begin with a slow onset that scales in duration with the overall source duration (89~. The first arriving seismic waves of moderate to large events in the near field often show an initial phase of irregular growth (90~.
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From page 287... ...
Ihmle, and T Jordan, Time-domain observations of a slow precursor to the 1994 Romanche Transform earthquake, Science, 274, 82-85, 1996.
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From page 289... ...
v v / v Ho (1992) v / Magnitude 0 2 4 6 8 1 1 1 1 1 I I I I I I I I I I I T I 1o8 1o1o 1012 1014 1016 1018 1o2o 1022 Seismic Moment (N-m)
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From page 290... ...
These factors have conspired to impede progress in understanding the mechanics of earthquake rupture; nevertheless, such an understanding is central to many of the most important goals of earthquake science, such as predicting the level and variability of strong ground motion, characterizing the nature of large earthquake recurrence, and understanding the extent to which earthquakes might be predictable.
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From page 291... ...
The average offset on a fault u and its characteristic dimension L are related to the static stress drop /\`r by the formula /\`r = csG u /L, where CS is a constant determined from crack theory, which depends on the fault type. In crack theory, the rupture velocity is a function of the fracture energy near the crack tip (96~.
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From page 292... ...
If supershear rupture propagation should prove common, however, it would have important implications for strong ground motion. During an episode of supershear rupture propagation, an earthquake will form a Mach cone, the seismic equivalent of a sonic boom, but in the case of earthquakes, a high-amplitude wavefront will result (107)
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From page 293... ...
, as well as smaller silent earthquakes on the San Andreas fault system (115~. A study of the Earth's longest-period free oscillations found excitations of the Earth's free oscillations that were not accounted for by known earthquake activity (116~.
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From page 294... ...
Because our understanding of strong ground motion is based primarily on a limited number of moderate earthquakes (M < 7.0) , an increased understanding of the factors that control strong ground motion, such as the rise time, is essential in efforts to extrapolate observations of strong ground motion in moderate earthquakes to larger earthquakes.
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From page 295... ...
Areas of strong high-frequency generation are observed to correlate with areas of strong slip variations (141) , and there are stochastic models of earthquake rupture that lead to realistic strong ground motions (142~.
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From page 296... ...
The possibility of multiple-segment ruptures has been included explicitly in assessments of earthquake probabilities in California (150~. One factor that determines how effectively a discontinuity will act to limit fault rupture is the distance between the offset fault segments.
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From page 297... ...
A possible interpretation is that these areas of the fault accumulate shear stress, while parts of the fault that are bordered by lower-velocity material may slip aseismically. The observation that the creeping section of the San Andreas and Calaveras faults in California have areas of micro-earthquake activity interspersed with small zones that fail repeatedly in small earthquakes (157)
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From page 298... ...
Deep Earthquakes Earthquakes below 70 kilometers present special research problems because fault ruptures at these depths cannot be explained by brittle fracture or friction (Figure 5.8; see Section 2.5~. Although their depths limit the seismic hazard (164)
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From page 299... ...
Copyright 1995 American Geophysical Union. Reproduced by permission of American Geophysical Union.
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From page 300... ...
, and instabilities associated with recrystallization during polymorphic phase transitions (172~. Dehydration embrittlement, which involves the lowering of the effective normal stress by water pressure from dehydration, is a leading contender for at least some intermediate-focus events (173)
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From page 301... ...
· What is the nature of fault friction under slip speeds characteristic of large earthquake ruptures? How can data on fault friction from laboratory experiments be reconciled with the earthquake energy budget observed from seismic radiation and near-fault heat flow?
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From page 302... ...
Seismic shaking is influenced heavily by the details of how seismic waves propagate through complex geological structures. In particular, strong ground motions can be amplified by trapping mechanisms in sedimentary basins and by wave multipathing along sharp geologic boundaries at basin edges, as well as by amplifications due to near-site properties.
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From page 303... ...
The normal-mode representation forms the basis for recovering earthquake source parameters from surface-wave and other low-frequency data. For three-dimensional Earth models where the deviations from spherical symmetry are relatively small, normal-mode perturbation theory provides general and efficient methods for computing theoretical seismograms, and it has been applied in many global tomographic studies to invert low-freauencv seismic waveforms for three-dimensional Earth structure.
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From page 304... ...
The critical reflections from the Moho (SmS phases) that dominate the seismograms shown in Figure 5.9 have an important effect on the attenuation of strong ground motion from earthquakes.
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From page 305... ...
Stead, and H Kanamori, Impact of broadband seismology on the understanding of strong motions, Bull.
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From page 306... ...
Herrmann, and D.V. Helmberger, The effect of crustal structure on strong ground motion attenuation relations in eastern North America, Bull.
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From page 307... ...
The synthetic seismograms calculated using a simple point source time function and a one-dimensional velocity model for the region provide a remarkably close fit to both the long-period and the short-period components of the Pnl and Snl body-wave phases generated by the crustal waveguide. Effects of Sedimentary Basins For many years, it has been known that ground motions on soil sites are typically stronger than those on rock sites due to the low shear moduli of the near-surface (upper 30 meters)
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From page 308... ...
These and other simulations of basin waves (194) demonstrate that it is now possible to perform simulations of strong ground motions in basin structures, and these demonstrations form the basis for the simulation of ground motions from scenarios of future earthquakes (195~.
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From page 309... ...
The enormous destructive potential of near-fault ground motions was manifested in the 1994 Northridge and 1995 Hyogoken Nanbu earthquakes. In each of these earthquakes, peak ground velocities as high as 175 centimeters per second were recorded (Figure 5.13~.
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From page 310... ...
Nonlinear Site Effects Soil response to strong shaking is a complex, nonlinear phenomenon that has long been investigated in laboratory experiments and in the field following large earthquakes (199~. Laboratory tests clearly demonstrate nonlinear strain behavior in soils under dynamic loading.
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From page 311... ...
Pitarka, and P.G. Somerville, Ground motion amplification in the Santa Monica area: Effects of shallow basin edge structure, Bull.
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From page 312... ...
312 LIVING ON AN ACTIVE EARTH: PERSPECTIVES ON EARTHQUAKE SCIENCE FIGURE 5.13 Recorded near-fault acceleration and velocity time histories and acceleration response spectra of the 1994 Northridge earthquake (top) and 1995 Hyogo-ken Nanbu earthquake (bottom)
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From page 313... ...
High-Frequency Ground Motions Ground motions at frequencies above 1 hertz are the most damaging motions for small- and moderate-sized structures, and they also contain important information about the seismic source and details of stress on the fault plane. The character of high-frequency ground motions was documented from the analysis of the first strong-motion accelerograms (203~.
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From page 314... ...
a better understanding of the geological controls on strong ground motions. The challenge is to recast the methodology of seismic hazard analysis in a way that more explicitly accounts for the dependence of earthquake phenomena on time.
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From page 315... ...
These points are illustrated below in discussions of characteristic earthquakes, seismic gaps, moment-rate budgeting, clustering, and stress-transfer effects. Characteristic Earthquakes The characteristic earthquake hypothesis states that seismic moment release on an individual fault segment is dominated by a characteristic earthquake rupturing the entire length of the segment (i.e., the largest possible earthquake for that segment)
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From page 316... ...
Because of these simplifications, characteristic earthquakes have been incorporated into a large number of seismic hazards analyses (209~. Seismic Gap Hypothesis Building on the characteristic earthquake hypothesis, the seismic gap hypothesis addresses the distribution of these large events through time.
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From page 317... ...
Using these methods to calculate tectonic stress accumulations is more complex because it requires assumptions about strain partitioning throughout fault and plate boundary zones. Application to Seismic Hazard Analysis There have been continuing efforts to utilize the understanding of earthquake forecasting to improve the capabilities of PSHA.
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From page 318... ...
Prediction of Strong Ground Motions Seismic waves travel through a medium having a free surface, strong variations (usually increases) of seismic velocity with depth, large-scale lateral variations in seismic velocities related to mountains and sedimentary basins, small-scale lateral variations (scatterers)
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From page 319... ...
Characterization of Site Response Local geological conditions have a primary influence on the amplitude and frequency content of strong ground motions. In particular, the vertical gradient in shear-wave velocity (which generally increases rapidly with depth just below the surface)
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From page 320... ...
(Bottom) Peak acceleration attenuation relations for crustal earthquakes showing the dependence on magnitude and site category.
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From page 321... ...
In these codes and provisions, site response is represented by period- and amplitude-dependent factors derived from sets of recorded data and from analyses of site response based on nonlinear or equivalentlinear models of soil response. Ground-Motion Prediction Using Seismological Models Based on developments in theoretical and computational seismology and on strongmotion recordings from a large number of major earthquakes that began with the 1979 Imperial Valley earthquake, much progress has been made in understanding the origin and composition of strong ground motion.
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From page 322... ...
The use of different methods in these two vibrational period ranges is necessitated by the observation that ground motions are much more stochastic at short periods than at long periods. An example of broadband simulation of strong ground motions is shown in Figure 5.15, which compares the recorded and simulated ground motions at Arleta from the 1994 Northridge earthquake.
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From page 323... ...
Wald, and R Graves, Implications of the Northridge earthquake for strong ground motions from thrust faults, Bull.
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From page 324... ...
Can the shaking from large earthquakes be predicted accurately from smaller events? · How important is the nonlinear seismic response of stable soils in estimating strong ground motion?
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From page 325... ...
Boatwright, Fractal analysis applied to characteristic segments of the San Andreas fault, J Geophys.
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From page 326... ...
Schwartz and K.J. Coppersmith, Fault behavior and characteristic earthquakes: Examples from the Wasatch and San Andreas fault zones, J
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From page 327... ...
Sykes, Change in the state of stress on the southern San Andreas fault resulting from the California earthquake sequence of April to June 1992, Science, 258, 1325-1328, 1992; R.S. Stein, G.C.P.
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From page 328... ...
Harris (Introduction to special section: Stress triggers, stress shadows, and implications for seismic hazard, J Geophys.
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From page 329... ...
Egbert, and J Booker, High-resolution electromagnetic imaging of the San Andreas fault in central California, J
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From page 330... ...
Evans, and R.L. Biegel, Internal structure and weakening mechanisms of the San Andreas fault, J
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From page 331... ...
Chester, Ultracataclasite structure and friction processes of the Punchbowl fault, San Andreas system, California, Tectonophysics, 295,199-221, 1998.
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From page 332... ...
Wentworth, New evidence of the state of stress on the San Andreas fault system, Science, 238, 1105-1111, 1987.
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From page 333... ...
in their study of seven moderate earthquakes in California. In the case of the 1989 Loma Prieta earthquake, the constraint from borehole strainmeter observations is that any precursory slip could not have accounted for more than about 0.3 percent of the co-seismic moment.
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From page 334... ...
It is difficult to test whether this scaling applies to the aseismic part of the nucleation process because the preceding signals would be undetectable using seismic waves. The seismic moment of the seismic nucleation phase was found to average about 0.5 percent of the mainshock seismic moment.
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From page 335... ...
From these they found that the foreshocks were unlikely to have triggered the mainshock through shear, normal, or mean stress changes. The same study also found that the dimension of the foreshock zones followed the same scaling, inferred independently for the seismic nucleation phase, suggesting that the two phenomena may be related.
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From page 336... ...
Abrahamson, Modification of empirical strong ground motion attenuation relations to include the amplitude and duration effects of rupture directivity, Seis.
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From page 337... ...
Kanamori, The origin of the tsunami excited by the 1989 Loma Prieta earthquake Faulting or slumping, Geophys.
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From page 338... ...
discovered an extremely slow earthquake sequence of approximately one-week duration in strainmeter and creepmeter records of the San Andreas fault near San Juan Bautista, California. The sequence was punctuated by several episodes of accelerated slip and a few microearthquakes.
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From page 339... ...
Examples of quasi-dynamic models with short rise times include the following: H Quin, Dynamic stress drop and rupture dynamics of the October 15,1979 Imperial Valley, California, earthquake, Tectonophysics, 175, 93-117, 1990; T
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From page 340... ...
Heaton, Inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 Imperial Valley, California earthquake, Bull.
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From page 341... ...
Am., 37,19-31, 1947; G.W. Housner, Properties of strong ground motion earthquakes, Bull.
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From page 342... ...
146. For example, fault segmentation was used to estimate the size of future earthquakes in the report by the Working Group on California Earthquake Probabilities (Probabilities of Large Earthquakes Occurring in California on the San Andreas Fault, U.S.
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From page 343... ...
Lett., 25, 45494552, 1998; T.E. Tullis, Perspective Deep slip rates on the San Andreas fault, Science, 285, 671-672, 1999.
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From page 344... ...
that killed 60,000 people. Seismic waves propagate efficiently below the asthenosphere (i.e., below about 300-kilometer depth)
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From page 345... ...
K.B. Olsen, Site amplification in the Los Angeles Basin from 3D modeling of ground motions, Bull.
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From page 346... ...
191. Moho reflections were partially responsible for the strong ground motions that damaged San Francisco after the 1989 Loma Prieta earthquake (P.G.
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From page 347... ...
Archuleta, and J.R. Matarese, Magnitude 7.75 earthquake on the San Andreas fault; Three-dimensional ground motion in Los Angeles, Science, 270,1628-1632, 1995; K.B.
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From page 348... ...
Hanks (b values and BAY seismic source models; Implications for tectonic stress variations along active crustal fault zones and the estimation of highfrequency strong ground motion, J Geophys.
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From page 349... ...
213. See Working Group on California Earthquake Probabilities, Seismic hazards in southern California: Probable earthquakes, 1994-2024, Bull.
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