Postseismic relaxation along the San Andreas fault at Parkfield from continuous seismological observations

Seismic velocity changes and nonvolcanic tremor activity in the Parkfield area in California reveal that large earthquakes induce long-term perturbations of crustal properties in the San Andreas fault zone. The 2003 San Simeon and 2004 Parkfield earthquakes both reduced seismic velocities that were measured from correlations of the ambient seismic noise and induced an increased nonvolcanic tremor activity along the San Andreas fault. After the Parkfield earthquake, velocity reduction and nonvolcanic tremor activity remained elevated for more than 3 years and decayed over time, similarly to afterslip derived from GPS (Global Positioning System) measurements. These observations suggest that the seismic velocity changes are related to co-seismic damage in the shallow layers and to deep co-seismic stress change and postseismic stress relaxation within the San Andreas fault zone.     

Fine structure of the landers fault zone: segmentation and the rupture process

Observations and modeling of 3- to 6-hertz seismic shear waves trapped within the fault zone of the 1992 Landers earthquake series allow the fine structure and continuity of the zone to be evaluated. The fault, to a depth of at least 12 kilometers, is marked by a zone 100 to 200 meters wide where shear velocity is reduced by 30 to 50 percent. This zone forms a seismic waveguide that extends along the southern 30 kilometers of the Landers rupture surface and ends at the fault bend about 18 kilometers north of the main shock epicenter. Another fault plane waveguide, disconnected from the first, exists along the northern rupture surface. These observations, in conjunction with surface slip, detailed seismicity patterns, and the progression of rupture along the fault, suggest that several simple rupture planes were involved in the Landers earthquake and that the inferred rupture front hesitated or slowed at the location where the rupture jumped from one to the next plane. Reduction in rupture velocity can tentatively be attributed to fault plane complexity, and variations in moment release can be attributed to variations in available energy.     

San andreas fault zone head waves near parkfield, california

Microearthquake seismograms from the borehole seismic network on the San Andreas fault near Parkfield, California, provide three lines of evidence that first P arrivals are "head" waves refracted along the cross-fault material contrast. First, the travel time difference between these arrivals and secondary phases identified as direct P waves scales linearly with the source-receiver distance. Second, these arrivals have the emergent wave character associated in theory and practice with refracted head waves instead of the sharp first breaks associated with direct P arrivals. Third, the first motion polarities of the emergent arrivals are reversed from those of the direct P waves as predicted by the theory of fault zone head waves for slip on the San Andreas fault. The presence of fault zone head waves in local seismic network data may help account for scatter in earthquake locations and source mechanisms. The fault zone head waves indicate that the velocity contrast across the San Andreas fault near Parkfield is approximately 4 percent. Further studies of these waves may provide a way of assessing changes in the physical state of the fault system.     

Clustering and periodic recurrence of microearthquakes on the san andreas fault at parkfield, california

The San Andreas fault at Parkfield, California, apparently late in an interval between repeating magnitude 6 earthquakes, is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters (<20-meter radius) of microearthquakes. Within these clusters, which account for 63% of the earthquakes in a 1987-92 study interval, virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes. Sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second. The rate of earthquake occurrence, particularly at depth, increased significantly during the study period, but the fraction of earthquakes that were cluster members decreased.     

Stress diffusion along the san andreas fault at parkfield, california

Beginning in January 1990, the epicenters of microearthquakes associated with a 12-month increase in seismicity near Parkfield, California, moved northwest to southeast along the San Andreas fault. During this sequence of events, the locally variable rate of cumulative seismic moment increased. This increase implies a local increase in fault slip. These data suggest that a southeastwardly diffusing stress front propagated along the San Andreas fault at a speed of 30 to 50 kilometers per year. Evidently, this front did not load the Parkfield asperities fast enough to produce a moderate earthquake; however, a future front might do so.     

A high-frequency secondary event during the 2004 Parkfield earthquake

By using seismic records of the 2004 magnitude 6.0 Parkfield earthquake, we identified a burst of high-frequency seismic radiation that occurred about 13 kilometers northwest of the hypocenter and 5 seconds after rupture initiation. We imaged this event in three dimensions by using a waveform back-projection method, as well as by timing distinct arrivals visible on many of the seismograms. The high-frequency event is located near the south edge of a large slip patch seen in most seismic and geodetic inversions, indicating that slip may have grown abruptly at this point. The time history obtained from full-waveform back projection suggests a rupture velocity of 2.5 kilometers per second. Energy estimates for the subevent, together with long-period slip inversions, indicate a lower average stress drop for the northern part of the Parkfield earthquake compared with that for the region near its hypocenter, which is in agreement with stress-drop estimates obtained from small-magnitude aftershocks.     

Dipping low-velocity layer in the mid-lower mantle: evidence for geochemical heterogeneity

Data from western United States short-period seismic networks reveal a conversion from an S to a P wave within a low seismic velocity layer (greater than or equal to the 4 percent velocity difference compared to the surrounding mantle) in the mid-lower mantle (1400 to 1600 kilometers deep) east of the Mariana and Izu-Bonin subduction zones. The low-velocity layer (about 8 kilometers thick) dips 30 degrees to 40 degrees southward and is at least 500 kilometers by 300 kilometers. Its steep dip, large velocity contrast, and sharpness imply a chemical rather than a thermal origin. Ancient oceanic crust subducted into the lower mantle is a plausible candidate for the low-velocity layer because of its broad thin extent.     

Oroville earthquakes: normal faulting in the sierra nevada foothills

Aftershocks of the Oroville, California, earthquake of 1 August 1975 define a 16- by 12-kilometer fault plane striking north-south and dipping 60 degrees to the west to a depth of 10 kilometers. Focal mechanisms from P-wave first motions indicate normal faulting with the western, Great Valley side downdropped relative to the Sierra Nevada block. The northward projection of the fault plane passes beneath Oroville Dam and crops out under the reservoir.     

Recognition of paleoearthquakes on the Puente Hills blind thrust fault,California

Borehole data from young sediments folded above the Puente Hills blind thrust fault beneath Los Angeles reveal that the folding extends to the surface as a discrete zone (相似文献   

The aftershock signature of supershear earthquakes

Recent studies show that earthquake faults may rupture at speeds exceeding the shear wave velocity of rocks. This supershear rupture produces in the ground a seismic shock wave similar to the sonic boom produced by a supersonic airplane. This shock wave may increase the destruction caused by the earthquake. We report that supershear earthquakes are characterized by a specific pattern of aftershocks: The fault plane itself is remarkably quiet whereas aftershocks cluster off the fault, on secondary structures that are activated by the supershear rupture. The post-earthquake quiescence of the fault shows that friction is relatively uniform over supershear segments, whereas the activation of off-fault structures is explained by the shock wave radiation, which produces high stresses over a wide zone surrounding the fault.     

Variation of P-Wave Velocity before the Bear Valley, California, Earthquake of 24 February 1972

Residuals for P-wave traveltimes at a seismnograph station near Bear Valley, California, for small, precisely located local earthquakes at distances of 20 to 70 kilometers show a sharp increase of nearly 0.3 second about 2 months before a magnitude 5.0 earthquake that occurred within a few kilometers of the station. This indicates that velocity changes observed elsewhere premonitory to earthquakes, possibly related to dilatancy, occur along the central section of the San Andreas fault system.     

Stress anomaly accompanying the 1979 lytle creek earthquake: implications for earthquake prediction

An unusual stress transient was recorded 15 kilometers from the epicenter of the Lytle Creek earthquake in southern California. It was observed at the recording site as an increased shear stress parallel to the fault surface and with the proper sense of shear to have triggered the earthquake. The anomaly began 2 to 4 weeks before the earthquake and lasted for 3 months.     

The parkfield, california, earthquake prediction experiment

Five moderate (magnitude 6) earthquakes with similar features have occurred on the Parkfield section of the San Andreas fault in central California since 1857. The next moderate Parkfield earthquake is expected to occur before 1993. The Parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process; observations and reports of seismicity and aseismic slip associated with the last moderate Parkfield earthquake in 1966 constitute much of the basis of the design of the experiment.     

The 1990 to 1991 Sudan earthquake sequence and the extent of the East african rift system

One of the largest earthquakes ever recorded in Africa (surface wave magnitude M(s) = 7.2) occurred about 50 kilometers east of the Upper River Nile on 20 May 1990. Four days later, two more large earthquakes (M(s) = 6.4 and 7.0) occurred about 50 kilometers to the northwest in the Nile Valley. In the following months, a further 60 events were recorded by seismic stations worldwide. The earthquakes are associated with two fault systems: one east of the Nile with azimuth southeast and one along the Nile Valley with azimuth north-northeast. The activity alternated between the two fault systems and indicates that the northern extremity of the western branch of the East African Rift System extends at least 350 kilometers north of Lake Albert.     

Natural and experimental evidence of melt lubrication of faults during earthquakes

Melt produced by friction during earthquakes may act either as a coseismic fault lubricant or as a viscous brake. Here we estimate the dynamic shear resistance (tau(f)) in the presence of friction-induced melts from both exhumed faults and high-velocity (1.28 meters per second) frictional experiments. Exhumed faults within granitoids (tonalites) indicate low tau(f) at 10 kilometers in depth. Friction experiments on tonalite samples show that tau(f) depends weakly on normal stress. Extrapolation of experimental data yields tau(f) values consistent with the field estimates and well below the Byerlee strength. We conclude that friction-induced melts can lubricate faults at intermediate crustal depths.     

Temporally unstable recurrence of earthquakes due to breaks in fractal scaling

Observed sequences of large earthquakes are not consistent in either recurrence time or energy release; long-term prediction has been impossible even in areas, such as Parkfield, with well-defined recurrence intervals. The seismic gap hypothesis, which predicts characteristic earthquakes in areas of the circum-Pacific belt that have not produced recent great earthquakes, has also failed to predict the observed clustering of high-energy events. Models in which fractal scaling is broken at high magnitude predict that characteristic events and recurrence behavior will be unstable in time. The central predictions of these models are supported by recent observations at Landers and Big Bear in California.     

Relations among fault behavior, subsurface geology, and three-dimensional velocity models

The development of three-dimensional P-wave velocity models for the regions surrounding five large earthquakes in California has lead to the recognition of relations among fault behavior and the material properties of the rocks that contact the fault at seismogenic depths; regions of high moment release appear to correlate with high seismic velocities whereas rupture initiation or termination may be associated with lower seismic velocities. These relations point toward a physical understanding of why faults are divided into segments that can fail independently, an understanding that could improve our ability to predict earthquakes and strong ground motion.     

Earthquakes near Parkfield, California: Comparing the 1934 and 1966 Sequences

Moderate-sized earthquakes (Richter magnitude M(L) 5(1/2)) have occurred four times this century (1901, 1922, 1934, and 1966) on the San Andreas fault near Parkfield in central California. In many respects the June 1966 sequence was a remarkably detailed repetition of the June 1934 sequence, suggesting a recurring recognizable pattern of stress and fault zone behavior.     

Seismic-wave attenuation associated with crustal faults in the new madrid seismic zone

The attenuation of upper crustal seismic waves that are refracted with a velocity of about 6 kilometers per second varies greatly among profiles in the area of the New Madrid seismic zone in the central Mississippi Valley. The waves that have the strongest attenuation pass through the seismic trend along the axis of the Reelfoot rift in the area of the Blytheville arch. Defocusing of the waves in a low-velocity zone and/or seismic scattering and absorption could cause the attenuation; these effects are most likely associated with the highly deformed rocks along the arch. Consequently, strong seismic-wave attenuation may be a useful criterion for identifying seismogenic fault zones.     

Seismic potential revealed by surface folding: 1983 coalinga, california, earthquake

The 2 May 1983 Coalinga, California, earthquake (magnitude 6.5) failed to rupture through surface deposits and, instead, elastically folded the top few kilometers of the crust. The subsurface rate of fault slip and the earthquake repeat time are estimated from seismic, geodetic, and geologic data. Three larger earthquakes (up to magnitude 7.5) during the past 20 years are also shown to have struck on reverse faults concealed beneath active folds.