Annual modulation of seismicity along the San Andreas Fault near Parkfield, CA (original) (raw)
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Pure and Applied Geophysics PAGEOPH, 1990
We analyse spectral ratio of the coda of doublets of microearthquakes. Our purpose is to find evidence for temporal changes of the attenuation in the crust before a large magnitude earthquake. A Moving Window Cross Spectral analysis of the coda of doublets gives a plot of the spectral ratio as a function of lapse time along the seismogram, for several frequency bands (SR(T,f) plot). From a certain pattern in the SR(T,f) plot, we should infer a temporal change in coda attenuation. Several doublets recorded in Central California by the USGS network are analysed.
Increased mean depth of earthquakes at Parkfield
Geophysical Research Letters, 1991
The mean focal depth of earthquakes in the northern and southern parts of the Parkfield segment of the San Andreas fault was greater than long term average by 1 km during the last 4 years approximately. It is proposed that this is a precursor to the next characteristic Parkfield earthquake, because the same phenomenon occurred before at least three other mainshocks along the San Andreas fault. In the northern segment the increase in mean depth is mostly due to reduction of shallow activity, in the southern segment deeper than usual activity seems to have increased. Within about 8 km from the 1966 epicenter no significant increase of mean depth is observed, but seismic quiescence [Wyss et al., 1990a] and a decrease in deformation rate [Wyss et al., 1990b] was documented. These observations are interpreted to suggest that the precursory process at Parkfield varies along strike, and that the next rupture may extend further north than the last one. Introduction Most earthquakes are found in a thin top layer of our planet, because only this layer is cool enough to be brittle. This seismogenic layer, approximately 15 km thick in California, is cut vertically by the San Andreas fault, where most earthquakes are confined to depths between 4 and 8 km. Plates are assumed to move past one another by aseismic creep below the seismogenic layer, because of the elevated temperatures. Several authors [e.g., Wesson et al., 1973; Smart and Mavko, 1979; Li and Rice, 1983] have proposed
Bulletin of the Seismological Society of America, 2017
This study explores methods to assess the seismic potential of a fault based on geodetic measurements, geological information of fault slip rate and seismicity data. The methods are applied to the Parkfield section along the San Andreas Fault (SAF) at the transition zone between the SAF creeping segment in the North and the locked section of Cholame to the south, where Mw~6 earthquakes occurred every 24.5 years on average since the M7.7 Fort Tejon earthquake of 1857. We compare the moment released by the known earthquakes and associated postseismic deformation with the moment deficit accumulated during the interseismic period derived from geodetic measurement of interseismic strain. We find that the recurring M6 earthquakes are insufficient to balance the slip budget. We discuss and evaluate various possible scenarios which might account for the residual moment deficit and implications of the possible magnitude and return Manuscript Click here to download Manuscript Parkfield_Clean.docx 2 period of Mw>6 earthquakes on that fault segment. The most likely explanation is that this fault segment hosts M6.5 to M7.5 earthquakes, with a return period of 140 to 300 years. Such events could happen as independent earthquakes in conjunction with ruptures of the Carrizo plain segment of the SAF. We show how the results from our analysis can be formally incorporated in probabilistic seismic hazard assessment assuming various magnitudefrequency distribution and renewal time models.
Stress perturbations influence earthquake recurrence and are of fundamental importance to understanding the earthquake cycle and determining earthquake hazard. The large population of repeating earthquakes on the San Andreas fault at Parkfield, California, provides a unique opportunity to examine the response of the repeating events to the occurrence of moderate earthquakes. Using 187 M 0:4 to ∼1:7 repeating earthquake sequences from the High Resolution Seismic Network catalog, we find that the time to recurrence of repeating events subsequent to nearby M 4-5 earthquakes is shortened, suggesting triggering by major events. The triggering effect is found to be most evident within a distance of ∼5 km, corresponding to static coseismic stress changes of > 0:6 26:6 kPa, and decays with distance. We also find coherently reduced recurrence intervals from 1993 to 1998. This enduring recurrence acceleration over several years reflects accelerated fault slip and thus loading rates during the early 1990s.
2006
The 1985 prediction of a characteristic magnitude 6 Parkfield earthquake was unsuccessful, since no significant event occurred in the 95% time window (1985-1993) anywhere near Parkfield. The magnitude 6 earthquake near Parkfield in 2004 failed to satisfy the prediction not just because it was late; it also differed in character from the 1985 prediction and was expectable according to a simple null hypothesis. Furthermore, the prediction was too vague in several important respects to meet the accepted definition of an earthquake prediction. An event occurring by chance and meeting the general description of the predicted one was reasonably probable. The original characteristic earthquake model has failed in comprehensive tests, yet it is still widely used. Modified versions employed in recent official seismic hazard calculations allow for interactions between segments and uncertainties in the parameters. With more adjustable parameters, the modified versions are harder to falsify. The characteristic model as applied at Parkfield and elsewhere rests largely on selected data that may be biased because they were taken out of context. We discuss implications of the 2004 event for earthquake prediction, the characteristic earthquake hypothesis, and earthquake occurrence in general.
Within 24 hr after the Landers earthquake, there were three magnitude 3.4+ events in western Nevada and an unexpected, widespread increase in the rate of small events. Based on combined catalogs for northern Nevada, southern Nevada, and southern California, and a model that assumes statistical independence of events in these regions, the probability of this happening in a 24-hr period by random chance is less than ~10 -~2 per day. Therefore, there is high statistical confidence that the increased seismicity was triggered by the Landers event. Based on the statistical model, we develop a list of 227 earthquakes in the first 83 days following the Landers earthquake, each of which has no more than 10% probability of occurring by random chance. The suspect events are broadly distributed in regions that correlate with historical activity in the Great Basin. The events are not uniquely associated with known volcanic activity, or with zones that were previously active with microearthquakes or aftershock sequences. The magnitudes of the largest triggered events appear to decrease with distance. With time, the number of suspect events decreases at a rate comparable to the rate of decrease of aftershocks of the Landers and Big Bear earthquakes.
Tectonophysics, 2006
In this work, we apply the Pattern Informatics technique for evaluating one surface expression of the underlying stress field, the seismicity, in order to study the Parkfield-Coalinga interaction over the years preceding the 1983 Coalinga earthquake. We find that significant anomalous seismicity changes occur during the mid-1970s in this region prior to the Coalinga earthquake that illustrate a reduction in the probability of an event at Parkfield, while the probability of an event at Coalinga is seen to increase. This suggests that the one event did not trigger or hinder the other, rather that the dynamics of the earthquake system are a function of stress field changes on a larger spatial and temporal scale.
Journal of Geophysical Research, 1993
Three-dimensional finite element calculations are employed to study interactions in space and time between the creeping segment of the San Andreas fault in central California and the adjacent currently locked zones of the 1857 and 1906 great earthquakes. Vertically, the model consists of an elastic upper crust over a Maxwell viscoelastic region, representing the entire lower crust or a narrower horizontal detachment layer, and a stiffer and more viscous upper mantle. The crust has a single vertical fault extending to the top of the mantle at 25 km depth. In zones along strike corresponding to the 1857 and 1906 events, the top 12.5 km of the fault is locked against slip, except in great earthquakes. Below the locked zones and everywhere along the creeping region between them, the fault is freely slipping. The model parameters are compatible with seismological and geological observations, and with a ratio of Maxwell relaxation time to the relaxing layer thickness in the range 1 to 2 yr/km, as established by Li and Rice (1987) and Fares and Rice (1988) based on fits to geodetic data along the San Andreas fault. An imposed constant far field shear motion and periodic 1857-and 1906 -type earthquakes generate slip rates along the creeping fault segment that evolve in time throughout the entire earthquake cycle. Shortly after an adjacent great earthquake, slip rates in the creeping zone are higher than the far field velocity, while later in the cycle they are lower. Hence, time dependency should be accounted for when measurements of fault slip are used to estimate the plate motion. If Parkfield earthquakes are a response to a time dependent loading of the type simulated here, their recurrence interval would tend to lengthen with time since the 1857 event. Thus, the hypothesis of characteristic periodic earthquakes at Parkfield may not provide the best estimate of the occurrence time of the next event. Using, for example, the statistics of past events and assuming that Parkfield earthquakes are a response to a slip deficit near Middle Mountain, and that the elastic crustal layer is 17.5 km thick, we find that the next event is predicted for about 1992 __ 9 years if the lower crust is a 7.5 km thick layer having a material relaxation time of 15 years, and 1995 ñ 11 years if the 7.5 km thick lower crust is characterized by a relaxation time of 7.5 years. These values may be compared to the 1988 ñ 7 years estimate based on periodicity in time. The modeling results also indicate that the interaction between the 1857 and 1906 rupture zones is small. 1906 ;URFACE EAK BQutl•tg Hollister K• Kine oClty 50 J Porkfield GH