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User Tools. Sign In. Advanced Search. Article Navigation. Close mobile search navigation Article navigation. Volume , Number 3. Previous Article Next Article. Research Article June 01, The San Andreas fault system through time J. Crowell J. Google Scholar. Journal of the Geological Society 3 : Article history received:. Abstract The active San Andreas fault system today lies at the splintered boundary between the Pacific and North American lithospheric plates, a tectonic arrangement that originated in California in the Oligocene.
This content is PDF only. Please click on the PDF icon to access. First Page Preview. Close Modal. You do not currently have access to this article. Because the algorithm used for atmospheric corrections works best for the gradually varying deformation signals, there is a certain amount of smearing of steps and rapid changes in surface velocities with time e. This may be responsible for the less impulsive onset of slip transient in the InSAR time series compared, for example, to the creepmeter observations, which have a much higher temporal resolution Figure 3 a ; however, we cannot rule out a possibility that at some locations along the fault, a slow slip event was already underway at the time of the Chiapas earthquake.
In places that exhibit a good agreement between the magnitudes of surface slip measured by InSAR sampling surface displacements at distances of a few hundred meters away from the fault trace and creepmeter and geologic observations Figure 9 , most of slip is inferred to have occurred on a highly localized interface, possibly as narrow as a few millimeters Figure 5 , similar to earlier observations from the Superstition Hills Fault Wei et al. At other locations where displacements recorded by creepmeters and geological measurements are lower than those observed with InSAR, even if one takes into account uncertainties of each measurement Figure 9 , only a fraction of the total shear motion involved in the triggered slip event occurs within a few meters from the fault trace, and the remainder must be distributed in a broader shear zone.
The width of the shear zone also has implications for paleoseismic studies of fault slip, such that piercing point estimates of coseismic offsets may underestimate slip if the latter is distributed in a broader zone. Results are shown in Figures S15 and S Interestingly, inversions of surface displacements indicate that the magnitude of surface slip appears to anticorrelate with the depth extent of slip. This may be due to the spatiotempral variability in slip history in the shallow conditionally stable layer.
San Andreas Fault Study Leads to Better Understanding of Earthquakes
The slip depths suggested by our inversions are comparable to the range of thicknesses of the sedimentary layer and the depth extent of creep reported by Sieh and Williams The inversions also suggest systematic increases in the fault dip angle from the Coachella Valley toward the Bombay Beach, consistent with models of Tymofyeyeva and Fialko The attitude of the SSAF in the uppermost crust has been the subject of some debate.
Fuis et al. Assuming that the slip interface that produces shallow creep connects to the seismogenic part of the SSAF, results shown in Figure 10 favor an eastward dipping geometry of the SSAF throughout all of the upper crust. The distribution of surface slip that occurred following the Chiapas earthquake Figure 9 does not follow that observed between and Lindsey, Fialko, et al. Also, Lindsey, Fialko, et al. The slow slip event triggered by the Chiapas earthquake does not follow the same pattern.
In particular, we observe a fairly localized surface slip along the transtensional North Shore segment Figure 9. This is also supported by the creepmeter data that showed a sizeable triggered slip event Figure 3 a for the first time since the recording began in The comparison of surface displacements that occurred over different epochs e.
This is similar to the shallow slip histories reported at other locations e. The variability in the amplitude and depth extent of triggered creep on the SSAF may have several explanations. First, the frictional properties of the uppermost crust may be variable both downdip and along the strike, possibly with some stochastic distribution, similar to a model proposed by Jiang and Fialko for a transitional zone at the bottom of the seismogenic layer. Second, the dynamic perturbations on different parts of the shallow fault may be more variable than suggested by surface seismic observations.
That, combined with the high sensitivity of such faults to stress perturbations, may lead to variability in triggered creep along the fault. Third, the behavior of triggered creep also depends on the past fault creep history at a given location. The timing of stress perturbations and the characteristic times of fault recovery to the unperturbed state may strongly influence the triggering thresholds, the magnitude of triggered slip, and the subsequent slip evolution.
The two types of models can both produce the spontaneous and triggered creep near the surface. Combining more advanced models with the increasingly rich observational data sets will help further illuminate physical mechanisms of shallow creep and rheological properties of the uppermost crust for major faults such as the SSAF.
We presented geodetic and geologic measurements of a transient creep event on the SSAF that ruptured the shallow part of the fault from Bombay Beach to the Mecca Hills. The timing of the event suggests that it was triggered by dynamic stress changes from the M w 8. At these locations e. A comparison of the new data with previously published results suggests that the rates of shallow creep on the SSAF may be essentially constant on time scales on the order of 10 years or greater but can vary considerably on shorter time scales. Such models may ultimately help refine our knowledge of potential connections between shallow creep events and earthquakes.
We thank two anonymous reviewers for insightful and detailed reviews. Digital Elevation data were provided by DLR. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries other than missing content should be directed to the corresponding author for the article. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account. If the address matches an existing account you will receive an email with instructions to retrieve your username.
Open access. Journal of Geophysical Research: Solid Earth. Research Article Free Access. Ekaterina Tymofyeyeva Corresponding Author E-mail address: katia. Tymofyeyeva, E-mail address: katia. Thomas K. Shahram Moafipoor Geodetics Inc. Tools Request permission Export citation Add to favorites Track citation.
Share Give access Share full text access. Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Figure 1 Open in figure viewer PowerPoint.
Map of the study area. Green dots denote seismicity Hauksson et al. The red rectangle outlines a section of the southern San Andreas fault involved in a recent shallow slow slip event. Figure 2 Open in figure viewer PowerPoint. Schematic view of buried creepmeter installed obliquely across a fault. Figure 3 Open in figure viewer PowerPoint. Dates of aseismic subevents are indicated. A southward propagating slip event on 30 November that may have nucleated in the Mecca Hills was recorded by all five extensometers, albeit weakly by the East Shoreline extensometer.
The 7 November slip event also propagated southward but was recorded only at Ferrum and Salt Creek. Figure 4 Open in figure viewer PowerPoint. Fault locations where triggered slip was investigated. Minor cracking was noted at Salt Creek, and fracturing, interpreted as possible surface cracking, was found near North Shore.
Figure 5 Open in figure viewer PowerPoint. Example of en echelon surface rupture associated with creep event showing the small dilational steps used to measure displacement. Figure 6 Open in figure viewer PowerPoint.
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Figure 7 Open in figure viewer PowerPoint. Figure 8 Open in figure viewer PowerPoint. The point locations are shown in Figure 7. The dashed vertical line denotes the time of the Chiapas earthquake. Figure 9 Open in figure viewer PowerPoint.
Distribution of triggered slip along the fault. White triangles represent measurements made during the field survey, purple squares represent creepmeter data, and black circles represent Interferometric Synthetic Aperture Radar data.
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The location of the Shoreline creepmeter is plotted as a brown circle. Figure 10 Open in figure viewer PowerPoint. Results of inversions of surface displacements for the amplitude, depth, and dip of shallow creep. Shaded blue areas denote the model uncertainties. The respective model parameters are presented below each model.
San Andreas Fault
Figure 11 Open in figure viewer PowerPoint. Dynamic stress perturbations from passing seismic waves. Surface traces of holocene and quaternary faults are marked in black, and the Salton Sea is drawn in blue. Waveforms are shifted to account for the difference in surface wave arrival times before averaging. Figure 12 Open in figure viewer PowerPoint.
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Simple models for fault slip and rheology of the southern San Andreas fault. We impose fault slip rates in regions marked in black and solve for the fault stress, slip rate, and state variables in regions marked in blue. Figure 13 Open in figure viewer PowerPoint. The responses of these shallow fault areas to dynamic perturbations are depth and history dependent. White arrows mark the time of applied dynamic stress perturbations. The waveforms white in g are drawn to illustrate stress changes at corresponding time. Figure 14 Open in figure viewer PowerPoint.
The vertical bars in gray indicate the timing of applied dynamic stress perturbations. Acknowledgments We thank two anonymous reviewers for insightful and detailed reviews.
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