Southern San Andreas Fault Research Articles

Evidence of non extensivity in the evolution of seismicity along the San Andreas Fault, California, USA: An approach based on Tsallis statistical physics

We examine the nature of the seismogenetic system along the San Andreas Fault (SAF), California, USA, by searching for evidence of complexity and non-extensivity in the earthquake record. We use accurate, complete and homogeneous earthquake catalogues in which aftershocks are included (raw catalogues), or have been removed by a stochastic declustering procedure (declustered catalogues). On the basis of Non-Extensive Statistical Physics (NESP), which generalizes the Boltzmann–Gibbs formalism to non-equilibrating (complex) systems, we investigate whether earthquakes are generated by an extensive self-excited Poisson process or by a non-extensive complex process. We examine bivariate cumulative frequency distributions of earthquake magnitudes and interevent times and determine the size and time dependence of the respective magnitude and temporal entropic indices, which indicate the level on non-equilibrium (correlation). It is shown that the magnitude entropic index is very stable and corresponds to proxy b-values that are remarkably consistent with the b-values computed by conventional means. The temporal entropic index computed from the raw catalogues indicate moderately to highly correlated states during the aftershock sequences of large earthquakes, progressing to quasi-uncorrelated states as these die out and before the next large event. Conversely, the analysis of the declustered catalogues shows that background seismicity exhibits moderate to high correlation that varies significantly albeit smoothly with time. This indicates a persistent sub-extensive seismogenetic system. The degree of correlation is generally higher in the southern SAF segment, which is consistent with the observation of shorter return periods for large earthquakes. A plausible explanation is that because aftershock sequences are localized in space and time, their efficient removal unveils long-range background interactions which are obscured by their presence! Our results indicate complexity in the expression of background seismicity along the San Andreas Fault, with criticality being a very likely mechanism as a consequence of the persistent non-equilibrium inferred from the temporal entropic index. However, definite conclusions cannot be drawn until the earthquake record is exhaustively studied in all its forms.

Paleoearthquakes at Frazier Mountain, California delimit extent and frequency of past San Andreas Fault ruptures along 1857 trace

AbstractLarge earthquakes are infrequent along a single fault, and therefore historic, well‐characterized earthquakes exert a strong influence on fault behavior models. This is true of the 1857 Fort Tejon earthquake (estimated M7.7–7.9) on the southern San Andreas Fault (SSAF), but an outstanding question is whether the 330 km long rupture was typical. New paleoseismic data for six to seven ground‐rupturing earthquakes on the Big Bend of the SSAF restrict the pattern of possible ruptures on the 1857 stretch of the fault. In conjunction with existing sites, we show that over the last ~650 years, at least 75% of the surface ruptures are shorter than the 1857 earthquake, with estimated rupture lengths of 100 to <300 km. These results suggest that the 1857 rupture was unusual, perhaps leading to the long open interval, and that a return to pre‐1857 behavior would increase the rate of M7.3–M7.7 earthquakes.

Seismic velocity structures in the southern California plate-boundary environment from double-difference tomography

SUMMARY We present tomographic images of crustal structures in the southern California plate-boundary area, with a focus on the San Jacinto fault zone (SJFZ), based on double-difference inversions of earthquake arrival times. Absolute arrival times of 247 472 P and 105 448 S wave phase picks for 5493 earthquakes recorded at 139 stations in southern California are used. Starting with a layered 1-D model, and continuing in later iterations with various updated initial models, we invert the data for Vp and Vs in a 270 km long, 105 km wide and 35 km deep volume around the SJFZ using a spatially variable grid with higher density around the SJFZ. The examined volume stretches from Cajon Pass to the northernmost Imperial Fault Zone and includes portions of the southern San Andreas Fault (SAF), the Elsinore Fault and the Brawley Seismic Zone in the Salton Trough. Because differential traveltimes used in the double-difference inversions are most sensitive to near-source structures, we obtain high resolution around the earthquake sources. After 30 iterations we improve the average traveltime misfit by a factor of 16. Though ray coverage is limited at shallow depths, we obtain detailed images of seismic velocities from 3 to 20 km throughout much of the study area. Our final velocity results show zones of low-velocity and anomalous Vp/Vs ratios associated with various fault strands and sedimentary basins, along with clear velocity contrasts across the SJFZ and the southern SAF. The velocity reductions in fault zone regions are generally highest in geometrically complex areas (up to 30–50 per cent in the top few kilometres), are higher for Vs than for Vp, and follow a flower-type pattern with depth. In the central section of the SJFZ, from the San Jacinto valley to the trifurcation area, the northeast side of the fault has generally higher seismic velocities than the southwest block. The obtained contrasts of Vp are more persistent and higher (up to 20 per cent) than the contrasts of Vs (up to 15 per cent), although the differences may stem (at least partially) from the higher resolution of Vp images. In the SJFZ sections to the northwest and southeast, there are patches with reversed velocities contrasts especially in the shallow crust near the San Jacinto Valley and other basins. Along the Banning fault there is no clear velocity contrast. For the southern SAF, the northeast side has generally lower seismic velocities in the seismogenic zone with patches of contrast reversals in the shallow crust. In the Brawley Seismic Zone, the northeast side has somewhat lower velocities in the ∼20 km section near the southern SAF and higher velocities farther to the southwest. The imaged features have important implications for various aspects of earthquake and crustal dynamics in the region.

Adjoint analysis of the source and path sensitivities of basin-guided waves

SUMMARY Simulations of earthquake rupture on the southern San Andreas Fault (SAF) reveal large amplifications in the San Gabriel and Los Angeles Basins (SGB and LAB) apparently associated with long-range path effects. Geometrically similar excitation patterns can be recognized repeatedly in different SAF simulations (e.g. Love wave-like energy with predominant period around 4 s, channelled southwestwardly from the SGB into LAB), yet the amplitudes with which these distinctive wavefield patterns are excited change, depending upon source details (slip distribution, direction and velocity of rupture). We describe a method for rapid calculation of the sensitivity of such predicted wavefield features to perturbations of the source kinematics, using a time-reversed (adjoint) wavefield simulation. The calculations are analogous to those done in adjoint tomography, and the same time-reversed calculation also yields path-sensitivity kernels that give further insight into the excitation mechanism. For rupture on the southernmost 300 km of SAF, LAB excitation is greatest for slip concentrated between the northern Coachella Valley and the transverse ranges, propagating to the NE and with rupture velocities between 3250 and 3500 m s–1 along that fault segment; that is, within or slightly above the velocity range (between Rayleigh and S velocities) that is energetically precluded in the limit of a sharp rupture front, highlighting the potential value of imposing physical constraints (such as from spontaneous rupture models) on source parametrizations. LAB excitation is weak for rupture to the SW and for ruptures in either direction located north of the transverse transverse ranges, whereas Ventura Basin (VTB) is preferentially excited by NE ruptures situated north of the transverse ranges. Path kernels show that LAB excitation is mediated by surface waves deflected by the velocity contrast along the southern margin of the transverse ranges, having most of their energy in basement rock until they impinge on the eastern edge of SGB, through which they are then funnelled into LAB. VTB amplification is enhanced by a similar waveguide effect.

Inception of the eastern California shear zone and evolution of the Pacific‐North American plate boundary: From kinematics to geodynamics

The San Andreas Fault (SAF) is the transform boundary between the Pacific and the North American plates, yet up to 25% of the relative plate motion is now accommodated by the eastern California shear zone (ECSZ). Here we investigate the inception of the ECSZ and its geodynamic interactions with the SAF using a 3‐D viscoelastoplastic finite element model. For a given fault configuration of the plate boundary zone, the model simulates long‐term slip on the faults and plastic strain outside the faults. Our results show that the formation of the Big Bend of the SAF around 5–12 Ma impeded fault slip and localized strain along the ECSZ, causing its inception. Development of the ECSZ was further enhanced by the activation of the Garlock Fault (GF) and lithospheric weakening caused by the encroachment of the Basin and Range extension. Similarly, the San Jacinto Fault (SJF) in southern California developed along a belt of localized strain, which resulted from the formation of the restraining bend along the San Bernardino Mountains segment of the SAF ∼2 Myr ago. Once activated, the SJF reduced slip on both the southern SAF and the ECSZ across the Mojave Desert. These results indicate causative relationship between the SAF and the ECSZ. The inception of the ECSZ and other young faults is the consequence of the evolving SAF plate boundary zone that continuously adjusts itself to accommodate the relative plate motion.

San Andreas Fault Rupture Scenarios from Multiple Paleoseismic Records: Stringing Pearls

We present a new method to objectively combine paleoseismic event data from multiple sites into rupture scenarios and apply it to the southern San Andreas fault (SSAF) of California. First, a pool is constructed of all ruptures between sites allowed by fault geometry and available event age probability distribution functions (PDFs). Scenarios constructed by drawing from this pool are evaluated by the average quality of agreement event dating evidence, by the degree of misfit of cumulative displacement over all ruptures compared to a prediction from the fault slip rate and elapsed time, and by the number of events in the scenario. Three slip-rate models from the 2008 Working Group on California Earthquake Probabilities (WGCEP) were considered. Scenarios with full fault length ruptures tend to be inconsistent with low slip rates through the San Bernardino and San Gorgonio sections of the SSAF. Favorable scenarios tend to include 1857-like ruptures for three of the most recent five prehistoric ruptures in the northern half of the SSAF from Carrizo to approximately Wrightwood. They also include one to two ruptures that involve the southern half of the SSAF, but most earthquakes there appear shorter and exhibit less consistency from event to event. By combining paleoseismic data into ensembles of scenarios and selecting viable scenarios using external constraints, our method provides rupture histories useful for seismic hazard assessment without having to first settle which event at a site correlates with those at adjoining sites. This opens a way for paleoseismic data to be used with greater power to understand the seismic hazard posed by faults like the southern San Andreas.

Applying a three‐dimensional velocity model, waveform cross correlation, and cluster analysis to locate southern California seismicity from 1981 to 2005

We compute high‐precision earthquake locations using southern California pick and waveform data from 1981 to 2005. Our latest results are significantly improved compared to our previous catalog by the following: (1) We locate events with respect to a new crustal P and S velocity model using three‐dimensional ray tracing, (2) we examine six more years of waveform data and compute cross‐correlation results for many more pairs than our last analysis, and (3) we compute locations within similar event clusters using a new method that applies a robust fitting method to obtain the best locations satisfying all the differential time constraints from the waveform cross correlation. These results build on the relocated catalogs of Hauksson and Shearer (2005) and Shearer et al. (2005) and provide additional insight regarding the fine‐scale fault structure in southern California and the relationship between the San Andreas Fault (SAF) and nearby seismicity. In particular, we present results for two regions in which the seismicity near the southern SAF seems to align on dipping faults.

Initiation of the San Jacinto Fault and its Interaction with the San Andreas Fault: Insights from Geodynamic Modeling

The San Andreas Fault (SAF) is the Pacific-North American plate boundary, yet in southern California a significant portion of the relative plate motion is accommodated by the San Jacinto Fault (SJF). Here we investigate the initiation of the SJF and its interaction with the SAF in a three-dimensional visco-elasto-plastic finite-element model. The model results show that the restraining bend of the southern SAF causes strain localization along the SJF, thus may have contributed to its initiation. Slip on the SJF tends to reduce slip rate on the SAF and enhance deformation in the Eastern California Shear Zone. The initiation of the SJF and its interaction with the SAF reflect the evolving plate boundary zone as it continuously seeks the most efficient way to accommodate the relative plate motion.

Evolution of stress in Southern California for the past 200 years from coseismic, postseismic and interseismic stress changes

SummaryThe seismicity of southern California results from stresses that arise from the relative motion of the Pacific and North American Plates being accommodated along the San Andreas Fault (SAF) system and the Eastern California Shear Zone (ECSZ). Here we calculate how the stress field in southern California has evolved over the past two centuries due to interseismic loading, as inferred from current GPS observations of surface velocities, from redistributions of static stress induced by large (Mw ≥ 6.5) earthquakes since the 1812 Wrightwood quake, and postseismic viscoelastic relaxation associated with these events that serves to transfer coseismic stresses from the deep, warm, lower crust and upper mantle to the overlying seismogenic upper crust. We calculate Coulomb stress changes on vertical strike-slip faults striking parallel to the SAF and at the hypocenters on the rupture planes of all Mw ≥ 6 events over the past two centuries. Our results suggest that the 1857 Mw = 8.2 Fort Tejon earthquake, by far the largest event to have occurred in the region over the past two centuries, had a profound influence on the state of stress in Southern California during the 19th century, inducing significant stress increases to the north (Parkfield region and adjoining creeping SAF) and south (southern SAF and San Jacinto fault), and stress relief across the southern ECSZ. These stress changes were then greatly magnified by postseismic relaxation through the early part of the 20th century. Slow interseismic build-up of stress further loads all major strike-slip faults and works to reload the areas of the ECSZ where stress was relieved by the 1857 quake. Our calculations suggest that only 56% of hypocenters were pushed closer to failure by preceding coseismic stress changes, suggesting that the occurrence of large earthquakes is not strongly determined by coseismic Coulomb stress changes. This percentage rises to 70% when postseismic stress changes are also considered. Our calculations demonstrate the importance of postseismic viscoelastic relaxation in the redistribution of stress following large earthquakes. We find, however, that postseismic processes associated with events more than about a decade old are near completion and thus do not significantly influence the regional velocity field presently observed in southern California.

Geometrical impact of the San Andreas Fault on stress and seismicity in California

Most large earthquakes in northern and central California clustered along the main trace of the San Andreas Fault (SAF), the North American‐Pacific plate boundary. However, in southern California earthquakes were rather scattered. Here we suggest that such along‐strike variation of seismicity may largely reflect the geometrical impact of the SAF. Using a dynamic finite element model that includes the first‐order geometric features of the SAF, we show that strain partitioning and crustal deformation in California are closely related to the geometry of the SAF. In particular, the Big Bend is shown to reduce slip rate on southern SAF and cause high shear stress and strain energy over a broad region in southern California, and a belt of high strain energy in the Eastern California Shear Zone.