Geology topics

Research about San Andreas Fault

Source-linked reports with geographic coverage including San Andreas Fault.

At least 37 records · Page 2Linked to original sources

USGS2013

Volatile ﬂuxes through the Big Bend section of the San Andreas Fault, California: helium and carbon-dioxide systematics

To investigate the source of volatiles and their relationship to the San Andreas Fault System (SAFS), 18 groundwater samples were collected from wells near the Big Bend section of the SAFS in southern California and analyzed for helium and carbon abundance and isotopes. Concentrations of 4 He, corrected for air-bubble entrainment, vary from 4.15 to 62.7 (× 10 − 8 ) cm 3 STP g − 1 H 2 O. 3 He/ 4 He ratios vary from 0.09 to 3.52 R A (where R A = air 3 He/ 4 He), consistent with up to 44% mantle helium in samples. A subset of 10 samples was analyzed for the major volatile phase (CO 2 ) — the hypothesized carrier phase of the helium in the mantle–crust system: CO 2 / 3 He ratios vary from 0.614 to 142 (× 10 11 ), and δ 13 C (CO 2 ) values vary from − 21.5 to − 11.9‰ (vs. PDB). 3 He/ 4 He ratios and CO 2 concentrations are highest in the wells located in the Mil Potrero and Cuddy valleys adjacent to the SAFS. The elevated 3 He/ 4 He ratios are interpreted to be a consequence of a mantle volatile flux though the SAFS diluted by radiogenic He produced in the crust. Samples with the highest 3 He/ 4 He ratios also had the lowest CO 2 / 3 He ratios. The combined helium isotope, He–CO 2 elemental relationships, and δ 13 C (CO 2 ) values of the groundwater volatiles reveal a mixture of mantle and deep crustal (metamorphic) fluid origins. The flux of fluids into the seismogenic zone at high hydrostatic pressure may cause fault rupture, and transfer volatiles into the shallow crust. We calculate an upward fluid flow rate of 147 mm a − 1 along the SAFS, up to 37 times higher than previous estimates (Kennedy et al., 1997). However, using newly identified characteristics of the SAFS, we calculate a total flux of 3 He along the SAFS of 7.4 × 103 cm 3 STP a − 1 (0.33 mol 3 He a − 1 ), and a CO 2 flux of 1.5 × 10 13 cm 3 STP a − 1 (6.6 × 10 8 mol a − 1 ), ~ 1% of previous estimates. Lower fluxes along the Big Bend section of the SAFS suggest that the flux of mantle volatiles alone is insufficient to cause the super hydrostatic pressure in the seismogenic zone; however, results identify crustal (metamorphic) fluids as a major component of the CO 2 volatile budget, which may represent the additional flux necessary for fault weakening pressure in the SAFS.

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USGS2012

Strain‐rate profile across the Elsinore, San Jacinto, and San Andreas Faults near Palm Springs, California, 1973‐81

A profile of the strain accumulation rate along a line trending N50°E across the subparallel Elsinore, San Jacinto, and San Andreas faults near Palm Springs, California, has been constructed from trilateration surveys in the 1973‐81 interval. The strain accumulation is principally right‐lateral shear across a vertical plane parallel to fault strike (N40°W). The strain rate profile for that component exhibits two clearly resolved maxima, one centered on the San Jacinto fault (γ max = 0.35 ± 0.02 µrad/a) and the other on the San Andreas fault (γ max = 0.40 ± 0.02 µrad/a); no maximum is associated with the Elsinore fault. This result clearly implies that slip at depth on both the San Andreas and San Jacinto faults is loading the shallower sections of these faults, and that eventually rupture can be expected on both.

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USGS2012

Determination of three‐dimensional velocity anomalies under a seismic array using first P arrival times from local earthquakes: 1. A homogeneous initial model

Geiger's method of locating local earthquakes has been extended to include the effect of P velocity variation along the ray paths in three dimensions. The crustal structure was modeled by rectangular blocks, and a parameter was assigned to each block describing the perturbation of P wave slowness in the block. On the basis of an initial model, a set of linear equations for the observed first P arrival times was formulated in terms of the source and medium parameters. The source parameters for all the earthquakes in the data set and medium parameters for all the blocks penetrated by the seismic rays were then determined simultaneously by the damped least squares method. A computer program has been written for the case of a homogeneous initial medium model with constant P velocity. It has been tested with artificial data and applied to a set of local earthquakes recorded by a dense seismic array in Bear Valley, California. The resultant velocity distribution in the top 5 km shows a narrow low‐velocity zone of about 5 km/s in the San Andreas fault zone sandwiched between high‐velocity regions of about 6 km/s.

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USGS2012

Relocation of local earthquakes by seismic ray tracing

Seismic ray tracing is used to relocate a set of local earthquakes recorded by a dense seismic array in Bear Valley, California. The crustal velocity structure is two-dimensional and incorporates most of the known and inferred velocity differences in the region. Relocated hypocenters fall within the San Andreas fault zone, and P residuals, without elevation or station corrections, now appear to be more internally consistent in the rms sense. Ray takeoff angles and azimuths are distinctly different from those of a uniformly layered earth model. Focal mechanism solutions using only traced data are well determined and have few inconsistent data. Zones of convergence and divergence of rays suggest large variations in observed amplitudes across the local network.

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USGS2012

Aftershocks halted by static stress shadows

Earthquakes impart static and dynamic stress changes to the surrounding crust. Sudden fault slip causes small but permanent—static—stress changes, and passing seismic waves cause large, but brief and oscillatory—dynamic—stress changes. Because both static and dynamic stresses can trigger earthquakes within several rupture dimensions of a mainshock, it has proven difficult to disentangle their contributions to the triggering process 1–3 . However, only dynamic stress can trigger earthquakes far from the source 4,5 , and only static stress can create stress shadows, where the stress and thus the seismicity rate in the shadow area drops following an earthquake 6–9 . Here we calculate the stress imparted by the magnitude 6.1 Joshua Tree and nearby magnitude 7.3 Landers earthquakes that occurred in California in April and June 1992, respectively, and measure seismicity through time. We show that, where the aftershock zone of the first earthquake was subjected to a static stress increase from the second, the seismicity rate jumped. In contrast, where the aftershock zone of the first earthquake fell under the stress shadow of the second and static stress dropped, seismicity shut down. The arrest of seismicity implies that static stress is a requisite element of spatial clustering of large earthquakes and should be a constituent of hazard assessment.

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USGS2012

A new perspective on the geometry of the San Andreas Fault in southern California and its relationship to lithospheric structure

The widely held perception that the San Andreas fault (SAF) is vertical or steeply dipping in most places in southern California may not be correct. From studies of potential‐field data, active‐source imaging, and seismicity, the dip of the SAF is significantly nonvertical in many locations. The direction of dip appears to change in a systematic way through the Transverse Ranges: moderately southwest (55°–75°) in the western bend of the SAF in the Transverse Ranges (Big Bend); vertical to steep in the Mojave Desert; and moderately northeast (37°–65°) in a region extending from San Bernardino to the Salton Sea, spanning the eastern bend of the SAF in the Transverse Ranges. The shape of the modeled SAF is crudely that of a propeller. If confirmed by further studies, the geometry of the modeled SAF would have important implications for tectonics and strong ground motions from SAF earthquakes. The SAF can be traced or projected through the crust to the north side of a well documented high‐velocity body (HVB) in the upper mantle beneath the Transverse Ranges. The north side of this HVB may be an extension of the plate boundary into the mantle, and the HVB would appear to be part of the Pacific plate.

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USGS2011

Deep rock damage in the San Andreas Fault revealed by P- and S-type fault-zone-guided waves

Damage to fault-zone rocks during fault slip results in the formation of a channel of low seismic-wave velocities. Within such channels guided seismic waves, denoted by Fg, can propagate. Here we show with core samples, well logs and Fg-waves that such a channel is crossed by the SAFOD (San Andreas Fault Observatory at Depth) borehole at a depth of 2.7 km near Parkfield, California, USA. This laterally extensive channel extends downwards to at least half way through the seismogenic crust, more than about 7 km. The channel supports not only the previously recognized Love-type- (FL) and Rayleigh-type- (FR) guided waves, but also a new fault-guided wave, which we name FF. As recorded 2.7 km underground, FF is normally dispersed, ends in an Airy phase, and arrives between the P- and S-waves. Modelling shows that FF travels as a leaky mode within the core of the fault zone. Combined with the drill core samples, well logs and the two other types of guided waves, FF at SAFOD reveals a zone of profound, deep, rock damage. Originating from damage accumulated over the recent history of fault movement, we suggest it is maintained either by fracturing near the slip surface of earthquakes, such as the 1857 Fort Tejon M 7.9, or is an unexplained part of the fault-creep process known to be active at this site.

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USGS2011

Tremor reveals stress shadowing, deep postseismic creep, and depth-dependent slip recurrence on the lower-crustal San Andreas fault near Parkfield

The 2003 magnitude 6.5 San Simeon and the 2004 magnitude 6.0 Parkfield earthquakes induced small, but significant, static stress changes in the lower crust on the central San Andreas fault, where recently detected tectonic tremor sources provide new constraints on deep fault creep processes. We find that these earthquakes affect tremor rates very differently, consistent with their differing transferred static shear stresses. The San Simeon event appears to have cast a "stress shadow" north of Parkfield, where tremor activity was stifled for 3-6 weeks. In contrast, the 2004 Parkfield earthquake dramatically increased tremor activity rates both north and south of Parkfield, allowing us to track deep postseismic slip. Following this event, rates initially increased by up to two orders of magnitude for the relatively shallow tremor sources closest to the rupture, with activity in some sources persisting above background rates for more than a year. We also observe strong depth dependence in tremor recurrence patterns, with shallower sources generally exhibiting larger, less-frequent bursts, possibly signaling a transition toward steady creep with increasing temperature and depth. Copyright 2011 by the American Geophysical Union.

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USGS2011

Scientific drilling into the San Andreas Fault Zone

This year, the world has faced energetic and destructive earthquakes almost every month. In January, an M = 7.0 event rocked Haiti, killing an estimated 230,000 people. In February, an M = 8.8 earthquake and tsunami claimed over 500 lives and caused billions of dollars of damage in Chile. Fatal earthquakes also occurred in Turkey in March and in China and Mexico in April.

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USGS2011

The ShakeOut scenario: A hypothetical Mw7.8 earthquake on the Southern San Andreas Fault

In 2008, an earthquake-planning scenario document was released by the U.S. Geological Survey (USGS) and California Geological Survey that hypothesizes the occurrence and effects of a M w 7.8 earthquake on the southern San Andreas Fault. It was created by more than 300 scientists and engineers. Fault offsets reach 13 m and up to 8 m at lifeline crossings. Physics-based modeling was used to generate maps of shaking intensity, with peak ground velocities of 3 m/sec near the fault and exceeding 0.5 m/sec over 10,000 km 2 . A custom HAZUS ®MH analysis and 18 special studies were performed to characterize the effects of the earthquake on the built environment. The scenario posits 1,800 deaths and 53,000 injuries requiring emergency room care. Approximately 1,600 fires are ignited, resulting in the destruction of 200 million square feet of the building stock, the equivalent of 133,000 single-family homes. Fire contributes $ 87 billion in property and business interruption loss, out of the total $ 191 billion in economic loss, with most of the rest coming from shake-related building and content damage ( $ 46 billion) and business interruption loss from water outages ( $ 24 billion). Emergency response activities are depicted in detail, in an innovative grid showing activities versus time, a new format introduced in this study.

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USGS2011

Preparing for a "Big One": The great southern California shakeout

The Great Southern California ShakeOut was a week of special events featuring the largest earthquake drill in United States history. On November 13, 2008, over 5 million Southern Californians pretended that the magnitude-7.8 ShakeOut scenario earthquake was occurring and practiced actions derived from results of the ShakeOut Scenario, to reduce the impact of a real, San Andreas Fault event. The communications campaign was based on four principles: 1) consistent messaging from multiple sources; 2) visual reinforcement: 3) encouragement of “milling”; and 4) focus on concrete actions. The goals of the ShakeOut established in Spring 2008 were: 1) to register 5 million people to participate in the drill; 2) to change the culture of earthquake preparedness in Southern California; and 3) to reduce earthquake losses in Southern California. Over 90% of the registrants surveyed the next year reported improvement in earthquake preparedness at their organization as a result of the ShakeOut.

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USGS2011

Scientific drilling into the San Andreas Fault Zone - an overview of SAFOD's first five years

The San Andreas Fault Observatory at Depth (SAFOD) was drilled to study the physical and chemical processes controlling faulting and earthquake generation along an active, plate-bounding fault at depth. SAFOD is located near Parkfield, California and penetrates a section of the fault that is moving due to a combination of repeating microearthquakes and fault creep. Geophysical logs define the San Andreas Fault Zone to be relatively broad (~200 m), containing several discrete zones only 2–3 m wide that exhibit very low P- and S-wave velocities and low resistivity. Two of these zones have progressively deformed the cemented casing at measured depths of 3192 m and 3302 m. Cores from both deforming zones contain a pervasively sheared, cohesionless, foliated fault gouge that coincides with casing deformation and explains the observed extremely low seismic velocities and resistivity. These cores are being now extensively tested in laboratories around the world, and their composition, deformation mechanisms, physical properties, and rheological behavior are studied. Downhole measurements show that within 200 m (maximum) of the active fault trace, the direction of maximum horizontal stress remains at a high angle to the San Andreas Fault, consistent with other measurements. The results from the SAFOD Main Hole, together with the stress state determined in the Pilot Hole, are consistent with a strong crust/weak fault model of the San Andreas. Seismic instrumentation has been deployed to study physics of faulting—earthquake nucleation, propagation, and arrest—in order to test how laboratory-derived concepts scale up to earthquakes occurring in nature.

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USGS2011

Low strength of deep San Andreas fault gouge from SAFOD core

The San Andreas fault accommodates 28–34 mm yr −1 of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7 km (ref. 1 ). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault 2,3 . These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms 1 . The combination of these measurements of fault core strength with borehole observations 1,4,5 yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust.

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USGS2010

Scale-invariant stress orientations and seismicity rates near the San Andreas Fault

We analyzed measurements of the direction of maximum horizontal compressive stress as a function of depth in two scientific research wells near the San Andreas Fault in central and southern California. We found that the stress orientations exhibit scale-invariant fluctuations over intervals from tens of cm to several km. Similarity between the scaling of the stress orientation fluctuations and the scaling of earthquake frequency with fault size suggests that these fluctuations are controlled by stress perturbations caused by slip on faults of various sizes in the critically-stressed crust adjacent to the fault. The apparent difference in stress scaling parameters between the two studies wells seem to correspond to differences in the earthquake magnitude-frequency statistics for the creeping versus locked sections of the fault along which these two wells are located. This suggests that stress heterogeneity adjacent to active faults like the San Andreas may reflect variations in stresses and loading conditions along the fault.

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USGS2010

Analysis of nonvolcanic tremor on the San Andreas Fault near Parkfield, CA using U.S. Geological Survey Parkfield Seismic Array

Reports by Nadeau and Dolenc (2005) that tremor had been detected near Cholame Valley spawned an effort to use UPSAR (U. S. Geological Survey Parkfield Seismic Array) to study characteristics of tremor. UPSAR was modified to record three channels of velocity at 40–50 sps continuously in January 2005 and ran for about 1 month, during which time we recorded numerous episodes of tremor. One tremor, on 21 January at 0728, was recorded with particularly high signal levels as well as another episode 3 days later. Both events were very emergent, had a frequency content between 2 and 8 Hz, and had numerous high-amplitude, short-duration arrivals within the tremor signal. Here using the first episode as an example, we discuss an analysis procedure, which yields azimuth and apparent velocity of the tremor at UPSAR. We then provide locations for both tremor episodes. The emphasis here is how the tremor episode evolves. Twelve stations were operating at the time of recording. Slowness of arrivals was determined using cross correlation of pairs of stations; the same method used in analyzing the main shock data from 28 September 2004. A feature of this analysis is that 20 s of the time series were used at a time to calculate correlation; the longer windows resulted in more consistent estimates of slowness, but lower peak correlations. These values of correlation (peaks of about 0.25), however, are similar to that obtained for the S wave of a microearthquake. Observed peaks in slowness were traced back to source locations assumed to lie on the San Andreas fault. Our inferred locations for the two tremor events cluster near the locations of previously observed tremor, south of the Cholame Valley. Tremor source depths are in the 14–24 km range, which is below the seismogenic brittle zone, but above the Moho. Estimates of error do not preclude locations below the Moho, however. The tremor signal is very emergent but contains packets that are several times larger than the background tremor signal and lasts about 5 s. These impulsive wavelets are similar to low-frequency earthquakes signals seen in Japan but appear to be broader band rather than just higher in low-frequency energy. They may be more appropriately called high-energy tremor (HET). HET signals at UPSAR correlate well with the record of this event from station GHIB of the HRSN borehole array at Parkfield and HETs typically have a higher cross-correlation coefficient than the rest of the tremor event. The amplitudes of a large HET are consistent with a magnitude of 0.1 when compared with a M2.3 event that had about the same epicenter. Polarizations of the tremor episode at UPSAR are mostly just north of east. Both linearity and azimuth evolve over time suggesting a change in tremor source location over time and linearity is typically higher at the HETs.

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USGS2010

Precise tremor source locations and amplitude variations along the lower-crustal central San Andreas Fault

We precisely locate 88 tremor families along the central San Andreas Fault using a 3D velocity model and numerous P and S wave arrival times estimated from seismogram stacks of up to 400 events per tremor family. Maximum tremor amplitudes vary along the fault by at least a factor of 7, with by far the strongest sources along a 25 km section of the fault southeast of Parkfield. We also identify many weaker tremor families, which have largely escaped prior detection. Together, these sources extend 150 km along the fault, beneath creeping, transitional, and locked sections of the upper crustal fault. Depths are mostly between 18 and 28 km, in the lower crust. Epicenters are concentrated within 3 km of the surface trace, implying a nearly vertical fault. A prominent gap in detectible activity is located directly beneath the region of maximum slip in the 2004 magnitude 6.0 Parkfield earthquake.

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USGS2010

Low-altitude aerial color digital photographic survey of the San Andreas Fault

Ever since 1858, when Gaspard-Félix Tournachon (pen name Félix Nadar) took the first aerial photograph (Professional Aerial Photographers Association 2009), the scientific value and popular appeal of such pictures have been widely recognized. Indeed, Nadar patented the idea of using aerial photographs in mapmaking and surveying. Since then, aerial imagery has flourished, eventually making the leap to space and to wavelengths outside the visible range. Yet until recently, the availability of such surveys has been limited to technical organizations with significant resources. Geolocation required extensive time and equipment, and distribution was costly and slow. While these situations still plague older surveys, modern digital photography and lidar systems acquire well-calibrated and easily shared imagery, although expensive, platform-specific software is sometimes still needed to manage and analyze the data. With current consumer-level electronics (cameras and computers) and broadband internet access, acquisition and distribution of large imaging data sets are now possible for virtually anyone. In this paper we demonstrate a simple, low-cost means of obtaining useful aerial imagery by reporting two new, high-resolution, low-cost, color digital photographic surveys of selected portions of the San Andreas fault in California. All pictures are in standard jpeg format. The first set of imagery covers a 92-km-long section of the fault in Kern and San Luis Obispo counties and includes the entire Carrizo Plain. The second covers the region from Lake of the Woods to Cajon Pass in Kern, Los Angeles, and San Bernardino counties (151 km) and includes Lone Pine Canyon soon after the ground was largely denuded by the Sheep Fire of October 2009. The first survey produced a total of 1,454 oblique digital photographs (4,288 x 2,848 pixels, average 6 Mb each) and the second produced 3,762 nadir images from an elevation of approximately 150 m above ground level (AGL) on the southeast leg and 300 m AGL on the northwest leg. Spatial resolution (pixel size or ground sample distance) is a few centimeters. Time and geographic coordinates of the aircraft were automatically written into the exchangeable image file format (EXIF) data within each jpeg photograph. A few hours after acquisition and validation, the photographs were uploaded to a publicly accessible Web page. The goal was to obtain quick-turnaround, low-cost, high-resolution, overlapping, and contiguous imagery for use in planning field operations, and to provide imagery for a wide variety of land use and educational studies. This work was carried out in support of ongoing geological research on the San Andreas fault, but the technique is widely applicable beyond geology.

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USGS2010

Migrating tremors illuminate complex deformation beneath the seismogenic San Andreas fault

The San Andreas fault is one of the most extensively studied faults in the world, yet its physical character and deformation mode beneath the relatively shallow earthquake-generating portion remain largely unconstrained. Tectonic ‘non-volcanic’ tremor, a recently discovered seismic signal probably generated by shear slip on the deep extension of some major faults , can provide new insight into the deep fate of such faults, including that of the San Andreas fault near Parkfield, California . Here I examine continuous seismic data from mid-2001 to 2008, identifying tremor and decomposing the signal into different families of activity based on the shape and timing of the waveforms at multiple stations . This approach allows differentiation between activities from nearby patches of the deep fault and begins to unveil rich and complex patterns of tremor occurrence. I find that tremor exhibits nearly continuous migration, with the most extensive episodes propagating more than 20 kilometres along fault strike at rates of 15–80 kilometres per hour. This suggests that the San Andreas fault remains a localized through-going structure, at least to the base of the crust, in this area. Tremor rates and recurrence behaviour changed markedly in the wake of the 2004 magnitude-6.0 Parkfield earthquake , but these changes were far from uniform within the tremor zone, probably reflecting heterogeneous fault properties and static and dynamic stresses decaying away from the rupture. The systematic recurrence of tremor demonstrated here suggests the potential to monitor detailed time-varying deformation on this portion of the deep San Andreas fault, deformation which unsteadily loads the shallower zone that last ruptured in the 1857 magnitude-7.9 Fort Tejon earthquake.

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