San Andreas Fault Research Articles

Chemical and isotope compositions of drilling mud gas from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: Implications on gas migration and the permeability structure of the San Andreas Fault

In this contribution we present results from two individual gas monitoring experiments which were conducted during the drilling of the SAFOD (San Andreas Fault Observatory at Depth) boreholes. Gas from circulating drilling mud was monitored during the drilling the SAFOD III side tracks and was later analyzed for δ 13C (CH 4, C 2H 6 and C 3H 8), H/D (CH 4) and noble gas isotopes. Furthermore, gas accumulations induced by drill pipe retrieval (“trip gas”) from the SAFOD MH and the SAFOD III boreholes were also investigated. The data are interpreted in the context of gas migration processes and the permeability structure of the San Andreas Fault (SAF) around two actively deforming zones at 3194 m and 3301 m borehole depth. Helium isotope ratios of 0.86 R a at 3203 m and between 0.51 and 0.88 R a at 3262 m (R a is the atmospheric 3He/ 4He ratio) indicate an improved flow of mantle volatiles between both fault strands. Much lower values were observed at 3147 m (0.26 R a) and 3312 m (0.22 R a). Hydrocarbon concentrations coincide with the occurrence of shale at ~ 3150–3200 m and below ~ 3310 m depth. The molecular and isotope composition of hydrocarbons and their spatial distributions imply hydrocarbon generation by thermal degradation of organic matter followed by extensive diffusion loss. Carbon isotope data furthermore suggest a thermal maturity of the source rock of approx. 1.4%R 0. The concentration of trip gas is generally low in the interval 3100 m–3450 m but exhibits high spatial variability. At 3128 m and 3223 m depth, the trip gas concentrations are as low as in the granite section of the SAFOD Main Hole. Considerable variations of R a values, trip gas concentrations, and the molecular composition of hydrocarbons when penetrating the active fault strands let us conclude that the permeability of the fault transverse to the fault direction is limited and that the active fault has not been breached over many earthquake cycles such that little or no fluid exchange took place. Diffusion is the dominant mechanism controlling hydrocarbon migration through the fault strands. The elevated R a values between both fault strands may reflect either episodic or continuous flow of mantle-derived fluids, suggestive of some limited permeability parallel to the fault direction.

Isotopic evidence for the infiltration of mantle and metamorphic CO 2–H 2O fluids from below in faulted rocks from the San Andreas Fault system

To characterize the origin of the fluids involved in the San Andreas Fault (SAF) system, we carried out an isotope study of exhumed faulted rocks from deformation zones, vein fillings and their hosts and the fluid inclusions associated with these materials. Samples were collected from segments along the SAF system selected to provide a depth profile from upper to lower crust. In all, 75 samples from various structures and lithologies from 13 localities were analyzed for noble gas, carbon, and oxygen isotope compositions. Fluid inclusions exhibit helium isotope ratios ( 3He/ 4He) of 0.1–2.5 times the ratio in air, indicating that past fluids percolating through the SAF system contained mantle helium contributions of at least 35%, similar to what has been measured in present-day ground waters associated with the fault (Kennedy et al., 1997). Calcite is the predominant vein mineral and is a common accessory mineral in deformation zones. A systematic variation of C- and O-isotope compositions of carbonates from veins, deformation zones and their hosts suggests percolation by external fluids of similar compositions and origin with the amount of fluid infiltration increasing from host rocks to vein to deformation zones. The isotopic trend observed for carbonates in veins and deformation zones follows that shown by carbonates in host limestones, marbles, and other host rocks, increasing with increasing contribution of deep metamorphic crustal volatiles. At each crustal level, the composition of the infiltrating fluids is thus buffered by deeper metamorphic sources. A negative correlation between calcite δ 13C and fluid inclusion 3He/ 4He is consistent with a mantle origin for a fraction of the infiltrating CO 2. Noble gas and stable isotope systematics show consistent evidence for the involvement of mantle-derived fluids combined with infiltration of deep metamorphic H 2O and CO 2 in faulting, supporting the involvement of deep fluids percolating through and perhaps weakening the fault zone. There is no clear evidence for a significant contribution from meteoric water, except for overprinting related to late weathering.

Fault rocks from the SAFOD core samples: Implications for weakening at shallow depths along the San Andreas Fault, California

The drilling of a deep borehole across the actively creeping Parkfield segment of the San Andreas Fault Zone (SAFZ), California, and collection of core materials permit direct geological study of fault zone processes at 2–3 km depth. The three drill cores sample both host and fault rocks and pass through two currently active, narrow (1–2 m wide) shear zones enclosed within a broader (ca. 240 m wide) region of inactive foliated gouges. The host rocks preserve primary sedimentary features and are cut by numerous minor faults and small, mainly calcite-filled veins. The development of Fe-enriched smectitic phyllosilicate networks following cataclasis is prevalent in the presently inactive foliated gouges of the main fault zone and in minor faults cutting clay-rich host rocks. Calcite, anhydrite and minor smectitic phyllosilicate veins are interpreted to have formed due to local fluid overpressuring events prior to, synchronous with and after local gouge development. By contrast, the active shear zone gouges lack mineral veins (except as clasts) and contain numerous clasts of serpentinite. Markedly Mg-rich smectitic phyllosilicates are the dominant mineral phases here, suggesting that the fault zone fluids have interacted with the entrained serpentinites. We propose that weakening of the SAFZ down to depths of at least 3 km can be attributed to the pervasive development of interconnected networks of low friction smectitic phyllosilicates and to the operation of stress-induced solution-precipitation creep mechanisms.

Seismic imaging using microseismic events: Results from the San Andreas Fault System at SAFOD

We have developed a new passive seismic imaging approach that consists of two steps. First, the hypocenter of the microseismic event is precisely located. Second, this event is treated as a “pseudo‐active” seismic source and the reflections within the recorded wavefield are processed by using a directional migration algorithm in order to construct a high‐resolution image of the illuminated subsurface region. In this paper we demonstrate the application of our approach to a number of microseismic events recorded by a borehole array in the San Andreas Fault Observatory at Depth main hole. Results obtained were high‐resolution 3‐D images of different SE–NW–oriented reflectors related to the San Andreas Fault (SAF) system in the close vicinity of the borehole. To support our approach, we compared our findings with other active and passive seismic images and analyzed the correlation with borehole lithology. We revealed a predominantly satisfactory agreement for both juxtapositions. Furthermore, the stacked image of several microearthquakes provides a spatial characterization of the complex internal structure of the SAF, with much higher resolution than can be obtained from surface seismic reflection data.

Remotely triggered microearthquakes and tremor in central California following the 2010 Mw 8.8 Chile earthquake

We examine remotely triggered microearthquakes and tectonic tremor in central California following the 2010 Mw 8.8 Chile earthquake. Several microearthquakes near the Coso Geothermal Field were apparently triggered, with the largest earthquake (Ml 3.5) occurring during the large‐amplitude Love surface waves. The Chile mainshock also triggered numerous tremor bursts near the Parkfield‐Cholame section of the San Andreas Fault (SAF). The locally triggered tremor bursts are partially masked at lower frequencies by the regionally triggered earthquake signals from Coso, but can be identified by applying high‐pass or matched filters. Both triggered tremor along the SAF and the Ml 3.5 earthquake in Coso are consistent with frictional failure at different depths on critically‐stressed faults under the Coulomb failure criteria. The triggered tremor, however, appears to be more phase‐correlated with the surface waves than the triggered earthquakes, likely reflecting differences in constitutive properties between the brittle, seismogenic crust and the underlying lower crust.

Asymmetric deformation across the San Francisco Bay Area faults from GPS observations in Northern California

We show that geodetic data from the Bay Area Regional Deformation (BARD) network indicate asymmetric motion across the San Andreas fault in the San Francisco Bay Area (SFBA), resulting from a strong contrast in rigidity across the fault, as determined previously from seismological data. Assuming asymmetric motion across the fault, we determine the location and size of the maximum strain rate in the region. We find that, compared to the determination using a symmetric model of deformation, it is shifted eastward and its value increases from ∼0.4 μstrain/yr to ∼0.65 μstrain/yr. Such strain rate amplitudes are consistent with previous geodetic slip rate estimates. We confirm that the geological units located east of SAF are entrained by the motion of the Pacific Plate and that the San Andreas fault (SAF) is the real rheological limit between the Pacific and North-American Plates. The asymmetry of rheology constrained in this study implies the strain rate maximum in SFBA is likely located between SAF and the Hayward fault system. This also has implications for hazards in the northern SFBA, in particular on the Rodgers creek fault.

Diversity of fault zone damage and trapping structures in the Parkfield section of the San Andreas Fault from comprehensive analysis of near fault seismograms

SUMMARY We perform comprehensive analyses of trapped waves and signals of damaged fault zone rocks associated with time delays of body waves along the Parkfield section of the San Andreas Fault (SAF). Waveforms generated by thousands of earthquakes and recorded by near fault stations in several permanent and temporary deployments are examined, with attention to the possible influence of a lithology contrast across the fault on signals of the low velocity damage zone. Clear candidate trapped waves are identified at only three stations, MMNB and two of the other near fault stations (FLIP and PIES) further to the NW. Clear candidate trapped waves are not seen at any of the near fault stations to the SE of MMNB. The locations of the events generating good candidate trapped waves at MMNB and the other two stations are not distributed broadly in space, but clustered in a small number of locations. Moreover, events that generate clear candidate trapped waves at one station do not typically generate trapped waves at the other two stations. These observations imply that the damage zone is highly variable along strike and that a coherent connected waveguide does not exist for distances along strike larger than at most 3–5 km (the distance between stations). Synthetic waveform fits for observed trapped waves at stations MMNB and FLIP indicate that the most likely parameters of the trapping structures at these locations are widths of about 150 m, depths of about 3 km, velocity reductions of 30–40 per cent, and Q values of 10–40. Synthetic calculations of trapped waves demonstrate that if there is a contrast of seismic velocity across the fault, the trapped waves are delayed relative to the S wave arrival. Trapped waves at station MMNB, and to a lesser extent also at stations FLIP and PIES in the NW section, show this characteristic. This suggests a lithology contrast in the top few km at these locations, in agreement with results from tomography and studies of head waves in the Parkfield area. At several mini across-fault arrays, where trapped waves are not observed, a low velocity damage zone is detected from the delay in the arrival time of body wave phases relative to a nearby off-fault station. The observed delay of the S wave is greater than the P wave delay, consistent with the existence of a damage zone with Poisson ratio of about 0.33. The observations of time delays without trapped waves indicate that parts of the damage zone are insufficiently coherent to generate trapped waves. A broader damage zone may exist in the region between the SAF and the South West Fracture Zone. The results highlight the diversity of damage structures along the ∼40 km of the SAF examined in this study, and imply that fault imaging based on data at single sites does not necessarily apply to a larger section.

Locating non-volcanic tremor along the San Andreas Fault using a multiple array source imaging technique

Non-volcanic tremor (NVT) has been observed at several subduction zones and at the San Andreas Fault (SAF). Tremor locations are commonly derived by cross-correlating envelope-transformed seismic traces in combination with source-scanning techniques. Recently, they have also been located by using relative relocations with master events, that is low-frequency earthquakes that are part of the tremor; locations are derived by conventional traveltime-based methods. Here we present a method to locate the sources of NVT using an imaging approach for multiple array data. The performance of the method is checked with synthetic tests and the relocation of earthquakes. We also applied the method to tremor occurring near Cholame, California. A set of small-aperture arrays (i.e. an array consisting of arrays) installed around Cholame provided the data set for this study. We observed several tremor episodes and located tremor sources in the vicinity of SAF. During individual tremor episodes, we observed a systematic change of source location, indicating rapid migration of the tremor source along SAF.

Surface-Wave Potential for Triggering Tectonic (Nonvolcanic) Tremor

Source processes commonly posed to explain instances of remote dynamic triggering of tectonic (nonvolcanic) tremor by surface waves include frictional failure and various modes of fluid activation. The relative potential for Love- and Rayleigh-wave dynamic stresses to trigger tectonic tremor through failure on critically stressed thrust and vertical strike-slip faults under the Coulomb–Griffith failure criteria as a function of incidence angle is anticorrelated over the 15- to 30-km-depth range that hosts tectonic tremor. Love-wave potential is high for strike-parallel incidence on low-angle reverse faults and null for strike-normal incidence; the opposite holds for Rayleigh waves. Love-wave potential is high for both strike-parallel and strike-normal incidence on vertical, strike-slip faults and minimal for ∼45° incidence angles. The opposite holds for Rayleigh waves. This pattern is consistent with documented instances of tremor triggered by Love waves incident on the Cascadia megathrust and the San Andreas fault (SAF) in central California resulting from shear failure on weak faults (apparent friction, μ *≤0.2). However, documented instances of tremor triggered by surface waves with strike-parallel incidence along the Nankai megathrust beneath Shikoku, Japan, is associated primarily with Rayleigh waves. This is consistent with the tremor bursts resulting from mixed-mode failure (crack opening and shear failure) facilitated by near-lithostatic ambient pore pressure, low differential stress, with a moderate friction coefficient ( μ ∼0.6) on the Nankai subduction interface. Rayleigh-wave dilatational stress is relatively weak at tectonic tremor source depths and seems unlikely to contribute significantly to the triggering process, except perhaps for an indirect role on the SAF in sustaining tremor into the Rayleigh-wave coda that was initially triggered by Love waves.

Fault zone structure from topography: Signatures of en echelon fault slip at Mustang Ridge on the San Andreas Fault, Monterey County, California

[1] We used high-resolution topography to quantify the spatial distribution of scarps, linear valleys, topographic sinks, and oversteepened stream channels formed along an extensional step over on the San Andreas Fault (SAF) at Mustang Ridge, California. This location provides detail of both creeping fault landform development and complex fault zone kinematics. Here, the SAF creeps 10–14 mm/yr slower than at locations ∼20 km along the fault in either direction. This spatial change in creep rate is coincident with a series of en echelon oblique-normal faults that strike obliquely to the SAF and may accommodate the missing deformation. This study presents a suite of analyses that are helpful for proper mapping of faults in locations where high-resolution topographic data are available. Furthermore, our analyses indicate that two large subsidiary faults near the center of the step over zone appear to carry significant distributed deformation based on their large apparent vertical offsets, the presence of associated sag ponds and fluvial knickpoints, and the observation that they are rotating a segment of the main SAF. Several subsidiary faults in the southeastern portion of Mustang Ridge are likely less active; they have few associated sag ponds and have older scarp morphologic ages and subdued channel knickpoints. Several faults in the northwestern part of Mustang Ridge, though relatively small, are likely also actively accommodating active fault slip based on their young morphologic ages and the presence of associated sag ponds.

A time-series SAR observation of surface deformation at the southern end of the San Andreas Fault Zone

The San Andreas Fault (SAF) is a continental transform fault that exists as a consequence of the relative motion of the Pacific and North American Plates. Differential SAR interferometry (DInSAR) particularly with a time-series analysis is an effective tool for long-term monitoring of the SAF. Radar interferometric phase, however, contains unwanted noises originated from atmosphere as well as observation system. For studying characteristics of interferometric phase at the southern end of the SAF zone, ERS-1/2 data, acquired from 1992 to 1999, and ENVISAT data, from 2003 to the present, were utilized. Troposphere interference and orbit error were the most significant sources of errors. Since the phase noise highly correlates with topographic elevation, a simulated interferogram from topographic elevation was utilized to compensate for the tropospheric signal delay. After subtracting the simulated phase from each DInSAR interferogram, more reliable results were obtained. These results are supported by comparison with the geodetic GPS data set.

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.

Where is the real transform boundary in California?

The zone of highest geodetically defined strain rate in California does not everywhere coincide with the surface trace of the San Andreas Fault (SAF). To determine whether this reflects the pattern of long‐term, permanent deformation, we analyzed the velocity field on swaths across the transform, located so as to avoid intersections among the major fault strands. Slip rates and flexural parameters for each fault were determined by finding the best fit to the velocity profile using a simple arctan model, representing the interseismic strain accumulation. Our slip rates compare well with current geologic estimates, which suggests that the present‐day velocity field is representative of long‐term motions. We find that the transform is a zone of high strain rate up to 80 km wide that is straighter than the SAF and has an overall trend closer to the relative plate motion vector than the SAF. Most sections of the SAF take up less than half of the total slip rate, and slip is transferred from one part of the system to another in a way that suggests that the SAF should not be considered as a unique locator of the plate boundary. Up to half of the total displacement takes place on faults outside the high strain rate zone, distributed over several hundred kilometers on either side. Our findings substantiate previous suggestions that the transform has the characteristics of a macroscopic ductile shear zone cutting the continental lithosphere, around which stress and strain rate decrease on a length scale controlled by the length of the transform.

Correction to “Constraints on the stress state of the San Andreas Fault with analysis based on core and cuttings from San Andreas Fault Observatory at Depth (SAFOD) drilling phases 1 and 2”

[1] Analysis of field data has led different investigators to conclude that the San Andreas Fault (SAF) has either anomalously low frictional sliding strength (μ 0.6). Arguments for the apparent weakness of the SAF generally hinge on conceptual models involving intrinsically weak gouge or elevated pore pressure within the fault zone. Some models assert that weak gouge and/or high pore pressure exist under static conditions while others consider strength loss or fluid pressure increase due to rapid coseismic fault slip. The present paper is composed of three parts. First, we develop generalized equations, based on and consistent with the Rice (1992) fault zone model to relate stress orientation and magnitude to depth-dependent coefficient of friction and pore pressure. Second, we present temperature- and pressure-dependent friction measurements from wet illite-rich fault gouge extracted from San Andreas Fault Observatory at Depth (SAFOD) phase 1 core samples and from weak minerals associated with the San Andreas Fault. Third, we reevaluate the state of stress on the San Andreas Fault in light of new constraints imposed by SAFOD borehole data. Pure talc (μ≈0.1) had the lowest strength considered and was sufficiently weak to satisfy weak fault heat flow and stress orientation constraints with hydrostatic pore pressure. Other fault gouges showed a systematic increase in strength with increasing temperature and pressure. In this case, heat flow and stress orientation constraints would require elevated pore pressure and, in some cases, fault zone pore pressure in excess of vertical stress.

Fault type analysis along the San Andreas Fault zone: A numerical approach

Finite Element (FE) modeling under plane stress condition is used to analyze the fault type variation with depth along and around the San Andreas Fault (SAF) zone. In this simulation elastic rheology was used and was thought justifiable as the variation in depth from 0.5 km to 20 km was considered. Series of calculations were performed with the variation in domain properties. Three types of models were created based on simple geological map of California, namely, 1) single domain model considering whole California as one homogeneous domain, 2) three domains model including the North American plate, Pacific plate, and SAF zone as separate domains, and 3) Four domains model including the three above plus the Garlock Fault zone. Mohr-Coulomb failure criterion and Byerlee’s law were used for the calculation of failure state. All the models were driven by displacement boundary condition imposing the fixed North American plate and Pacific plate motion along N34°W vector up to the northern terminus of SAF and N50°E vector motion for the subducting the Gorda and Juan de Fuca plates. Our simulated results revealed that as the depth increased, the fault types were generally normal, and at shallow depth greater strike slip and some thrust faults were formed. It is concluded that SAF may be terminated as normal fault at depth although the surface expression is clearly strike slip.

What drives short‐ and long‐term crustal deformation in the southwestern United States?

The driving forces for crustal deformation are often inferred from the crustal kinematics. In the Southwestern United States, however, crustal kinematics delineated by the Global Positioning System (GPS) measurements differs significantly from that reconstructed from the geological records. This discrepancy is likely due to the different timescales reflected in these datasets. Here we explore the geodynamic links between the short‐ and long‐term crustal deformation in the Southwestern US using regional‐scale three‐dimensional elastic and viscous finite element models. We show that the GPS‐measured short‐term crustal deformation results mainly from interseismic traction along the San Andreas Fault (SAF). Much of the short‐term strain is transient that does not contribute to permanent deformation. The long‐term traction on the SAF is ∼10 MPa lower than that constrained by the GPS data, which, together with gravitational spreading, can reproduce the geologically reconstructed long‐term crustal deformation.

Seismic picture of a fault zone. What can be gained from the analysis of fine patterns of spatial distribution of weak earthquake centers?

Association of earthquake hypocenters with fault zones appears more pronounced in cases with more accurately determined positions of the earthquakes. For complex, branched structures of major fault zones, it is assumed that some of the earthquakes occur at feathering fractures of smaller scale.It is thus possible to develop a «seismological» criterion for definition of a zone of dynamic influence of faults, i.e. the zone containing the majority of earthquakes associated with the fault zone under consideration. In this publication, seismogenic structures of several fault zones located in the San-Andreas fault system are reviewed. Based on the data from a very dense network of digital seismic stations installed in this region and with application of modern data processing methods, differential coordinates of microearthquakes can be determined with errors of about first dozens of meters.It is thus possible to precisely detect boundaries of the areas wherein active deformation processes occur and to reveal spatial patterns of seismic event localization. In our analyses, data from the most comprehensive seismic catalog were used. The catalogue includes information on events which occurred and were registered in North California in the period between January 1984 and May 2003. In this publication, the seismic data processing results and regularities revealed during the analyses are compared with the data obtained from studies of fault structures, modeling and numerical simulation results. Results of quantitative research of regularities of localization of seismic sources inside fault zones are presented. It is demonstrated by 3D models that seismic events are localized in the vicinity of an almost plain surface with a nearly constant angle of dip, the majority of events being concentrated at that conventional surface. Detection of typical scopes of seismicity localization may prove critical for solution of problems of technogenic impact on fault zones for the purpose of partial stress release. The obtained results suggest that the region, wherein active deformation takes place during preparation of medium earthquakes (M≤6.5÷7.0), includes a number of local «strips», each about 100 m in width. The latter size is comparable to a scope of technogenic capabilities of producing an impact on geo-environment. It is hoped that studies of both fine spatial and temporal patterns of seismicity in the vicinity of fault zones will allow to find reliable pinpoints for definition of both place and time for implementation of technogenic impacts. In our opinion, the implemented study demonstrates the burning need to establish test sites with dense and wellequipped local seismic networks in Russia.

Variations of the velocity contrast and rupture properties of M6 earthquakes along the Parkfield section of the San Andreas fault

SUMMARY We investigate the seismic velocity contrast across the San Andreas fault (SAF) in the Parkfield area using fault zone head waves (FZHW) that propagate along the bimaterial fault interface and direct P waves. We systematically analyse large data sets of near-fault waveforms recorded by several seismic networks over the period 1984–2005. Clear FZHW are observed at many stations on the NE side of the fault in the creeping section of the SAF north of Middle Mountain (MM). This indicates the presence of a sharp bimaterial interface and that the NE side of the fault has lower seismic velocities in that region. The obtained P-wave velocity contrast is about 5–10 per cent north of MM, and it systematically decreases to 0–2 per cent near Gold Hill (GH). The along-strike variations of the velocity contrast are consistent with geological observations of a sliver of high-velocity rock immediately to the NE of the SAF near GH, associated with the GH fault, and existing 3-D seismic tomography results. The obtained imaging results offer an explanation for the mixed rupture directions of the M6-type Parkfield earthquakes. The strong velocity contrast around MM is expected to produce a preferred propagation direction to the SE for earthquakes that nucleate near MM (e.g. the 1934 and 1966 Parkfield earthquakes). In contrast, the near-zero velocity contrast and multiple fault branches near GH imply that earthquakes that nucleate near GH (e.g. the 2004 Parkfield earthquake) are not expected to have a preferred propagation direction to the SE, and are likely to propagate in directions that are controlled by other factors such as structural and stress heterogeneities. The observed systematic reduction of the velocity contrast along the SAF from NW of MM to SE of GH provides a dynamic arrest mechanism for earthquakes that nucleate in the northern part of the Parkfield section and propagate to the SE, and a dynamic arrest mechanism for earthquakes that nucleate in the southern section and propagate to the NW.

Constraints on the stress state of the San Andreas Fault with analysis based on core and cuttings from San Andreas Fault Observatory at Depth (SAFOD) drilling phases 1 and 2

Analysis of field data has led different investigators to conclude that the San Andreas Fault (SAF) has either anomalously low frictional sliding strength (μ < 0.2) or strength consistent with standard laboratory tests (μ > 0.6). Arguments for the apparent weakness of the SAF generally hinge on conceptual models involving intrinsically weak gouge or elevated pore pressure within the fault zone. Some models assert that weak gouge and/or high pore pressure exist under static conditions while others consider strength loss or fluid pressure increase due to rapid coseismic fault slip. The present paper is composed of three parts. First, we develop generalized equations, based on and consistent with the Rice (1992) fault zone model to relate stress orientation and magnitude to depth‐dependent coefficient of friction and pore pressure. Second, we present temperature‐ and pressure‐dependent friction measurements from wet illite‐rich fault gouge extracted from San Andreas Fault Observatory at Depth (SAFOD) phase 1 core samples and from weak minerals associated with the San Andreas Fault. Third, we reevaluate the state of stress on the San Andreas Fault in light of new constraints imposed by SAFOD borehole data. Pure talc (μ≈0.1) had the lowest strength considered and was sufficiently weak to satisfy weak fault heat flow and stress orientation constraints with hydrostatic pore pressure. Other fault gouges showed a systematic increase in strength with increasing temperature and pressure. In this case, heat flow and stress orientation constraints would require elevated pore pressure and, in some cases, fault zone pore pressure in excess of vertical stress.

Joint inversion for Vp, Vs, and Vp/Vs at SAFOD, Parkfield, California

We refined the three‐dimensional (3‐D) Vp, Vs and Vp/Vs models around the San Andreas Fault Observatory at Depth (SAFOD) site using a new double‐difference (DD) seismic tomography code (tomoDDPS) that simultaneously solves for earthquake locations and all three velocity models using both absolute and differential P, S, and S‐P times. This new method is able to provide a more robust Vp/Vs model than that from the original DD tomography code (tomoDD), obtained simply by dividing Vp by Vs. For the new inversion, waveform cross‐correlation times for earthquakes from 2001 to 2002 were also used, in addition to arrival times from earthquakes and explosions in the region. The Vp values extracted from the model along the SAFOD trajectory match well with the borehole log data, providing in situ confirmation of our results. Similar to previous tomographic studies, the 3‐D structure around Parkfield is dominated by the velocity contrast across the San Andreas Fault (SAF). In both the Vp and Vs models, there is a clear low‐velocity zone as deep as 7 km along the SAF trace, compatible with the findings from fault zone guided waves. There is a high Vp/Vs anomaly zone on the southwest side of the SAF trace that is about 1–2 km wide and extends as deep as 4 km, which is interpreted to be due to fluids and fractures in the package of sedimentary rocks abutting the Salinian basement rock to the southwest. The relocated earthquakes align beneath the northeast edge of this high Vp/Vs zone. We carried out a 2‐D correlation analysis for an existing resistivity model and the corresponding profiles through our model, yielding a classification that distinguishes several major lithologies.