Seismic velocity variations on the San Andreas fault caused by the 2004 M6 Parkfield Earthquake and their implications (original) (raw)
2007, Earth, Planets and Space
Repeated earthquakes and explosions recorded at the San Andreas fault (SAF) near Parkfield before and after the 2004 M6 Parkfield earthquake show large seismic velocity variations within an approximately 200m-wide zone along the fault to depths of approximately 6 km. The seismic arrays were co-sited in the two experiments and located in the middle of a high-slip part of the surface rupture. Waveform cross-correlations of microearthquakes recorded in 2002 and subsequent repeated events recorded a week after the 2004 M6 mainshock show a peak of an approximately 2.5% decrease in seismic velocity at stations within the fault zone, most likely due to the co-seismic damage of fault-zone rocks during dynamic rupture of this earthquake. The damage zone is not symmetric; instead, it extends farther on the southwest side of the main fault trace. Seismic velocities within the fault zone measured for later repeated aftershocks in the following 3-4 months show an approximate 1.2% increase at seismogenic depths, indicating that the rock damaged in the mainshock recovers rigidity-or heals-through time. The healing rate was not constant but was largest in the earliest post-mainshock stage. The magnitude of fault damage and healing varies across and along the rupture zone, indicating that the greater damage was inflicted and thus greater healing is observed in regions with larger slip in the mainshock. Observations of rock damage during the mainshock and healing soon thereafter are consistent with our interpretation of the low-velocity waveguide on the SAF being at least partially softened in the 2004 M6 mainshock, with additional cumulative effects due to recurrent rupture.
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Repeated earthquakes and explosions recorded at the San Andreas fault near Parkfield before and after the 2004 M6 Parkfield earthquake show large seismic velocity variations within a ~200-m-wide zone along the fault to depths of ~6 km. The seismic arrays were co-sited in the two experiments, and located in the middle of a high-slip part of the surface rupture. Waveform cross-correlations of microearthquakes recorded in 2002 and subsequent repeated events recorded a week after the 2004 M6 mainshock show a peak ~2.5% decrease in seismic velocity at stations within the fault zone, most likely due to the co-seismic damage of fault-zone rocks during dynamic rupture of this earthquake. The damage zone is not symmetric, instead extending farther on the southwest side of the main fault trace. Seismic velocities within the fault zone measured for later repeated aftershocks in following 3-4 months show ~1.2% increase at seismogenic depths, indicating that the rock damaged in the mainshock recovers rigidity, or heals, through time. The healing rate was not constant but largest in the earliest stage of post-mainshock. The magnitude of fault damage and healing varies across and along the rupture zone, indicating that the greater damage was inflicted and thus greater healing is observed in regions with larger slip in the mainshock. Observations of rock damage during the mainshock and healing soon thereafter are consistent with our interpretation of the low-velocity waveguide on the SAF being at least partially softened in the 2004 M6 mainshock, with additional cumulative effects due to recurrent rupture.
Bulletin of the Seismological Society of America, 2006
We deployed a dense linear array of 45 seismometers across and along the San Andreas fault near Parkfield a week after the M 6.0 Parkfield earthquake on 28 September 2004 to record fault-zone seismic waves generated by aftershocks and explosions. Seismic stations and explosions were co-sited with our previous experiment conducted in 2002. The data from repeated shots detonated in the fall of 2002 and 3 months after the 2004 M 6.0 mainshock show ϳ1.0%-1.5% decreases in seismic-wave velocity within an ϳ200-m-wide zone along the fault strike and smaller changes (0.2%-0.5%) beyond this zone, most likely due to the coseismic damage of rocks during dynamic rupture in the 2004 M 6.0 earthquake. The width of the damage zone characterized by larger velocity changes is consistent with the low-velocity waveguide model on the San Andreas fault, near Parkfield, that we derived from fault-zone trapped waves . The damage zone is not symmetric but extends farther on the southwest side of the main fault trace. Waveform crosscorrelations for repeated aftershocks in 21 clusters, with a total of ϳ130 events, located at different depths and distances from the array site show ϳ0.7%-1.1% increases in S-wave velocity within the fault zone in 3 months starting a week after the earthquake. The velocity recovery indicates that the damaged rock has been healing and regaining the strength through rigidity recovery with time, most likely due to the closure of cracks opened during the mainshock. We estimate that the net decrease in seismic velocities within the fault zone was at least ϳ2.5%, caused by the 2004 M 6.0 Parkfield earthquake. The healing rate was largest in the earlier stage of the postmainshock healing process. The magnitude of fault healing varies along the rupture zone, being slightly larger for the healing beneath Middle Mountain, correlating well with an area of large mapped slip. The fault healing is most prominent at depths above ϳ7 km.
2006
We deployed a dense linear array of 45 seismometers across and along the San Andreas fault near the SAFOD site at Parkfield in 2003 to record fault-zone trapped waves generated by near-surface explosions and microearthquakes located within the fault zone. Observations and simulations of the fault-zone trapped waves show a ~150-200-m-wide low-velocity waveguide along the SAF, within which shear velocities are reduced by 20-40% from wall-rock velocities and the Q value of fault-zone rocks is 10-50, indicating the existence of a damage zone on the major plate boundary at Parkfield. The damage zone on the SAF extends across seismogenic depths to at least ~7 km and is not symmetric but extends farther on the southwest side of the main fault trace. The width and velocities of this zone delineated by fault-zone trapped waves recorded at surface arrays are consistent with the results from SAFOD drilling and logs that show high porosity and multiple slip planes in a ~200-m-wide lowvelocity zone with velocity reduction of ~20-30% on the main SAF at ~3.2 km depth [Hickman, 2005]. Recently, down-hole seismic stations within the main fault zone at this depth also registered prominent fault-zone guided waves from microearthquakes occurring below, indicating that the low-velocity waveguide on the SAF extends to the deeper seismogenic level [Malin, et al., 2006].
Active Seismic Monitoring of the San Andreas Fault at Parkfield
2008
A unique data set of seismograms for 720 source-receiver paths has been collected as part of a controlled source Vibroseis experiment San Andreas Fault (SAF) at Parkfield. In the experiment, seismic waves repeatedly illuminated the epicentral region of the expected M6 event at Parkfield from June 1987 until November 1996. For this effort, a large shear-wave vibrator was interfaced with the 3-component (3-C) borehole High-Resolution Seismic Network (HRSN), providing precisely timed collection of data for detailed studies of changes in wave propagation associated with stress and strain accumulation in the fault zone (FZ). Data collected by the borehole network were examined for evidence of changes associated with the nucleation process of the anticipated M6 earthquake at Parkfield These investigations reported significant traveltime changes in the S coda for paths crossing the fault zone southeast of the epicenter and above the rupture zone of the 1966 M6 earthquake. Analysis and modeling of these data and comparison with observed changes in creep, water level, microseismicity, slip-at-depth and propagation from characteristic repeating microearthquakes showed temporal variations in a variety of wave propagation attributes that were synchronous with changes in deformation and local seismicity patterns. The main lesson learned from Vibroseis experiment is that changes were clearly observable in the locked part of SAF, which has relatively little natural seismicity could otherwise be used for monitoring of travel-time and attenuation changes. The creeping part of the SAF northwest of Parkfield is not expected to accumulate stress and it is also heavily instrumented. Monitoring of this region revealed no significant changes in seismic signatures. Remarkably in 2004, the expected M6 earthquake at Parkfield occurred and nucleated well into the locked SAF section, well to the southeast of the Vibroseis/HRSN monitoring experiment which was primarily centered on Middle Mountain. This result suggests that active seismic monitoring can be a useful tool for detecting stress changes associated with the nucleation of larger earthquakes even when observations are made over the events nucleation zones with low natural seismicity. Numerical modeling studies and a growing number of observations have argued for the propagation of fault-zone guided waves (FZGW) within a SAF zone that is 100 to 200 m wide at seismogenic depths and with 20 to 40% lower shear-wave velocity than the adjacent unfaulted rock. FZGW are also capable of assessing the degree of fault continuity and other properties of complex FZ geometries such as fault jogs. The SAF in the Cholame valley where 2004 M6 earthquake nucleated, is characterized by such complexity and because FZGW also primarily propagate within the core of fault zones, active continuous seismic monitoring using guided waves is our proposed solution for earthquake studies in Parkfield area.
Bulletin of the Seismological Society of America, 2006
The 2004 Parkfield earthquake generated a unique set of near-field, high-resolution colocated measurements of acceleration, volumetric strain, and velocity at 11 stations in the General Earthquake Observation System (GEOS) array. The recordings indicate no precursory strain or displacement was discernable at sensitivities of 10 11מ strain and 5 ן 10 8מ m 25 sec prior to the earthquake at distances of 0.5 to 12 km of fault rupture. Coherent fault-parallel and fault-normal displacement pulses, observed along the fault north of the epicenter, are consistent with model predictions for "fling," directivity, and displacement for right-lateral, strike-slip fault rupture. The fault-parallel and fault-normal pulses imply apparent rupture velocities of 2.86 ע 0.15 and 3.03 ע 0.24 km/sec, respectively. Unprecedented high-resolution volumetric-strain recordings on opposite sides of the fault show that dynamic strains radiated from ruptured segments of the fault are more than an order of magnitude larger than final coseismic strain offsets associated with fault slip, suggesting that dynamic radiated strain may have contributed to the triggering of failure on unruptured segments. High-resolution recordings show that coseismic strain offsets occur abruptly over time intervals of less than 10 sec near the time of arrival of the dominant radiated fault-parallel and fault-normal displacements. Subsequent measurements show that the strain offsets continue to increase by as much as 69% in 5 min and 300% in 24 hr over that measured during initial fault slip at depth. Estimates of local material parameters from simultaneous measurements of volumetric strain and acceleration confirm seismic calibration factors previously measurable in situ only at tidal periods.
Seismic and geodetic evidence for extensive, long-lived fault damage zones
Geology, 2009
During earthquakes, slip is often localized on preexisting faults, but it is not well understood how the structure of crustal faults may contribute to slip localization and energetics. Growing evidence suggests that the crust along active faults undergoes anomalous strain and damage during large earthquakes. Seismic and geodetic data from the Calico fault in the eastern California shear zone reveal a wide zone of reduced seismic velocities and effective elastic moduli. Using seismic traveltimes, trapped waves, and interferometric synthetic aperture radar observations, we document seismic velocities reduced by 40%-50% and shear moduli reduced by 65% compared to wall rock in a 1.5-km-wide zone along the Calico fault. Observed velocity reductions likely represent the cumulative mechanical damage from past earthquake ruptures. No large earthquake has broken the Calico fault in historic time, implying that fault damage persists for hundreds or perhaps thousands of years. These fi ndings indicate that faults can affect rock properties at substantial distances from primary fault slip surfaces, and throughout much of the seismogenic zone, a result with implications for the amount of energy expended during rupture to drive cracking and yielding of rock and development of fault systems.
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