Yong-gang Li | University of Southern California (original) (raw)
Papers by Yong-gang Li
Bulletin of the Seismological Society of America
We present evidence for multiple fault branches of the West Napa fault zone (WNFZ) based on fault... more We present evidence for multiple fault branches of the West Napa fault zone (WNFZ) based on fault‐zone trapped waves (FZTWs) generated by two explosions that were detonated within the main surface rupture zone produced by the 24 August 2014 Mw 6.0 South Napa earthquake. The FZTWs were recorded by a 15‐kilometer‐long dense (100 m spacing) linear seismic array consisting of 155 4.5‐hertz three‐component seismometers that were deployed across the surface ruptures and adjacent faults in Napa Valley in the summer of 2016. The two explosions were located ∼3.5 km north and ∼5 km south of the 2016 recording array. Prominent FZTWs, with large amplitudes and long wavetrains following the P and S waves, are observed on the seismograms. We analyzed FZTW waveforms in both time and frequency domains to characterize the branching structure of subsurface rupture zones along the WNFZ. The 2014 surface rupture zone was ∼12 km in length along the main trace of the WNFZ, which appears to form an ∼40...
Acta Geologica Sinica - English Edition, 2014
An extensive seismic experiment using 100 three-component seismometers (40 broad-band and 60 shor... more An extensive seismic experiment using 100 three-component seismometers (40 broad-band and 60 short-period stations) were installed by researchers from USC, UCSD, and UCLA around the Calico fault in Mojave Desert in 2006 for a seismic investigation of fault-zone compliance shown by InSar anomalies (Fig. 1). All recorders were synchronized through GPS that also provide the coordinates of station locations. We successfully recorded the data from 3 shot-hole explosions and hundreds of aftershocks of the 1992 M7.4 Landers and 1999 M7.1 Hector Mine earthquakes as well as a quite bit of telemetry earthquakes (Fig. 2). The preliminary results from tomography and faultzone trapped wave modeling for the data show a 1 to 2 km wide low-velocity damage zone (LVZ) on the Calico fault, in good agreement with the InSAR-derived compliant zone [Fialko et al., 2002; Fialko, 2004]. The initial analysis also indicates along-strike variations in the degree of damage, with a wider, or perhaps more severe damage zone on the Calico fault to the northwest of the array than to the southeast [Cochran, Li et al., 2006; 2007]. These findings indicate that faults can affect rock properties at significant distances from the primary fault slip surfaces, a result with implications for the portion of energy expended during rupture to drive the cracking and yielding of rock. The data from more local and telemetry events allow us to obtain a more detailed delineation of the LVZ on the Calico fault along the strike and with depth. We thus aim to reconcile the seismic and InSAR anomalies to produce an integrated structural model of the earthquake damage zone around a major strike-slip fault in the Eastern California Shear Zone. Fig. 1 Left: Map shows locations of the square-type dense seismic array (red square) consisting of 100 PASSCAL RT130 seismometers with 40-T and L22 three-component sensors deployed at the Calico fault in Mojave Desert, Southern California in 2006. Red stars denote the 1992 M7.4 Landers earthquake and the 1999 M7.1 Hector Mine earthquake [Fialko, 2002]. 3 explosions SP1, SP2 and SP3 are detonated within and outside the fault zone. Right: Map shows InSar strain anomalies caused by the 1999 M7.1 Hector Mine Earthquake.
Repeated earthquakes and explosions recorded at the San Andreas fault near Parkfield before and a... more 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.
A mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the M7... more A mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the M7.4 Landers earthquake of June 28, 1992, with a maximum offset of 1 km from the trace. We found a distinct wave train with a relatively long period following the S waves that shows up only when both the stations and the events are close to the fault trace. This wave train is interpreted as a seismic guided wave trapped in a low-velocity fault zone. To study the distribution of amplitude of the guided waves with distance from the fault trace and also their attenuation with travel distance along the fault zone, we eliminated source and recording site effects by the coda normalization method. The normalized amplitudes of guided waves show a spectral peak at 3-4 Hz, which decays sharply with distance from the fault trace. Spectral amplitudes at high frequencies (8-15 Hz) show an opposite trend, increasing with distance from the fault trace. The normalized amplitudes of guided waves at 3-4 Hz ...
This Chapter is to review fault rock co-seismic damage and post-mainshock healing progressions as... more This Chapter is to review fault rock co-seismic damage and post-mainshock healing progressions associated with the 1992 M7.4 Landers, the 1999 M7.1 Hector Mine, and the 2004 M6.0 Parkfield earthquakes in California through observations and 3-D finite-difference modeling of fault-zone trapped waves (FZTWs) generated by explosions and aftershocks within and close to the rupture zone and recorded at linear seismic arrays deployed across and along the ruptured faults. These FZTWs allow us to delineate internal structures and physical properties of rupture zones at seismogenic depths with high-resolution. The spatial extent of damage zones, the spatial-varying structure, the damage zone segmentation, and the temporal changes in damage magnitude associated with the recent earthquakes are discussed in this Chapter. The knowledge of spatial and temporal (4-D) patterns in fault zone structure will help predict the behavior of future earthquakes on active faults, and will also help evaluate t...
Acta Geologica Sinica
:This article is to review results from scientific drilling and fault-zone trapped waves (FZTWs) ... more :This article is to review results from scientific drilling and fault-zone trapped waves (FZTWs) at the south Longman-Shan fault (LSF) zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan, China. Immediately after the mainshock, two Wenchuan Fault Scientific Drilling (WFSD) boreholes were drilled at WFSD-1 and WFSD-2 sites approximately 400 m and 1 km west of the surface rupture along the Yinxiu-Beichuan fault (YBF), the middle fault strand of the south LSF zone. Two boreholes met the principal slip of Wenchuan earthquake along the YBF at depths of 589-m and 1230-m, respectively. The slip is accompanied with a 100–200-m-wide zone consisting of fault gouge, breccia, cataclasite and fractures. Close to WFSD-1 site, the nearly-vertical slip of ∼4.3t-m with a 190-m wide zone of highly fractured rocks restricted to the hanging wall of the YBF was found at the ground surface after the Wenchuan earthquake. A dense linear seismic array was deployed across the surface rupt...
SEG Technical Program Expanded Abstracts 1990, 1990
ABSTRACT In October, 2002, coordinated by the Pre-EarthScope/SAFOD, we conducted an extensive sei... more ABSTRACT In October, 2002, coordinated by the Pre-EarthScope/SAFOD, we conducted an extensive seismic experiment at the San Andreas fault (SAF), Parkfield to record fault-zone trapped waves generated by explosions and microearthquakes using dense linear seismic arrays of 52 PASSCAL 3-channel REFTEKs deployed across and along the fault zone. We detonated 3 explosions within and out of the fault zone during the experiment, and also recorded other 13 shots of PASO experiment of UWM/RPI (Thurber and Roecker) detonated around the SAFOD drilling site at the same time. We observed prominent fault-zone trapped waves with large amplitudes and long duration following S waves at stations close to the main fault trace for sources located within and close to the fault zone. Dominant frequencies of trapped waves are 2-3 Hz for near-surface explosions and 4-5 Hz for microearthquakes. Fault-zone trapped waves are relatively weak on the north strand of SAF for same sources. In contrast, seismograms registered for both the stations and shots far away from the fault zone show a brief S wave and lack of trapped waves. These observations are consistent with previous findings of fault-zone trapped waves at the SAF [Li et al., 1990; 1997], indicating the existence of a well-developed low-velocity waveguide along the main fault strand (principal slip plan) of the SAF. The data from denser arrays and 3-D finite-difference simulations of fault-zone trapped waves allowed us to delineate the internal structure, segmentation and physical properties of the SAF with higher resolution. The trapped-wave inferred waveguide on the SAF Parkfield segment is ~150 m wide at surface and tapers to ~100 m at seismogenic depth, in which Q is 20-50 and S velocities are reduced by 30-40% from wall-rock velocities, with the greater velocity reduction at the shallow depth and to southeast of the 1966 M6 epicenter. We interpret this low-velocity waveguide on the SAF main strand as being the remnant of damage zone caused by M6 earthquake episode at Parkfield although it probably represents the accumulated wear from many previous great earthquakes and other kinematical processes. The width of low-velocity waveguide likely represents the damage extent in dynamic rupture, consistent with the scale of process zone size to rupture length as existing model predicted. The variation in velocity reduction along the fault zone indicates an inference of changes in on-fault stress, fine-scale fault geometry, and fluid content at depths. On the other hand, a less developed and narrower low-velocity waveguide is on the north strand that experienced minor breaks at surface in the 1966 M6 event probably due to energy partitioning, strong shaking and dynamic strain by the earthquake on the main fault.
ABSTRACT Coordinated by the SAFOD PIs, we used 96 PASSCAL short-period three-component seismomete... more ABSTRACT Coordinated by the SAFOD PIs, we used 96 PASSCAL short-period three-component seismometers in linear arrays deployed across and along the San Andreas fault (SAF) near the town of Parkfield and the SAFOD drilling site in 2002 and 2003, respectively. The data recorded for near-surface explosions detonated in the experiments (Li and Vidale), PASO project (Thurber and Roecker) and refraction profiling (Hole), and local earthquakes show fault-zone trapped waves clearly for the source and receivers located close to the fault. The time duration of the dominant trapped energy after S-arrivals increases with the event-to-array distance and focal depth progressively. Using a finite-difference code, we first synthesize fault-zone trapped waves generated by explosions to determine the shallowest 1 or 2 km fault zone structure with the velocity constraints from seismic profiling of the shallow SAF at Parkfield [Catchings et al., 2002]. We then strip shallow effects to resolve deeper structure of the fault zone, and synthesize trapped waves from earthquakes at depths between 2.5 and 11 km to complete a model of the SAF with depth-variable structure in 3-D. We also use the P-first arrivals and polarity as additional information in modeling of velocities and location of the material interface with the structural constraints from seismic tomography at Parkfield [Thurber et al., 2004] to the bed-rock velocities. In grid-search modeling, we tested various values for fault zone depth, width, velocity, Q, and source location. The best-fit model parameters from this study show evidence of a damaged core zone on the main SAF, which likely extends to seismogenic depths. The zone is marked by a low-velocity waveguide ~150 m wide, in which Q is 10-50 and shear velocities are reduced by 30-45% from wall-rock velocities. We also find some seismic energy trapped partitioned in the branching faults that connect to the San Andreas main fault at a shallow depth near Parkfield.
ABSTRACT We deployed dense linear arrays of PASSCAL REFTEK130s across and along the San Andreas F... more ABSTRACT We deployed dense linear arrays of PASSCAL REFTEK130s across and along the San Andreas Fault near the SAFOD drilling site at Parkfield to record fault-zone trapped waves in the fall of 2003. We acquired the data for 114 local earthquakes with M0-2.3 including 2 SAFOD target events (located by Thurber and Roecker; Nadeau), five USGS explosions in a fan geometry, and 47 small shots (Hole; Catchings and Rymer). Prominent fault-zone trapped waves with large amplitudes and long duration at 2-5 Hz were observed for events within or close to the fault zone. The data from ~25 on-fault events with the raypath incidence angles smaller than 10 degrees from vertical show that the time duration of the dominant trapped wave energy increases from ~0.8 s to ~2.0 s as the event depth increases from ~2.5 km to ~10 km progressively. These observations show the existence of a low-velocity waveguide on the SAF that likely extends to seismogenic depths in this region, although the velocity reduction in the deeper part of fault zone becomes smaller due to the larger confined stress at greater depths. This is consistent with the low-velocity waveguide in the rupture zone of the 1992 M7.4 Landers earthquake, which has been mapped across seismogenic depths [Li et al., 1994; 2000]. 3-D finite-difference waveform simulations of the data delineate the internal damaged structure of the SAF near SAFOD drilling site using a depth-dependent model with the low-velocity waveguide, which is 150 m wide at the top and narrower at 10 km depth, and has shear velocities reduced by 30-45% from wall-rock velocities and Q of 10-50. The model parameters are consistent with those obtained on the SAF near Parkfield in previous studied using fault-zone trapped waves [Li et al., 1990; 1997; 2004], showing that the low-velocity zone extends along the SAF strike from the rupture of the 1966 M6 earthquake to the NW creeping segment. We interpret that the trapped wave inferred low-velocity waveguide on the SAF is a remnant of repeated damage due to M6 earthquakes and other large historical earthquakes on the principal slip plane at Parkfield. We also find some seismic energy trapped in the Buzzard Canyon fault that is approximately 1 km SW to the SAF main trace and dips toward the NE to connect to the SAF at a shallow depth. Among many other methods, fault-zone trapped waves can be used to document fine-scale damaged fault zones with high-resolution.
ABSTRACT We are measuring shear wave anisotropy along the surface rupture of the Hector Mine faul... more ABSTRACT We are measuring shear wave anisotropy along the surface rupture of the Hector Mine fault zone. Portable seismic arrays deployed following the October 16, 1999 M7.1 Hector Mine earthquake recorded numerous aftershocks in 1999 and 2000. We are analyzing data from two arrays (northern and southern) deployed in 1999 and 2000 in approximately the same location, each with a line of stations approximately 500m long that crossed the surface rupture. Our analysis of the northern array data shows significant scatter with source location, receiver location, and calender time. The 1999 data clearly show the fast direction parallel to the regional maximum compressive stress direction. The 2000 data splitting results show less consistent splitting directions and delay. This evidence for a change in anisotropy suggests an evolution of the near-fault region after a major quake. Previous work has shown transient reduction of strength, an increase in scattering and fault normal extension following an earthquake. These phenomenon and our new observations may indicate the opening of isotropically oriented cracks at the time of an earthquake by dynamic stresses, followed by gradual collapse of the cracks, with the cracks perpendicular to the greatest compressive stress closing first. A few months after the Hector Mine earthquake, we may be seeing anisotropy from preferential closure of unfavorable oriented cracks, which has changed by a year later, when most cracks have closed. We will check this pattern by analysis of data from the southern array, and explore the distribution of cracks in the region of the fault in future work, possibly by December.
Tectonophysics, 2014
Darfield earthquake Multiple ruptures Fault rock co-seismic damage Fault-zone trapped waves To ch... more Darfield earthquake Multiple ruptures Fault rock co-seismic damage Fault-zone trapped waves To characterize the subsurface structure of the damage zones caused by the 2010-2011 Canterbury earthquake sequence in New Zealand's South Island, we installed two short linear seismic arrays; Array 1 across the Greendale Fault (GF) surface rupture and Array 2 over the surface projection of the blind Port Hills Fault (PHF) that ruptured in the 2010 M7.1 Darfield and 2011 M6.3 Christchurch earthquakes, respectively. We recorded 853 aftershocks for~4 months after the Christchurch earthquake. Fault-zone trapped waves (FZTWs) identified at Array 1 for aftershocks occurring on both the GF and the PHF show that the postS durations of these FZTWs increase as focal depths and epicentral distances from the array increase, suggesting an effective low-velocity waveguide formed by severely damaged rocks existing along the GF and PHF at seismogenic depths. Locations of aftershocks generating prominent FZTWs delineates the subsurface GF rupture extending eastward as bifurcating blind fault segments an additional~5-8 km beyond the mapped~30-km surface rupture into a zone with comparably lower seismic moment release west of the PHF rupture which extends westward to within 5.3 ± 1 km of the subsurface GF. The propagation of FZTW through the intervening 'gap' indicates moderate GF-PHF structural connectivity. We interpret this zone as a fracture mesh reflecting the interplay between basement faults and stress-aligned microcracks that enable the propagation of PHF-sourced FZTWs into the GF damage zone. Simulations of observed FZTWs suggest that the GF rupture zone is~200-250-m wide, consistent with the surface deformation width. Velocities within the zone are reduced by 35-55% with the maximum reduction in the~100-m-wide damage core zone corresponding with surface and shallow subsurface evidence for discrete fracturing. The damage zone extends down to depths of~8 km or deeper, consistent with hypocentral locations and geodetically-derived fault models.
Journal of Geophysical Research, 1990
A 600-m borehole vertical seismic profiling (VSP) survey conducted in crystalline rock in the Moj... more A 600-m borehole vertical seismic profiling (VSP) survey conducted in crystalline rock in the Mojave Block stress province, southern California, yielded 100 oriented three-component seismograms from a fan-shooting geometry with radius of 100 m using both vertical and horizontal vibrations and a vertical impact source. The seismograms show up to 12 ms of shear wave splitting and 5% lateral velocity
Journal of Geophysical Research, 1996
ABSTRACT
Journal of Geophysical Research, 2003
We studied the complex multiple-faulting pattern of the 40-km-long rupture zone of the 1999 M7.1 ... more We studied the complex multiple-faulting pattern of the 40-km-long rupture zone of the 1999 M7.1 Hector Mine, California, earthquake with fault zone trapped waves generated by near-surface explosions and aftershocks, and recorded by linear seismic arrays deployed across the surface rupture. The explosion excited trapped waves, with relatively large amplitudes at 3-5 Hz and a long duration of S coda waves, are similar to those observed for aftershocks but have lower frequencies and travel more slowly. Threedimensional finite difference simulations of fault zone trapped waves indicate a 75-to 100-m-wide low-velocity and low-Q zone (waveguide) along the rupture surface on the Lavic Lake fault (LLF) in the Bullion Mountains. The S velocity within the waveguide varies from 1.0 to 2.5 km/s at depths of 0-8 km, reduced by 35−4535-45% from the wall rock velocity, and Q is 35−4510-60. The pattern of aftershocks for which we observed trapped waves shows that this low-velocity waveguide has two branches in the northern and southern portions of the rupture zone, indicating a multiple-fault rupture at seismogenic depth. North of the Bullion Mountains, although only the rupture segment on the northwest LLF broke to the surface, a rupture segment on a buried fault also extended 15kminthemorenortherlydirectionfromthemainshockepicenter.Tothesouth,theruptureontheLLFintersectedtheBullionfault(BF)andbifurcated.TheruptureonthesouthLLFextended15 km in the more northerly direction from the main shock epicenter. To the south, the rupture on the LLF intersected the Bullion fault (BF) and bifurcated. The rupture on the south LLF extended 15kminthemorenortherlydirectionfromthemainshockepicenter.Tothesouth,theruptureontheLLFintersectedtheBullionfault(BF)andbifurcated.TheruptureonthesouthLLFextended10 km from the intersection and diminished while there was minor rupture on the southeast BF, which dips to the northeast and disconnects from the LLF at depth. Thus the analysis of fault zone trapped waves helps delineate a more complex set of rupture planes than the surface breakage, in accord with the complex pattern of aftershock distribution and geodetic evidence that the Hector Mine event involved several faults which may also rupture individually. Our simulations of dynamic rupture using a finite element code show that generic models are able to produce the general features of the northern part of the rupture, including slips on subparallel fault segments. The models indicate that such a faulting pattern is physically plausible and consistent with observations.
Bulletin of the Seismological Society of America
We present evidence for multiple fault branches of the West Napa fault zone (WNFZ) based on fault... more We present evidence for multiple fault branches of the West Napa fault zone (WNFZ) based on fault‐zone trapped waves (FZTWs) generated by two explosions that were detonated within the main surface rupture zone produced by the 24 August 2014 Mw 6.0 South Napa earthquake. The FZTWs were recorded by a 15‐kilometer‐long dense (100 m spacing) linear seismic array consisting of 155 4.5‐hertz three‐component seismometers that were deployed across the surface ruptures and adjacent faults in Napa Valley in the summer of 2016. The two explosions were located ∼3.5 km north and ∼5 km south of the 2016 recording array. Prominent FZTWs, with large amplitudes and long wavetrains following the P and S waves, are observed on the seismograms. We analyzed FZTW waveforms in both time and frequency domains to characterize the branching structure of subsurface rupture zones along the WNFZ. The 2014 surface rupture zone was ∼12 km in length along the main trace of the WNFZ, which appears to form an ∼40...
Acta Geologica Sinica - English Edition, 2014
An extensive seismic experiment using 100 three-component seismometers (40 broad-band and 60 shor... more An extensive seismic experiment using 100 three-component seismometers (40 broad-band and 60 short-period stations) were installed by researchers from USC, UCSD, and UCLA around the Calico fault in Mojave Desert in 2006 for a seismic investigation of fault-zone compliance shown by InSar anomalies (Fig. 1). All recorders were synchronized through GPS that also provide the coordinates of station locations. We successfully recorded the data from 3 shot-hole explosions and hundreds of aftershocks of the 1992 M7.4 Landers and 1999 M7.1 Hector Mine earthquakes as well as a quite bit of telemetry earthquakes (Fig. 2). The preliminary results from tomography and faultzone trapped wave modeling for the data show a 1 to 2 km wide low-velocity damage zone (LVZ) on the Calico fault, in good agreement with the InSAR-derived compliant zone [Fialko et al., 2002; Fialko, 2004]. The initial analysis also indicates along-strike variations in the degree of damage, with a wider, or perhaps more severe damage zone on the Calico fault to the northwest of the array than to the southeast [Cochran, Li et al., 2006; 2007]. These findings indicate that faults can affect rock properties at significant distances from the primary fault slip surfaces, a result with implications for the portion of energy expended during rupture to drive the cracking and yielding of rock. The data from more local and telemetry events allow us to obtain a more detailed delineation of the LVZ on the Calico fault along the strike and with depth. We thus aim to reconcile the seismic and InSAR anomalies to produce an integrated structural model of the earthquake damage zone around a major strike-slip fault in the Eastern California Shear Zone. Fig. 1 Left: Map shows locations of the square-type dense seismic array (red square) consisting of 100 PASSCAL RT130 seismometers with 40-T and L22 three-component sensors deployed at the Calico fault in Mojave Desert, Southern California in 2006. Red stars denote the 1992 M7.4 Landers earthquake and the 1999 M7.1 Hector Mine earthquake [Fialko, 2002]. 3 explosions SP1, SP2 and SP3 are detonated within and outside the fault zone. Right: Map shows InSar strain anomalies caused by the 1999 M7.1 Hector Mine Earthquake.
Repeated earthquakes and explosions recorded at the San Andreas fault near Parkfield before and a... more 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.
A mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the M7... more A mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the M7.4 Landers earthquake of June 28, 1992, with a maximum offset of 1 km from the trace. We found a distinct wave train with a relatively long period following the S waves that shows up only when both the stations and the events are close to the fault trace. This wave train is interpreted as a seismic guided wave trapped in a low-velocity fault zone. To study the distribution of amplitude of the guided waves with distance from the fault trace and also their attenuation with travel distance along the fault zone, we eliminated source and recording site effects by the coda normalization method. The normalized amplitudes of guided waves show a spectral peak at 3-4 Hz, which decays sharply with distance from the fault trace. Spectral amplitudes at high frequencies (8-15 Hz) show an opposite trend, increasing with distance from the fault trace. The normalized amplitudes of guided waves at 3-4 Hz ...
This Chapter is to review fault rock co-seismic damage and post-mainshock healing progressions as... more This Chapter is to review fault rock co-seismic damage and post-mainshock healing progressions associated with the 1992 M7.4 Landers, the 1999 M7.1 Hector Mine, and the 2004 M6.0 Parkfield earthquakes in California through observations and 3-D finite-difference modeling of fault-zone trapped waves (FZTWs) generated by explosions and aftershocks within and close to the rupture zone and recorded at linear seismic arrays deployed across and along the ruptured faults. These FZTWs allow us to delineate internal structures and physical properties of rupture zones at seismogenic depths with high-resolution. The spatial extent of damage zones, the spatial-varying structure, the damage zone segmentation, and the temporal changes in damage magnitude associated with the recent earthquakes are discussed in this Chapter. The knowledge of spatial and temporal (4-D) patterns in fault zone structure will help predict the behavior of future earthquakes on active faults, and will also help evaluate t...
Acta Geologica Sinica
:This article is to review results from scientific drilling and fault-zone trapped waves (FZTWs) ... more :This article is to review results from scientific drilling and fault-zone trapped waves (FZTWs) at the south Longman-Shan fault (LSF) zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan, China. Immediately after the mainshock, two Wenchuan Fault Scientific Drilling (WFSD) boreholes were drilled at WFSD-1 and WFSD-2 sites approximately 400 m and 1 km west of the surface rupture along the Yinxiu-Beichuan fault (YBF), the middle fault strand of the south LSF zone. Two boreholes met the principal slip of Wenchuan earthquake along the YBF at depths of 589-m and 1230-m, respectively. The slip is accompanied with a 100–200-m-wide zone consisting of fault gouge, breccia, cataclasite and fractures. Close to WFSD-1 site, the nearly-vertical slip of ∼4.3t-m with a 190-m wide zone of highly fractured rocks restricted to the hanging wall of the YBF was found at the ground surface after the Wenchuan earthquake. A dense linear seismic array was deployed across the surface rupt...
SEG Technical Program Expanded Abstracts 1990, 1990
ABSTRACT In October, 2002, coordinated by the Pre-EarthScope/SAFOD, we conducted an extensive sei... more ABSTRACT In October, 2002, coordinated by the Pre-EarthScope/SAFOD, we conducted an extensive seismic experiment at the San Andreas fault (SAF), Parkfield to record fault-zone trapped waves generated by explosions and microearthquakes using dense linear seismic arrays of 52 PASSCAL 3-channel REFTEKs deployed across and along the fault zone. We detonated 3 explosions within and out of the fault zone during the experiment, and also recorded other 13 shots of PASO experiment of UWM/RPI (Thurber and Roecker) detonated around the SAFOD drilling site at the same time. We observed prominent fault-zone trapped waves with large amplitudes and long duration following S waves at stations close to the main fault trace for sources located within and close to the fault zone. Dominant frequencies of trapped waves are 2-3 Hz for near-surface explosions and 4-5 Hz for microearthquakes. Fault-zone trapped waves are relatively weak on the north strand of SAF for same sources. In contrast, seismograms registered for both the stations and shots far away from the fault zone show a brief S wave and lack of trapped waves. These observations are consistent with previous findings of fault-zone trapped waves at the SAF [Li et al., 1990; 1997], indicating the existence of a well-developed low-velocity waveguide along the main fault strand (principal slip plan) of the SAF. The data from denser arrays and 3-D finite-difference simulations of fault-zone trapped waves allowed us to delineate the internal structure, segmentation and physical properties of the SAF with higher resolution. The trapped-wave inferred waveguide on the SAF Parkfield segment is ~150 m wide at surface and tapers to ~100 m at seismogenic depth, in which Q is 20-50 and S velocities are reduced by 30-40% from wall-rock velocities, with the greater velocity reduction at the shallow depth and to southeast of the 1966 M6 epicenter. We interpret this low-velocity waveguide on the SAF main strand as being the remnant of damage zone caused by M6 earthquake episode at Parkfield although it probably represents the accumulated wear from many previous great earthquakes and other kinematical processes. The width of low-velocity waveguide likely represents the damage extent in dynamic rupture, consistent with the scale of process zone size to rupture length as existing model predicted. The variation in velocity reduction along the fault zone indicates an inference of changes in on-fault stress, fine-scale fault geometry, and fluid content at depths. On the other hand, a less developed and narrower low-velocity waveguide is on the north strand that experienced minor breaks at surface in the 1966 M6 event probably due to energy partitioning, strong shaking and dynamic strain by the earthquake on the main fault.
ABSTRACT Coordinated by the SAFOD PIs, we used 96 PASSCAL short-period three-component seismomete... more ABSTRACT Coordinated by the SAFOD PIs, we used 96 PASSCAL short-period three-component seismometers in linear arrays deployed across and along the San Andreas fault (SAF) near the town of Parkfield and the SAFOD drilling site in 2002 and 2003, respectively. The data recorded for near-surface explosions detonated in the experiments (Li and Vidale), PASO project (Thurber and Roecker) and refraction profiling (Hole), and local earthquakes show fault-zone trapped waves clearly for the source and receivers located close to the fault. The time duration of the dominant trapped energy after S-arrivals increases with the event-to-array distance and focal depth progressively. Using a finite-difference code, we first synthesize fault-zone trapped waves generated by explosions to determine the shallowest 1 or 2 km fault zone structure with the velocity constraints from seismic profiling of the shallow SAF at Parkfield [Catchings et al., 2002]. We then strip shallow effects to resolve deeper structure of the fault zone, and synthesize trapped waves from earthquakes at depths between 2.5 and 11 km to complete a model of the SAF with depth-variable structure in 3-D. We also use the P-first arrivals and polarity as additional information in modeling of velocities and location of the material interface with the structural constraints from seismic tomography at Parkfield [Thurber et al., 2004] to the bed-rock velocities. In grid-search modeling, we tested various values for fault zone depth, width, velocity, Q, and source location. The best-fit model parameters from this study show evidence of a damaged core zone on the main SAF, which likely extends to seismogenic depths. The zone is marked by a low-velocity waveguide ~150 m wide, in which Q is 10-50 and shear velocities are reduced by 30-45% from wall-rock velocities. We also find some seismic energy trapped partitioned in the branching faults that connect to the San Andreas main fault at a shallow depth near Parkfield.
ABSTRACT We deployed dense linear arrays of PASSCAL REFTEK130s across and along the San Andreas F... more ABSTRACT We deployed dense linear arrays of PASSCAL REFTEK130s across and along the San Andreas Fault near the SAFOD drilling site at Parkfield to record fault-zone trapped waves in the fall of 2003. We acquired the data for 114 local earthquakes with M0-2.3 including 2 SAFOD target events (located by Thurber and Roecker; Nadeau), five USGS explosions in a fan geometry, and 47 small shots (Hole; Catchings and Rymer). Prominent fault-zone trapped waves with large amplitudes and long duration at 2-5 Hz were observed for events within or close to the fault zone. The data from ~25 on-fault events with the raypath incidence angles smaller than 10 degrees from vertical show that the time duration of the dominant trapped wave energy increases from ~0.8 s to ~2.0 s as the event depth increases from ~2.5 km to ~10 km progressively. These observations show the existence of a low-velocity waveguide on the SAF that likely extends to seismogenic depths in this region, although the velocity reduction in the deeper part of fault zone becomes smaller due to the larger confined stress at greater depths. This is consistent with the low-velocity waveguide in the rupture zone of the 1992 M7.4 Landers earthquake, which has been mapped across seismogenic depths [Li et al., 1994; 2000]. 3-D finite-difference waveform simulations of the data delineate the internal damaged structure of the SAF near SAFOD drilling site using a depth-dependent model with the low-velocity waveguide, which is 150 m wide at the top and narrower at 10 km depth, and has shear velocities reduced by 30-45% from wall-rock velocities and Q of 10-50. The model parameters are consistent with those obtained on the SAF near Parkfield in previous studied using fault-zone trapped waves [Li et al., 1990; 1997; 2004], showing that the low-velocity zone extends along the SAF strike from the rupture of the 1966 M6 earthquake to the NW creeping segment. We interpret that the trapped wave inferred low-velocity waveguide on the SAF is a remnant of repeated damage due to M6 earthquakes and other large historical earthquakes on the principal slip plane at Parkfield. We also find some seismic energy trapped in the Buzzard Canyon fault that is approximately 1 km SW to the SAF main trace and dips toward the NE to connect to the SAF at a shallow depth. Among many other methods, fault-zone trapped waves can be used to document fine-scale damaged fault zones with high-resolution.
ABSTRACT We are measuring shear wave anisotropy along the surface rupture of the Hector Mine faul... more ABSTRACT We are measuring shear wave anisotropy along the surface rupture of the Hector Mine fault zone. Portable seismic arrays deployed following the October 16, 1999 M7.1 Hector Mine earthquake recorded numerous aftershocks in 1999 and 2000. We are analyzing data from two arrays (northern and southern) deployed in 1999 and 2000 in approximately the same location, each with a line of stations approximately 500m long that crossed the surface rupture. Our analysis of the northern array data shows significant scatter with source location, receiver location, and calender time. The 1999 data clearly show the fast direction parallel to the regional maximum compressive stress direction. The 2000 data splitting results show less consistent splitting directions and delay. This evidence for a change in anisotropy suggests an evolution of the near-fault region after a major quake. Previous work has shown transient reduction of strength, an increase in scattering and fault normal extension following an earthquake. These phenomenon and our new observations may indicate the opening of isotropically oriented cracks at the time of an earthquake by dynamic stresses, followed by gradual collapse of the cracks, with the cracks perpendicular to the greatest compressive stress closing first. A few months after the Hector Mine earthquake, we may be seeing anisotropy from preferential closure of unfavorable oriented cracks, which has changed by a year later, when most cracks have closed. We will check this pattern by analysis of data from the southern array, and explore the distribution of cracks in the region of the fault in future work, possibly by December.
Tectonophysics, 2014
Darfield earthquake Multiple ruptures Fault rock co-seismic damage Fault-zone trapped waves To ch... more Darfield earthquake Multiple ruptures Fault rock co-seismic damage Fault-zone trapped waves To characterize the subsurface structure of the damage zones caused by the 2010-2011 Canterbury earthquake sequence in New Zealand's South Island, we installed two short linear seismic arrays; Array 1 across the Greendale Fault (GF) surface rupture and Array 2 over the surface projection of the blind Port Hills Fault (PHF) that ruptured in the 2010 M7.1 Darfield and 2011 M6.3 Christchurch earthquakes, respectively. We recorded 853 aftershocks for~4 months after the Christchurch earthquake. Fault-zone trapped waves (FZTWs) identified at Array 1 for aftershocks occurring on both the GF and the PHF show that the postS durations of these FZTWs increase as focal depths and epicentral distances from the array increase, suggesting an effective low-velocity waveguide formed by severely damaged rocks existing along the GF and PHF at seismogenic depths. Locations of aftershocks generating prominent FZTWs delineates the subsurface GF rupture extending eastward as bifurcating blind fault segments an additional~5-8 km beyond the mapped~30-km surface rupture into a zone with comparably lower seismic moment release west of the PHF rupture which extends westward to within 5.3 ± 1 km of the subsurface GF. The propagation of FZTW through the intervening 'gap' indicates moderate GF-PHF structural connectivity. We interpret this zone as a fracture mesh reflecting the interplay between basement faults and stress-aligned microcracks that enable the propagation of PHF-sourced FZTWs into the GF damage zone. Simulations of observed FZTWs suggest that the GF rupture zone is~200-250-m wide, consistent with the surface deformation width. Velocities within the zone are reduced by 35-55% with the maximum reduction in the~100-m-wide damage core zone corresponding with surface and shallow subsurface evidence for discrete fracturing. The damage zone extends down to depths of~8 km or deeper, consistent with hypocentral locations and geodetically-derived fault models.
Journal of Geophysical Research, 1990
A 600-m borehole vertical seismic profiling (VSP) survey conducted in crystalline rock in the Moj... more A 600-m borehole vertical seismic profiling (VSP) survey conducted in crystalline rock in the Mojave Block stress province, southern California, yielded 100 oriented three-component seismograms from a fan-shooting geometry with radius of 100 m using both vertical and horizontal vibrations and a vertical impact source. The seismograms show up to 12 ms of shear wave splitting and 5% lateral velocity
Journal of Geophysical Research, 1996
ABSTRACT
Journal of Geophysical Research, 2003
We studied the complex multiple-faulting pattern of the 40-km-long rupture zone of the 1999 M7.1 ... more We studied the complex multiple-faulting pattern of the 40-km-long rupture zone of the 1999 M7.1 Hector Mine, California, earthquake with fault zone trapped waves generated by near-surface explosions and aftershocks, and recorded by linear seismic arrays deployed across the surface rupture. The explosion excited trapped waves, with relatively large amplitudes at 3-5 Hz and a long duration of S coda waves, are similar to those observed for aftershocks but have lower frequencies and travel more slowly. Threedimensional finite difference simulations of fault zone trapped waves indicate a 75-to 100-m-wide low-velocity and low-Q zone (waveguide) along the rupture surface on the Lavic Lake fault (LLF) in the Bullion Mountains. The S velocity within the waveguide varies from 1.0 to 2.5 km/s at depths of 0-8 km, reduced by 35−4535-45% from the wall rock velocity, and Q is 35−4510-60. The pattern of aftershocks for which we observed trapped waves shows that this low-velocity waveguide has two branches in the northern and southern portions of the rupture zone, indicating a multiple-fault rupture at seismogenic depth. North of the Bullion Mountains, although only the rupture segment on the northwest LLF broke to the surface, a rupture segment on a buried fault also extended 15kminthemorenortherlydirectionfromthemainshockepicenter.Tothesouth,theruptureontheLLFintersectedtheBullionfault(BF)andbifurcated.TheruptureonthesouthLLFextended15 km in the more northerly direction from the main shock epicenter. To the south, the rupture on the LLF intersected the Bullion fault (BF) and bifurcated. The rupture on the south LLF extended 15kminthemorenortherlydirectionfromthemainshockepicenter.Tothesouth,theruptureontheLLFintersectedtheBullionfault(BF)andbifurcated.TheruptureonthesouthLLFextended10 km from the intersection and diminished while there was minor rupture on the southeast BF, which dips to the northeast and disconnects from the LLF at depth. Thus the analysis of fault zone trapped waves helps delineate a more complex set of rupture planes than the surface breakage, in accord with the complex pattern of aftershock distribution and geodetic evidence that the Hector Mine event involved several faults which may also rupture individually. Our simulations of dynamic rupture using a finite element code show that generic models are able to produce the general features of the northern part of the rupture, including slips on subparallel fault segments. The models indicate that such a faulting pattern is physically plausible and consistent with observations.