Yong-Gang Li

Seismic guided waves trapped in the fault zone of the Landers, California, earthquake of 1992

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Seismic trapped modes in the Oroville and San Andreas Fault zones

Three-component borehole seismic profiling of the recently active Oroville, California, normal fault and microearthquake event recording with a near-fault three-component borehole seismometer on the San Andreas fault at Parkfield, California, have shown numerous instances of pronounced dispersive wave trains following the shear wave arrivals. These wave trains are interpreted as fault zone-trapped seismic modes. Parkfield earthquakes exciting trapped modes have been located as deep as 10 kilometers, as shallow as 4 kilometers, and extend 12 kilometers along the fault on either side of the recording station. Selected Oroville and Parkfield wave forms are modeled as the fundamental and first higher trapped SH modes of a narrow low-velocity layer at the fault. Modeling results suggest that the Oroville fault zone is 18 meters wide at depth and has a shear wave velocity of 1 kilometer per second, whereas at Parkfield, the fault gouge is 100 to 150 meters wide and has a shear wave velocity of 1.1 to 1.8 kilometers per second. These low-velocity layers are probably the rupture planes on which earthquakes occur.

Seismic and geodetic evidence for extensive, long-lived fault damage zones

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.

Seismic Evidence for Rock Damage and Healing on the San Andreas Fault Associated with the 2004 M 6.0 Parkfield Earthquake

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 exper- iment 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 1.0%-1.5% decreases in seismic-wave velocity within an 200-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 (Li et al., 2004). The damage zone is not symmetric but extends farther on the southwest side of the main fault trace. Waveform cross- correlations for repeated aftershocks in 21 clusters, with a total of 130 events, located at different depths and distances from the array site show 0.7%-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 2.5%, 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 promi- nent at depths above 7 km.

Fine Structure of the Landers Fault Zone: Segmentation and the Rupture Process

Observations and modeling of 3- to 6-hertz seismic shear waves trapped within the fault zone of the 1992 Landers earthquake series allow the fine structure and continuity of the zone to be evaluated. The fault, to a depth of at least 12 kilometers, is marked by a zone 100 to 200 meters wide where shear velocity is reduced by 30 to 50 percent. This zone forms a seismic waveguide that extends along the southern 30 kilometers of the Landers rupture surface and ends at the fault bend about 18 kilometers north of the main shock epicenter. Another fault plane waveguide, disconnected from the first, exists along the northern rupture surface. These observations, in conjunction with surface slip, detailed seismicity patterns, and the progression of rupture along the fault, suggest that several simple rupture planes were involved in the Landers earthquake and that the inferred rupture front hesitated or slowed at the location where the rupture jumped from one to the next plane. Reduction in rupture velocity can tentatively be attributed to fault plane complexity, and variations in moment release can be attributed to variations in available energy.

Postseismic Fault Healing on the Rupture Zone of the 1999 M 7.1 Hector Mine, California, Earthquake

We probed the rupture zone of the October 1999 M 7.1 Hector Mine earthquake using repeated near-surface explosions in October 2000 and November 2001. Three dense linear seismic arrays were deployed across the north and south Lavic Lake faults (LLFs) that broke to the surface in the mainshock and across the Bullion fault (BF) that experienced minor slip in that event. Two explosions each year were detonated in the rupture zone, one on the middle and one on the south LLF. We found that P and S velocities of fault-zone rocks increased by 0.7%-1.4% and 0.5%-1.0% between 2000 and 2001, respectively. In contrast, the velocities for P and S waves in surrounding rocks increased much less. This trend indicates that the Hector Mine rupture zone has been healing by strengthening after the main- shock, most likely due to the closure of cracks that opened during the 1999 earth- quake. The observed fault-zone strength recovery is consistent with an apparent crack density decrease of 1.5% within the rupture zone. The ratio of travel-time decrease for P to S waves was 0.72, consistent with partially fluid-filled cracks near the fault zone. This restrengthening is similar to that observed after the 1992 M 7.4 Landers earthquake, which occurred 25 km to the west (Li and Vidale, 2001). We also find that the velocity increase with time varies from one fault segment to another at the Hector Mine rupture zone. We see greater changes on the LLFs than on the BF, and the greatest change is on the middle LLF at shallow depth. We tentatively conclude that greater damage was inflicted, and thus greater healing is observed, in regions with larger slip in the mainshock.

Depth-dependent structure of the Landers fault zone from trapped waves generated by aftershocks

We delineate the internal structure of the Johnson Valley and Kickapoo faults (Landers southern rupture) at seismogenic depth using fault zone trapped waves generated by aftershocks. Trapped waves recorded at the dense linear seismic arrays deployed across and along the surface breaks of the 1992 M7.5 Landers earthquake show large amplitudes and dispersive wave trains following the S waves. Group velocities of trapped waves measured from multiple band-pass-filtered seismograms for aftershocks occurring at different depths between 1.8 km and 8.2 km show an increase in velocity with depth. Velocities range from 1.9 km/s at 4 Hz to 2.6 km/s at 1 Hz for shallow events, while for deep events, velocities range from 2.3 km/s at 4 Hz to 3.1 km/s at 1 Hz. Coda-normalized amplitude spectra of trapped waves peak in amplitudes at 3–4 Hz for stations located close to the fault trace. The amplitude decays rapidly with the station offset from the fault zone. Normalized amplitudes also decrease with distance along the fault, giving an apparent Q of 30 for shallow events and 50 for deep events. We evaluated depth-dependent fault zone structure and its uncertainty from these measurements plus our previous results from near-surface explosion-excited trapped waves [Li et al., 1999] in a systematic model parameter-searching procedure using a three-dimensional (3-D) finite difference computer code [Graves, 1996]. Our best model of the Landers fault zone is 250 m wide at the surface, tapering to 100–150 m at 8.2 km depth. The shear velocity within the fault zone increases from 1.0 to 2.5 km/s and Q increases from 20 to 60 in this depth range. Fault zone shear velocities are reduced by 35 to 45% from those of the surrounding rock and also vary along the fault zone with an increase of ∼10% near ends of the southern rupture zone.

A delineation of the Nojima fault ruptured in the M7.2 Kobe, Japan, earthquake of 1995 using fault zone trapped waves

We used four linear seismic arrays of portable seismometers at the northern Awaji Island, Japan, to record fault zone trapped waves from aftershocks of the 1995 M7.2 Hyogoken Nanbu (Kobe) earthquake from April to June 1996. Three arrays were deployed across the Nojima fault, which ruptured during the mainshock, while one array was deployed across the Higashiura fault, which did not break recently. We observed significant fault zone trapped waves with relatively large amplitudes and the long wave train following S waves only when both the stations and aftershocks were located close to the Nojima fault. The coda-normalized spectral amplitudes of trapped waves show a maximum peak at 4–7 Hz, which decreases rapidly with distance from the fault trace. The normalized amplitudes of trapped waves also show a decrease with hypocentral distance along the fault, giving an apparent Q of approximately ∼25 at 4–7 Hz. In comparison, the array across the Higashiura fault recorded much shorter wave trains with higher frequencies after S arrivals for the same events. We simulate these trapped waves as S waves guided in a low-velocity waveguide sandwiched between high-velocity wall rocks. We find an adequate fit by using a waveguide 60 m wide at the northern site and 30–40 m wide elsewhere along the Nojima fault, a waveguide S velocity of 1.5–1.7 km/s, and a Q value of 25. For the Higashiura fault, the S velocity is 2.5 km/s, and the Q value is 80. The locations of aftershocks for which we observed fault zone trapped waves show that the Nojima waveguide is 9 km long and dips southeastward at 80°–85° to a depth of ∼16 km. It extends 6 km farther south westward along the Asano fault, though there are no obvious surface breaks along it. However, the waveguide is disconnected from the Suma fault on the main island, which was also ruptured during the Kobe earthquake, possibly because of the existence of an offset between the Nojima fault and the Suma fault.

Healing of the shallow fault zone from 1994–1998 After the 1992 M7.5 Landers, California, Earthquake

We conducted seismic surveys at the Johnson Valley fault in 1994, 1996, and 1998. We found that the shear velocity of the fault zone rock increased by ∼1.2% between 1994 and 1996, and increased further by ∼0.7% between 1996 and 1998. This trend indicates the Landers rupture zone has been healing by strengthening after the mainshock, most likely due to the closure of cracks that opened during the 1992 earthquake. The observed fault-zone strength recovery is consistent with a decrease of ∼0.03 in the apparent crack density within the fault zone. The ratio of decrease in travel time for P to S waves changed from 0.75 in the earlier two years to 0.65 in the later two years between 1994 and 1998, suggesting that cracks near the fault zone are partially fluid-filled and have became more fluid saturated with time.

Study of the 1999 M 7.1 Hector Mine, California, Earthquake Fault Plane by Trapped Waves

We recorded fault-zone trapped waves from aftershocks on portable seismometers in a tight linear array across the Lavic Lake fault, which was one of several faults that ruptured in the M 7.1 Hector Mine, California, earthquake on 16 October 1999. Trapped waves with large amplitudes and long duration at 4 to 7 Hz produced by aftershocks occurring within the rupture zone were recorded at stations close to the fault trace. However, the S waves registered at stations farther from the rupture zone for the same events were much briefer. Trapped waves recorded at the Hector Mine rupture zone are similar to those observed in the Landers rupture zone (Li et al., 1994a,b), but show higher frequencies. Simulations of these trapped waves indicate a 75 to 100-m-wide low-velocity and low-Q waveguide along the Hector Mine rupture zone in which the S velocity is reduced by about 40% to 50% from wall-rock velocities, and Q is 10 to 60 in the depth range from the surface to 10 km. We interpret this low-velocity waveguide as being a remnant of the process zone formed by inelastic deformation around the propagating crack tip during dynamic rupture in the 1999 Hector Mine earthquake. The reductions of velocities and Q within the Hector Mine rupture zone are similar to those within the Landers rupture zone, suggesting that the fault-zone rock was damaged to the same degree in the two earthquakes. The wave-guide width (75-100 m) on the Hector Mine rupture zone (40 km in the total length) is half that (150-250 m) of the Landers rupture zone (80 km in the total length), consistent with the scaling of process zone size to rupture length as predicted in some published dynamic rupture models. Locations of aftershocks for which we observed trapped waves show bifurcation of the northern Hector Mine rupture at depth, although only the west rupture branch broke to the surface.