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

Abstract

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 also show a systematic decrease with hypocentral distance along the fault zone, from which we infer an apparent Q of 50. In order to confirm the existence of the guided waves, a dense array of 31 stations was deployed at one of the 11 sites. The resultant records revealed unequivocal evidence for the existence of guided waves associated with the fault zone. By modeling the waveforms as S waves trapped in a low‐velocity waveguide sandwiched between two homogeneous half‐spaces with velocity Vs = 3.0 km/s, we infer a waveguide width of about 180 m, a shear velocity of 2.0–2.2 km/s, and a Q of ∼50. Hypocenters of aftershocks with clear guided waves show a systematic distribution both laterally and with depth delineating the extent of the low‐velocity fault zone in three dimensions. We find that the zone extends to a depth of at least 10 km. This zone apparently continues to the south across the Pinto Mountain fault because guided waves are observed at stations north of the Pinto Mountain fault for earthquakes with epicenters south of it. On the other hand, the zone appears to be discontinuous at the fault bend located about 20 km north of the mainshock epicenter; guided waves were observed for stations and epicenters which are located on the same sides of the fault bend but not for those on the opposite sides.

Wave Propagation from a Line Source Embedded in a Fault Zone

“…7c). Although Li et al (1994) did not infer the origin of the high frequency spectral components, each of the peaks in the low and high wavenumber range is interpreted from our simulation to be formed by the waves trapped in the low-velocity zone and the waves scattered by the cracks, respectively.…”

“…In this section, we compare the amplitude spectra observed by Li et al (1994) in the fault zone of the 1992 Landers earthquake with the synthesis calculated for the fault zone Model (5) of the low-velocity zone with densely distributed parallel cracks. Li et al (1994) deployed a seismic array across the fault trace of the M7.4 Landers earthquake of June 28, 1992.…”

“…Li et al (1994) deployed a seismic array across the fault trace of the M7.4 Landers earthquake of June 28, 1992. They found the distinct wave train with a relatively long period following the direct S waves that shows up only when both the stations and the events are close to the fault trace.…”

“…Moreover it is revealed from seismic observations that a fault zone is characterized as a lower velocity zone than the surrounding intact rocks (Mooney & Ginzburg, 1986) and low-Q area (Kurita, 1975;Li et al, 1994). When an earthquake occurs in the fault zone, the following seismic waveforms are observed in the fault zone: the P and S headwaves refracted along the cross-fault material contrast (Ben-Zion & Malin, 1991;Hough et al, 1994) and seismic waves trapped in a low-velocity zone (e.g., Li & Leary, 1990;Li et al, 1994). It is important to determine the fault zone structure for the purposes of earthquake prediction and strong motion prediction.…”

“…This method of analysis has advantages that multiple elastic wave scattering due to a large number of densely distributed cracks is easily treated and that the velocity contrast can be easily introduced. Finally, we try to simulate the fault zone trapped waves observed by Li et al (1994) and estimate the crack size and the density of crack distribution.…”