Shear wave anisotropy and stress direction in and near Long Valley Caldera, California, 1979–1988

Abstract

We observed shear wave splitting (birefringence) for two temporary surface deployments of three-component, digital seismographs, that were in place before and after M=6+ earthquakes near the Long Valley caldera, California. In one of these deployments, the data sample precedes the large events of the May 1980 Mammoth Lakes earthquake sequence by 6 months and cover the two most active months of the May 1980 aftershock sequence; one of the stations (WIT) from this deployment was reoccupied with identical instrumentation in 1988. Another deployment preceded the 1986 Chalfant Valley mainshock by 2 days and recorded events for 6 days. The polarization of the faster shear wave changes from N30øW at Mammoth to due north at Chalfant Valley and in both cases is parallel to the strike of nearby surface faults and to the mean direction of P axes determined from focal mechanism groupings. Observations from nearby stations yield fast directions nearly at 23 ø from each other, and time separations of fast and slow shear waves show considerable station-to-station variation, showing no correlation with earthquake-station distance or earthquake depth. These observations suggest that the observed anisotropy results primarily from near-station (presumably shallow) effects rather than from widespread aligned microcracks. The records at Long Valley station WIT show a slight variation of average fast shear wave polarization from 1979 to 1980, but with the limited amount of data available, this difference is not statistically significant. Time separation measurements of up to 0.48 s at station WIT yield estimates of 15-30% velocity anisotropy, among the highest reported for local earthquakes. Secular variation in the time separation of fast and slow shear waves between events recorded at station WIT in 1979 and 1980 is swamped by the scatter in the data, if it is present. The five events recorded in 1988 showed similar S wave polarizations and time separations to those in 1980, but there are not enough events for a statistical comparison. Unlocated events at Long Valley station MRS show a slight increase in time separation after the May 1980 M=6.0+ earthquakes, but the increase is not statistically significant. 1985]. Crampin et al. [1984] suggest a pervasive tendency in crustal rocks for differential opening and growth of existing 1Now at Office of Strong Motion Studies, California Division of Mines and Geology, Sacramento. cracks at stresses 1 or 2 orders of magnitude less than those at which conventional dilatancy [e.g., Brace et al., 1966] occurs. The existence of crustal anisotropy related to low 1987; Savage et al., 1989; McEvilly et al., 1988]. Our study area, the Long Valley caldera in the central eastern Sierran uplift of California provides a useful area to examine crustal seismic anisotropy (Figure 1). Intensely studied, the Long Valley caldera is seismically active. Ongoing studies of the Long Valley caldera have shown complicated patterns of stress based on focal mechanisms [e.g., Vetter and Ryall, 1983; Vetter, 1988], def...

Seismic anisotropy from local earthquakes in the transition region from a subduction to a strike‐slip plate boundary, New Zealand

“…Deep phases sample both the AB and the crustal anisotropy and yield fast polarizations ranging from those due to the AB (20 ø) to those due to cracks (east-west). We suggest that in the North Island Dextral Fault Belt, crustal anisotropy is aligned parallel with the surface faulting, which contributes to the observed anisotropy [Savage et al, 1990] and is, in turn, parallel to mantle anisotropy. However, outside this region, surface faulting is less prevalent and anisotropy in the crust is due to cracks aligned in the stress direction, which is not necessarily parallel to mantle anisotropy (Figure 12).…”

A reappraisal of shear wave splitting parameters from Italian active volcanic areas through a semiautomatic algorithm

“…Most of these data have been already analyzed (Bianco et al 1996(Bianco et al , 1998a(Bianco et al , b, 1999(Bianco et al , 2006Del Pezzo et al 2004), constituting a useful validation test for the new algorithm, as we applied it exactly on the same number of data previously selected by those authors, following the same selection rules. In order to compare the time delay values with the previous ones and also those of different volcanic areas, we estimated the anisotropy percentage ξ defined following Savage et al (1990):…”

“…Shear wave splitting parameters have been studied in several volcanic environments such as Hawaii (Booth et al 1992;Munson et al 1995) and Long Valley Caldera (Savage et al 1990). Changes in shear wave splitting have been observed during the 1989 and the 2001 eruption at Mt.…”

Crust and upper-mantle seismic anisotropy variations from the coast to inland in central and Southern Mexico

“…The expected effect of anisotropy is that Ps amplitude of the tangential component is minimum along the fast and slow polarization directions and reaches a maximum 45 • from either one of these axes (Bowman & Ando 1987;Silver & Chan 1988;Savage et al 1990;Levin & Park 1998). Moreover, in transversely anisotropic layers with horizontal symmetry axes, RFs tend to display a 180 • periodicity as a function of backazimuth for the arrival times of the Ps phases, as well as a change in polarity in its tangential component whenever there is a change from an azimuth of fast velocity to one of slow.…”