P wave velocity variations in the Coso Region, California, derived from local earthquake travel times

Abstract

Inversion of 4036 P wave travel time residuals from 429 local earthquakes using a tomographic scheme provides information about three-dimensional upper crustal velocity variations in the Indian Wells Valley-Coso region of southeastern California. The residuals are calculated relative to a Coso-specific velocity model, corrected for station elevation, weighted, and back-projected along their ray paths through models defined with layers of blocks. Slowness variations in the surface layer reflect local geology, including slow velocities for the sedimentary basins of Indian Wells and Rose valleys and relatively fast velocities for the Sierra Nevada and Argus Mountains. In the depth range of 3-5 km the inversion images an area of reduced compressional velocity in western and northern Indian Wells Valley but finds no major velocity variations beneath the Coso volcanic field to the north. These results are consistent with a recent study of anomalous shear wave attenuation in the Coso region. Between 5 and 10 km depth, low-velocity areas {7% slow) appear at the southern end of the Coso volcanics, reaching east to the Coso Basin. Numerical tests of the inversion's resolution and sensitivity to noise indicate that these major anomalies are significant and well-resolved, while other apparent velocity variations in poorly sampled areas are probably artifacts. The seismic data alone are not sufficient to uniquely characterize the physical state of these low-velocity regions. Because of the Coso region's history of Pleistocene bimodal volcanism, high heat flow, geothermal activity, geodetic deformation, and seismic activity, one possibility is to link the zones of decreased P velocity to contemporary magmatic activity.

Missing roots and mantle “drips”: Regional Pn and teleseismic arrival times in the southern Sierra Nevada and vicinity, California

“…Corrections are near zero for the temporary stations, which were on crystalline bedrock, but become significant in the vicinity of Indian Wells Valley (Figure 2), where residuals tend to reflect both sedimentary fill under some stations and upper crustal velocity anomalies previously imaged by Walck and Clayton [1987] from local earthquake travel times and associated by them with magmatic systems in the area.…”

“…If the crust is laterally homogeneous, then the Moho would dip 0° to about 2° south under the Sierra. Because of both the limited reversal of mantle paths in the Caso area and the known presence of velocity perturbations in this area [Walck and Clayton, 1987], it is difficult to infer the N-S component of Moho dip. If the Moho dips away from Coso both to the north and south, true Pn velocities might be as low as 7.8 km/s, but if it is nearly flat, the true Pn would be greater than 8.0 km/s.…”

Three‐dimensional P and S wave velocity structures of the Coso Geothermal Area, California, from microseismic travel time data

“…Present-day tectonic movements are evidenced by widespread microearthquakes in the region [Malin, 1994]. [Walck and Clayton, 1987]. However, between 5 and 10 km depth, a low-velocity zone of 7% perturbation anomaly was found at the southern end of the Coso geothermal region.…”

Seismic Traveltime Tomography of the Crust and Lithosphere

“…Studies have been carried out in many different regions including California (Aki & Lee, 1976;Thurber, 1983;Eberhart-Phillips, 1986;Walck & Clayton, 1987;Walck, 1988;Zhao & Kanamori, 1992;Eberhart-Phillips & Michael, 1993;Scott et al, 1994), Taiwan (Kao & Rau, 1999), Japan (Nakanishi, 1985;, South America (Bosch, 1997;Graeber & Asch, 1999) and the Mediterranean (Chiarabba et al, 1997;Luca et al, 1997;Papazachos & Nolet, 1997;Haslinger et al, 1999). Apart from the source-receiver geometry, one of the distinguishing features of LET compared to the other types of tomography discussed here, is the need to relocate earthquake hypocenters (spatial location and origin time) as part of the image reconstruction.…”

“…This kind of analysis is not commonly used in wide-angle studies, although recently, Zelt (1998), Zelt et al (1999Zelt et al ( , 2001 and Day et al (2001) implemented checkerboard tests for analyzing the resolution of velocity structure , also use it for interface structure) derived from wide-angle traveltime inversion. In local earthquake tomography, checkerboard tests have been implemented by Chiarabba et al (1997) and Graeber & Asch (1999); Walck & Clayton (1987) and Walck (1988) used synthetic reconstructions with anomalies positioned in key localities (i.e. regions of high geological interest).…”