Applications of attenuation tomography to Imperial Valley and Coso-Indian Wells Region, southern California

Spatial variations in seismic wave attenuation were tomographically imaged from observed S to P wave amplitude ratios in the Imperial Valley and Coso-Indian Wells regions of southern California. In the Coso-Indian Wells region a highly attenuating body (S wave quality factor Q _β ≈ 30) coincides with a slow P wave anomaly mapped by Walck and Clayton (1987). This coincidence suggests the presence of a 3- to 5-km depth magmatic or hydrothermal body in the Indian Wells region. In the Imperial Valley, slow P wave travel time anomalies and highly attenuating S wave anomalies were found in the Brawley seismic zone at a depth of 8–12 km. The effective S wave quality factor is very low (Q _β ≈ 20), and the P wave velocity is 10% slower than the surrounding areas. These results suggest either magmatic or hydrothermal intrusions, or fractures at depth, possibly related to active shear in the Brawley seismic zone. This attenuation tomographic technique is shown to be useful in delineating the spatial variations in seismic wave attenuation and in estimating the effective seismic quality factor of attenuation anomalies.