FAULT ZONE IMAGES IN THE CHINO-SAN BERNARDINO AREA, SOUTHERN CALIFORNIA FROM SEISMIC ATTENUATION

Earthquake waves can be amplified as they travel through sedimentary basins and interact to generate complex seismic wavefields near basin edges. It is generally accepted that basins in the ~55-km wide Chino-San Bernardino area located between the San Andreas fault and downtown Los Angeles may act as a waveguide producing large amplifications from earthquake ruptures. But information on how cross faults within the basins and faults near the basin edges may affect the pattern of wave propagation is less well known. Seismic energy can be trapped along fault zones due to fracturing, fluid saturation, and high pore-fluid pressure. Local seismicity and faults in the area that act as groundwater barriers can also affect rock properties. Here we show that seismic attenuation (scattering and absorption) is useful for characterizing such spatially variable rock properties near faults.

We develop the first high-resolution 3D attenuation model for the Chino-San Bernardino area using 4770 three-component recordings of 972 local earthquakes (M<3.6). Data were recorded by eight dense nodal arrays and the regional Southern California Seismic Network. We present peak delay and coda-attenuation tomography at 12 and 18 Hz (with horizontal and vertical grid sizes of 3 km and 1 km) as proxies of seismic scattering and absorption, respectively. Our results show strong scattering contrasts across the major faults interpreted as due to rock fracturing and compaction differences or contrasts in rock types across the faults. Low scattering values coincide with a seismicity cluster on the Fontana fault and may point to the presence of fluid-saturated rocks and increased pore pressure in that region. This area is also characterized by high absorption anomalies indicative of intense fracturing which may have led to higher permeability and enhanced fluid flow. We will present and discuss detailed images of faults down to 10 km depth.