SEISMIC ATTENUATION IMAGING LINKS ROCK-FLUID PROPERTIES TO OFF-FAULT EARTHQUAKE MIGRATION BENEATH METROPOLITAN LOS ANGELES

Seismicity in the Los Angeles metropolitan area has been primarily attributed to the regional stress loading. Below the urban areas, earthquake sequences have occurred over time showing migration off the faults and providing evidence that secondary processes may be involved in their evolution. Combining high-frequency seismic attenuation with other geophysical observations is a powerful tool for understanding off-fault sequences and which rock properties characterize regions with ongoing seismicity. We develop the first high-resolution 3D seismic attenuation models across the region east of downtown Los Angeles using 5,600 three-component seismograms from local earthquakes recorded by a dense seismic array. We present peak delay and coda-attenuation tomography (with horizontal and vertical grid spacings of 3 km and 1 km) as proxies of seismic scattering and absorption, respectively.

The scattering model shows high sensitivity to the seismicity along some of the major faults, such as the Cucamonga fault and the San Jacinto fault zone indicating highly fractured fault-rocks. Based on the low scattering volume bounded by the Cucamonga and Red Hill faults on the north and the Fontana fault on the south, we infer that the Perris block collapse occurred along these faults and involved a region we refer to as the downthrown edge of the Perris block. The source region of the 2019 Fontana seismic sequence is marked by a high absorption anomaly revealing possible fluid pathways into an intensely fractured zone in the basement. 3D fault plane analysis of the sequence shows it occurs in three main clusters with distinct fault orientations and moves off the primary fault plane to the south over time. Our attenuation results indicate complex 3D wave propagation effects at multiple scales within the study area which are controlled by the tectonic interactions and rock-fluid properties.