Using active source seismology to image the Palos Verdes Fault damage zone as a function of distance, depth, and geology

Authors

Travis Alongi, University of California
Emily E. Brodsky, University of California
Jared Kluesner, U.S. Geological Survey
Daniel Brothers, U.S. Geological Survey

Date of this Version

10-15-2022

Citation

Earth and Planetary Science Letters 600 (2022) 117871. https://doi.org/10.1016/j.epsl.2022.117871

Comments

Used by permission.

Abstract

Fault damage zones provide a window into the non-elastic processes of an earthquake. Geological and seismic tomography methods have been unable to measure damage zones at depth with sufficient spatial sampling to evaluate the relative influence of depth, distance, and lithological variations. Here, we identify and analyze the damage zone of the Palos Verdes Fault offshore southern California using two 3D seismic reflection datasets. We apply a novel algorithm to identify discontinuities attributed to faults and fractures in large seismic volumes and examine the spatial distribution of fault damage in sedimentary rock surrounding the Palos Verdes Fault. Our results show that damage through fracturing is most concentrated around mapped faults and decays exponentially to a distance of ∼2km, where fracturing reaches a clearly defined and relatively undamaged background for all examined depths and lithologies (450m to 2.2km). This decrease in fracturing with distance from the central fault strand exhibits similar functional form to outcrop studies. However, here we extend analysis to distances seldom accessible (∼10km lateral distance). Separating the data by geologic units we find that the damage decay and background level differs for each unit, with the older and deeper units having higher levels of background fracturing and shallower exponential decays of fracturing with distance from the fault. Surprisingly, these differences in damage decay and background level trade-off result in a consistent damage zone width regardless of lithology or depth. We find that the damage zone has similar decay trends on both sides of the fault. When examining the damage zone at shorter (4km vs 17km) along strike distances, the damage zone has a more complex decay trend and at least two strands are resolvable.