Using Active Source Seismology to Image a Strike-Slip Fault Damage Zone as a Function of Depth, Distance, and Geology

Fault damage zones provide a window into the non-elastic processes and products of an earthquake, but geological and seismic tomography methods have limited resolution and in particular have been unable to measure damage zones at depth with sufficient resolution to separately resolve depth, distance and lithological variations. Here, we identify and analyze the damage zone of the Palos Verdes Fault in offshore southern California using two existing 3D seismic reflection datasets. We use a novel algorithm applied to the seismic data to identify seismic discontinuities attributed to faults and fractures in large seismic volumes (~ 10^8 points) and examine the spatial distribution of damage. Our results from the Palo Verdes Fault zone show that damage is focused around mapped faults and that damage decays with distance from the mapped fault, reaching a clearly defined relatively undamaged background region at a distance of 2.5 km from the fault. The damaged zone obeys power law-like behavior at the outer edges of a 3-strand fault network, with a power law exponent of ~0.7, and this trend may be extrapolated to a probability of 1 at the mapped main (central) fault strand location. The power law like dependence of the damage zone with distance from fault is like outcrop studies. However, here we extend the study to distances seldom accessible. We find that fracturing in the damage zone increases with depth to around 500 m and at greater depths damage may be more strongly controlled by lithology, induration, and unit thickness than depth variations.