A large fault line underlies the Palos Verdes Peninsula adjacent to Los Angeles, shown here, with damage areas extending offshore. Photo by Dave Proffer via Flickr, CC BY 2.0
Large faults are often surrounded by a network of secondary faults, created when nearby rocks are deformed by the forces that caused the slip. This damaged area acts as a record of fault activity, but what we can learn from it, such as how the faults will behave during future earthquakes, has yet to be fully explored.
In a new study, Alonge and his colleagues used multiple seismic reflection data sets to provide new insights into the offshore portion of the Palos Verdes Fault near Los Angeles, extending from the ocean floor to about 400 meters below. The study is published in the journal AGU Advances.
In seismic reflection, researchers tow a sonar-like system behind a boat, emitting sound waves and recording the return. This signal travels through the water to the ocean floor, where it is reflected off boundaries between different rock layers, including faults.
The strength of the returning signal and the time it takes to return provide information about the density differences between the layers and the depth of each boundary. The team used this information to create around 100 images of the complex network of faults.
The authors mapped approximately 30 kilometers of the fault. In addition to demonstrating that this method can be used to map damage areas, their study identifies the factors that control the formation of damage areas and explains how these features control earthquake rupture.
The fault offset, or the displacement on either side of the fault, determines the width of the damage area, but the bends along the fault determine how the damage pattern changes along the length of the fault.
The authors also found that in the most damaged areas of the fault, the most fluid seeps out from the seafloor because fractured areas are more permeable. Other factors, such as sediment cover and crustal uplift, also influence where fluid seeps out.
These new data will be important for understanding earthquake hazard in this region and elsewhere, as the presence and extent of fluid damage along the fault will affect how earthquakes propagate.
More information: Travis Alongi et al., “Characterization of the fault damage zone from high-resolution seismic images along the Palos Verdes Fault, California.” AGU Advances (2024). DOI: 10.1029/2023AV001155
Courtesy of the American Geophysical Union
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Citation: Subsurface imaging reveals one of Los Angeles’ fault networks (July 30, 2024) Retrieved July 30, 2024 from https://phys.org/news/2024-07-imaging-surface-reveals-los-angeles.html
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