Earthquakes are the result of rapid movement of crustal blocks on faults, which are marked by a narrow fault zone (FZ) with a width of several hundred meters. Most earthquake rupture models, including the asperity and stick-slip paradigms, suggest that FZ structure (geometrical and material properties) controls earthquake rupture initiation, propagation, and termination. However, determining FZ structure at seismogenic depths where earthquakes nucleate and the majority of slip occurs has been shown to be extremely difficult. In this project, seismologists at Saint Louis University use high-frequency body-wave waveforms from aftershocks near the FZ to determine fine-scale. Waveform characteristics enables them to design a new strategy in which the FZ width and velocities are determined separately, thus eliminating the trade-off between the two. Furthermore, by using FZ reflected waves, they can pinpoint the locations in the FZ where they have obtained FZ width and velocity parameters. This gives them a unprecedented high depth resolution of FZ structure. In addition, they are able to relocate aftershocks with an accuracy of a few tens of meters relative to the FZ boundaries. They plan to apply the method to data sets of various fault zones in California, including the Landers, Hector Mine, San Jacinto, and San Andreas Fault at Parkfield to determine FZ parameters (strike, width, velocity and density drops, and Q) and possible temporary variations. Broader Impacts: The information obtained is crucial to developing models of the earthquake source. Experience gained in this study will help design future FZ seismic experiments and the methodologies developed can be easily applied to other regions and data sets, such as those from the on-going USArray.
