SCEC Award Number 19101 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Evaluation of Deep, Widespread Seismicity beneath Long Beach through Deep-Learning-based Noise Suppression and Reverse-Time Migration
Name Organization
Gregory Beroza Stanford University Lei Yang Stanford University
Other Participants
SCEC Priorities 3b, 3a, 3e SCEC Groups Seismology, FARM, CS
Report Due Date 04/30/2020 Date Report Submitted 11/16/2020
Project Abstract
We revisit the finding of widespread deep seismicity in the upper mantle imaged with a dense, temporary nodal seismic array in Long Beach, California using back-projection to detect candidate events, and trace randomization to develop a reliable imaging threshold for candidate detections. We find that nearly all detections of small events at depths greater than 20 km in the upper mantle fall below the reliability threshold. We find a modest number of small, shallower events in the crust that appear to align with the active Newport-Inglewood Fault. These events occur primarily at 15-20 km depth near the base of the seismogenic zone. Localized seismicity under fault zones is consistent with the notion that the deep extensions of active faults are localized and deforming, with stress concentration leading to a concentration of small events, near the seismic-aseismic transition.
Intellectual Merit The finding of widespread deep seismicity under LA has the potential to revolutionize the understanding of crust and mantle deformation and its relationship to earthquake processes. We find that the previously reported events are likely artifiacts, which is disappointing, but an important negative result.
Broader Impacts This project developed a novel method of processing dense array data and assisted in the training of postdoctoral associate Lei Yang.
Exemplary Figure Figure 2. Back-projection imaging/location. Left and center panels show the BP imaging results of Julian day 28, 2012 night time (local) on original and trace-randomized data, respectively; Right panel shows the BP imaging result on the same data using the new detection threshold. Nearly all previous detections, particularly deep detections, do not meet the detection threshold.