SCEC Award Number 18059 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Atmosphere Processes Coupling to the Solid Earth: Quantification of Seismic Signals Emerging from Weather Related Events
Name Organization
Frank Vernon University of California, San Diego Yehuda Ben-Zion University of Southern California
Other Participants Chris Johnson
Lei Qin - Graduate student
SCEC Priorities 3g, 3a, 5e SCEC Groups Seismology, GM
Report Due Date 03/15/2019 Date Report Submitted 03/14/2019
Project Abstract
Analysis of continuous seismic waveforms recorded by a temporary deployment at Sage Brush Flats on the San Jacinto fault zone reveal earthquake- and tremor-like signals generated by the interac-tion of wind with obstacles above the surface. Tremor-like waveforms are present at the site during wind velocities above 2 m/s, which occur for 70% of the deployment duration. The wind-related sig-nals produce ground velocities that exceed the average ground motions of M1.0-1.5 earthquakes for 6% of the day. Waveform spectra show a modulation of the amplitude that correlates with the wind velocity and distance from local structures shaken by the wind. The earthquake-like signals are present within the tremor-like waveforms as impulsive bursts. Earthquake-like signals are found to originate from the vegetation and local structures, and are modified on length scales of 10’s of me-ters. Transient signals originating beyond the study area are also observed with amplitudes greater than some microseismic events. The ground motions generated by the wind contribute to local high-frequency seismic noise that is different than the global ambient noise. During elevated wind condi-tions a borehole seismometer at a depth of 148 m shows increased energy in the 1-8 Hz band that is commonly used for earthquake and tremor detection. Some of the wind-related signals recorded by the borehole sensor may be associated with small failures of the subsurface material. The wind-related earthquake- and tremor-like signals should be accounted for in earthquake detection algo-rithms due to the similar features in both the time and frequency domain.
Intellectual Merit We address the SCEC5 Research Priorities by characterizing common signals in seismic waveforms emerging from atmosphere coupling to the solid Earth. While atmospheric coupling is not explicitly stated, the proposed project investigates seismic signals coupling to the shallowest portion of the crust and will improve the interpretation of signals extracted from data sets that are used to image structures and seismic sources. The research on non-tectonic signals observed in the data that could be misinterpreted as tremor or overlooked at station noise will contribute directly to research priorities of the seismology disciplinary group.
Broader Impacts The seismic hazards of active plate boundary faults will affect more individuals as the population of California continues to increase. This project improves the ability to identify new classes of wave-forms generated by wind interaction with the surface to characterize ground motions in the shallow crust. An improved understanding of shallow crustal properties will further our understanding of the rheology of the upper 100’s of meters of the surface. Identifying new classes of waveforms will aug-ment existing studies mining seismic waveforms to detect microseismicity.

The project has supported a USC graduate student and fostered collaborations for that student with Christopher W Johnson, a postdoc in the SCEC scientific community. The collaborations developed by young scientist is integral to the advancement of new ideas and research directions.
Exemplary Figure Figure 2. (a) Daily trace of vertical velocity filtered between 5-20 Hz for node 01.33 during DOY 50 shown with the 1-minute average wind velocity (green) and maximum wind velocity (blue) to display the tremor-like signals produced by wind interacting with the vegetation. The red lines correspond to the time of the east, north, and vertical waveforms shown at (b) 01:27:46 UTC and (c) 11:14:48 UTC which exhibit earthquake-like waveforms within the tremor-like signals.

Figure produced by Christopher W Johnson and submitted to JGR for publication.