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Title: Fortnightly modulation of San Andreas tremor and low-frequency earthquakes

Abstract

Earth tides modulate tremor and low-frequency earthquakes (LFEs) on faults in the vicinity of the brittle-ductile (seismic-aseismic) transition. Our response to the tidal stress carries otherwise inaccessible information about fault strength and rheology. We analyze the LFE response to the fortnightly tide, which modulates the amplitude of the daily tidal stress over a 14-d cycle. LFE rate is highest during the waxing fortnightly tide, with LFEs most strongly promoted when the daily stress exceeds the previous peak stress by the widest margin. This pattern implies a threshold failure process, with slip initiated when stress exceeds the local fault strength. Furthermore, variations in sensitivity to the fortnightly modulation may reflect the degree of stress concentration on LFE-producing brittle asperities embedded within an otherwise aseismic fault.

Authors:
 [1];  [2];  [3];  [2]
  1. United States Geological Survey, Pasadena, CA (United States). Earthquake Science Center
  2. Los Alamos National Lab. (LANL), Los Alamos, NM (United States). Geophysics Group
  3. United States Geological Survey, Pasadena, CA (United States). Volcanic Science Center
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1340914
Report Number(s):
LA-UR-15-27728
Journal ID: ISSN 0027-8424
Grant/Contract Number:
AC52-06NA25396
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 113; Journal Issue: 31; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; faults; low-frequency earthquakes; tidal triggering; fortnightly tides

Citation Formats

van der Elst, Nicholas J., Delorey, Andrew A., Shelly, David R., and Johnson, Paul A. Fortnightly modulation of San Andreas tremor and low-frequency earthquakes. United States: N. p., 2016. Web. doi:10.1073/pnas.1524316113.
van der Elst, Nicholas J., Delorey, Andrew A., Shelly, David R., & Johnson, Paul A. Fortnightly modulation of San Andreas tremor and low-frequency earthquakes. United States. doi:10.1073/pnas.1524316113.
van der Elst, Nicholas J., Delorey, Andrew A., Shelly, David R., and Johnson, Paul A. 2016. "Fortnightly modulation of San Andreas tremor and low-frequency earthquakes". United States. doi:10.1073/pnas.1524316113. https://www.osti.gov/servlets/purl/1340914.
@article{osti_1340914,
title = {Fortnightly modulation of San Andreas tremor and low-frequency earthquakes},
author = {van der Elst, Nicholas J. and Delorey, Andrew A. and Shelly, David R. and Johnson, Paul A.},
abstractNote = {Earth tides modulate tremor and low-frequency earthquakes (LFEs) on faults in the vicinity of the brittle-ductile (seismic-aseismic) transition. Our response to the tidal stress carries otherwise inaccessible information about fault strength and rheology. We analyze the LFE response to the fortnightly tide, which modulates the amplitude of the daily tidal stress over a 14-d cycle. LFE rate is highest during the waxing fortnightly tide, with LFEs most strongly promoted when the daily stress exceeds the previous peak stress by the widest margin. This pattern implies a threshold failure process, with slip initiated when stress exceeds the local fault strength. Furthermore, variations in sensitivity to the fortnightly modulation may reflect the degree of stress concentration on LFE-producing brittle asperities embedded within an otherwise aseismic fault.},
doi = {10.1073/pnas.1524316113},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 31,
volume = 113,
place = {United States},
year = 2016,
month = 7
}

Journal Article:
Free Publicly Available Full Text
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  • A 24% precursory change in apparent electrical resistivity was observed before a magnitude 3.9 earthquake of strike-slip nature on the San Andreas Fault in central California. The experimental configuration and numerical calculations suggest that the change is associated with a volume at depth rather than some near-surface phenomenon. The character and duration of the precursory period agree well with those of other earthquake studies and support a dilatant earthquake mechanism model.
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  • Tidal triggering of earthquakes is hypothesized to provide quantitative information regarding the fault's stress state, poroelastic properties, and may be significant for our understanding of seismic hazard. To date, studies of regional or global earthquake catalogs have had only modest successes in identifying tidal triggering. We posit that the smallest events that may provide additional evidence of triggering go unidentified and thus we developed a technique to improve the identification of very small magnitude events. We identify events applying a method known as inter-station seismic coherence where we prioritize detection and discrimination over characterization. Here we show tidal triggering ofmore » earthquakes on the San Andreas Fault. We find the complex interaction of semi-diurnal and fortnightly tidal periods exposes both stress threshold and critical state behavior. Lastly, our findings reveal earthquake nucleation processes and pore pressure conditions – properties of faults that are difficult to measure, yet extremely important for characterizing earthquake physics and seismic hazards.« less
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