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Pervasive Foreshock Activity Across Southern California

Trugman, Daniel T. and Ross, Zachary E. (2019) Pervasive Foreshock Activity Across Southern California. Geophysical Research Letters, 46 (15). pp. 8772-8781. ISSN 0094-8276. doi:10.1029/2019GL083725.

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Foreshocks have been documented as preceding less than half of all mainshock earthquakes. These observations are difficult to reconcile with laboratory earthquake experiments and theoretical models of earthquake nucleation, which both suggest that foreshock activity should be nearly ubiquitous. Here we use a state‐of‐the‐art, high‐resolution earthquake catalog to study foreshock sequences of magnitude M4 and greater mainshocks in southern California from 2008–2017. This highly complete catalog provides a new opportunity to examine smaller magnitude precursory seismicity. Seventy‐two percent of mainshocks within this catalog are preceded by foreshock activity that is significantly elevated compared to the local background seismicity rate. Foreshock sequences vary in duration from several days to weeks, with a median of 16.6 days. The results suggest that foreshock occurrence in nature is more prevalent than previously thought and that our understanding of earthquake nucleation may improve in tandem with advances in our ability to detect small earthquakes.

Item Type:Article
Related URLs:
URLURL TypeDescription
Trugman, Daniel T.0000-0002-9296-4223
Ross, Zachary E.0000-0002-6343-8400
Additional Information:© 2019 American Geophysical Union. Received 14 MAY 2019; Accepted 25 JUL 2019; Accepted article online 30 JUL 2019; Published online 8 AUG 2019. The two earthquake catalogs analyzed in the manuscript are publicly available online. The QTM catalog and the SCSN catalog are both archived by the Southern California Earthquake Data Center ( We use publicly available heat flow data from Blackwell et al. (2011). Our calculations use open source Python software packages, including a wrapper of original Okada (1992) code (Thompson, 28 May 2014/2019). D. Trugman acknowledges institutional support from the Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory under Project 20180700PRD1. We are grateful to P. Johnson, I. McBrearty, and N. Lubbers for discussions while formulating the study, and we thank two anonymous reviewers and editor Gavin Hayes for insightful comments and suggestions that improved the manuscript.
Group:Seismological Laboratory
Funding AgencyGrant Number
Los Alamos National Laboratory20180700PRD1
Subject Keywords:foreshocks; earthquake catalogs; statistical seismology; earthquake triggering; earthquake nucleation
Issue or Number:15
Record Number:CaltechAUTHORS:20190820-133631627
Persistent URL:
Official Citation:Trugman, D. T., & Ross, Z. E. (2019). Pervasive foreshock activity across southern California. Geophysical Research Letters, 46, 8772–8781.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:98043
Deposited By: Tony Diaz
Deposited On:20 Aug 2019 21:05
Last Modified:16 Nov 2021 17:36

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