What 25+ years of "Did You Feel It" intensities tell us about shaking in California

Seismological Research Letters
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Abstract

“When will the Big One happen?” is a question that people often have for earthquake scientists. But while waiting for the “Big One” to occur, people will usually experience frightening or damaging shaking from multiple relatively smaller‐magnitude earthquakes. Given this context, it raises the question: “Where does most of the damage come from?” Could smaller, yet more frequent, earthquakes account for the majority of reported impactful shaking? To explore this question, we consider reports of earthquake damage and felt shaking experiences from a catalog of community‐collected intensity values from the U.S. Geological Survey’s “Did You Feel It?” system. Comparing these intensities to expectations from a ground‐motion model, we find that earthquakes of magnitudes smaller than expected are responsible for most reported intensities of community decimal intensities (CDI) 4.5 and above (moderate and higher shaking intensity levels). (Here “expected value” is meant in its mathematical sense of the mean or equivalently median shaking intensity.) We also present a regional analysis of observed earthquake shaking for specific areas, detailing the maximum intensity experienced within a predetermined area. We identify several instances of M < 4.5 events that generated maximum intensities of CDI > 5 in regions in California surrounding the cities of Eureka, San Francisco, Los Angeles, and San Diego. Our results motivate the need to include smaller‐magnitude earthquakes in communications about earthquake hazard and risk reduction.

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Publication type Article
Publication Subtype Journal Article
Title What 25+ years of "Did You Feel It" intensities tell us about shaking in California
Series title Seismological Research Letters
DOI 10.1785/0220240393
Volume 96
Issue 4
Publication Date January 29, 2025
Year Published 2025
Language English
Publisher Seismological Society of America
Contributing office(s) Earthquake Science Center, Geologic Hazards Science Center
Description 13 p.
First page 2625
Last page 2637
Country United States
State California
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