News Release 

Cataloging Southern California's tiny hidden earthquakes

American Association for the Advancement of Science

Nearly 1.8 million tiny tremblors have been added to the catalog of total seismic events in Southern California over the past decade, reports a new study, which details the most comprehensive earthquake catalog to date. While many of these new earthquakes are small - as low as 0.3 magnitude and virtually imperceptible on the surface - the results help to fill gaps in the earthquake record, as well as our understanding of geophysical processes that make them. There is a well-known empirical relationship between the size and frequency of earthquakes; for every unit decrease in magnitude, they become nearly ten times more frequent. However, cataloging low-magnitude tremblors is challenging as their seismic signal is often indistinguishable from background noise and the overlapping seismic energy of near-simultaneous earthquakes. Not only does the power-law size relationship illustrate the unrelenting tectonic forces grinding below our feet, but it also suggests that a vast majority of smaller earthquakes evade detection and remain unrecognized in the earthquake catalogs that form the basis for much of what we know about seismic processes. According to the authors, the tiny hidden quakes may be the key to resolving some of the most vexing problems in seismic science, including fault geometry and the triggering and nucleation of larger earthquakes. Zachary Ross and colleagues used a template matching algorithm, which can tease out subtle earthquake waveforms within continuous streams of seismic data, to search for hidden earthquakes in Southern California. The analysis by Ross et al. identified over 1.8 million earthquakes occurring between 2008 and 2017 - a ten-fold increase over the original Southern California Seismic Network earthquake catalog. The results suggest that about 495 earthquakes occur each day in Southern California - nearly one every 174 seconds.

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