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How Many Earthquakes Are There?

October 8, 2008
Seismological Society of America
Authors explore how ground motion measures scale with magnitude and explore the question: How many earthquakes are there?

A new method for estimating the capability of a network to detect earthquakes suggests that the seismic monitoring network for Southern California, as an example, does not accurately reflect all earthquakes that register a magnitude of 3.3 or smaller within southern California, thereby giving seismologists an incomplete picture of recent and current seismicity.

The study, published in the October issue of the Bulletin of the Seismological Society of America, provides a new empirically-based approach for seismologists to understand the detection capabilities of seismic networks.

While today's improved seismic networks detect earthquakes down to low magnitudes in regions of the densest coverage, seismologists need to estimate the completeness magnitude which varies in space and time. This magnitude of completeness indicates the magnitude below which the earthquake catalog does not contain all events that occurred..

D. Schorlemmer of the University of Southern California (USC) and J. Woessner of the Swiss Seismological Service present a new approach to estimate the magnitude of completeness which will enable scientists to develop a richer understanding of the distribution of smaller earthquakes. The authors' new approach uses an analysis based on the actual performance of seismic stations rather than a theoretical assessment based on sampling of earthquakes.

This advancement is important because one way scientists estimate the number of large, damaging earthquakes is to study the distribution of small earthquakes. Without an accurate understanding of how likely the seismic networks are detecting earthquakes of different magnitudes, scientists may obtain incorrect seismic hazard estimates for an area.

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Seismological Society of America. "How Many Earthquakes Are There?." ScienceDaily. ScienceDaily, 8 October 2008. <>.
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