News Release

Tip sheet for October issue of BSSA

Peer-Reviewed Publication

Seismological Society of America

Please cite BSSA as the source of this information.

Ground motion near the epicenter

One of the most important questions in seismic hazard assessment is how ground motion measures scale with magnitude. Data sets from near the source of very large earthquakes are sparse since the events are rare and the seismic network systems do not provide full coverage of all areas. So how do seismologists explain the observation that peak ground acceleration, which is a measurement of how hard the earth shakes in a given geographic area, does not on average increase with magnitude near the rupture plane of large earthquakes? Seismologists Jan Schmedes and Ralph J. Archuleta of the Institute for Crustal Studies at the University of California,Santa Barbara computed the ground motion based on simulations of very large earthquakes and determined that locally ground motion does increase near the source but not equally along the rupture. The maximum ground motion for a given distance from the fault does increase with magnitude of even large earthquakes, but at a given distance range there is a lot of variation in the ground motion and as the fault length increases the maximum value is less likely to be sampled by a seismic network.

A pdf of this paper is found below. Contact information for the authors: Jan Schmedes can be reached at Ralph Archuleta can be reached at

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.

A pdf of this paper can be found below. The authors' contact details: Jochen Woessner, Swiss Seismological Service,; Danijel Schorlemmer, Assistant Professor (research) of Earth Sciences at University of Southern California,


BSSA is published by the Seismological Society of America, which is a scientific society devoted to the advancement of seismology and its applications in understanding and mitigating earthquake hazards and in imaging the structure of the earth.

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