[ Back to EurekAlert! ] Public release date: 6-Aug-2009
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Contact: Nan Broadbent
nan0604@msn.com
408-431-9885
Seismological Society of America

Seismology tip sheet

Please cite the Bulletin of the Seismological Society of America as the source of this information. A copy of each paper can be found below.

No daily or weekly pattern to earthquakes in Western U.S.

Daily traffic and noisy machines mask the vibrations caused by earthquakes, making seismic stations unable to detect many M >1 earthquakes. As a result, more earthquakes appear to happen on Sundays and late at night when people and machines are at rest, according to a new study of the apparent daily and weekly periodicity of seismic activity.

"People may think they feel more earthquakes at night or on Sundays, but that's just because it's quiet," says Stephen S. Gao, professor of geological sciences at the Missouri University of Science and Technology (Missouri S&T) and co-author of a study published today in the August issue of the Bulletin of the Seismological Society of America (BSSA). He co-authored the study with Ali H. Atef and Kelly H. Liu, also of Missouri S&T.

The study used a catalogue of 790,232 earthquakes for the period 1963-2008 which were recorded in Nevada, California and the adjacent Pacific Ocean and found significant peaks for Sundays and the early morning hours. While cyclical patterns do exist in nature, such as the Earth rotating around the Sun, there are no known cyclical events that would explain daily or hourly variations for earthquakes in Western United States. The authors looked to human activity to explain the apparent temporal pattern and found that when cultural noise – or sound made by human activity – was high, such as during the morning rush hour, the ability of seismic networks to detect earthquake activity is low. Moving vehicles on freeways were not the sole source of noise. Busy factories and mines, machinery such as lawn mowers and shaking buildings, caused by the movement of people, contribute to the ambient background noise.

The BSSA paper, "Apparent Weekly and Daily Earthquake Periodicities in the Western United States," is available upon request at press@seismosoc.org.

Authors: Ali H. Atef, Kelly H. Liu and Stephen S. Gao, Missouri University of Science and Technology.

Contact: Stephen S. Gao, Missouri University of Science and Technology, sgao@mst.edu, 573-341-6676.

Mineral deposits in caves offer accurate record of quakes

Historic earthquakes generated by the New Madrid seismic zone represent some of the largest recorded in the United States. Speleothems, or mineral deposit formations, found in caves may provide accurate recordings of major historic earthquakes generated by the New Madrid seismic zone, providing a new method for documenting and interpreting prehistoric earthquakes.

Researchers explored the potential seismic records of three caves within 300 kilometers of the New Madrid seismic zone. The timing of the initiation and regrowth of stalagmites in the caves is consistent with the historic and prehistoric record of several known seismic events. The method may be applicable to any region around the world in the vicinity of major seismic zones where caves exist.

The BSSA paper, "Major Earthquakes Recorded by Speleothems in Midwestern U.S. Caves," is available upon request at press@seismosoc.org.

Author: Samuel V. Panno, Illinois State Geological Survey.

Contact: Samuel V. Panno, panno@isgs.illinois.edu, 217-244-2456.

Revised understanding of San Andreas fault geometry near Desert Hot Springs

The Mission Creek and Banning faults are two principal strands of the San Andreas fault zone in the northern Coachella Valley of southern California. The two faults merge at depth to form one fault zone, according to a new analysis of the fault geometry near Desert Hot Springs. The refined understanding of the fault zone has implications for regional earthquake hazards and local groundwater resources, according to a paper to be published in the August issue of BSSA.

The structural characteristics of the San Andreas fault in the northern Coachella Valley strongly influence the seismic hazard of the area. In contrast to previous studies, this analysis by Rufus D. Catchings, et al., suggests the Mission Creek fault is a near-vertical or slightly southwest-dipping fault, implying that the northeastern part of the city of Desert Hot Springs is not on a hanging wall, suggesting that the northeastern part of the city should not experience increased shaking due to the hanging wall effect -- or the increased shaking associated with the upper wall of an inclined fault. In contrast, the thick accumulation of low-velocity sediments southwest of the Mission Creek fault will likely amplify seismic waves, resulting in strong shaking in the southwestern part of the city of Desert Hot Springs and much of the northern Coachella Valley.

The authors of this paper suggest a more complete understanding of the fault geometry and shallow basin structures are needed in order to better mitigate the hazard to lifeline structures (natural gas lines, I-10 freeway, electrical lines, Colorado River Aqueduct, etc.) that serve large population centers in southern California.

A copy of the BSSA paper, "San Andreas Fault Geometry at Desert Hot Springs, California, and Its Effects on Earthquake Hazards and Groundwater," is available upon request by contacting press@seismosoc.org.

Authors: Rufus D. Catchings, Michael J. Rymer, Mark R. Goldman and Gini Gandhok, U.S. Geological Survey.

Contact: Rufus D. Catchings, catching@usgs.gov; 650-329-4749

Uniform California Earthquake Rupture Forecast, Version 2 (UCERF 2)

California's 35 million people live among some of the most active earthquake faults in the United States. Public safety demands credible assessments of the earthquake hazard to maintain appropriate building codes for safe construction and earthquake insurance for loss protection. Seismic hazard analysis begins with an earthquake rupture forecast—a model of probabilities that earthquakes of specified magnitudes, locations, and faulting types will occur during a specified time interval. This paper describes Version 2 of the Uniform California Earthquake Rupture Forecast (UCERF 2; see Table 1 for list of acronyms), which estimates the long-term rate of earthquakes with magnitudes greater than five (M ≥ 5.0) and the conditional time-dependent probability of large earthquakes in California and its boundary zones.

The BSSA paper, "Uniform California Earthquake Rupture Forecast, Version 2 (UCERF 2)" is available upon request at press@seismosoc.org.

Authors: E. H. Field, K.R. Felzer, A.D. Frankel, T. Parsons, R. S. Stein and M.D. Petersen of U.S. Geological Survey; T. E. Dawson and C.J. Wills of California Geological Survey; V. Gupta and T. H. Jordan of the University of Southern California; and R. J. Weldon II of the University of Oregon.

Contact: Edward H. Field, U.S. Geological Survey, field@usgs.gov, (626) 583-7814

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