Seismology tip sheet for Dec. 2011: BSSA

Dec. 2011 BSSA: Monitoring the earthquake source process in North America; fractured bedrock may amplify shaking; and historic 1909 quake near Montana/Canada re-assessed

Please cite the Bulletin of the Seismological Society of America (BSSA) as the source of this information.

Earthquake monitoring has yielded a rich database quantifying the last 30 years of activity, particularly moderate and larger quakes in active seismic zones. Now scientists have implemented procedures to routinely determine earthquake source information for the smaller magnitude earthquakes typically observed in central and eastern North America.

The benefits of the approach are reduced uncertainty in earthquake magnitude, improved source depth estimates and information about the tectonic stresses that cause the earthquakes. These parameters, in turn, contribute to assessing seismic hazards by constraining ground motion and source scaling relations impact assessment products such as PAGER -- Prompt Assessment of Global Earthquakes for Response.

This study applied two techniques for source inversion to earthquakes with reported magnitude greater than 3.5 in continental North America to create a catalog that includes 400 new well-calibrated moment magnitudes and source parameters for earthquakes. The catalog provides new detailed information on systematic variations in faulting styles and geometry for central and eastern regions of North America.

"Monitoring the Earthquake Source Process in North America," by Robert B. Herrmann at Saint Louis University, Harley Benz at U.S. Geological Survey and Charles J. Ammon at Penn State. Corresponding author: Robert B. Herrmann, Saint Louis University, rbh@eas.slu.edu, 314-977-3120.

Strong earthquakes may induce fracturing of bedrock that amplifies shaking during subsequent seismic events, increasing the likelihood of future rockslides and other earthquake-triggered slope failures, suggests a study of a large, unstable rock slope in Randa, Switzerland.

Unconsolidated soil and sedimentary basins are known to amplify shaking, but bedrock is generally thought to be stable. But at Randa, located within the most seismically active area in Switzerland, scientists evaluated ground motion recordings during small earthquakes and found that fracturing of the rock mass can amplify ground motion by up to a factor of ten.

"Site Effects in Unstable Rock Slopes: Dynamic Behavior of the Randa Instability (Switzerland)," by Jeffrey Moore, Valentin Gischig, Jan Burjanek, Simon Loew and Donat Faeh of ETH Zurich.

Corresponding author: Jeffrey Moore, jeffrey.moore@erdw.ethz.ch

A new study locates the May 16, 1909 earthquake near the Montana-Saskatchewan border along an extension of the mapped 186-mile long Hinsdale fault in Montana, suggesting local seismic hazards may be greater than previously recognized.

The largest historical earthquake in the northern Great Plains of North America, the estimated magnitude 5.3-5.4 earthquake was felt in the states of Minnesota, Montana, North Dakota, South Dakota and Wyoming in the United States and in the provinces of Alberta, Manitoba, Ontario and Saskatchewan in Canada.

The refined location of the quake may have seismic-hazard implications for the region, particularly for the earth-filled Fort Peck Dam constructed in the 1930's on the Missouri River in northeast Montana.

"The 16 May 1909 Northern Great Plains Earthquake," by W. H. Bakun of the U.S. Geological Survey, M. C. Stickney of Montana Bureau of Mines and Geology and G. C. Rogers of Geological Survey of Canada.

Corresponding author: W. H. Bakun, bakun@usgs.gov, 650-329-4793