BSSA tipsheet for October 2007

Santa Rosa, California, suffered surprisingly severe damage from both the 1906 San Francisco earthquake on the San Andreas Fault and the 1969 earthquakes near Santa Rosa along the Healdsburg-Rogers Creek Fault. New research findings by D.K. McPhee and colleagues suggest the city sits atop a complex basin configuration that makes the area susceptible to future violent shaking from a wide variety of seismic activity. Taking advantage of new geophysical investigations, researchers were able to identify and study the correlation between 3D basin geometry and observed shaking in previous earthquakes. The refined understanding of the unique geology and basin configuration beneath Santa Rosa may lead to more accurate seismic hazard assessments and improved construction to better withstand future earthquakes.

The Seismic Alert System (SAS) for Mexico City is a public early warning system intended to signal the imminent arrival of seismic waves that may cause damaging ground shaking. In operation since 1991, SAS provides an approximate 60-second warning time for earthquakes located some 180 miles away, usually off the Pacific coast. The efficacy of the SAS is dependent upon reliable estimation of an earthquake's magnitude based on the analysis of a few seconds of close-to-source ground motion recordings. Seismologist A. Iglesias of Instituto de Geofísica, UNAM, México and colleagues evaluated the SAS performance and determined that the system may miscalculate the magnitudes of earthquakes and issued a high number of false warnings and missed alerts. Authors note two basic problems with the current SAS: it provides only partial coverage of areas that can generate damaging earthquakes for Mexico City, and the algorithm used to detect ground motion is inadequate. To improve the area coverage of the SAS and its performance, Iglesias et al., propose adding stations and an alternate method to detect earthquakes that may cause damage in Mexico City. Finally, the authors propose a single alert system rather than the current two-tiered method.

The geological properties beneath an area influence the intensity of ground shaking caused by earthquakes. Therefore, the evaluation of site conditions (particularly soil verses rock) is an important, yet time-consuming, part of determining seismic hazard and risk assessment. David J. Wald and Trevor I. Allen of the U.S. Geological Survey in Golden, Colorado, detail a new simple and quick method for estimating site conditions for very large areas, which may prove quite valuable when needing to evaluate areas where accurate site condition maps do not yet exist. This contribution is a significant advancement because its relative low cost makes its use within reach to all earthquake-prone regions of the world.

* This paper is not embargoed.

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