Geology Reports⌕ Search

Geology topics

M.D. Petersen

Publications and source records attributed to M.D. Petersen.

21 records · Page 2Linked to original sources
USGS2000

Discrepancy between earthquake rates implied by historic earthquakes and a consensus geologic source model for California

We examine the difference between expected earthquake rates inferred from the historical earthquake catalog and the geologic data that was used to develop the consensus seismic source characterization for the state of California [California Department of Conservation, Division of Mines and Geology (CDMG) and U.S. Geological Survey (USGS) Petersen et al., 1996; Frankel et al., 1996]. On average the historic earthquake catalog and the seismic source model both indicate about one M 6 or greater earthquake per year in the state of California. However, the overall earthquake rates of earthquakes with magnitudes (M) between 6 and 7 in this seismic source model are higher, by at least a factor of 2, than the mean historic earthquake rates for both southern and northern California. The earthquake rate discrepancy results from a seismic source model that includes earthquakes with characteristic (maximum) magnitudes that are primarily between M 6.4 and 7.1. Many of these faults are interpreted to accommodate high strain rates from geologic and geodetic data but have not ruptured in large earthquakes during historic time. Our sensitivity study indicates that the rate differences between magnitudes 6 and 7 can be reduced by adjusting the magnitude-frequency distribution of the source model to reflect more characteristic behavior, by decreasing the moment rate available for seismogenic slip along faults, by increasing the maximum magnitude of the earthquake on a fault, or by decreasing the maximum magnitude of the background seismicity. However, no single parameter can be adjusted, consistent with scientific consensus, to eliminate the earthquake rate discrepancy. Applying a combination of these parametric adjustments yields an alternative earthquake source model that is more compatible with the historic data. The 475-year return period hazard for peak ground and 1-sec spectral acceleration resulting from this alternative source model differs from the hazard resulting from the standard CDMG-USGS model by less than 10% across most of California but is higher (generally about 10% to 30%) within 20 km from some faults.

Bulletin of the Seismological Society of America↗
USGS2000

A test of various site-effect parameterizations in probabilistic seismic hazard analyses of southern California

We evaluate the implications of several attenuation relationships, including three customized for southern California, in terms of accounting for site effects in probabilistic seismic hazard studies. The analysis is carried out at 43 sites along a profile spanning the Los Angeles basin with respect to peak acceleration, and 0.3-, 1.0-, and 3.0-sec response spectral acceleration values that have a 10% chance of being exceeded in 50 years. The variability among currently viable attenuation relationships (espistemic uncertainty) is an approximate factor of 2. Biases between several commonly used attenuation relationships and southern California strong-motion data imply hazard differences that exceed 10%. However, correcting each relationship for the southern California bias does not necessarily bring hazard estimates into better agreement. A detailed subclassification of site types (beyond rock versus soil) is found to be both justified by data and to make important distinctions in terms of hazard levels. A basin depth effect is also shown to be important, implying a difference of up to a factor of 2 in ground motion between the deepest and shallowest parts of the Los Angeles basin. In fact, for peak acceleration, the basin-depth effect is even more influential than the surface site condition. Questions remain, however, whether basin depth is a proxy for some other site attribute such as distance from the basin edge. The reduction in prediction error (sigma) produced by applying detailed site and/or basin-depth corrections does not have an important influence on the hazard. In fact, the sigma reduction is less than epistemic uncertainties on sigma itself. Due to data limitations, it is impossible to determine which attenuation relationship is best. However, our results do indicate which site conditions seem most influential. This information should prove useful to those developing or updating attenuation relationships and to those attempting to make more refined estimates of hazard in the near future.

California↗