| ABSTRACT
Recent parallel development of improved quantitative methods to analyze intensity distributions for historical earthquakes and of Web-based systems for collecting intensity data for modern earthquakes provides an opportunity to reconsider not only important individual historical earthquakes but also the overall characterization of intensity distributions for historical events. In this talk I compare be intensity distributions of historical earthquakes with those from modern earthquakes for which intensities have been determined by the U.S. Geological Survey “Did You Feel It?” Web site. Traditional modified Mercalli intensity (MMI) values for historical earthquakes are higher, on average, than those from recent events of comparable magnitude for distances ≤500 km but comparable or lower on average at larger distances, with a smaller overall felt extent. I conclude that the discrepancy can be explained by fundamental reporting biases associated with written earthquake accounts. A detailed consideration of the DYFI intensity distribution for the 2011 Mineral, VA, earthquake illustrates how reporting and sampling biases can account for historical earthquake intensity biases as high as two intensity units, and for the qualitative difference in intensity distance decays for modern versus historical events. Thus, intensity maps for historical earthquakes tend to imply more widespread damage patterns than are revealed by intensity distributions of modern earthquakes of comparable magnitude. I further use the separate intensity-attenuation relations that have been developed to characterize intensities for historical and modern earthquakes in California to develop a correction-curve approach estimate equivalent DYFI values for historical earthquakes . This allows the growing wealth of well-calibrated DYFI data to be used as calibration events in the analysis of historical earthquakes. As a test case I present a new analysis of the 1868 Hayward fault earthquake, and discuss the implications of the result for seismic hazard assessment on partially creeping faults.
|
