HR: 08:05h
AN: S41C-01    [Abstracts]
TI: Anomalous Intensities in the Gangetic Plain Following Great Himalayan Earthquakes
AU: * Bilham, R
EM: bilham@colorado.edu
AF: CIRES, University of Colorado, Boulder, CO 80309 United States
AU: Hough, S
EM: hough@gps.caltech.edu
AF: USGS, South Wilson Road, Pasadena, CA 91106 United States
AU: Ambraseys, N
EM: n.ambraseys@imperial.ac.uk
AF: Imperial College, Prince Consort Road, London, SW7 2BU United Kingdom
AB: We have examined details of the intensity distribution of the sediments of the Punjab, Ganges and Brahmaputra river valleys following the Ms=8.1 Shillong (1897), Ms=7.8 Kangra (1905), and Ms=8.2 Bihar/Nepal (1934) earthquakes. For each earthquake we subtract the observed MSK intensities from a synthetic intensity derived from an inferred planar rupture model of the earthquake, combined with a homogeneous attenuation function derived from instrumentally recorded earthquakes. The resulting residuals are contoured to identify regions of anomalous intensity caused by local site effects. Observations indicative of liquefaction are treated separately from other indications of shaking severity lest they inflate inferred residual shaking estimates. Despite this precaution we find that intensites are 1-3 intensity units higher near the major rivers, as well as at the edges of the Ganges basin. We find evidence for a post-critical Moho reflection from the 1897 and 1905 earthquakes that raises intensities 1-2 units at distances of the order of 150 km from the rupture zone, and we find that the 1905 and 1934 earthquakes may have respectively triggered earthquakes at Dehra Dun and, more speculatively, near Agra. The 1934 earthquake shows an intensity low to the east suggestive of westward directivity. Four or more M=8 earthquakes are apparently overdue in the region based on seismic moment summation in the past 500 years. Results from the current study permit anticipated intensities in these future earthquakes to be refined to incorporate site effects derived from dense macroseismic data.
DE: 1734 Seismology
SC: Seismology [S]
MN: 2004 AGU Fall Meeting