HR: 10:35h
AN: T32B-02 INVITED     [Abstracts]
TI: Unprecedented Massive earthquakes in the Himalaya driven by Elastic Strain stored within the Tibetan Plateau?
AU: * Bilham, R
EM: bilham@colorado.edu
AF: Geol Sci, University of Colorado, Boulder, CO 80309-0399 United States
AU: Feldl, N
EM: feldl@colorado.edu
AF: Geol Sci, University of Colorado, Boulder, CO 80309-0399 United States
AB: In view of the absence of historical precedent for the 1600 km long Sumatra/Andaman rupture of the eastern edge of the Indian plate in 2004, we question whether similar earthquakes without historical precedent could also occur in the Himalaya. A seismic slip deficit corresponding to a single Mw=8.7 earthquake currently exists, and although no great earthquake has ruptured the Himalayan frontal thrusts in the past 300 years, evidence that larger earthquakes have done so between 900 and 1505 is now available in the form of trench investigations of slip in the range 5-21 m, over rupture lengths of 200-300 km. The magnitude of these and possibly larger earthquakes is unknown. We investigate possibly magnitudes for Himalayan earthquakes by developing a synthetic scaling law relating renewal time to rupture length. We first emulate the accumulation of present-day elastic strain near the southern edge of the Tibet plateau using 3D boundary element methods, and then, after incrementing this strain by a time interval of many hundreds of years, we investigate the amount of slip that corresponds to different rupture lengths. Our interseismic model uses the inferred subsurface geometry of the Indian plate (20-40 km depth) and the observed GPS velocity field to 1000 km north of the Himalaya as constraints, with minor adjustments to the shallow geometry beneath the Greater Himalaya as our only variable. Assuming that frictionless aseismic slip occurs beneath the plateau, we find that its rate decays to zero 25 km south of what we have previously considered the locking line, with dip increasing to 15 deg. at 12-18 km depth, consistent with focal mechanisms in this region. An unexpected result is that vertical deformation is twice the width (70 km) & half the amplitude (3 mm/yr) of that assoicated with uniform-slip elastic models, though still consistent with the lower error envelope of leveling data. To account for the 14-21 mm/yr long term advance of the Himalaya over India we find that elastic strain stored more than 300 km north of the edge of the Tibetan plateau must participate in driving the largest Himalayan earthquakes (8.2<Mw<8.7). We find that a renewal time of &~ 500 years is consistent with the length/Mw scaling of recent 6.5<Mw<8.2 earthquakes, but that a doubling of this interval must occur to account for the largest slip events. We find also that, just as in the Andaman/Nicobar islands, regions of the Himalaya that have experienced recent Mw=7.8 earthquakes are vulnerable to much larger earthquakes sooner than would be anticipated by seismic gap theory.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Tectonophysics [T]
MN: Fall Meeting 2005