HR: 1330h
AN: S52A-0128    [PDF]
TI: Characterizing Near-Field Rupture Directivity Effects at Low to Intermediate Frequencies
AU: Fukushima, Y
EM: yf@ori.shimz.co.jp
AF: Ohsaki Research Institute, Fukoka Seimei Bldg. 2-2-2 Uchisaiwai-cho Chiyoda-ku, Tokyo, 100-0011 Japan
AU: * Ichinose, G A
EM: gene_ichinose@urscorp.com
AF: URS Corporation, 566 El Dorado St, 2nd Floor, Pasadena, CA 91101-2560 United States
AU: Somerville, P G
EM: paul_somerville@urscorp.com
AF: URS Corporation, 566 El Dorado St, 2nd Floor, Pasadena, CA 91101-2560 United States
AU: Koketsu, K
EM: koketsu@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1 Yayoi Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Analysis of near-fault ground motions of recent large crustal earthquakes have shown that the forward-rupture direction is significant in recorded strong motions. There are only a few near-fault observations and therefore they are under-represented in empirical relationships. Ground motion predictions based on simulations using point source or far-field approximations may be underestimating the ground motion levels because they cannot fully capture near-fault directivity effects. We show that the "directivity pulse" results in amplifications at low frequencies and does not result in a shift of the corner frequency and addition of high frequency radiation as predicted by the ($\omega ^{-2}$) model. We examined the directivity effects using a finite-fault simulation with a simple uniform slip model. The rupture parameters include sub-fault grid spacing, rupture velocity, source-time function, and dislocation rise-time. The waveform synthetics are computed using the reflectivity method and a layered earth model. These simple slip models generated directivity effects at low and intermediate frequencies that fit reasonably with observed near-fault ground motions from the 1992 Landers, 1994 Northridge, and 1995 Kobe earthquakes. Amplifications in near-fault Fourier accelerations for the forward-rupture directions were amplified by a factor of 10 to 50 between 0.05 to 0.6 Hz relative to the reverse-rupture directions. These amplifications were most dominant on the fault normal component for strike-slip and dip-slip faults near the terminal edge of the rupture. The amplification for dip-slip ruptures are significant between $\pm 45\deg $ from the forward rupture direction. Directivity effects resulting from the simulations are consistent with observations made by Somerville et al. [1997] and analytical models by Bernard et al. [1996]. This amplification is not an artifact of the simulation parameterization or rupture parameters and we show examples where changes in these parameters caused changes at only higher frequencies ($f > $0.5 $Hz$).
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 7223 Seismic hazard assessment and prediction
SC: Seismology [S]
MN: 2003 Fall Meeting