HR: 08:15h
AN: NS21A-02    [Abstracts]
TI: Near-Surface Factors Affecting Earthquake Ground-Motion Amplification Due to Basin Edges: Modeling of the Haifa Urban Basin, Israel
AU: * Louie, J N
EM: louie@seismo.unr.edu
AF: Nevada Seismological Laboratory, University of Nevada 174, Reno, NV 89557, United States
AU: Gvirtzman, Z
EM: zohar@gsi.gov.il
AF: Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem, 95501, Israel
AB: A fault-bounded basin up to 2 km deep underlies the low-elevation sections of the city of Haifa. Extensive seismic- reflection sections and an unusually dense H/V data set of single-station microtremor recordings define basin geometry and stratigraphy in detail and allow constraint of shear velocities. This basin appears as a contrast in shear velocity of 2:1 against surrounding older sedimentary rocks, even in its deeper reaches. We conducted 2-D and 3-D finite-difference modeling studies of the basin using S. Larsen's E3D code up to frequencies of 6 Hz. Our modeling included sensitivity studies using simple cross sections, and comparisons of 2-D results against 1-D modeling conducted in ProShake. A prominent 2-D effect not seen in the 1-D modeling is a narrow zone of high PGV amplification a few hundred meters inside the basin from its edges, which also appears in spectral accelerations at shorter periods. Spectral accelerations at longer periods are more amplified where the basin is deeper. Tracking wave propagation within the synthetic sections, this "basin-edge effect" appears where two wave phases interfere constructively: 1) the Rayleigh wave propagating slowly horizontally into the basin from the basin's edge, after conversion from the upgoing S wave outside the basin, at its edge; and 2) the upgoing S wave within the basin, delayed by the low shear velocities in the basin. This effect is not related to any upward refraction or propagation along the (often) steep wall of the basin, nor to any 1-D or 2-D resonances. The edge effect locates instead within the basin at the distance from the edge where the delay of slow, shallow Rayleigh-wave propagation equals the delay of the upgoing S wave in the basin, relative to the S arrival outside the basin. In Haifa, with 0.3 km/s shear velocities near the surface, the basin-edge effect locates to several hundred meters from the edge. With this explanation for the basin-edge effect, we surmise that it will appear only at periods T < t₀, where t₀ is the one-way vertical shear-wave travel time through the basin. Our synthetic spectral accelerations confirm this, with the basin-edge effect appearing only at periods T < 1.6 sec.
DE: 0902 Computational methods: seismic
DE: 6334 Regional planning (1880)
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7255 Surface waves and free oscillations
DE: 7290 Computational seismology
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting