FIGURE 5.12 Basin effects in Santa Monica from the 1994 Northridge earthquake. Upper left panel is a geological map with the locations of seismic stations (triangles) and the scarp of the Santa Monica fault (arcuate black lines). Lower left panel is a sheer velocity model of the upper crust along map profile kk’. Upper panels compare the observed seismograms (left) with seismograms simulated from the model (right). The simulation indicates that the large pulse at station smch is caused by the focusing at the rock-sediment interface along the Santa Monica fault. SOURCE: Modified from R.W. Graves, A. Pitarka, and P.G. Somerville, Ground motion amplification in the Santa Monica area: Effects of shallow basin edge structure, Bull. Seis. Soc. Am., 88, 1224-1242, 1998. Copyright Seismological Society of America.

damping as shear strain levels increase beyond about 10–4 to 10–5 or as ground acceleration becomes greater than about 0.1g. This softening causes the fundamental period of the soil layer to lengthen. At higher frequencies, cyclic pore pressure increases may produce cyclic strain hardening, which is manifested by high-frequency spikes toward the end of the record, increasing the duration of the record and sometimes producing the largest accelerations. These effects have been demonstrated in the small number of data sets where surface and subsurface seismic data are available (200) to enable direct comparison between the motion in the rock or stiff soil (which is assumed to be linear) and the resulting motion

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