LONG-PERIOD GROUND MOTIONS AND DYNAMIC STRAIN FIELD OF LOS-ANGELES BASIN DURING LARGE EARTHQUAKES

During a big earthquake along the San Andreas fault in southern Califo rnia, high excitation and low attenuation of long-period (3 to 10 sec) strong ground motions will cause wave motions to propagate efficientl y far from the epicentral area. These ground motions could potentially be destructive to large-dimension structures in the Los Angeles basin . We performed calculations using the surface-wave Gaussian beam metho d for a 3D southern California crustal structure. Displacement field a s well as the associated dynamic strain field produced by large propag ating ruptures along the San Andreas fault are evaluated. Results indi cate that in the presence of lateral heterogeneity, focusing and multi pathing interference contribute significantly to a complex pattern of the displacement field and the associated dynamic strain field. For a big event on the San Andreas fault with a seismic moment of 1.8 x 10(2 8) dyne-cm, long-period displacement in the Los Angeles basin could re ach a maximum amplitude of meters in places. Since this calculation is fast, we have evaluated the displacement field for a dense grid of po ints; a differentiation gives the corresponding effective horizontal d ynamic strain field. At times, the maximum effective dynamic strains m ay reach mid-10(-3) to even 10(-2)-high enough to be of engineering co ncern. This computational result probably gives the upper bound values due to the large source assumed. For events of smaller seismic moment release along less extensive ruptures, these results can easily be sc aled down proportionally. Different scenarios are considered in this s tudy with different slip distributions. It is found that with a given seismic moment, a more evenly distributed fault slip over the rupture surface will result in lower peak values on both displacements and dyn amic strains. Our displacement results give similar values to those ob tained by Kanamori using empirical Green's functions but substantially higher than Bouchon and Aki's results.