Empirical Analyses of Near-Source Ground Motion

摘要

The distance dependence of ground motion relationships derived from close-in data is very sensitive to the ambiguous interpretation of distance when the station is a source dimension or less from the fault. The choice of the method for measuring distance can have a dramatic impact on the resulting ground motion curves. The straightforward application of ground motion curves based on certain distance measures may lead to unconservative predictions of peak ground motion. The magnitude dependence of close-in peak ground motion data does not show evidence of a saturation, or decreased magnitude dependence, at large magnitudes. Neither is a saturation observed if the earthquake size, or strength, is parameterized by seismic moment. It is possible that this reflects an inadequate data set. Unusually high peak vertical accelerations recorded at close-in distances may be separated into two categories. In the first high vertical acceleration may be directly attributed to the source radiation. In the second the site sedimentary structure is the most important factor. In the latter case, the high vertical acceleration occurs in a vertically-polarized converted or channeled phase arriving between the P and S arrivals. The high vertical accelerations recorded in the recent Imperial Valley and Coyote Lake earthquakes are of this type.