Site Hazard Analysis Methods with Empirical and Geophysical Ground Motion Models

A stochastic site hazard model is presented based on the geophysical ground motion model. The hazard model is developed in two steps. In the first step, the occurrence of earthquakes is represented as a time-dependent stochastic model. In the second step, a method for ground motion estimation is developed. The earthquake occurrence model is based on the slip-predictable hypothesis. For the ground motion estimation model, empirical and theoretical ground motion attenuation models are used. Theoretical ground motion attenuation relationships are obtained by simulation of ground motion based on the normal mode method. The hazard model forecasts probabilities of exceeding peak ground acceleration and acceleration response of a structure at a specified site conditional on the time of occurrence of the last large earthquake.

In order to demonstrate the usefulness and applicability of this model, the seismic hazard is estimated at sites near the Middle America Trench off the Pacific coast of Mexico. The data from this section of the Middle America Trench have suggested that slip-predictable behavior may be appropriate.

First, the probability of the occurrence of the September 19, 1985, Mexico earthquake is estimated based on the stochastic slip-predictable model. The results show that the probability of occurrence of the September 19 event was very high because of the long period of inactivity in the source region. It is also found that consideration of the seismic gap is very important in estimating the hazard.

The stochastic site hazard model represents a considerable improvement over the existing seismic hazard model. However, since earthquake occurrence patterns and characteristics of ground motions are strongly dependent on the geophysical mechanisms and wave propagation effects, further studies in these areas are necessary for estimating the seismic site hazard with high reliability.