A STUDY IN THE VARIABILITY OF SEISMICITY: OGATA MODEL MODIFIEDA. A. Lyubushin, Jr., and V. F. Pisarenko
The purpose of this paper is to analyze the time variation of seismicity with the help of a nonstationary selfexciting point process model. The model parameters are evaluated in a moving time window containing the fixed number of events (thus the time length of the window varies). From each window a parameter vector is obtained, and so a "cloud" of parameter vectors results for the seismic region. This "cloud" is analyzed with a view to detecting anomalies and different "modes" of seismicity. The results of processing the seismic catalog of the western USA (1963--1990) for various subregions are presented. The most interesting are the variations of the model parameter $\beta$, which controls the influence of the energy of events on the rate of subsequent earthquakes. This parameter approaches negative values in some regions before strong earthquakes, which can be interpreted as a lack of aftershocks for moderate earthquakes before strong ones.