A Testable Five-Year Forecast of Moderate and Large Earthquakes in Southern California Based on Smoothed Seismicity

Abstract

We present a five-year forecast of southern California earthquakes with magnitudes 5.0 and greater. The forecast uses earthquake data only, with no explicit use of tectonic, geologic, or geodetic information. The forecast is based on observed regularity of earthquake occurrence rather than on any physical model. The earthquake rate density (probability per unit area, time, and magnitude) is assumed constant in time. We estimate it as the sum of contributions from all magnitude 5 and larger earthquakes in our catalog, which for large quakes extends from 1800 to the present. The contribution from each quake is inversely proportional to epicentral distance and directly dependent on the magnitude. We use the same model to estimate the probable focal mechanisms of future earthquakes, using a weighted sum of the moment tensors of previous quakes. We assume that the rate density is proportional to a smoothed version of past seismicity, using a recently published (Kagan et al. 2006) catalog of California earthquakes. That catalog includes all known earthquakes larger than magnitude 7.5 since 1800 and smaller earthquakes as the lower threshold of completeness has decreased with time. We treat earthquakes as point sources, except that large earthquakes ( M ≥ 6.5) are represented by multiple rectangular dislocation patches (see figure 2 in Kagan et al. 2006). For the forecast that is the subject of this paper, we represent each of the patches by a point source at its center. The estimated rate density depends linearly on the magnitude of past earthquakes and inversely as epicentral distance out to a few hundred kilometers. We assume that 2% of all earthquakes are surprises , assumed uniformly likely in those areas with no earthquakes. Foreshocks and aftershocks are treated as any other earthquake. The method is further described in Kagan and Jackson (1994). We have used the …