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

Yan Y. Kagan, David D. Jackson, & Yufang Rong

Published 2007, SCEC Contribution #985

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 same method to forecast earthquakes for the northwest and southwest Pacific regions (Jackson and Kagan 1999; Kagan and Jackson 2000), shown on our Web site, http://scec.ess.ucla.edu/~ykagan/predictions_index.html. We have also applied this technique since 1992 (Jackson et al. 1995; Kagan et al. 2003) in an experimental long-term forecast for southern California. The latest version is on the Web at http://moho.ess.ucla.edu/~kagan/s_cal_haztbl.tmp.

The model contains just a few adjustable parameters, which we normally determine so that a forecast based on the first half of a catalog best fits the second half. At any given time we can revise the parameter estimates and the forecast. For the California testing experiment, we will fix all of the parameters and suspend revisions for the five-year duration of the test.

Our forecast differs from some other forecasts presented in this issue (Bird and Liu 2007, this issue; Helmstetter et al. 2007, this issue; Shen et al. 2007, this issue) and elsewhere in several important respects.

1. By using the longest possible time span for our "learning" catalog, we have tried to construct a model that will be valid over several decades. Some other models are based on smaller earthquakes over shorter periods; these may not capture effects that are only evident in the long term.
2. We use an extended source representation for large earthquakes: We replace each epicenter point by a series of rectangular dislocations that cover the whole rupture area. Therefore, we forecast non-negligible earthquake rates for the whole 1857 Fort Tejon rupture extent, though some of these areas currently exhibit almost no seismic activity.
3. We use only moderate and large (M ≥ 5.0) earthquakes in our forecast. Thus we largely avoid the complications of smaller earthquakes caused by volcanism, geothermal activity, or fault creep.
4. Our forecasting algorithm predicts not only the rate, size distribution, and location of future earthquakes, as almost all other forecast programs do, but also focal mechanisms of these events with an indication of forecast uncertainty.

Key Words
United States, California, Southern California, geologic hazards, seismicity, earthquake prediction, seismic risk, risk assessment, algorithms, earthquakes

Kagan, Y. Y., Jackson, D. D., & Rong, Y. (2007). A Testable Five-Year Forecast of Moderate and Large Earthquakes in Southern California Based on Smoothed Seismicity. Seismological Research Letters, 78(1), 94-98. doi: 10.1785/gssrl.78.1.94.