Comparison of short-term and long-term earthquake forecast models for southern California

A. Helmstetter (1) , Yan Y. Kagan (2) , David D. Jackson (2)

1 LDEO - Lamont-Doherty Earth Observatory
2 IGPP - Institute of Geophysics and Planetary Physics [Los Angeles]

We consider the problem of forecasting earthquakes on two different time scales: years, and days. We evaluate some published forecast models on these time scales, and suggest improvements in forecast models on both time scales. For time scales of several years, we modify the smoothed seismicity method of Kagan and Jackson [1994; 2000] by smoothing the locations of magnitude 2 and larger earthquakes. Kagan and Jackson used only magnitude 5 and larger. The new long-term model outperforms the best known published models in a retrospective test on magnitude 5 and larger, primarily because it has much higher spatial resolution. We have also developed a model to estimate daily earthquake probabilities in southern California, using the Epidemic Type Earthquake Sequence model [Kagan and Knopoff, 1987; Ogata, 1988; Kagan and Jackson, 2000]. The forecasted seismicity rate is the sum of a constant background seismicity and of the aftershocks of all past earthquakes. The background rate is estimated by smoothing past seismicity. Each earthquake triggers aftershocks with a rate that increases exponentially with its magnitude and decreases with time following Omori's law. We use an isotropic kernel to model the spatial distribution of aftershocks for small (m 5.5) mainshocks, and by smoothing the location of early aftershocks for larger mainshocks. The model also assumes that all earthquake magnitudes follow the Gutenberg-Richter law with a uniform b-value. We use a maximum likelihood method to estimate the model parameters and test the short-term and long-term forecasts. A retrospective test using a daily update of the forecasts between 1985/1/1 and 2004/3/10 shows that the short-term model increases the average probability of an earthquake occurrence by a factor 11.5 compared to the long-term time-independent forecast.

Domaines: Géomorphologie

A. Helmstetter, Yan Y. Kagan, David D. Jackson. Comparison of short-term and long-term earthquake forecast models for southern California. Bulletin of the Seismological Society of America, 2006, 96 (1), pp.90-106. ⟨10.1785/0120050067⟩. ⟨hal-00195371⟩