Application of the EEPAS Model to Forecasting Earthquakes of Moderate Magnitude in Southern California

Abstract

The EEPAS (Every Earthquake a Precursor According to Scale) model is a method of long-range forecasting that uses the previous minor earthquakes in a catalog to forecast the major ones. It is based on the precursory scale increase (Ψ) phenomenon, which involves an increase in the magnitude and rate of occurrence of minor earthquakes close to the source region of a major event in preparation, including most recent major earthquakes in California (Evison and Rhoades 2002, 2004). The period of time occupied by the increase scales with magnitude, but it is on the order of 15 years for an M 7 event, five years for an M 6 event, and one or two years for an M 5 event. With a one-year time horizon as specified for the Regional Earthquake Likelihood Models (RELM) testing in southern California, it is therefore feasible to consider using the model to forecast earthquakes of M 5 and above. The model has previously been fitted to the New Zealand earthquake catalog using earthquakes of magnitudes exceeding 3.95 to forecast those exceeding magnitude 5.75. It was shown to explain the data much better than a baseline model that is in principle time-invariant and has a location distribution based on proximity to the epicenters of past earthquakes. In the same form, and with the same magnitude thresholds, it was tested on California over the period 1975-2001 and again performed much better than the baseline model (Rhoades and Evison 2004). In the same form, but with magnitude thresholds one unit higher, it was tested on Japan over the period 1965-2001 and produced a similar result (Rhoades and Evison 2005), albeit with a smaller advantage over the baseline model. In order to fit well to lower magnitudes down to M 6.25, some adjustment of parameters, and in particular …