- © 2009 by the Seismological Society of America
To construct and test certain hypotheses of earthquake occurrence, we've compiled a new catalog covering the whole of California, which lists all known earthquakes at magnitude 4.7 and above and provides focal mechanism information for each earthquake. Accounting for magnitude errors, the catalog should be complete at magnitude 5.0 and above for most of California after 1940 and at specified higher thresholds otherwise.
Earthquake magnitudes play an important role in earthquake forecasting and prediction, but their use is complicated by the fact that there are many magnitude types, measured in different ways with different uncertainties. Even a single catalog may include moment magnitude, local magnitude, surface wave magnitude, and so on. Here we adopt moment magnitude as the standard and convert other types into moment magnitude using regression relationships.
Catalog data have errors in location, magnitude, and focal mechanism that can influence the results of earthquake studies. Neglecting these errors, or estimating them poorly, could cause valid hypotheses to be rejected or invalid ones to be accepted. The uncertainties decrease with time, except that they increase temporarily after large earthquakes. Therefore we've conducted an extensive uncertainty study.
Many studies assume a distinction between mainshocks, aftershocks, and foreshocks, and some catalogs are “declustered” using somewhat arbitrary definitions (e.g., Gardner and Knopoff 1974; Reasenberg and Oppenheimer 1985). Here we take a somewhat different approach. Using a stochastic epidemic-type aftershock (ETAS) model (Zhuang et al. 2002, 2004; Zhuang et al. 2005), we assigned to each event a probability that it occurred independently. The independence probability for each earthquake can then be employed by any user to decluster the catalog to fit his or her needs. Our method is still arbitrary, but we feel it is more transparent and flexible than other methods.
Kagan, Jackson, and Rong (2006) compiled an …