Publication Date:
2008-10-01
Description:
The possibility that a moderate earthquake may be followed by an equal or larger one (foreshock probability) increases the hazard in its immediate vicinity for a short time by an order of magnitude or more. Thus, foreshock probabilities are of interest for time-dependent seismic hazard forecasts. We calculate the probability of an initial earthquake (a foreshock) being followed by a mainshock in New Zealand, considering the parameters of elapsed time and distance and magnitude differences between foreshock and mainshock. We use nonaftershock events between 1964 and 2007, with magnitude 〉 or =4.0 and shallower than 40 km, separating the catalog into events within and outside the Taupo volcanic zone (TVZ). We provide a model for the probability P(t, r, delta M) that at time t after a potential foreshock (FS) of magnitude M (sub FS) and at distance r, a mainshock with magnitude M (sub FS) +delta M will occur: P(t, r, delta M) = P (sub 0) X10 (super (-Bdelta M)) (t+c (sub t) ) (super -pt) (r+c (sub r) ) (super -pr) , where P (sub 0) , B, p (sub t) , c (sub t) , p (sub r) , and c (sub r) are constants to be determined. We find that (1) binning data using fixed intervals of time or space before fitting the parameters returns different values than a more robust approach of fitting directly the entire range, (2) foreshock probabilities decrease with increasing interevent time as described by a modified Omori law with an exponent p (sub t) close to 1 (0.9+ or -0.2 [TVZ] and 0.8+ or -0.1 elsewhere--uncertainty estimates are 95% confidence intervals throughout this study), (3) foreshock probabilities decrease with increasing epicentral distance also following a modified Omori type decay with exponent p (sub r) of 0.9+ or -0.2 (non-TVZ) and 1.7+ or -0.6 (TVZ), and (4) the mainshock magnitude distribution follows the Gutenberg-Richter relationship (B = 1.0+ or -0.17 [non-TVZ] and 1.5+ or -0.5 [TVZ]). The differences between the TVZ and the rest of New Zealand are consistent with higher attenuation in the region, deduced from previous studies.
Print ISSN:
0037-1106
Electronic ISSN:
1943-3573
Topics:
Geosciences
,
Physics
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