ARRHENIUS BEHAVIOR OF AFTERSHOCK SEQUENCES

Arrhenius expression of static fatigue Is used to model aftershock seq uences. Assuming that the initial stress condition sigma(o) at the mai nshock origin time t(o) is the superposition of the stress before the mainshock and of a stress step sigma(od) produced by the dynamic ruptu re of the mainshock, we may express a general Arrhenius aftershock mod el as Delta Sigma(tau(i)) = sigma(od) + (RT/gamma)Delta t(i), where De lta Sigma(tau(i)) is the cumulative stress drop of mainshock and after shocks at time tau(i) = t(i) - t(o), t(i) is the origin time of ith af tershock and Delta t(i) = In t(i) - In t(o). The fit of the model to t he aftershock sequences of May 6, 1976, M(L) = 6.3 Friuli earthquake ( northeastern Italy) and of November 23, 1980, M(L) = 6.6 Campania-Basi licata earthquake (southern Italy) is rather good: the coefficient of variation obtained by regression analysis is around 3% and indicates t hat, at least for the time window length considered (around 780 hours since the mainshock), the cumulative stress drop is entirely coseismic . The present aftershock model is derived from the empirical model t = s exp ((U - gamma sigma)/RT), considering the aftershock origin time to be equivalent to time to fracture t. Results show the validity of t his approach. As an example, the derivatives of stress with respect to time to fracture obtained by the analysis of the two sequences are in good agreement with those obtained on laboratory samples; in particul ar, when the laboratory conditions (confining pressure, characteristic s of the specimen) are similar to field conditions, the scatter is bet ween 5 and 50%. The model appears also to be consistent with experimen tal evidences of direct correlation of p (exponent of Omori law) on su rface heat flow.