Renormalization of earthquake aftershocks

Abstract

Together with the Gutenberg-Richter distribution of earthquake magnitudes, Omori's law is the best established empirical characterization of earthquake sequences and states that the number of smaller earthquakes per unit time triggered by a main shock decays approximately as the inverse of the time ($1/t^p$, with $p \approx 1$) since the main shock. Based on these observations, we explore the theoretical hypothesis in which each earthquake can produce a series of aftershock independently of its size according to its ``local'' Omori's law with exponent $p=1+\theta$. In this scenario, an aftershock of the main shock produces itself other aftershocks which themselves produce aftershocks, and so on. The global observable Omori's law is found to have two distinct power law regimes, the first one with exponent $p_-=1 - \theta$ for time $t < t^* \sim \kappa^{-1/\theta}$, where $0<1-\kappa <1$ measures the fraction of triggered earthquakes per triggering earthquake, and the second one with exponent $p_+=1 + \theta$ for larger times. The existence of these two regimes rationalizes the observation of Kisslinger and Jones [1991] that the Omori's exponent $p$ seems positively correlated to the surface heat flow: a higher heat flow is a signature of a higher crustal temperature, which leads to larger strain relaxation by creep, corresponding to fewer events triggered per earthquake, i.e. to a larger $\kappa$, and thus to a smaller $t^*$, leading to an effective measured exponent more heavily weighted toward $p_+>1$.Comment: 13 pages, in press in Geophys. Res. Let