Space‐time model for regional seismicity and detection of crustal stress changes

Abstract

The regional occurrence rate of earthquakes is modeled as a function of previous activity, the specific form of which is based on empirical laws in time and space such as the modified Omori formula and the Utsu‐Seki scaling law of aftershock area against magnitude, respectively. Its parameters, including the p‐value of the aftershock decay rate, vary from place to place in such a way that, for example, the regional p‐value at the locations around the main shock is similar to the one estimated from the corresponding individual aftershock sequence. The estimated space‐time model is used to visualize features of the regional seismic activity. The utility is demonstrated by applying the model to the events of M ≥ 5 in and around Japan for the period 1926–1995. Among the parameters of the model, this paper is particularly concerned with the normalized K‐value of the modified Omori function indicating the aftershock productivity. This takes high values around the boundaries of asperities that are estimated in the wide area off the East Coast of Tohoku District, Japan, by the inversion of historical strong motion seismograms. Furthermore, this space‐time model enables us to identify the anomalous period and regions where the actual occurrence rates deviate systematically from the modeled one. Such an anomaly (activation and quiescence) relative to the model could sensitively reveal the change of stresses in the regions. We attempt to relate this relative activation and lowering of the seismicity to the pattern of Coulomb stress changes due to a rupture or silent slip elsewhere. For example, such anomalies seen in the seismic activity in the central Japan (M ≥ 2.5) during 1995–1999 and during 2001 are respectively likely the consequence of the Coulomb stress changes due to the 1995 Kobe rupture and the interplate aseismic slip during 2001 beneath the western Tokai region.