Numerical modelling of space and time variations of seismic activity before major earthquakes

Abstract

The spatial and temporal variations of seismic activity before large earthquakes, including precursory swarm earthquakes, clustering, pre-seismic quiescence in the epicentral area, doughnut patterns and foreshocks, are numerically modelled by quasi-static and dynamical rupture processes on three-dimensional heterogeneous faults with various types of non-uniform strength distributions. The main physical parameters that could yield these precursory changes of seismicity patterns are the form of distributions of static and sliding frictional strengths and their spatial variations over the fault, and also the rate of increase of tectonic stress. For weakly heterogeneous faults, a large-scale main shock rupturing the entire fault surface occurs without any precursory indications. If the fault has moderately to heavily non-uniform strengths on a small-scale, gradually increasing activity tends to occur over a long period prior to a main event, with no swarms or quiescence. For extremely heterogeneous faults including a number of small to moderate-size asperities with medium to high strengths, a typical sequence appears with swarms, quiescence, foreshocks and a main shock event. The temporal behaviour of these events seems to depend primarily on the form of distribution of the strengths. If the fault includes a large-size asperity with medium to high strengths in its central part, a doughnut pattern of seismicity repeatedly appears with high activity and quiescence in the surrounding and central asperity regions. If a belt-like barrier region with extremely high strengths lies in the central part of the fault, dividing the entire region into two sections, large-scale shocks take place successively in the adjacent unruptured regions at slightly different times, and a main shock event rupturing the entire fault occurs after a long period of quiescence. These results from numerical simulations could provide possible explanations of observed seismicity patterns prior to major earthquakes.