Fractal properties of seismicity in regions affected by large, shallow earthquakes in western Japan: Implications for fault formation processes based on a binary fractal fracture network model

Abstract

The temporal variation in the b value (which is defined as the slope of the Gutenberg‐Richter frequency N–magnitude M relationship), log N = a − bM, and the multifractal dimension of the spatial distribution Dq of seismic activity are analyzed using earthquake catalog data for regions affected by two recent large earthquakes in Japan. An anomalous decrease in Dq and an increase in b value appear as a common seismic rupture response during the period of decreasing seismic energy release in the lead up to these large, shallow earthquakes. The change in fractal dimensions of faults inferred from sequential seismic events reflects the transition of fracture from a dispersive rupture process to an anomalous rupture process and shows that the growth of the fault network through fault linkage is abruptly restrained in the fault formation process leading up to the main shock. The anomalies in the fractal and multifractal dimensions prior to large earthquakes indicate that the anomalous activation and clustering of smaller earthquakes represents criticality of the state around asperities along a fault.