Fault strength dependency of natural earthquake-size distribution based on the precise focal mechanism data

“b-value”, which degree of power law decay in the earthquake-size distribution, is known showing spatial and temporal variation based on observational studies. Especially, the temporal variation sometime have detected before a large earthquake occurrence, showing that it can be an indicator for the occurrence and may help earthquake hazard mitigation. The b-value changes due to tectonic stress regime. In addition, laboratory experiments have revealed acoustic emission size distribution depends on differential stress magnitude and criticality of failure condition. However, it is unclear in natural earthquake activity that which factor controls b-value condition. In this study, we show b-value change of small earthquake sequences in normalized shear and normal stress.

We carried out dense seismic observation composed by over 1000 stations deployed in hypocentral area (with diameter about 35 km) of the 2000 Western Tottori Earthquake (M7.3). About one year observation enabled us to obtain hypocenters and focal mechanisms about 5000 small earthquakes. Relative stress tensor have been inverted by the focal mechanism data in spatial bins and relative shear and normal stress for individual earthquake also estimated.  We investigated b-value change in relative shear and normal stress condition of small earthquake dataset. The b-value dependency on relative shear stress were detected, showing that the b-value decrease with increasing shear stress. In addition, the b-value takes minimum value at normal stress at critical point where line following Coulomb Failure condition with friction coefficient of 0.6 touches the unit Mohr circle. This suggests that the b-value become small in case of fault plane in optimal direction.