Source Parameter Scaling Relations for Shallow Crustal Earthquakes: Exploration With the Single Asperity Model

Understanding the scaling laws of source parameters is a fundamental subject in seismology. This study conducts spectral ratio analysis to estimate the source parameters for 409 shallow crustal earthquakes with Mw 3.2-6.0 in Japan. Subsequently, the source parameter scaling relations are investigated for a wide magnitude range by combining the results of this and previous studies. The spectral ratio method applied in this study provides the finite source properties of the localized area with large slip. The single asperity model, a simple heterogeneous source model with a single localized area with large slip, is used to estimate the stress drop since it can be more suitable to characterize earthquake sources than the standard homogeneous circular source. This investigation calibrates the constant to link the corner frequency and the source radius by comparing the corner frequency and source area estimated by the spectral ratio analyses. This calibrated constant is used to re-calculate the stress drop of small earthquakes from the corner frequency estimated in previous studies. The stress drop and apparent stress increase with increasing magnitude up to Mw similar to 5 and become magnitude-independent for larger earthquakes. The radiation efficiency ranges typically from 0.1 to 1.0 and is independent of magnitude. The slip dependences of the stress drop, apparent stress, radiation efficiency, and fracture energy observed in this study are explained by the ones predicted from the slip-weakening model incorporating the thermal pressurization effect. Thermal pressurization is one of the possible mechanisms explaining the observed source parameter scaling relations. This study investigates the relationship between earthquake sizes and parameters characterizing the earthquake sources (source parameter scaling relations) to understand the difference between small and large earthquakes. The source parameters are estimated for shallow crustal earthquakes in Japan. Then, the source parameter scaling relations are obtained over a broad magnitude range by combining the results of this and previous studies. This investigation is based on a simple heterogeneous source model rather than the homogeneous circular source model, which has been widely used. A familiar hypothesis is that the sources of small and large earthquakes are physically similar (earthquake similarity). The results suggest that the earthquake similarity holds for earthquakes with a magnitude larger than about 5.0 but does not for small earthquakes. This study demonstrates that the observed source parameter scaling relations can be explained by the source parameter scaling relations predicted from the stress relaxation model incorporating the effect of the pore pressure rise within the fault core by frictional heating during seismic faulting (thermal pressurization of pore fluids). Thermal pressurization of pore fluids can be one of the significant mechanisms explaining the differences in the source parameter scaling relations for small and large earthquakes. The single asperity model, a simple heterogeneous source model, is used rather than the standard homogeneous circular source model The apparent stress and stress drop increase with magnitude up to Mw similar to 5 and become magnitude-independent for larger earthquakes Thermal pressurization is one of the possible mechanisms explaining the observed source parameter scaling relations