タイトル：

Laboratory earthquake seismology: non-self-similar earthquakes generated by a controlled fault asperity

要旨：

A fundamental observation in earthquake source physics is that most earthquakes follow self-similar scaling, with source duration proportional to the cube root of seismic moment. However, some earthquake clusters exhibit nearly constant source duration regardless of seismic moment. Although several models have been proposed to explain such non-self-similar behavior, observational uncertainties in natural earthquakes, particularly path effects, have hindered validation of their underlying mechanisms. To investigate the origin of non-self-similar earthquakes, we analyzed microearthquakes generated by size- and shape-controlled fault asperities on a 4-m-long laboratory fault. We set the asperities by placing circular gouge patches on the fault. The gouge was produced by pulverizing the same rock as the host specimen, thereby minimizing differences in frictional properties and introducing localized stress heterogeneity. After carefully correcting the observed waveforms for the amplitude and frequency responses of acoustic emission sensors and for intrinsic attenuation, the events exhibit non-self-similar behavior characterized by a nearly constant source duration of approximately 2.5 μs across magnitudes from Mw -7.3 to -6.0. Based on these observations, we developed a dynamic rupture model quantitatively consistent with the asperity configuration and the inferred source parameters. The model comprises a fixed source-patch size, variable stress drop, and self-healing friction, and represents a plausible mechanism for non-self-similar earthquakes, complementing existing models. Future work will combine larger-scale experiments and source modeling under varying fault and stress conditions to further investigate the conditions under which non-self-similar behavior emerges and whether it can serve as an indicator of the physical state of faults.