Slow initial phase and nucleation process of microearthquakes in western Nagano, central Japan

We investigated the slow initial phase of small earthquakes ( M 0.0–3.6) in western Nagano, central Japan, by deconvolution with causal attenuation operators and empirical Green's functions. To determine whether the slow initial phase is caused by propagation effects along the raypath or source effects, we investigated whether all earthquakes begin as shear cracks with a constant stress drop and rupture velocity, whether the path effect can be represented by a constant attenuation operator, and whether the seismograms of the smallest earthquakes are empirical Green's functions for the path effect of larger earthquakes. We found that the larger earthquakes ( M ≥ 1.9) have complicated source processes in combination with linear growth, while smaller earthquakes show simple linear increases in moment accelerations. It is possible that the larger earthquakes are fundamentally different from the smaller earthquakes. It also appears that the complex waveforms observed in the larger earthquakes can be explained by multiple rupture processes, although the slow initial phase of M ≥ 1.9 events, at the very least, must be explained as a source effect.