Direct Evidence for Diverse Source Complexity in Small Earthquakes (Mw 3.3–5.0) Obtained from Near-Source Borehole Seismic Data

Small earthquakes (Mw <5) may have a similar degree of complexity as large earthquakes. However, their seismic waveforms are strongly distorted during wave propagation, making their complexity challenging to resolve. In many cases, the source parameters of small events are determined based on models that assume their source patterns are simple. In this study, to directly examine the source complexities in small events, we examined high-quality near-source (<8 km) seismic waveforms recorded by two excellent downhole sensors in Japan. The results show that the P waveforms of microearthquakes (Mw <2) are always simple at the sensors and agree well with the synthetic waveforms based on a 1D structure up to 20 Hz. The microearthquake waveforms in this frequency band essentially represent path effects besides the static source effect, suggesting that the contribution of structural inhomogeneity to the observed waveforms is small. Taking advantage of this, we inferred the moment rate functions of 164 Mw 3.3–5.0 events from the shapes of the direct P waves. They showed diversity in their complexity, and even conservatively estimated, 25% of the events had multiple subevents. The results suggest that methods that account for complexity, rather than those that assume a simple source pattern, are required to characterize even small events.