Small magnitude slow earthquakes remain largely undetected in geodetic data due to noise levels. However, tremor and low-frequency earthquakes (LFE) may manifest slowly slipping fault motion as a cluster of events, i.e., a slow earthquake. Here, we identify >900 slow earthquakes in southwest Japan via data-driven clustering of tremor and LFE catalogs. We establish a more complete database for slow earthquakes in southwest Japan and demonstrate their characteristics and long-term behavior. While sometimes sub-episodes of well-known episodic tremor and slip, the small slow earthquake clusters share similar scaling properties–energy, duration, and rupture area–with larger magnitude fast and slow earthquakes. The small slow earthquake clusters tend to rupture faster when migrating in their preferred rupture direction, but inter-event rupture speeds are on average similar to those of larger slow earthquakes. This suggests that rupture speed does not necessarily control slow earthquake spatial extent or eventual magnitude.

Plain Language Summary

Slow earthquakes release a similar amount of energy as fast earthquakes but over a much longer time scale. While this difference makes them less dangerous to the public, their frequent occurrence signals that faults move almost continuously. In this study, we apply a technique to group seismic events in southwest Japan into clusters that represent slow earthquake phenomena. We use these slow earthquake clusters to investigate fault behavior in the long-term and their characteristics as compared to other slow earthquake phenomena. We found that the slow earthquakes are small in size, but their properties scale similarly to larger slow earthquakes and fast earthquakes in energy, duration, and ruptured area, suggesting that the slow earthquakes are fractal in nature. However, how fast the slow earthquakes rupture does not indicate how big they can become, in either their energy released or spatial spread.