Phenomenology of deep slow earthquake family in southwest Japan: Spatiotemporal characteristics and segmentation

[1] The phenomenology of deep slow earthquakes, including low-frequency tremors, very low frequency earthquakes, and short-term slow slip events, on the subducting plate interface in southwest Japan is investigated on the basis of their spatiotemporal characteristics. The belt-like distribution of tremors is divided into segments bounded by gaps. The repetition of tremor episode depends on the magnitude of the episode, which is defined by the number of detected tremors within each episode. Major tremor episodes with a large number of detected tremors recur at an interval of approximately 6 months, accompanying the short-term slow slip events in large segments. In small segments, tremor episodes with small numbers of tremors have recurrence intervals of a few months. Some of them occasionally accompany the slight ground tilting caused by a small-size slow slip event. Even in the same segment, there exist different recurrence intervals according to the magnitude of the tremor episode. The migration pattern varies greatly in initiation, termination, direction, and speed; however, the migration direction has a general tendency for each segment. On both sides of the gap, the tremor starts next to the gap and propagates to the other side. Assuming all tremor episodes are caused by slow slip events, the equivalent moment and slip are estimated from the number of detected tremor counts and the area of each tremor episode. The equivalent slip history estimated from all tremor episodes is almost constant in each segment. This indicates that tremor activity is a good proxy for slow slips.