Long-term SSEs in the Tokai Area, Central Japan, and Their Relationships Between Released Moment and Duration

Abstract:

The long-term slow slip event (LSSE) in the Tokai area, central Japan, had continued five years from mid-2000 to mid-2005 [Suito and Ozawa (2009)]. LSSEs detected along many subduction zones and were considered to have some relationship between the released moments and their durations [e.g. Schwartz and Rokosky (2007)]. The duration of the Tokai LSSE in the first half of 2000s, expressed 'the latest event' hereafter, is very long and it is interesting to find an answer whether the latest event in the Tokai area is anomalous or not. In order to reveal this question, I analyzed a leveling data from 1981 to 1999 -- before the era of GNSS -- and the length change data between 1974 and 1992.

The deduced rates show two sorts of spatial patterns and the patterns appear alternately. One of them resembles the pattern during the latest event. Summing up the results and accounting for the estimation errors, it is clear that there are at least three events in the last thirty years; 2000-2005 (already and well known event), 1987-1990, and around 1983. The 2000-2005 event is the biggest and longest one, while 1983 event is the smallest and shortest one in the three events. It suggests that the duration of the event is longer, the interval to the next event becomes longer, and that there are some relationships between the size, duration time, and the recurrence interval of the events.

The second event, from 1987 to 1990, made clearer signal in the leveling data than the the first one made. I examined a forward modeling for the second event, assuming uniform slip on a rectangular fault that located the same place as the latest SSE occurred on. The results of the modeling show that the amount of the slip during the second event was around 5 to 15 mm/yr and the corresponding released moment is about Mw 6.6, which was smaller than the latest SSE done. It suggests that the duration of LSSEs in the Tokai Area is longer than that was expected from the relationship between the released moment and the duration for the LSSEs in the other area.