Depth-dependent behavior of deep low frequency tremor in Cascadia and Nankai

Episodic tremor and slip (ETS) is well observed in the transition zone along the subducting plate interface in Nankai and Cascadia. ETS activity is characterized by along-strike segmentation and depth-dependent properties. Recurrence behavior is different for each segment and is also controlled by the depth. Even within the same segment, tremor swarms occur more frequently with increasing depth (Wech and Creager, 2011; Obara et al., 2010). This is interpreted by depth-dependent interplate properties such as frictional strength and/or pore fluid pressure. In this presentation, the tremor activity in both regions is compared in terms of depth-dependent behavior by using tremor catalogs from NIED (Obara et al., 2010) and PNSN (Wech and Creager, 2008).

Depth-dependent behavior is simply investigated by dividing the tremors into shallow and deep parts. In Cascadia, infrequent and frequent tremor swarms in the shallower and deeper parts, respectively, are commonly observed in the whole region. In the shallower part, there are few inter-ETS tremor swarms; however, the deeper tremor activity seems to be rather constant associated with temporal variation including quiescence just after major ETS (Wech et al., 2010).

In Nankai, the depth-dependent behavior of tremor is generally similar to Cascadia; however, there are two differences. One is the existence of sweet spots, which are characterized by a spatial concentration of tremor locations within very short time intervals. The number of sweet spots is larger in the deeper part. Another is long-term variation of tremor mainly caused by long-term SSEs. This triggering phenomenon is only observed in the shallower part. Clear interaction between tremor and long-term SSEs are observed in the Bungo Channel. Moreover, slight activation of tremor is detected just before the occurrence of long-term SSEs. This might indicate a precursory slip in the ETS region affected by stress accumulation in the updip long-term SSE source fault.