9:00 AM - 9:15 AM
[SCG39-19] Systematics and heterogeneity of deep and shallow slow earthquakes
Keywords:Slow earthquake, tremor, very low frequency earthquake, slow slip event
During the last two decades from the first discovery of slow earthquakes in southwest Japan around 2000, many types of slow earthquakes associated with different source depths and time scales have been discovered in such a way to fill gaps in space and spectral domains because of deployment of observation networks, continuous monitoring, and development of analyzing technology. As a result, composition of slow earthquakes in shallow and deep parts became common at present although different at the beginning. On the other hand, more quantitative and comprehensive analyses revealed more heterogeneous activity pattern of slow earthquakes rather than homogeneous. In this presentation, I would like to review the slow earthquake activity from points of view on systematics and heterogeneity.
Slow earthquakes in the Nankai trough region are separated into shallow and deep sides of the huge earthquake seismogenic zone along the subducting Philippine Sea plate interface. Deep slow earthquakes are systematically separated into Episodic Tremor and Slip (ETS), which is composed of tremor, very low frequency earthquake (VLF), and short-term slow slip event (SSE) at the deepest part and long-term SSE in between ETS and locked zones. Long-term SSEs have been detected in some places in this intervened zone as to fill the spatial gap recently although originally detected at two limited areas. These deep slow earthquakes are distributed at their own depths along-strike horizontally separated into some segments with different spatiotemporal pattern. This suggests that the type of slow earthquakes is systematically controlled by the environment as a function of depth.
On the other hand, the distribution pattern of shallow slow earthquakes is different from those of deep slow earthquakes. Although shallow VLF were originally detected at some spatiotemporal clusters along the Nankai trough, very recently shallow SSE, VLF and tremor has been detected simultaneously southeast off Kii Peninsula. This is an evidence for the occurrence of ETS in shallow side. Moreover, a long-term SSE was detected by acoustic GNSS measurement system near the Nankai trough. As a result, we have a common set of slow earthquakes in both shallow and deep sides; however, detail distribution pattern is different. Near the Nankai trough, shallow ETS and long-term SSE are aligned along-strike on the same depth contour line of the subducting plate. This indicates along-strike strong heterogeneity including different types of slow earthquakes and also strong/weak coupling portions. This along-strike heterogeneity is similar to the Japan trench.
Regarding spatiotemporal activity of slow earthquakes, there are two characteristic properties: migration and recurrence. Migration of ETS including slow speed main front migration and Rapid Tremor Reversal is common for shallow and deep sides. On the other hand, recurrence property is different. Deep ETS indicates beautiful periodic pattern at each segment; however, there is no clear periodicity in shallow ETS activity. This might be interpreted that shallow strong heterogeneity affects interaction between different neighboring phenomena and disturbs regular recurrence behavior. However, migration indicating rupture propagation of slow slip event is interpreted as a common process after initiation in both shallow and deep sides.
Slow earthquakes in the Nankai trough region are separated into shallow and deep sides of the huge earthquake seismogenic zone along the subducting Philippine Sea plate interface. Deep slow earthquakes are systematically separated into Episodic Tremor and Slip (ETS), which is composed of tremor, very low frequency earthquake (VLF), and short-term slow slip event (SSE) at the deepest part and long-term SSE in between ETS and locked zones. Long-term SSEs have been detected in some places in this intervened zone as to fill the spatial gap recently although originally detected at two limited areas. These deep slow earthquakes are distributed at their own depths along-strike horizontally separated into some segments with different spatiotemporal pattern. This suggests that the type of slow earthquakes is systematically controlled by the environment as a function of depth.
On the other hand, the distribution pattern of shallow slow earthquakes is different from those of deep slow earthquakes. Although shallow VLF were originally detected at some spatiotemporal clusters along the Nankai trough, very recently shallow SSE, VLF and tremor has been detected simultaneously southeast off Kii Peninsula. This is an evidence for the occurrence of ETS in shallow side. Moreover, a long-term SSE was detected by acoustic GNSS measurement system near the Nankai trough. As a result, we have a common set of slow earthquakes in both shallow and deep sides; however, detail distribution pattern is different. Near the Nankai trough, shallow ETS and long-term SSE are aligned along-strike on the same depth contour line of the subducting plate. This indicates along-strike strong heterogeneity including different types of slow earthquakes and also strong/weak coupling portions. This along-strike heterogeneity is similar to the Japan trench.
Regarding spatiotemporal activity of slow earthquakes, there are two characteristic properties: migration and recurrence. Migration of ETS including slow speed main front migration and Rapid Tremor Reversal is common for shallow and deep sides. On the other hand, recurrence property is different. Deep ETS indicates beautiful periodic pattern at each segment; however, there is no clear periodicity in shallow ETS activity. This might be interpreted that shallow strong heterogeneity affects interaction between different neighboring phenomena and disturbs regular recurrence behavior. However, migration indicating rupture propagation of slow slip event is interpreted as a common process after initiation in both shallow and deep sides.