5:15 PM - 6:30 PM
[SSS06-P03] Aftershock activity of the 2015 M7.1 earthquake and back-arc rifting in the northern Okinawa Trough
Keywords:OBS observation, the northern Okinawa Trough, back-arc rifting, aftershock activity
A magnitude (M) 7.1 earthquake occurred on November 14, 2015 at the western margin of the northern Okinawa Trough (OT). The northern OT is assumed to be in an incipient stage of the back-arc spreading [e.g., Iwasaki et al., 1990; Nakahigashi et al., 2004] and the mainshock–aftershock sequence of the 2015 event might be caused by the back-arc rifting. Although the aftershock activity seems to be distributed along the transition area from the East China Sea shelf to the OT [Nishizawa et al., 2019], their focal depths were not well constrained because of the onshore seismic station coverage. In order to better understand the tectonic process which controls the aftershock sequences, we precisely relocated their hypocenters and determined focal mechanisms using long-term ocean bottom seismometer (LOBS) data.
We used data from seven onshore stations operated by Kagoshima University and Kyushu University and five LOBSs. We selected the aftershocks equal to or larger than M2.0 (M2.5 in part) in the Japan Meteorological Agency (JMA) catalog data from April 12, 2016 to April 16, 2017. Initial hypocenters were determined by HYPOMH program [Hirata and Matsu’ura, 1987] with manual picks of the P and S phases. Station corrections for the LOBSs were calculated using a time difference between P- and PS-converted wave arrivals for intermediate-deep earthquakes which occurred around Satsuma Peninsula at depths of 80–180 km. We applied the station correction values for all stations to both P and S arrivals, and then obtained the 453 final hypocenters. Due to the station correction, the mean of median values of travel-time residuals for P and S waves decreased from -43 msec to -1 msec and from 421 msec to -158 msec, respectively. We also determined 122 focal mechanisms based on estimating the strike and dip of the two nodal planes satisfying P-wave first-motion polarities.
Final relocated hypocenter distribution shows sharp linear trends, while the JMA one shows two broad clusters. From our results, we can clearly recognize the aftershock activity as three earthquake alignments: a 70-km-long trend along the NNE-SSW direction (Segment A), a 30-km-long trend along the WNW-ESE direction (Segment B) located on the north of Segment A, and a 20-km-long trend along the NNE-SSW trend (Segment C). The relocated focal depths are mainly distributed around 10 km in Segments A and B, and in the range of 0–15 km in Segment C, while the JMA ones are widely distributed within the range of 0–20 km. In Segment A, the spatiotemporal variation of seismicity indicates that the aftershocks seem to migrate from south to north, and most of the focal mechanisms show normal-fault types with NW-SE tension axes. In Segment B, the seismicity is relatively steady during the observation period, and left-lateral strike-slip-fault types with NW-SE tension axes are largely confirmed. In Segment C, an earthquake swarm activity started about half a year after the mainshock and lasted for about two months, and normal-fault types with NW-SE tension axes are dominant.
The activities in Segments A and C are mainly concentrated in the upper crust. These aftershocks might be caused by crustal extension related to the back-arc rifting in the northern OT because their focal mechanisms and the directions of the alignments are consistent with the existing normal faults imaged by the MCS survey [Nishizawa et al., 2019]. The left-lateral strike-slip faulting type events in Segment B could be explained by along-axis variation in the back-arc spreading rate and presence of a transcurrent fault [Kakuta and Goto, 2002].
We used data from seven onshore stations operated by Kagoshima University and Kyushu University and five LOBSs. We selected the aftershocks equal to or larger than M2.0 (M2.5 in part) in the Japan Meteorological Agency (JMA) catalog data from April 12, 2016 to April 16, 2017. Initial hypocenters were determined by HYPOMH program [Hirata and Matsu’ura, 1987] with manual picks of the P and S phases. Station corrections for the LOBSs were calculated using a time difference between P- and PS-converted wave arrivals for intermediate-deep earthquakes which occurred around Satsuma Peninsula at depths of 80–180 km. We applied the station correction values for all stations to both P and S arrivals, and then obtained the 453 final hypocenters. Due to the station correction, the mean of median values of travel-time residuals for P and S waves decreased from -43 msec to -1 msec and from 421 msec to -158 msec, respectively. We also determined 122 focal mechanisms based on estimating the strike and dip of the two nodal planes satisfying P-wave first-motion polarities.
Final relocated hypocenter distribution shows sharp linear trends, while the JMA one shows two broad clusters. From our results, we can clearly recognize the aftershock activity as three earthquake alignments: a 70-km-long trend along the NNE-SSW direction (Segment A), a 30-km-long trend along the WNW-ESE direction (Segment B) located on the north of Segment A, and a 20-km-long trend along the NNE-SSW trend (Segment C). The relocated focal depths are mainly distributed around 10 km in Segments A and B, and in the range of 0–15 km in Segment C, while the JMA ones are widely distributed within the range of 0–20 km. In Segment A, the spatiotemporal variation of seismicity indicates that the aftershocks seem to migrate from south to north, and most of the focal mechanisms show normal-fault types with NW-SE tension axes. In Segment B, the seismicity is relatively steady during the observation period, and left-lateral strike-slip-fault types with NW-SE tension axes are largely confirmed. In Segment C, an earthquake swarm activity started about half a year after the mainshock and lasted for about two months, and normal-fault types with NW-SE tension axes are dominant.
The activities in Segments A and C are mainly concentrated in the upper crust. These aftershocks might be caused by crustal extension related to the back-arc rifting in the northern OT because their focal mechanisms and the directions of the alignments are consistent with the existing normal faults imaged by the MCS survey [Nishizawa et al., 2019]. The left-lateral strike-slip faulting type events in Segment B could be explained by along-axis variation in the back-arc spreading rate and presence of a transcurrent fault [Kakuta and Goto, 2002].