[S-CG55] Ocean Floor Geoscience

[SCG55-P21] Seismicity at an NTO between the first and second segments of the central Indian ridge using the matched filter method

Shiho Yamada¹, *Toshinori Sato², Tomoaki Yamada³, Masanao Shinohara³ (1.Graduate School of Science and Engineering, Chiba University, 2.Graduate School of Science, Chiba University, 3.Earthquake Research Institute, University of Tokyo)

Keywords:Central Indian ridge, NTO massif, Matched filter method

The first segment of the central Indian ridge, which extends north of the Rodrigues triple junction, contains a hydrothermal vent area called the Kairei hydrothermal vent field (KHF). To reviel the crustal structure and seismicity in this area, we conducted seismic surveys using ocen bottom seismometers (Sato et al. Tectonophysics, 2023). There is an area called the Yokoniwa Rise between the first and second segments of the ridge. At the Yokoniwa Rise, rocks that exist deep under the ocean are exposed on the ocean floor, making it topographically elevated (Okino et al.,2015). The Yokoniwa Rise is considered to be an NTO massif. NTO massif is a topographical uplift that can be seen near non-transform offsets (NTOs) between segments of slow-spreading ridge systems, where no transform faults are observed. It is said that multiple faults cause rocks deep underground to rise to near the ocean floor (Eulalia et al., 2000). In Sato et al. (Tectonophysics, 2023), natural earthquakes were detected manually by visual inspection. This method is limited in the number of earthquakes that can be detected, and many earthquakes may still exist. In this study, we focused on the region between the first and second segments of the ridge, where there were relatively few earthquake events, to detect more earthquakes and investigate the causes of the NTO massif.
The matched filter method was used to detect seismic events. The hypocenters of the detected events were determined using the hypoDD program. The focal mechanisms of the detected events were determined using the FPFIT program.
As a result, 1349 new events were detected from 268 template earthquakes. The number of seismic events whose hypocenter could be determined by hypoDD was 487, and the mechanism could be determined for 5. Looking at the hypocenter distribution, we found many earthquake events, especially near the east side of the first segment and the west side of the second segment. Considering that the NTO massif is uplifted and formed by multiple faults, it is consistent that more earthquakes are detected at the edge of the segments. Considering the mechanism determined by Sato et al. (Tectonophysics, 2023), left-lateral strike-slip faults and normal faults are predominant in this region.
Acknowledgments
We thank the captain and crew of JAMSTEC's R/V Yokosuka for their support.