Long-term ocean bottom seismic array observation at the Chile Triple Junction

The Chile Triple Junction (CTJ) locates in southern Chile off the Taitao Peninsula, where the active Chile ridge is subducting eastward at the Chile trench beneath the South American plate (SA). Its anomalous heat of this ridge subduction would cause some affect in the mantle and crust of the SA above. To study crustal activities associated with subduction of the active ridge and seismic structures, we have conducted seafloor seismic array observations twice around the CTJ. In the first one, 5 long-term ocean bottom seismometers (LTOBS) were installed from 2009 to 2010 with about 30 km spacing. Both of micro-seismic activities near the ridge axis and the Darwin fracture zone (Shinohara et al. 2010) and non-volcanic tremors along the ridge axis (Sáez et al. 2019) were found. We conducted the second observation from 2019 to 2021 with shorter spacing so as to constrain the hypocenter depth precisely. This presentation introduces the outline of the second observation and preliminary results of seismic activities.

We used 8 broadband ocean bottom seismometers (BBOBSs) and 5 LTOBSs, and 2 BBOBSs equipped with the differential pressure gauges (BBOBSPs). We deployed 13 OBSs in January 2019 using the R/V "Mirai". The average spacing was about 10 km, and the observation period was set as two years. The recovery was carried out in January 2021 by the Chile Navy's patrol boat "Cirujano Videla". Although all 13 OBSs were successfully released from the seafloor, one BBOBS was lost unfortunately during recovery due to sudden stormy weather in the dark. Finally, we retrieved almost full of two-years-long seismic data from 12 OBSs.

Local earthquakes were detected by the STA/LTA method, then we manually picked the P- and S-wave arrivals and determined hypocenters using a 1-D velocity model. In the past 10 years, only dozens of earthquakes were detected by the global seismic network. However, we could determine over 2000 small earthquakes in the two-years observation, including three M4 class events. Hypocenters show a systematic difference in northern and southern areas bounded at 46.4°S, where looks as a seismicity gap. Based on this earthquake distribution, we suggest a key control of the hot ridge subducting at this location. A detailed study of the focal mechanism solutions will carry out to analyze the seismic activity around the CTJ further.