Characteristics of Slow Slip Event in March 2020 Revealed From Borehole and DONET Observatories

Keisuke Ariyoshi,Takane Hori,Toshinori Kimura,Masaru Nakano,Yuya Machida,Kensuke Suzuki,Takehiro Nishiyama,Narumi Takahashi,Kentaro Sueki,Eiichiro Araki,Takeshi Iinuma,Shuichi Kodaira,Shuichiro Yada

doi:10.3389/feart.2020.600793

Abstract

We have detected an event of pore pressure change (hereafter, we refer it to “pore pressure event”) from borehole stations in real time in March 2020, owing to the network developed by connecting three borehole stations to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) observatories near the Nankai Trough. During the pore pressure event, shallow very low-frequency events (sVLFEs) were also detected from the broadband seismometers of DONET, which suggests that the sVLFE migrated toward updip region along the subduction plate boundary. Since one of the pore pressure sensors have been suffered from unrecognized noise after the replacement of sensors due to the connecting operation, we assume four cases for crustal deformation component of the pore pressure change. Comparing the four possible cases for crustal deformation component of the volumetric strain change at C0010 with the observed sVLFE migration and the characteristic of previous SSEs, we conclude that the pore pressure event can be explained from SSE migration toward the updip region which triggered sVLFE in the passage. This feature is similar to the previous SSE in 2015 and could be distinguished from the unrecognized noise on the basis oft-test. Our new finding is that the SSE in 2020 did not reach very shallow part of the plate interface because the pore pressure changes at a borehole station installed in 2018 close to the trough axis was not significant. In the present study, we estimated the amount, onset and termination time of the pore pressure change for the SSE in 2020 by fitting regression lines for the time history. Since the change amount and duration time were smaller and shorter than the SSE in 2015, respectively, we also conclude that the SSE in 2020 had smaller magnitude that the SSE in 2015. These results would give us a clue to monitor crustal deformation along the Nankai Trough directly from other seafloor observations.

Highlights

In the wake of the 2004 Sumatra-Andaman earthquake (e.g., Wiseman and Bürgmann, 2011), the Japanese government has established seafloor networks of cable-linked observatories around Nankai Trough (Figure 1): DONET (Dense Oceanfloor Network system for Earthquakes and Tsunamis along the Nankai Trough) which has sensors of broadband seismogram, strong ground motion, and hydraulic pressure (e.g., Kaneda et al, 2015; Kawaguchi et al, 2015)
The borehole observatories in holes C0002 (Figure 2), C0010, and C0006 were successfully connected to DONET: C0002 in January 2013 (Kitada et al, 2013), C0010 in April 2016 (Saffer et al, 2017), and C0006 in March 2018. This operation has enabled us to monitor crustal deformation along the Nankai Trough in real time. In the latter part of March 2020 (Figure 3), significant change of pore pressure and five shallow very low-frequency events (sVLFEs) were detected by the borehole site C0002 and broadband seismometers of DONET, respectively
We found the five sVLFEs between March 23 and 24, 2020 from the broadband velocity seismograms of the DONET stations along Nankai Trough

Summary

INTRODUCTION

In the wake of the 2004 Sumatra-Andaman earthquake (e.g., Wiseman and Bürgmann, 2011), the Japanese government has established seafloor networks of cable-linked observatories around Nankai Trough (Figure 1): DONET (Dense Oceanfloor Network system for Earthquakes and Tsunamis along the Nankai Trough) which has sensors of broadband seismogram, strong ground motion, and hydraulic pressure (e.g., Kaneda et al, 2015; Kawaguchi et al, 2015). This operation has enabled us to monitor crustal deformation along the Nankai Trough in real time Afterward, in the latter part of March 2020 (Figure 3), significant change of pore pressure (we refer to “pore pressure event”) and five sVLFEs were detected by the borehole site C0002 and broadband seismometers of DONET, respectively. This feature could not be explained from the oceanic loading.

25–36 Intermediate

18 March 14:00

Findings

DATA AVAILABILITY STATEMENT