Abstract
In our recent study, we detected the pore pressure change due to the slow slip event (SSE) in March 2020 at the two borehole stations (C0002 and C0010), where the other borehole (C0006) close to the Nankai Trough seems not because of instrumental drift for the reference pressure on the seafloor to remove non-crustal deformation such as tidal and oceanic fluctuations. To overcome this problem, we use the seafloor pressure gauges of cabled network Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) stations nearby boreholes instead of the reference by introducing time lag between them. We confirm that the time lag is explained from superposition of theoretical tide modes. By applying this method to the pore pressure during the SSE, we find pore pressure change at C0006 about 0.6 hPa. We also investigate the impact of seafloor pressure due to ocean fluctuation on the basis of ocean modeling, which suggests that the decrease of effective normal stress from the onset to the termination of the SSE is explained by Kuroshio meander and may promote updip slip migration, and that the increase of effective normal stress for the short-term ocean fluctuation may terminate the SSE as observed in the Hikurangi subduction zone.
Highlights
Inland dense networks of seismic and geodetic observations have revealed that slow earthquakes occur on the subduction plate boundaries in the shallower and deeper extensions of megathrust earthquake source regions worldwide (e.g., Schwartz and Rokosky, 2007; Obara and Kato, 2016)
For C0006, where pore pressure change is expected to be negligible due to the slow slip event (SSE) (Ariyoshi et al, 2021), it is quite stable in case of raw data of nearby DONET station and fluctuates in case of Pc0_raw for the reference seafloor pressure
Ii) We demonstrate that the reference seafloor pressures at boreholes can be substituted with nearby DONET seafloor pressure gauge data
Summary
Inland dense networks of seismic and geodetic observations have revealed that slow earthquakes occur on the subduction plate boundaries in the shallower and deeper extensions of megathrust earthquake source regions worldwide (e.g., Schwartz and Rokosky, 2007; Obara and Kato, 2016). Low-frequency tremor (LFT) and very low-frequency earthquake (VLFE) are detectable from seismometers because of their dominant frequencies of several hertz (Obara, 2002) and 10–100 s period (Ito et al, 2007), respectively. Slow slip events (SSE) are thought to release aseismic slip for days to years (e.g., Ide et al, 2007), which would be detected from geodetic observations. Dense observation networks have revealed the relations between SSE, VLFE and LFT activity. Some of SSE accompany VLFE and LFT as “episodic tremor and slip” (ETS) (Dragert et al, 2001; Obara and Kato, 2016), where SSE fault zones overlap the source regions of VLFE and LFT
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