Shallow slow slip and associated VLFE observed by seafloor fiber optic strainmeter in the Nankai Trough.

Observations in boreholes have revealed that slow slip occurs repeatedly in the shallow part of the plate subduction boundary region in the Nankai Trough (Araki et al., 2017). To investigate its activity in wider area, we developed and installed a seafloor fiber-optic strainmeter connected to a submarine cable observation network (DONET) in west of the previous observation network in the borehole in 2019. Continuous observation is performed by the seafloor fiber-optic strainmeter to date.

In January-February, 2022, the seafloor fiber-optic strainmeter observed slow transient extension of about 0.7 μstrain over 3 weeks. Very low-frequency earthquakes (VLFEs) were observed in the vicinity during this period. The fiber-optic strainmeter was able to observe slow step-like changes for many of the VLFEs that occurred near DONET-2F node. For example, for a VLFE of Mw 4.4 determined by seismic waveform inversion of DONET seismometer records, the observed strain change by the fiber-optic strainmeter was about 20 n strain The observed strain change polarity and transitions were consistent with those expected from the focal mechanisms of the VLFEs, suggesting the possibility to investigate slip processes of each individual VLFE by seafloor fiber strain record.

We also consider that it is difficult to explain the amount of slow changes observed by the fiber optic strainmeter during the 3-week period with fault movement by these VLFEs in the same area. We propose that the slow strain variations may have been caused by a shallow slow slip that may have straddled the DONET-2F and 2G areas where the VLFEs were observed, and that the slow slip may have loaded individual VLFE faults into a burst-like cluster of VLFEs that are contemporaneous with the shallow slow slip. The fact that the active period of the VLFE is slightly later than the observed shallow slow slip also supports such a mechanism of VLFE generation.