Characteristics of shallow tremor waveforms observed by distributed acoustic sensing using offshore fiber-optic cable at the Nankai Trough, southwest Japan

Distributed acoustic sensing (DAS) measurement, which uses a fiber-optic cable as a strain sensor, allows us spatially high-density observation than seismometers; therefore, DAS has been widely used for seismic observations recently. We conduct DAS measurement with an offshore fiber-optic cable off the Cape Muroto, along the Nankai Trough in southwest Japan. This area is a typical area with adjacent occurrences of slow and megathrust earthquakes. As the relationship between slow and megathrust earthquakes is pointed out (e.g., Obara and Kato, 2016), monitoring of slow earthquakes with a high resolution is necessary to understand tectonic conditions in subduction zones. Although many studies have observed regular earthquakes by DAS measurement recently, there are few studies which observed slow earthquakes with DAS. We observed shallow tremors, a type of slow earthquake in a frequency range of 2–10 Hz, by DAS measurement.

We detected 28 shallow tremor signals off Cape Muroto by using DAS from January 30 to February 8, 2022. The signals of these tremors were also observed in broadband seismograms of the Dense Oceanfloor Network system for Earthquake and Tsunami (DONET) data. We manually picked the arrivals of the tremor signals in root-mean-square envelopes of DAS and DONET waveforms and located the tremor events at the point where the residual between synthetic and observed arrival times is the least by the grid search. Synthetic travel times were calculated based on a one-dimensional S-wave velocity structure model representing the area near the Nankai Trough (Nakano et al., 2013). The tremors were located mainly around 135.7ºE and 33.8ºN, which corresponds to a subducted seamount peak indicated by Nakamura et al. (2022).

The amplitude of tremor signals in the frequency range of 2–10 Hz observed in a DAS channel is 1–2 nstrain. Assuming a plane wave, the velocity waveforms can be calculated by multiplying the apparent velocity by the strain waveform (e.g., Daley et al., 2016). The apparent velocity of the tremor signal propagation was estimated to be ~4 km/s in DAS data; therefore, the amplitudes of the velocities were estimated to be 4000–8000 nm/s, which is similar to or one order larger than that in broadband seismometers of the nearest DONET stations. In detail, tremor signals in DAS data are composed of several phases with variable apparent velocities, and these phases are coherent within only 50–100 m. Generally, the duration of tremors observed in the DAS channels (40–60 s) is longer than that observed in velocity waveforms of DONET broadband seismometers (30–50 s). Comparing the waveforms of regular earthquakes in DAS and ocean bottom seismometers (OBSs) at the same location in December 2019, we found that the duration of the regular earthquakes in the DAS strain waveform is also longer than that in the OBS velocity waveform. The difference in waveform characteristics may be caused by that in the sensitivity of the incident angle between DAS and OBS.