Shallow slow earthquake activity in the Nankai Trough after the 2024 M7.6 Noto Peninsula earthquake analyzed by distributed acoustic sensing data

Distributed acoustic sensing (DAS), one of the fiber-optic sensing technologies, measures the temporal change in strain or strain rate along a fiber-optic cable from the backscattering phase differences. Because DAS can measure spatial variation in strain along a fiber-optic cable with a meter-scale interval, DAS has been used for seismic observation recently. We have been conducting a DAS observation off the Cape Muroto in the Nankai Trough since January 2022. Off the Cape Muroto, the coseismic slip area of historical megathrust earthquakes and slow earthquake area is adjacent along the dip direction. Slow earthquakes can have the potential to trigger megathrust earthquakes; therefore, the investigation of slow earthquake activity is important to understand tectonic conditions on the plate boundary. Baba et al. (2023) detected shallow tremors, a type of slow earthquake observed in 2–8 Hz, by the DAS observation off the Cape Muroto. They located these tremors using DAS and broadband seismogram data from Dense Oceanfloor Network system for Earthquake and Tsunami (DONET). They located these tremors mainly around 134.7ºE and 32.8ºN, which is updip of a subducted seamount.

On January 1st, 2024, the M7.6 Noto Peninsula earthquake occurred at 16:10:22 (JST). After this earthquake, 13 tremors occurred off the Cape Muroto between 19:00 on January 1st and 4:00 on January 2nd. These tremors were observed by DAS data with a 120-km long fiber-optic cable and G-node stations of DONET. The signals of these tremors propagated from south to north and amplitudes are larger in the southern part near the trench of the DAS cable. We located these tremors based on the amplitude variation at each channel. For tremor activity in 2022, the maximum velocity amplitudes of tremors in DONET decreased by approximately 2.6 orders as the hypocentral distance increased by one order. We converted maximum strain amplitudes measured by DAS in tremors in 2024 to velocity amplitudes based on the amplitude difference between DONET and DAS at a similar hypocentral distance for tremors in 2022. Then we located the tremors in 2024 at the grid point where the attenuation by distance is closest to the case of tremors in 2022 by a grid search. As a result, tremors in 2024 were located in 134.2–134.6ºE and 32.3–32.5ºN near the Nankai Trough. In the future, we discuss the mechanism of the activation of these tremors after the Noto Peninsula earthquake.

This tremor activity after the Noto Peninsula earthquake was reported in a risk assessment by the Japanese government. Now we are constructing an earthquake monitoring system using DAS and DONET data in the Nankai Trough based on the automatic location of regular and slow earthquakes. In this presentation, we will also introduce the conception of this system.