Imaging shallow subsurface structures is important for urban development, including building design, use of underground spaces, and seismic simulation. In fault zone areas, ground rupture and the amplification of seismic motion can cause significant damage, and information on shallow structures is necessary to evaluate soil properties. In addition, the shallow subsurface S-wave velocity structure of a fault zone can be significantly heterogeneous (Chimoto et al., 2015). Taylor et al. (2019) used ambient noise to investigate the near-surface structure of the North Anatolian Fault Zone. The authors observed a strong seismic velocity gradient across the fault along the distributed acoustic sensing (DAS) line. Such velocity variations are also observed in the Xiaojiang fault zone (Liang et al., 2023).
On April 14, 2016, an earthquake of magnitude 6.5 occurred in Kumamoto Prefecture, Japan, at a depth of 11 km, according to Japan Meteorological Agency (JMA). Twenty-eight hours later, in the early morning of April 16, 2016, a large earthquake occurred at a depth of 12 km with a JMA magnitude of 7.3. Both earthquakes measured a maximum intensity of seven in the Kumamoto Prefecture and caused extensive damage to residents, housing, and infrastructure. The two earthquakes were caused by different faults (Fig. 1): the April 14 foreshock occurred at the northern end of the Hinagu Fault, which extends southwest from Mashiki City to the southern Yatsushiro Sea, and the April 16 mainshock occurred on the Futagawa Fault, which extends westward from the Aso region through Mashiki City to the Uto Peninsula. The foreshock of the Kumamoto earthquake ruptured mainly the northern part of the Hinagu fault. Laboratory experiments (Scholz, 2015) show that stress accumulation decreases the slope of the straight line in the Gutenberg-Richter relation (Gutenberg and Richter 1944). Nanjo et al. (2019) found that the southern Hinagu and Yatsushiro Sea sections, which showed lower b-values, were not ruptured by the two large Kumamoto earthquakes. Therefore, these areas have the potential for large earthquakes in the future. In particular, the Hinagu and Yatsushiro Sea sections are among the most active fault zones in Japan, with a 6% and 16% probability of earthquakes within the next 30 years, respectively (The Headquarters for Earthquake Research Promotion, 2024).
The Japan Seismic Hazard Information Stations (J-SHIS) were established in 2005 by the National Research Institute for Earth Science and Disaster Resilience (NIED) and has been operational since then (Fujiwara et al., 2006). The J-SHIS provides average S-wave velocities of the upper 30 m (AVS30) and the ground amplification factor is calculated using the microtopography classification data with a resolution of 250 m mesh. To determine the detailed structure around a fault, it is necessary to obtain actual observations and a more detailed velocity model.
Spatiotemporal observations using seismic networks have been common since the 2000s. Areal observations using a large number of spatially dense seismometers, such as the large-N array, have been developed later (e.g., Schmandt et al., 2013; Matsumoto et al., 2020). Microtremor surveys, which use arrays with several seismometers, have been conducted to estimate shallow structures. However, this method is labor-intensive and difficult to perform in urban areas. Recently, DAS, which uses fiber optic cables as sensors to measure the strain or strain rate in time series, has attracted attention as an observational method in geosciences (see Zhan, 2019 for a review). The DAS technique involves installing a measurement device, called an interrogator, at the end of a fiber-optic cable and injecting optical pulses into the cable from the device. The optical pulse is then scattered by the impurities in the cable (Rayleigh backscattering), and the backscattered wave returns to the interrogator. When the cable is subjected to vibrations, the phase of the scattered wave changes as the cable expands or contracts. The strain in this section is determined by measuring this change between two points along the cable. DAS has the advantage of using existing cables. This makes it easy to connect the interrogator to a fiber-optic cable and set the parameters, allowing observations to be conducted in urban areas and densely populated areas along national highways where observations using conventional seismometers are difficult. In addition, DAS enables ultra-high-density multi-point observations because the observation points are spaced several meters apart along several tens of kilometers of the cable. Owing to these advantages, DAS has been used to estimate inland fault locations (Atterholt et al., 2022), epicenters (Lentaset al., 2023), and subsurface structures using various microtremors (e.g., Song et al., 2021; Shao et al., 2022; Jiang et al., 2023; Cheng et al., 2023). Additionally, it has been used for fault zone imaging (Yang et al., 2022) and submarine cable studies (e.g., Lior et al., 2022; Shinohara et al., 2022). In volcanic regions, DAS has been used for the source studies of volcanic earthquakes (Klaasen et al., 2021), site characterization (Nishimura et al., 2021), and tomography studies of the volcanic basement (Biondi et al., 2023).
Several studies have applied seismic interferometry to DAS data to obtain velocity structures. Song et al. (2021) applied seismic interferometry to DAS observations conducted using fiber-optic cables running under two roads in China. The results showed that one road had a shallow velocity structure, whereas, for the other road, which had heavy vehicle traffic, the noise sources were distributed in a non-uniform and non-isotropic manner, making it impossible to extract surface waves from seismic interferometry. Song et al., (2022) applied three-station interferometry (TSI) in the above section to solve this problem. The TSI method could extract surface waves, and the shallow velocity structure at this road was clarified, although the noise source distribution was complex. Stehly et al. (2008) reconstructed the surface wave signal between two stations by calculating the cross-correlation between the coda waves of the noise cross-correlation function (NCF) obtained from the two stations and a third station (C3: Correlation of Coda of Correlation). By analyzing a continuous record of 150 stations, Froment et al. (2011) demonstrated that C3 can sufficiently suppress the effects caused by the distribution of anisotropic noise sources. TSI for direct waves was proposed by Curtis et al. (2010), which uses the entire NCF instead of the coda waves of the NCF, and the cross-correlation and convolution are calculated based on the locations of the virtual sources and receiver points, as described above. Qiu et al. (2021) improved the signal-to-noise ratio of surface waves extracted from the NCFs of a linear seismometer array using the TSI method.
In this study, DAS observations were conducted using fiber-optic cables laid underground along National Route 3 in Kumamoto. Seismic interferometry and the TSI methods were then applied to the observed data. Finally, surface wave and inversion analyses were performed to determine the shallow S-wave velocity structure of the national route along the Hinagu Fault.