We mapped temporal seismic velocity variations during the Kumamoto earthquake due to fault rupture and a volcanic eruption.

Many significant geological features in central Japan reflect the history of tectonic events such as volcanic eruptions, fault ruptures, and the collision of the Izu-Bonin arc. Because most previous studies focused on deep, large-scale structures at relatively low resolution or on local structures using high-resolution data (e.g., active source seismic data), tectonic structures over a wide area throughout central Japan have not been characterized in detail. In this study, we estimated 3D S wave velocity structures with high spatial resolution by extracting surface waves from 1-year-long ambient seismic data recorded by the Hi-net high-sensitivity seismograph network. We computed cross spectra of ambient noise data between station pairs and then extracted phase velocity dispersion curves using a frequency domain method. We developed an algorithm to calculate phase velocity and obtained numerous dispersion data. We then estimated the 3D S wave velocity structure by applying a direct surface-wave inversion method. Our results clearly mapped heterogeneous features such as tectonic lines, sedimentary plains, and volcanic systems. We observed some low-velocity anomalies below volcanoes. Active fault zones identified as low-velocity zones were consistent with features on seismic reflection profiles. Several geological blocks that are related to the Izu collision were observed around the Izu Peninsula. We observed previously unreported geological features of the island of Honshu. Our high-resolution S wave velocity model can be used for not only interpretation of geological structures but also hazard assessment of earthquake.Plain Language Summary Techniques based on the speed of seismic waves are used to observe features of the Earth's interior in Japan, but most studies using them have focused on deep structures (tens or hundreds of kilometers deep) over a wide area or on local structures with high-resolution data. The tectonic structures over a wide area throughout central Japan have therefore not been well characterized. Studies of detailed, shallow structures typically use artificial seismic waves; however, these are difficult to do over wide areas due to the cost, safety considerations. In this study, we used only passive sources, generated by ambient (background) vibrations of the Earth, to extract details of the shallow crust and construct a three-dimensional geologic model based on seismic wave speeds in central Japan. Our results clearly mapped geological features such as tectonic lines, sedimentary plains, and volcanoes. An active fault zone was identified as a low-speed zone that agreed with features identified in previous studies using active sources. Several geological blocks were identified around the Izu Peninsula, and the features observed on our results reflected processes of the tectonic evolution. Our results can also be used to predict how earthquake waves propagate and thereby contribute to disaster evaluation.

Seismic ambient noise with frequencies > 1 Hz includes noise related to human activities. A reduction in seismic noise during the COVID-19 pandemic has been observed worldwide, as restrictions were imposed to control outbreaks of the SARS-CoV-2 virus. In this context, we studied the effect of changes in anthropogenic activities during COVID-19 on the seismic noise levels in the Tokyo metropolitan area, Japan, considering time of day, day of the week, and seasonal changes. The results showed the largest reduction in noise levels during the first state of emergency under most conditions. After the first state of emergency was lifted, the daytime noise reverted to previous levels immediately on weekdays and gradually on Sundays. This was likely because economic activities instantly resumed, while non-essential outings on Sundays were still mostly avoided. Furthermore, the daytime noise level on Sundays was strongly reduced regardless of changes on weekdays after the second state of emergency, which restricted activities mainly at night. Sunday noise levels gradually increased from the middle of the second state of emergency, suggesting a gradual reduction in public concern about COVID-19 following a decrease in the number of infections. Our findings demonstrate that seismic noise can be used to monitor social activities.

Seismic noise of frequencies >1 Hz includes noise that is strongly related to human activities. Reduction in seismic noise during the COVID-19 pandemic has been observed worldwide as restrictions were imposed on numerous human activities to control outbreaks of the virus. In this context, we studied the effect of reduced anthropogenic activities during COVID-19 on the noise levels in the Tokyo metropolitan area, Japan, considering seasonal variation. A significant reduction in noise was observed during the emergency, including that of frequencies >20 Hz, which was associated with school activities. After lifting the state of emergency, noise reverted to previous levels immediately for weekdays, but gradually for Sunday. This was likely because economic activities instantly resumed post-emergency on weekdays; however, most people still continued to avoid non-essential outings on Sunday. We also observed seasonal variation related to school holidays, energy consumption, and industrial activity. Noise levels in the frequency range of 1–5 Hz were found to be related to construction activity, which increased in winter and gradually decreased from 2017. Our findings demonstrate that seismic noise can be used to monitor economic activities and movement of people at a local scale.

Earthquake hazard assessment is vital to protect people and infrastructure facilities in the case of an earthquake, especially for densely populated metropolitan areas such as Tokyo, Japan, which is located in the earthquake-prone Kanto Basin composed of large-scale thick sedimentary formations (Denolle et al., 2018;Koketsu & Kikuchi, 2000;Sato et al., 2005). A major earthquake of magnitude 7 is predicted to have 70% possibility of directly hitting the Kanto Basin within the next 30 years (Cabinet office, 2013). The subsurface structure models are used to estimate damage due to such a major earthquake and make disaster prevention plan (Tokyo, 2022). Constructing an accurate subsurface structure is important for more accurate earthquake damage prediction and mitigating possible damage from future earthquakes. In this respect, an estimation of the S-wave velocity structure from the epicenter to the surface is essential for accurately assessing earthquake hazards (Denolle et al., 2014;Kano et al., 2017). Several studies on resource development and disaster prevention have contributed to the assessment of subsurface structures of the Kanto region (

Subsurface structure survey based on horizontal-to-vertical (H/V) spectral ratios is widely conducted. The major merit of this survey is its convenience to obtain a stable result using a single station. Spatial variations of H/V spectral ratios are well-known phenomena, and it has been used to estimate the spatial fluctuation in subsurface structures. It is reasonable to anticipate temporal variations in H/V spectral ratios, especially in areas like geothermal fields, carbon capture and storage fields, etc., where rich fluid flows are expected, although there are few reports about the temporal changes. In Okuaizu Geothermal Field (OGF), Japan, dense seismic monitoring was deployed in 2015, and continuous monitoring has been consistent. We observed the H/V spectral ratios in OGF and found their repeated temporary drops. These drops seemed to be derived from local fluid activities according to a numerical calculation. Based on this finding, we examined a coherency between the H/V spectral ratios and fluid activities in OGF and found a significance. In conclusion, monitoring H/V spectral ratios can enable us to grasp fluid activities that sometimes could lead to a relatively large seismic event.

Research interest in the Kinki region, southwestern Japan, has been aroused by the frequent occurrence of microearthquake activity that do not always coincide with documented active fault locations. Previous studies in the Kinki region focused mainly on deep, large-scale structures and could not efficiently resolve fine-scale (~ 10 km) shallow crustal structures. Hence, characterization of the upper crustal structure of this region at an improved spatial resolution is required. From the cross-correlation of the vertical components of the ambient seismic noise data recorded by a densely distributed seismic array, we estimated Rayleigh wave phase velocities using a frequency domain method. Then, we applied a direct surface wave tomographic method for the measured phase velocity dispersion data to obtain a 3D S-wave velocity model of the Kinki region. The estimated velocity model reveals a NE–SW trending low-velocity structure coinciding with the Niigata–Kobe Tectonic Zone (NKTZ) and the active Biwako-seigan Fault Zone (BSFZ). Also, we identified fine-scale low-velocity structures coinciding with known active faults on the eastern side of the NKTZ, as well as sets of low-velocity structures across the Tanba region. Furthermore, sedimentary basins manifest as low-velocity zones extending to depths ranging from ~ 1.5 to 2 km, correlating with those reported in previous studies. Our results therefore contribute towards fundamental understanding of earthquake faulting as well as tectonic boundary and will be useful for hazard assessment and disaster mitigation. Graphical abstract

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