Three‐Dimensional S Wave Velocity Structure of Central Japan Estimated by Surface‐Wave Tomography Using Ambient Noise

다국어 초록 (Multilingual Abstract)

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.
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.


We estimated a 3D S wave velocity structure for central Japan using surface waves extracted from ambient noise
Our velocity model reflects small‐scale geological features and deformation over a wide area
Low‐velocity anomalies were observed that continue down the dip direction of active reverse faults