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Extraction of Multimodal Surface Wave Dispersion Curves in Volcanic Area: Case Study on Mt. Ontake, Central Japan

Misaki Asai1, Yuta Maeda1, Toshiki Watanabe1

Abstract

Over the past quarter-century, ambient noise tomography (ANT) has been proposed for imaging subsurface seismic velocity structures. In ANT, extracting surface wave dispersion curves from the cross-correlation functions (CCFs) of ambient noise is an important process. However, artifacts in the dispersion energy plot and non-uniqueness in the inversion of subsurface structures are notable challenges. To solve these problems, the frequency-Bessel transform (F-J) method and high-resolution linear radon transform (HTLRT) have been developed. However, they have been examined only in flat fields with a scale of several hundred kilometers. To examine their applicability in volcanic regions where steep topography is common, we applied the F-J method to seismic data at Mt. Ontake, central Japan. We used 43 stations within a 50 × 50 km region centered on the summit of Mt. Ontake from January 1 to December 31, 2019. The maximum and average interstation distances were 42.0 km and 12.7 km, respectively. The calculation of the CCFs was successful for multiple components and showed the same tendency as that on flat land. The modified F-J spectrogram shows dispersive energy without cross-artifacts, but the second and third artifacts remain and overtones are unclear. Next, we apply the modified multicomponent F-J method and HTLRT to solve them. More constrained dispersive energy is expected by these processes; however, the effects of the topography and heterogeneity of a volcano should be considered. We plan to perform a simulation using synthetic data, including these conditions, to ensure the practicality of ANT in volcanic areas.