[S-SS11] Strong Ground Motion and Earthquake Disaster

[SSS11-08] Investigation of Deep Subsurface Structures in Central Wajima City, Ishikawa Prefecture, Using Microtremor Array Surveys

*Takumi Hayashida¹, Hiroto Nakagawa¹ (1.International Institute of Seismology and Earthquake Engineering, Building Research Institute)

Keywords:Microtremor, 2024 Noto Peniusula earthquake, Wajima City, Deep subsurface structures

During the 2024 Noto Peninsula Earthquake (Mj 7.6, Mw 7.5), Wajima City experienced extensive damage, particularly in urban areas such as the Kawai and Fugeshi districts. Microtremor surveys using miniature arrays conducted in the city center suggest the presence of soft to slightly soft sedimentary layers with S-wave velocities (Vs) below 200 m/s, extending from the surface to depths of several tens of meters (Hayashida et al., 2024 JAEE Annual Meeting). At the K-NET Wajima station (ISK003), located at the foot of a hill on the city's eastern side, the Vs in the shallow layer is larger than that in the city center, and the ground motion characteristics during the mainshock and aftershocks, including peak ground velocity and response spectra in the 1–2 s period range, tend to be slightly lower than the city center. However, there are concerns regarding the suitability of ISK003 as a reference site for sedimentary rock conditions (Kawase et al., 2024 JAEE Annual Meeting). To better understand the ground motion characteristics in this region, it would be crucial to consider the influence of deeper subsurface structures as well as shallow structures. In this study, we conducted microtremor array surveys with radii ranging from 110 m to 770 m to estimate the deep subsurface structure in the central urban area of Wajima City (Figure 1). The surveys employed four portable broadband seismometers (Guralp CMG-40T) and four geophones (McSEIS-AT) capable of measuring low-frequency bands. The Rayleigh wave phase velocity, estimated using the spatial autocorrelation (SPAC) method, was larger in the frequency range above 0.7 Hz in the hilly area in the west compared to values obtained in the city center (Horikawa et al., 2010). However, in the east of the city center, where ISK003 is located, no significant differences were observed in the 0.3–1 Hz range compared to the city center, suggesting minimal variation in the deep subsurface structure between the city center and the eastern area. In this presentation, we will further discuss the spatial variability of the deep subsurface structure by analyzing the spatial distribution of the microtremor horizontal-to-vertical spectral ratio (MHVSR) derived from single-point three-component microtremor records.