Subsurface structure identification at the blind prediction site of ESG6 based on the earthquake-to-microtremor ratio method and diffuse field concept for earthquakes

• Published in: Earth, Planets and Space Vol. 75; no. 1; pp. 35 - 13
• Main Authors: Nagashima, Fumiaki, Kawase, Hiroshi, Nakano, Kenichi, Ito, Eri
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Science and Business Media LLC 13.03.2023; Springer Berlin Heidelberg; Springer; Springer Nature B.V; SpringerOpen
• Subjects: 4. Seismology; Boreholes; Data logging; Diffuse field concept; Earth and Environmental Science; Earth Sciences; Earthquake prediction; Earthquake-to-microtremor ratio; Earthquakes; Effects of Surface Geology on Seismic Motion (ESG): General State-of-Research; Frequency ranges; G; Geodesy; Geography. Anthropology. Recreation; Geology; Geophysics/Geodesy; Horizontal-to-vertical spectra ratio; Methods; P waves; Peak frequency; QB275-343; QE1-996.5; S waves; Seismic activity; Seismic wave velocities; Seismic waves; Seismological research; Subsurface structure; Wave propagation; Wave velocity; Japan; Horizontal-to-vertical spectra ratio; Subsurface structure; Diffuse field concept; Earthquake-to-microtremor ratio
• ISSN: 1880-5981; 1343-8832; 1880-5981
• DOI: 10.1186/s40623-023-01791-x

We participated in the blind prediction exercise organized by the committee of the blind prediction experiment during the 6th International Symposium on Effects of Surface Geology on Seismic Motion (CBP-ESG6). In response to the committee's request, we identified the ground velocity structure from microtremors observed at a target site as the first step of the exercise. First, we calculated the horizontal-to-vertical spectral ratio of microtremors (MHVR) at the target site from the distributed microtremor data collected in the vicinity of the target site in Kumamoto Prefecture. Then, we converted the MHVR into a pseudo horizontal-to-vertical spectral ratio of earthquake (pEHVR) using the previously proposed and validated earthquake-to-microtremor ratio (EMR) method, where an empirically obtained EMR is used to convert MHVR into pEHVR. Next, we inverted the S-wave and P-wave velocity structures based on the pEHVR and the diffuse field concept for earthquakes. The theoretical EHVR calculated from the identified velocity structure reproduced the pEHVR quite well in the frequency range of 0.1–22 Hz. After the collection of the blind prediction results by all the participants, the CBP-ESG6 released the observed earthquake records, a preferred model based on the P–S logging data from the in-situ borehole measurement combined with the generic deeper structure, and the average of all the predicted structures by the participants. Notably, our inverted structure was found to be close to the preferred model and the averaged one of all the blind prediction participants, despite some minor differences in the horizontal site amplification factor around the maximum peak frequency at 0.8–1 Hz. Graphical Abstract