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

Abstract

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

Developing a Region-Specific Predictive Model for Shear Wave Velocity to Evaluate Seismic Site Response and Liquefaction Potential of Silty Soils

“…This novel dataset fills an important gap, as previous research in the study area has primarily focused on sandy soils [14] [15]. The consistent Gs values across the soil profile also provide valuable insights into the spatial homogeneity of the site, which is crucial for accurately assessing the seismic response and liquefaction susceptibility of the local soil conditions [36] [37]. The comprehensive Gs characterization, along with the subsequent analyses of shear wave velocity and liquefaction potential, offers a robust framework for geotechnical site assessment and hazard mitigation in the Niger Delta region.…”

“…Eurocode 8 requires soil-specific Vsliquefaction correlations considering age, cementation, and stress history [35]. Similar requirements exist in other codes [36] [37], evolving with new research. Vs can be measured by field tests or estimated from SPT, CPT, and shear wave logging.…”

Unified approach for evaluation of horizontal site amplification factors with special reference to history of studies on the effects of surface geology on seismic motion

“…If a borehole sensor is available, then a joint inversion of the EHVR and SBR can be performed. As shown in Nagashima and Kawase (2019), we can also determine the damping (Q values) of the layers above the borehole sensor depth in the joint inversion. The EHVR inversion method is significantly robust and more than 100 sites in Japan have verified its feasibility and efficiency (e.g., Ducellier et al 2013;Fukihara et al 2015;Kawase et al 2018).…”

“…19 Based on the distributed data of microtremors for the blind prediction experiment during the ESG6 symposium in 2021, we compare the observed MHVR (green), the converted pEHVR from MHVR (red), and the observed EHVR (blue) from aftershock records disclosed after the blind prediction on the left. On the right, we compare the resultant S-wave velocity profiles inverted for pEHVR (red) and EHVR (blue), shown in Nagashima et al (2023). The profile with an orange line is the preferred model distributed by the committee of the blind prediction (Matsushima et al 2021(Matsushima et al , 2022 constructed by the boring data down to 38 m and the deep structure proposed by Senna et al (2018).…”

“…The tables of EMRs with five categories, the detailed procedure for applying the EMR method, and the validation exercises are reported by Kawase et al (2018). The effectiveness of using the pEHVR in the inversion method of the EHVR proposed by Nagashima et al (2014) was presented (Nagashima et al 2023) in the blind prediction experiment Step 1 during the ESG6 (Matsushima et al 2021(Matsushima et al , 2022Chimoto et al 2021Chimoto et al , 2022. Comparison of the predicted (pEHVR) and observed EHVR from the aftershock records opened after the blind prediction and the resultant velocity structures based on the inversions from pEHVR and EHVR are shown in Fig.…”