4:30 PM - 4:45 PM
[SSS11-05] Dense microtremor observation at Midori area of the Neodani fault
Keywords:Neodani fault, Midori area, microtremor observation, predominant period, peak of MHVSR
Dense microtremor observations with six seismometers (JU410 manufactured by Hakusan Corporation) were conducted at the surface fault displacement section of the Midori area of the Neodani Fault that passes through Motosu City, Gifu Prefecture. We carried out three-component observations at a total of 18 points: 6 points at 5-meter intervals in the vicinity of the surface fault, and 6 points each from north and south at 10-meter intervals from either end. Sampling was at 200Hz, and observations were conducted for approximately 10 minutes or more at each point.
The horizontal/vertical spectral ratio (MHVSR) is calculated from the observed data, and a heat map of the MHVSR arranged on the survey line is attached. On the north portion of the surface earthquake fault, there are a series of peaks with a period of about 0.1 seconds and a spectral ratio of about two, and on the south portion there are a series of clear peaks with a period of slightly shorter than 0.1 seconds and a spectral ratio of more than two. It can also be seen that the peak of this periodic band becomes unclear just above the surface fault. This is consistent with the results of previous research at the same location by Tsuji and Ohmachi (2001). Furthermore, the MHVSR near the surface earthquake fault showed azimuth dependence, with dominant vibrations in a specific direction around the peak period. These phenomena have also been reported on the Yoshioka-Shikano fault that appeared in the 1943 Tottori earthquake (Noguchi et al., 2025) and the surface earthquake fault in the 2016 Kumamoto earthquake (Kagawa et al., 2017). They are of interest when considering earthquake ground motion in the immediate vicinity of surface faults.
In order to estimate the typical ground structure on both portions divided by the surface fault, two L-shaped array observations of different sizes were carried out in front of Midori Station and in the parking lot of the Neodani Earthquake Fault Observation Museum, as shown in the figure, and phase velocity were derived. The short-period phase velocity, which corresponds to surface S-wave velocity, is 400 m/s at Midori Station on the north portion and 200 m/s at the Museum on the south portion, showing a remarkable difference. We would like to estimate the underground velocity structure at each array location and report on more detailed analysis results.
The Neodani Earthquake Fault Observation Museum allowed us to make the array observation in their parking lot. In order to estimate the phase velocity from observed array data, we used BIDOver3.2 developed by Dr. Cho I in AIST. This study was supported by JSPS KAKENHI Grant Number 24K071562.
REFERENCE: Tsuji and Ohmachi, 2001, JSCE, 26EES. Kagawa et al., 2017, JSCE A1. Noguchi et al., 2025, JAEE Journal.
The horizontal/vertical spectral ratio (MHVSR) is calculated from the observed data, and a heat map of the MHVSR arranged on the survey line is attached. On the north portion of the surface earthquake fault, there are a series of peaks with a period of about 0.1 seconds and a spectral ratio of about two, and on the south portion there are a series of clear peaks with a period of slightly shorter than 0.1 seconds and a spectral ratio of more than two. It can also be seen that the peak of this periodic band becomes unclear just above the surface fault. This is consistent with the results of previous research at the same location by Tsuji and Ohmachi (2001). Furthermore, the MHVSR near the surface earthquake fault showed azimuth dependence, with dominant vibrations in a specific direction around the peak period. These phenomena have also been reported on the Yoshioka-Shikano fault that appeared in the 1943 Tottori earthquake (Noguchi et al., 2025) and the surface earthquake fault in the 2016 Kumamoto earthquake (Kagawa et al., 2017). They are of interest when considering earthquake ground motion in the immediate vicinity of surface faults.
In order to estimate the typical ground structure on both portions divided by the surface fault, two L-shaped array observations of different sizes were carried out in front of Midori Station and in the parking lot of the Neodani Earthquake Fault Observation Museum, as shown in the figure, and phase velocity were derived. The short-period phase velocity, which corresponds to surface S-wave velocity, is 400 m/s at Midori Station on the north portion and 200 m/s at the Museum on the south portion, showing a remarkable difference. We would like to estimate the underground velocity structure at each array location and report on more detailed analysis results.
The Neodani Earthquake Fault Observation Museum allowed us to make the array observation in their parking lot. In order to estimate the phase velocity from observed array data, we used BIDOver3.2 developed by Dr. Cho I in AIST. This study was supported by JSPS KAKENHI Grant Number 24K071562.
REFERENCE: Tsuji and Ohmachi, 2001, JSCE, 26EES. Kagawa et al., 2017, JSCE A1. Noguchi et al., 2025, JAEE Journal.