[S-SS11] Strong Ground Motion and Earthquake Disaster

[SSS11-02] Long-term fluctuation of microtremor at a permanent observation site in Tottori University

*Takao Kagawa¹, Soma Nishigaki¹, Tatsuya Noguchi¹ (1.Tottori University, Faculty of Engineering)

Keywords:Microtremor, Long-term fluctuation, H/V, Natural phenomenon, Human activity

Long-term fluctuations of microtremors were analyzed using continuous seismic observation records installed on the premises of Tottori University. In recent years, systems that combine an accelerometer and a high-resolution recording system have become mainstream. It is important to verify the stability of the microtremor H/V spectrum ratio considering decrease in sensitivity of accelerometer especially at low frequencies. Therefore, it was verified by comparing with the records by a velocity type sensor. In low frequency band below 1 Hz, the velocity type sensor shows a stable peak for long-term, however the acceleration type sensor shows an unstable peak depending on the strength of the microtremor sources such as sea waves (see attached Figure). Due to the self-restraint of activities due to the COVID-19 from the spring of 2020, microtremor amplitude of horizontal components in the frequency band of 5 Hz or higher is smaller than the previous year on the premises of Tottori University. Microtremor amplitude also changed due to a shift in traffic flow caused by the opening of a new motorway near the observation site in May 2019. Since the stability of the peak of microtremor H/V spectral ratios captured by accelerometer in low frequency band is important for the study of ground motion characteristics, we would like to continue to analyze trends at other sites.