Effects of Underground Installation of Seismometers on Three Dimensional Attenuation Structure Inversion

- Utilization of MeSO-net Data -

Abstract

In the Kanto district, Metropolitan Seismic Observation network (MeSO-net) of highly dense seismic network has accumulated continuous waveform data since 2008. It is expected that higher resolution of three-dimensional attenuation structures can be obtained by using these high-density observation records. However, seismometers of MeSO-net is installed at a depth of 20 m and there is a problem to handle each waveform as a record at a free surface. First, we examined characteristics of the spectrum of waveform records and it was found that troughs in the spectrum were formed due to underground installation. Next, we carried out simultaneous inversion of site amplification and three dimensional Q using MeSO-net, K-NET and KiK-net records. The stations are classified into eight groups based on the predominant period calculated from logging data for MeSO-net, K-NET and KiK-net stations were divided into two groups based on the predominant period estimated from AVS30 by J-SHIS. If the amplifications at all the observation points are added to unknown parameters, the trade-off will be caused with the attenuation structure. We grouped the stations to avoid this problem and assumed that the same group has a same amplification. As a result, the obtained attenuation structure was consistent with previous studies. It was confirmed that the effect of the underground installation could be obtained by properly considering the amplification factor. In addition, it was found that the amplification of the MeSO-net record is close to one at the free surface of hard rock site.