Estimation of subsurface structure in the Shimabara Peninsula using microtremor H/V spectral ratio

In recent times, the estimation of long-period strong ground motions has become more important in the construction of large-scale buildings. From the contour map of peak period for long-period ground motions in Japan (the Central Disaster Prevention Council, 2008), it has been estimated that long-period ground motions are amplified not only in sedimentary basins like the Kanto Plain in the central Japan, but also in volcanic areas such as the Shimabara Peninsula in Kyushu, Japan. In particular, in the Yadake region of the Shimabara Peninsula, the long-period ground motions are amplified to the same extent as in the Kanto Plain. In order to estimate the ground structure in the Shimabara Peninsula using microtremor H/V spectra (horizontal-to-vertical spectral ratio), we carried out microtremor observations at 60 sites throughout the Shimabara Peninsula. Using data derived from these observation sites, we traced a contour map of primary natural peak period (the longest peak period that ranges from 1 to 10 s). Peak periods of 5–6 s in the H/V spectra were observed at many of the observation sites to the east of the Shimabara Peninsula, where thick volcanic sediments are distributed. It is thought that the thick volcanic sediment layer is the cause of such long peak periods in the H/V spectra. In the central western area of the Shimabara Peninsula, there are no remarkable peaks in the observed H/V spectra. According to explosion seismic research (Explosion seismic research group of Unzen Volcano, 1995), this area corresponds to a rock layer having Vp = 3.5 km/s; this is a solid lava layer that extends to the ground surface. This structure is reflected in the shape of the H/V spectra; in this region, the value of H/V spectral ratio remains nearly constant in the frequency of microtremors. We also estimated subsurface structures in the peninsula using the observed H/V spectra. Using P-wave velocity obtained from the explosion seismic research, S-wave velocity and density were calculated according to Ludwig et al. (1970). Using a trial-and-error estimation process, S-wave velocity, P-wave velocity, and density were fixed, and the thickness of the sedimentary layers was adjusted to find a reasonable fit between the primary natural peak period of the calculated H/V spectra and the observed H/V spectra in order to determine the ground structure. The depth to the Vs = 600 m/s layer is estimated as 1.2 km at boring Site USDP2 which lies to the east of the Shimabara Peninsula. Our result is consistent with borehole sample data. The horizontal component of long-period microtremors in the Yadake area commonly exceeds that in the central part of the Shimabara Peninsula. If the ground structure is determined using the same parameters as the surrounding sites, the depth to the basement at Site Yadake should be approximately 1000 m. However, local tectonic maps and the results of explosion seismic research do not indicate such a steep basin structure under Site Yadake. Therefore, we varied the S-wave velocity of the shallow part of the underground structure and found that a very low-velocity layer exists beneath the area surrounding Site Yadake. It is thought that this low-velocity layer is related to the existence of numerous hot-spring sources in and around Site Yadake and causes an increase in long-period strong ground motions

Three component seismic array observation at galleries in Nakatatsu mine, Fukui, Japan

We carried out seismic array observation in mining tunnels of Nakatatsu mine, Fukui, Japan. This observation site is located at about 20 kilometers away from the earthquake fault of Nobi earthquake (1891, M8.0). The major purposes in this observation are: 1) detection of small-scale heterogeneity around the fault; and 2) development of technique to explore structure by using seismic array data. Eighty seismometers which have three components with natural frequency of two hertz were deployed. The observation continued from September 2009 until the middle of February 2010. The sensors were connected to the recording system by cables. The data were converted to digital data by 24bit delta-sigma converter with maximum sampling interval of 1 ms and extracted from memory to storage by self-triggering algorithm in the system. The recording time is 16 – 64 sec. More than 100 earthquakes were recorded in the observation. We were able to obtain seismograms of not only local earthquakes but also distant earthquakes. The good environment for observation with hard rock site and low artificial noise allowed us to obtain high quality data. Some records of the earthquakes have adequate S/N ratios up to hertz. In addition, many scattered phases which could be generated by small-scale heterogeneity in the mid- to lower crust in this area were observed with sufficiently large amplitude

中竜鉱山における３成分地震計アレイ観測

We carried out seismic array observation in mining tunnels of Nakatatsu mine, Fukui, Japan. This observation site is located at about 20 kilometers away from the earthquake fault of Nobi earthquake (1891, M8.0). The major purposes in this observation are: 1) detection of small-scale heterogeneity around the fault; and 2) development of technique to explore structure by using seismic array data. Eighty seismometers which have three components with natural frequency of two hertz were deployed. The observation continued from September 2009 until the middle of February 2010. The sensors were connected to the recording system by cables. The data were converted to digital data by 24bit delta-sigma converter with maximum sampling interval of 1 ms and extracted from memory to storage by self-triggering algorithm in the system. The recording time is 16 – 64 sec. More than 100 earthquakes were recorded in the observation. We were able to obtain seismograms of not only local earthquakes but also distant earthquakes. The good environment for observation with hard rock site and low artificial noise allowed us to obtain high quality data. Some records of the earthquakes have adequate S/N ratios up to hertz. In addition, many scattered phases which could be generated by small-scale heterogeneity in the mid- to lower crust in this area were observed with sufficiently large amplitude