Observation of Microtremors in the Tsukuba Area, Japan, using a Portable Broadband Seismometer

Studies of microtremors have been advanced by different approaches, that is, a variety of observational studies and analyses, for both the short-period and the long-period ranges since the microtremors for each range have their own source and site characteristics in time and space. We, therefore, conducted microtremor observation around Tsukuba City, Ibaraki, Japan, in July and August, 1991, in order to clarify site characteristics of six locations around the Tsukuba Mountain, deploying a portable broadband seismometer (Streckeisen STS-2) in the field of subsurface structure and studying ground motion in both frequency ranges, simultaneously. By a comparison with an STS-1 seismometer, the STS-2 gives reliable frequency ranges higher than 0.09 Hz and 0.05 Hz in horizontal and vertical components, respectively. The correlation of the reference site and the other sites implies that the source of microtremors shares common characteristics for the lower range (< 1 Hz) and changes temporally by human activities for the higher range (> 1 Hz), particularly 1.2 ～ 2.5 Hz, in this area. Three types of dominant peak frequencies for the range of 0.1 ～ 1 Hz are revealed: (1) the peak frequencies ranging from 0.2 to 0.3 Hz observed near Mt.Tsukuba can be explained by the topographic high model of Bard (1982). Two frequency peaks ranging (2) from 0.2 to 0.4 Hz and (3) from 0.5 to 0.8 Hz, observed at the stations on alluvials, are related to any vertical resonance in soft layers, consistent with other geological information. Amplitude ratios at sedimentary sites with respect to TSK (Mt.Tsukuba), a rock site, are greater than unity over 5 Hz where the ratios are reported to be smaller than unity in many areas, which implies relatively hard sedimentary layers in the Tsukuba area.