9:45 AM - 10:00 AM
[HQR05-04] Mapping of concealed bedrock rises in the northwestern adjacent zone of Mt. Tsukuba by means of dense H/V measurements
Keywords:Single station microtremor measurement, H/V spectral ratio, Near surface geology, Kanto Plain, Mt. Tsukuba
We conducted dense microtremor measurements in the northwestern adjacent zone of the Tsukuba Mountain for estimating near-surface 3-D S-wave velocity structure with special focus on the delineation of concealed bedrock rises at the near surfaces. The field surveys consisted of single point microtremor measurements up to 300 points to calculate H/V spectral ratio (HVSR) curves, and small array microtremor surveys to derive dispersion curves. Surface geography of the surveyed area, 6 km long and 6 km wide, is mainly classified into two terraces of different stages and two lowlands along Sakura-gawa and Kan-non-gawa streams. It is characteristic that a small stock of granitic rock, 80 m long by 50 m wide, exposes at Miya-yama in the surveyed area surrounded by terrace sediments. Similar granitic rocks outcrop widely on the northwestern flank of Mt. Tsukuba, and the riverbed of Sakura-gawa. The major purpose of our survey was to clarify whether the Miya-yama granitic stock is a hummocky hill or the basement rock.
Characteristic HVSR curves having peaks in relatively high-frequency portions were identified at the flank of Mt. Tsukuba and the northwestern extent, in addition at the surrounding zone of the granitic stock. The geotechnical drill database presented by Ibaraki Prefectural Government was useful to make 1-D velocity models from the HVSR curves. Dispersion curves derived from small array data were also utilized to make Vs models. Finally, 3-D near surface velocity structure was reconstructed with aid to neural network analysis. It successfully delineated the buried bedrock rises around Miya-yama and the northwestern extent of a ridge of Mt. Tsukuba.
Characteristic HVSR curves having peaks in relatively high-frequency portions were identified at the flank of Mt. Tsukuba and the northwestern extent, in addition at the surrounding zone of the granitic stock. The geotechnical drill database presented by Ibaraki Prefectural Government was useful to make 1-D velocity models from the HVSR curves. Dispersion curves derived from small array data were also utilized to make Vs models. Finally, 3-D near surface velocity structure was reconstructed with aid to neural network analysis. It successfully delineated the buried bedrock rises around Miya-yama and the northwestern extent of a ridge of Mt. Tsukuba.