3Dトレンチ地質情報を用いた数値地質モデルの構築と可視化

抄録

We proposed a new method on 3D geological modeling and visualization of strike-slip fault structure using previously reported three-dimensional trenching data. The trenching data was acquired from the survey on the Tanna fault of the Northern Izu active fault system, which generated the Kita-Izu earthquake in 1930 AD. The geological information exposed on the walls was recorded as the logs with boundaries of sedimentary units and faults in the field, and they are converted to raster graphics with RGB color information and geographical coordinates on the computer. The raster information was extracted as point cloud data in rectangular coordinate system, then 2-cm-grid depth data for sedimentary unit boundaries and fault planes were produced with complementary spline methods. Based on this grid data, surface modeling for both sedimentary unit and fault planes was carried out using ArcGIS and its extensions. In order to distribute these 3D geological models and animations for readers, we converted them to U3D format files and PDF files which have high coverage. As a result, we could successfully re-construct three-dimensional geological structure and re-confirmed the existence of penultimate paleoearthquake event prior to the 1930 Kita-Izu earthquake on the Tanna fault. Since the penultimate event occurred ca.500-600 y. B. P and had shorter recurrence interval, the event had been considered as low-reliability. However, the fact that we could re-identify the existence of this event arises a possibility of future surface deformation on the Tanna fault associated with its activity or with triggered slip by adjacent seismic sources. Because the tunnel of railway line penetrates into the Tanna fault, this possibility is not ignorable. Thus, our proposed method for modeling and visualization of three-dimensional trenches is useful for quantitative analysis and representation of more objective and practical geological data.