近畿圏における大大特プロジェクトIの地下構造調査

Abstract

近畿地方の南北縦断測線(新宮―舞鶴測線)において，制御震源を用いた大規模な広角反射法・屈折法地震探査による大深度弾性波探査を行った。この結果，フィリピン海プレートの構造と地殻全体の速度構造が求められた。プレートの沈み込みは，近畿北部まで達しており，地域によってプレート面からの反射率が変化することがわかった。紀伊半島の南部で反射率が弱くなるところがあるが，これはプレート境界面のアスペリティ構造に関連する可能性がある。さらに，低周波地震発生域付近では反射波が部分的に見えなくなる。また，有馬高槻構造線を横断する測線で反射法探査を実施し，断層に沿ってグラーベン(溝)構造があることが明らかになった。震源断層が2重になることは強震動の予測に取り入れる情報として有用だと思われる。

A program was started by the Headquarters for Earthquake Research Promotion Japan in 2002 for the purpose of the reduction of seismic hazard in the metropolitan areas. As a part of this program, deep seismic profiling was begun in order to reveal the regional characterization of metropolitan areas, from 2002 as a five-year project. The final goal of the project is to produce a map of more reliable estimation of strong ground motions in the Kanto and Kinki areas. The surveys required for this project are accurate determinations of a source, a propagation path and a site response for earthquakes. More specifically, purposes of the surveys are to determine the accurate positions and geometry of source faults, subducting plates and mega-thrust faults, crustal structure, seismogenic zone, sedimentary basins, and 3D velocity structures. Reconstructions of source fault and velocity models leads to more realistic 3D estimation of strong motion seismic waves. The surveys were conducted in the fiscal year of 2002-2003, in the Kanto area. Deep seismic profiling was extensively carried out along the 4 lines. In the fiscal year of 2004, the survey area was shifted to the Kinki area. Surveys along two measure lines, Osaka-Suzuka (E-W) and Shingu-Maiduru (N-S) lines were conducted by ERI, Univ. of Tokyo and DPRI, Kyoto Univ. This is a very preliminary report for the Shingu-Maiduru (N-S) line of about 240km, including technical procedure of the survey. In the profile, 13 shots (100-700kg) of dynamite and 3 multivibrator (a few hundreds of sweeps) with 4 vibroseis trucks were used for refraction and wide-angle reflection surveys. More than 2000 seismometer stations were set at intervals of 50-100m all along the line. Clear first arrivals and reflections from the Philippine Sea plate and reflectors in the crust were obtained at almost of the stations. The reflections from the Philippine Sea plate are very clear beneath Kii Peninsula and are still visible under the northern Kinki district at depth of about 50-70km. A reflection survey was also carried out across the Arima-Takatuki Tectonic Line. The survey line is about 30km long and a graben-like structure of the fault was clearly revealed.