Report on DELP 1989 Cruise in the TTT Junction Areas : Part 2: Upper Crustal Structure Near the Trench-Trench-Trench Triple Junction off the Boso Peninsula, Japan | 1989年度DELP海溝三重会合点海域調査研究航海報告 : 第2部:房総沖海溝三重会合点付近の上部地殻構造

1989年,房総沖海溝三重会合点付近において,上部地殻を精密に求めるため,エアガン-海底地震計(OBS)を用いた地震波探査が行われた. 22台のOBSを設置して,更に,気象庁の常時観測システムのOBS4台とあわせて,計26台でアレイ観測を行った.このうち3台は, 7000mを超える深海に設置された.長さ300km,巾80kmの調査海域に, 3本のエアガンの測線(全長450km)を設けた.東西に引かれた測線では,水深が数100mから9000mまで変化している.同時に,房総半島においても臨時観測点を5箇所設けた.

In July 1989 twenty-two ocean bottom seismographs (OBSs) from 5 Japanese institutions were deployed in an array on the deep sea floor in the vicinity of the trench-trench-trench triple junction off the Boso Peninsula, Japan. The array is 300 km long and 80 km wide, including 3 profiles for study of crustal structure. A permanent OBS array telemetered by a cable system (Japan Meteorological Agency) was used as a part of the array. The sea floor array was complemented by five temporary land stations on the Boso Peninsula. Water depth along the seismic profile varies considerably from a few 100 m to 9000 m or more. We placed three OBSs specially designed for the very deep ocean. We conducted airgun-OBS profiling for the refraction survey. Two 16-liter airguns were fired every 40-50s spaced about 100m apart. Airgun signals were observed as far as 30-40 km from the OBS. To obtain the sedimentary and shallow crustal structure we transformed seismograms of airgun shots into the intercept time (τ)-ray parameter (p) domain and inverted them in a P-wave speed structure with depth by the τ-sum inversion method. We derived one-dimensional models for all OBSs. Both the sedimentary thickness and the P-wave speed in the sedimentary layer change along the profiles. The difference in sedimentary structure between the oceanic side of the trench axis and the continental side is evident: the sediment on the pacific plate consists of a vertically homogeneous unconsolidated layer (0.32 s-1 of vertical wave speed gradient) and a consolidated layer with a large gradient of 4.8s-1, while on the Northeast Japan plate the gradient is 0.65s-1 in the topmost and 0.8s-1 in the lower part of the sediment.