Crustal structure across the Kurile Trench off south-eastern Hokkaido by airgun-OBS profiling

Abstract

Summary. Seismic refraction profiles were shot across the Kurile Trench, south-east of Hokkaido, Japan. The experiment in a typical island arc-trench system, where the Pacific Plate subducts beneath Hokkaido, was designed to investigate the transition of crustal structure from oceanic basin to continental slope across the trench. High resolution data were obtained from a 9 litre airgun and multi-OBS system. They were processed through a series of operations: digitization, filtering, slant stacking, t-sum inversion, two-dimensional ray tracing, and comparison with synthetic seismograms. A typical oceanic structure composed of an unconsolidated sedimentary layer (P wavespeed of 1.8–2.2 kms−1, thickness of 0.5–1 km), layer 2 (3.1–5.8 kms−1, 1–2 km) with a large wavespeed gradient, and relatively homogeneous layer 3 (6.0–6.8 kms−1, 5 km) is found on the oceanic side of the trench. Observations of shear waves converted at the boundary between the sedimentary layer and layer 2 gave compressional to shear wavespeed ratios of about 7.0 for the topmost sedimentary layer and 1.73–1.77 for layers 2 and 3. The transition of structure occurs slightly landward of the trench axis and thick low wavespeed materials (P wavespeed of 2.5–4.7 km s−1) persist beneath the continental slope. The density profile derived from our wavespeed structure is consistent with published free-air gravity anomaly data. The general crustal structure in the southern part of the Kurile Trench is similar to that of the Japan Trench off Sanriku. However, the distribution of the thick low wavespeed materials on the continent side differs between the two trench regions. These materials are more abundant in the Kurile Trench region, as is also inferred from the gravity anomaly pattern. The variability of the gravity anomaly on the continental side along the trench axis suggests that the distribution of these materials changes even within a single trench system. The seismic activity inferred from S–P time distribution at each OBS is high near the trench, and low in the oceanic basin beyond about 30 km from the trench axis as well as on the continental side where thick low wavespeed materials exist. An activity which seems to occur exactly in the narrow transition zone dividing the typical oceanic crust and the thick low wavespeed materials in the continental side structure is found.