5-10. Multidisciplinary research on deformation process of island arc crust

 

 Crustal study in Japan using controlled seismic sources was stared in 1950. ERI has been a responsible institute for this research activity. Results from this research have been referred as basic information of crustal structure in Japanese islands by researchers in various fields of geosciences.

  The physical mechanism of earthquake occurrence is controlled by a series of processes including stress accumulation by plate motion, stress concentration at fault zones, rupture nucleation and dynamic rupture propagation. ERI started a project of “the deformation process of the island arc crust” in the earthquake prediction program. The research of this project aims at elucidating the physical relation between crustal inhomogeneity and crustal activity by integrated seismic observations.

  In 1997-1998, an extensive seismic expedition was undertaken in Northern Honshu Arc. This expedition revealed a detailed crustal section seriously deformed by the Miocene back arc spreading of the Sea of Japan.

  A multidisciplinary project in 1999-2000 was intended to reveal various scale structural heterogeneity the Hidaka collision zone, Hokkaido, where the Kuril forearc (KA) has been collided to the Northeast Japan Arc (NJA) since Miocene (Fig. 1). A 227-km long refraction　profile was undertaken to determine the whole crustal structure from NJA to KA. In the eastern part of the profile (Fig.2). eastward dipping reflectors are imaged in a depth range of 10-20 km, probably representing obducting curst of KA. Beneath these reflectors, another almost flat reflector is situated at a 25 km depth. The reflective patterns obtained forms a wedge-like (crocodile) structure expressing the crustal delamination of KA. The western part of the profile, which belongs to the fold-and-thrust belt of the collision zone, is characterized by a very thick (more than 5-10 km) sedimentary package including two or more velocity reversals,  beneath which the crystalline crust of NJA is traced with a slight eastward dip down to 20-25 km.

  In 2001-2002, intensive onshore-offshore seismic refraction/wide-angle reflection expeditions were conducted under the cooperation with JAMSTEC. These experiments recorded very strong reflection from the subducted Philippine Sea Plate, which will provide important constraints on the structure and physical properties of the plate boundary. In SW Japan, a large and dense seismic array is operating to study crust and upper mantle structure and crustal inhomogeneity of inland seismogenic regions.

Fig. 1. Map of 1999-2000 experiments in Hokkaido. Stars and solid lines indicate shot points and profile lines of seismic survey, respectively.

 

Fig. 2. Crustal model from the seismic refraction experiment.