Deep Seismic Surveys in the Kinki District : Shingu-Maizuru Line

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Seismic surveys of the crust and the upper mantle structure have been conducted under the Special Project for Earthquake Disaster Mitigation in Urban Areas (DAIDAITOKU). In 2004 deep seismic surveys were carried out in the Kinki district southwest Japan. Seismic refraction and wide-angle-reflection surveys were carried out along the Shingu-Maizuru line, which crosses Honshu Island from the Pacific side to the Japan Sea side nearly coincident with the direction of the subduction of the Philippine Sea plate (PHS). In the profile, 13 shots (100-100 kg) of dynamite and 3 multi-sweeps (a few hundreds of sweeps) with 4 vibroseis trucks were used as sources of the surveys. More than 2300 observation sites were set at intervals of 50-100m all along the line. Clear first arrivals and reflections from PHS and from reflectors in the crust were obtained at almost all the stations. In particular, waves from dynamite were well recorded at all stations. As for the stacked data of multisweeps, waves can be recorded in the offset range of 30-40km from the source. The multi-vibration method is particularly useful for the surveys in the urban areas, where dynamite cannot be used as a source. The reflections from PHS are very clear beneath Kii Peninsula and are still visible under the northern Kinki district at depth of about 50-10 km, where no earthquakes occur. The reflectors seem to indicate aseismic slab in the northern Kinki district. A clear two parallel reflectors dipping towards north associated with PHS are seen beneath Kii Peninsula. The lower reflector is coincident with the upper boundary of earthquake distribution in the mantle, which has been considered as the plate boundary. The boundary seems to be the oceanic Moho. The upper reflector is determined to be about 1-10km shallower than the lower one and this is the plate boundary of the subducting plate. The layer between the two reflectors is thought to be the oceanic crust subducting in the mantle. Low frequency earthquakes and5or tremors occur at 30-40km deep in the upper reflector, which is less distinct at the low frequency events. In the inland area, many clear reflections were obtained at depth of about 15 and 25 km. These reflectors decline towards north, which seem to relate with the large active faults. The reflector at depth of about 15km is well corresponded to the base of the seismogenic zone in the crust and seems to play an important role to the nucleation process of large inland earthquakes. Another reflector at depth of about 25km may possibly be the impermeable layer, which may cause pre seismic slip for large inland earthquakes.

Deep Seismic Surveys in the Kinki District : Shingu-Maizuru Line

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