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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS015 Poster presentation 2337
P-Wave velocity structures, reflectors and seismic activity from seismic
surveys in Kinki district, Southwest Japan
Prof. Kiyoshi Ito
Disaster Prevention Research Institute Kyoto University IASPEI
Issei Hirose, Yasuhiro Umeda, Hiroshi Sato, Naoshi Hirata, Tanio Ito, Susumu Abe,
Taku Kawanaka, Takeshi Ikawa
A large-scale seismic survey was conducted across the Kinki district from the Pacific to the Sea of Japan
(Shingu-Maizuru line) in 2004, under the Special Project for the Strong Motion Evaluation in the Urban
Areas. 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 along a line of 220km
long. More than 2200 seismometer stations were set at intervals of 50-100m all along the line. Clear
first arrivals and reflections from the Philippine Sea plate (PSP) and reflectors in the crust were obtained
at almost stations. Shallow P-wave velocity structure less than about 7km was well determined from
refraction analyses and the resultant velocity changes of the surface layer are well correspond to the
large-scale geological features. P-wave velocity structure of the whole crust and upper mantle was also
determined from refracted and reflected waves. The reflections from the PSP are very clear beneath Kii
Peninsula and are still visible under the northern Kinki district at depth of about 50-70km. Double
reflectors with related to the subducting PSP were well recorded and the lower reflector is well
coincident with the upper boundary of the mantle earthquakes. Therefore, the upper boundary, which
seems to be the plate boundary, is shallower than the depth that has been determined from earthquake
distributions by 7-10km. Besides, many north-dipping seismic reflectors in the lower crust in the middle
and southern Kinki district were detected and some of them seem to be connected to the large active
faults, such as the Median and the Arima-Takatsuki Tectonic Lines. In northern Kinki district, two near
horizontal reflectors of 15 and 25 km deep were found and the shallower reflector is coincident to the
base of the crustal seismicity. Thus the results are useful for the estimation of strong motion as well as
the nucleation process of the large earthquakes.
Keywords: seismic velocity structure, controlled source seismology, philippine sera plate