P-wave velocity structure at a summit area of Kusatsu-Shirane volcano is investigated by using first arrival time data of a controlled seismic experiment conducted in 2003. The objectives of this study are to reveal a shallow P-wave velocity structure of Shirane pyroclastic cone and to make correlation of the resultant velocity model to geological information. The first arrival times for two dynamite shots recorded by 59 temporal seismic stations are used for investigating a two-layered velocity model. The first layer velocity is estimated by using arrivals at stations close to a shot point. The second layer velocity and depths of the interface are inferred by the time-term analysis. Estimated velocities of the first and second layers are 1.1 km/s and 2.9 km/s, respectively. Thickness of the first layer varies from almost 0 m at surface exposures of lavas to about 100 m at the summit of the pyroclastic cone. The first layer is interpreted as pyroclastic deposits covering the surface of the Shirane pyroclastic cone and the second is the uppermost subsurface lavas, by comparing to surface and borehole geological information and P-wave velocity of borehole core samples.

Key words:
Kusatsu-Shirane volcano, Shirane pyroclastic cone, controlled seismic experiment, P-wave velocity structure