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1 Acta Geophysica Polonica

1 2002

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Analysis of deflection for identification of epicenter migration directions of swarm activity from Hida Mountains in Japan

Lasocki S., Kustowski B.

Acta Geophysica Polonica

2002

Vol. 50, nr 1

23-34

Migration trends of an earthquake swarm which occurred in volcanically active Hida Mountains in Japan in 1998 are studied by means of a recently developed technique called the analysis of deflection. The mutual spatial orientation of every two consecutive seismic events from a time-ordered seismic series is parameterized by the deflection angle of a straight line connecting their epicenters, measured from the N direction. The probability density function (PDF) of deflection is estimated from the sample of deflections using the nonparametric kernel method. Modes of the deflection PDF assign dominant migration trends and their modal values roughly indicate the strength of the trends. In average, for the whole period of the swarm activity, two preferable directions of migration have been identified. The dominant trend was close to the N direction and the secondary trend was close to the E-W direction. These directions are in agreement with local mechanisms of the majority of the events. For studying time changes of migration trends, the swarm is divided into 22 consecutive series and the analysis of deflection is performed separately for every series. The directional structure of the swarm turns out to be highly variable in time. Two rapid, nearly perpendicular changes of the dominant migration tendency have bean indicated. The first shift took place in less than 24 hours time. Its course can be derived from a detailed integrated interpretation of the results of analysis of deflection and epicenter distribution maps.