Experimental observation of acoustic waves by a hydrophone above the focal area of the earthquake swarm.

Abstract

On August 30 1984, an earthquake swarm activity occurred at sea off the east coast of Izu Peninsula. An experimental observation of the acoustic wave of the swarm was carried out by the survey ship TENYO with a hydrophone at sea just above the focal area during the active period of the swarm. The hydrophone was hanged from the ship down to a deep water, to a few hundred meters above the seabottom. A large number of seismic events were recorded and their arrival times were measured precisely. The hypocenteres determined from the land observation data and additional hydrophone data were compared with the tentative hypocenters determined from only the land observation data by JMA (Japan Meteorological Agency). The comparison clearly pointed out that the hydrophone data had largely contributed to improve the accuracy of the determined hypocenters, especially in the accuracy of depth determinations. The most beneficial point of the hydrophone observation is its capability of real-time monitoring which allows evaluations of the activity of earthquake swarms on a real-time basis. The other significant advantage is its mobility which enables to keep the observation point within the area just close to the epicenters, even when they migrate considerably. These advantages seem to pave the way to establish a new method of real-time monitoring of earthquake swarms at sea.