東海沖海底地震計 (TKOBS) による観測結果

抄録

Observations of the four permanent ocean bottom seismograph station (TKOBS) operated off the south coast of Tokai area, Central Japan are summarized here. Earthquake detection capability of local earthquake at each station is identical to those of sensitive land station, although wind generating micro-tremor and artificial noise due to ships disturbed the observation sometimes. Events of magnitude 3 (MJ) are detectable at ranges of up to 150km from the system.
Seismograms from near events resemble to those obtained at land station. However, features of seismogram from local events are generally much different. Separation of compressional waves on the vertical component from transverse waves on the horizontal components are so prominent that it is almost impossible to see S phase onset on the vertical record, on the other hand, P phase onset on the horizontal seismogram always indicates delay of few tenth to 2 second.
The seismograms especially from deep focus events are more impulsive for both P and S phases than those obtained at land stations. These features of seismogram imply that crustal structure under the station may be less heteorogeneous than those of land station.
Because of reflection from the sea surface the seismograms are much more reverberatory than those obtained on land-based system. In spite of these differences of seismogram in wave forms, observed wave amplitude is almost same as those obtained at land stations.
The four stations show prominent travel time anomalies up to 2 seconds for both P and S phases. P arrivals at TK1 station are generally 1sec faster than those expected from JMA standard travel time table. As distance from station to the coast becomes shorter, P arrival becomes later and delays about 1 second at the TK4 station. These travel time anomalies may be qualitatively explained by the shape of crustal structure that the Moho discontinuity becomes shallow and thickness of crustal layer decreases as continental crust approach the trench.
Micro-earthquake activities which are not detectable from land stations around the TKOBS are located. Earthquake swarm often occurs around the north eastern margin of aftershock area of Tonankai earthquake (1944) and micro earthquake occurs sometime in the region between the western margin of Nankai-Suruga trough and the TK3 station. Seismicity in the trough is quiescent and no earthquake is observed. On the opposite side of the trough, seismic activity along the Oshima Zenisu ridge, echelon ridge of Izu Ogasawara inner arc, extends to southwest of Nishi Shitito fault zone. Generally, observation confirmed that micro-earthquake activity is very low around the array where large inter-plate earthquake is expected by many seismologists.