Crustal structure and seismicity of the Havre Trough at 26 degrees S

An ocean bottom seismographic (OBS) experiment was carried out to investiga te the present-day tectonics at the northern end of the Havre Trough, a bac karc basin in the southwestern Pacific. Hypocenter distribution shows no se ismicity within 50 km of the OBS array but two active regions outside the T rough; one is to the northeast of the array at a bathymetric high between t he Lau Basin and Havre Trough; the other is to the southeast of the array b eneath the landward slope of the Kermadec Trench. P wavespeed structure obt ained from the refraction survey is characterized by a thinned island are c rust with a thickness of 9-10 km. Many volcanic intrusions shown in reflect ion profiles that support extension tectonics are covered by undisturbed se diments. These suggest little present activity related to rifting and/or sp reading.