A TILT AND SEISMICITY EPISODE IN THE NEW HEBRIDES (VANUATU) ISLAND-ARC

Tilt and seismicity have been monitored in the central New Hebrides island arc since 1978 using bench mark arrays, long tube water tiltmeters, borehole tiltmeters, and a local seismometer network. Releveling of the bench mark array on Efate island in late November 1986 revealed a 10-mu-rad tilt up to the NNW since the previous leveling in April 1986. the tilt event was preceded by a magnitude 5.9 thrust event that occurred on October 25, 1986, at a depth of 48 km and about 11 km NW of the tiltmeter instruments. Six days later, a shallow (< 20 km) swarm of earthquake occurred 12 km NNW of the tiltmeter instruments and 5 km north of the epicenter of the magnitude 5.9 earthquake. Closely coincident in time with the swarm, a 5-mu-rad tilt up to the NNW that occurred over a period of 5 days was recorded on both the 100 m baseline water tube tiltmeter and the borehole bubble level tiltmeter. A composite focal mechanism of 191 earthquakes selected from the swarm indicates a thrust mechanism with some component of strike slip. Calculations show that the seismic slip associated with a swarm of this magnitude is apparently inadequate to cause the observed surface deformation. Two similar shallow swarms in November 1987 and July 1988 have occurred within 15 km of the 1986 swarm but with no apparent surface deformation. The most likely explanation, supported by simple modeling, is that the swarm and tilt are the result of a magmatic intrusion from island arc volcanism. An alternate hypothesis is that both the seismicity and the tilt are due to an episode of largely aseismic creep in the upper crust.