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Wide‐band long‐period seismographs with peak magnification up to 500,000 at 30–40 sec have been installed in an airtight chamber in a deep mine. The rigid environmental control allows for relatively noise‐free output at high magnifications. The seismograms of the central South Pacific event of August 24, 1968, recorded by these instruments are compared with long‐period seismograms from, the WWSSN instruments at the same site. The high‐magnification systems recorded several body phases and the dispersed Rayleigh waves, whereas the WWSSN instruments recorded only the Rayleigh waves. The body waves on the high‐magnification system enabled the computation of an epicentral distance of 87°±5° for this small‐magnitude (CGS mb = 5.0) event. The surface wave magnitude (M8) computed by using the high‐gain records is 4.4.

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 μrad 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 earthquakes 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 μrad 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.