The October 1980 Earthquake Sequence near the New Hebrides

Four large earthquakes occurred in 1980 in a seismic gap near the Loyalty Islands in the New Hebrides. At 3:25 UT on October 24 an event with M, = 6.7 initiated the sequence. Three events, M, = 6.7, 7.2, and 6.5, followed on the next day. We investigated this sequence by using the seismicity, first-motion, and wavefonn data and long-period surface waves. The first-motion data constrain one of each pair of nodal planes. With this constraint, inversion of Rayleighand Love-wave spectra at 256 seconds determines the other nodal plane. The mechanisms of all four events are almost pure thrust on a plane dipping about 20 degrees east and striking parallel to the local strike of the New Hebrides trench. The first-day aftershocks indicate an initial rupture zone of about 2,000 km2, which is consistent with the estimated seismic moment of 3 x 1027 dyne-cm.' During the next two days, the aftershock activity expanded to an area of 10,000 to 20,000 km2 in the directions both along and perpendicular to the trench. Within 5 hours after the third and largest event, the initial rupttire zone had become mostly quiescent. Modeling of wa'i'eforms suggests a body-wave moment of between 0.5 and 1.0 x 1027 dyne-cm and a source process time of 11 seconds. This pattern suggests that the initial rupture zone represents a zone of increased strength (i.e. an asperity), and the stress change due to failure of this asperity subsequently migrated outward. During the two-year period before the main event, seismicity in the initial rupture zone was very low except near the point where the first mainshock initiated. A very tight clustering of activity occurred there. This pattern indicates gradual stress concentration near the asperity which finally failed during the mainshock sequence.