M6.5 - SOUTHEAST OF THE LOYALTY ISLANDS

Tectonic Summary

The sequence of earthquakes that has been occurring southeast of the Loyalty Islands since September 27, 2007, is a geophysical phenomenon known as an earthquake "swarm"-- an episode of high earthquake activity in which the largest earthquake does not occur at the beginning of the episode and in which the largest earthquake is not substantially larger than other earthquakes of the episode.

The locations and focal mechanisms of most shocks in the current Loyalty Islands swarm are consistent with their occurring on the thrust-fault interface between the Australia plate and the New Hebrides plate. In the epicentral region of the earthquakes, the Australia plate moves east-northeast relative to the New Hebrides plate with a velocity of about 11 cm/yr. The Australia plate subducts beneath the New Hebrides plate and is seismically active to a depth of about 170 km down-dip of the swarm region.

Worldwide, earthquake swarm activity is commonly associated with tectonic regions in which both strike-slip faulting and normal faulting occurs and where magmatic activity occurs at shallow depths in the earth’s crust, but swarms also occur in regions where these conditions are not met. The current swarm is an example of a swarm that principally involves thrust-faulting and that is not at a location that is known to be experiencing crustal magmatism.

The current Loyalty Islands swarm is occurring just north of a swarm of shocks in December 25, 2003 - January 03, 2004, that contained twelve shocks of magnitude 6 or greater and that included two earthquakes of magnitude greater than 7. The 2003 - 2004 swarm included both reverse-faulting earthquakes and normal-faulting earthquakes.