Spatio-temporal distribution of microseismicity along the Eastern Lau Spreading Center, Lau Basin, SW Pacific

A 12-month deployment of 51 ocean bottom seismometers (OBS) on the Eastern Lau spreading center (ELSC) of the Lau back-arc basin recorded more than 4500 local micro-earthquakes within the first five months of observations between December 2009 and April 2010. Preliminary observations are based on a catalog of ~1600 well-located events with a minimum of two P-S phase pairs, seven or more defining phases and an azimuthal gap of less than 165 deg. Seismicity is closely correlated with geological features identified using high-resolution seafloor imagery. Two distinct spatial clusters of hypocenters are located off axis on the eastern flank of the ELSC, near 20.58S, 175.98W and 21.31S, 176.15W. The swarm at 20.58S appears to be associated with a cluster of volcanic cones located between the ELSC and the active Tofua volcanic arc. This swarm dominates, containing 44% of the well-located events in the catalog, and it is characterized by two stages of high event rate followed by a steady decline over the five-month duration. The second cluster near 21.31S contains 14% of the events and also was marked by two stages of high event rate. Inspection of the multibeam bathymetric data from this area shows that this swarm lies beneath flat topography, likely draped by volcano-clastic sediments from the arc. Two small swarms containing 2% and 11% of the cataloged microearthquakes are located at overlapping spreading centers (OSC) near 20.19S and 20.54S along the ELSC. Three spatial clusters of intermittent seismic activity are located on the ridge axis. One near 19.92S contains 3% of the cataloged events and is distributed on the saddle of a rifted volcano. The others are positioned near 20.87S (5% of events) and 21.15S (4% of the events).