The Santa Cruz Islands region has high seismicity near a 90° bend in the boundary between the Pacific and Australian plates. Southward, along the Vanuatu island arc, the Australian plate under-thrusts the Pacific plate in a well-defined subduction zone. Westward, a transpressional, predominantly transform boundary extends to the southern Solomon Islands subduction zone. The Santa Cruz Islands region has upper plate strike-slip and normal faulting, plate boundary under-thrusting, outer rise extensional faulting, and intraplate compressional faulting. On February 6, 2013 the largest under-thrusting earthquake (Mw 8.0) that has been instrumentally recorded in the region ruptured the megathrust. The epicenter (10.738°S, 165.138°E) is about 1° north of epicenters of prior large shallow under-thrusting events in Vanuatu in 1934 (M~7.8) and 1966 (MS 7.9), and there is overlap of all three events' aftershock zones, but not their large-slip regions. At least 10 lives were lost, with 6 more missing, due to tsunami ~1.5m high striking the town of Lata and several villages on the main Santa Cruz Island of Nendö (Ndeni) and a nearby small island Nibanga. Inundation of 500m flooded the Lata airport. The tsunami was well-recorded by DART buoys spanning an unusually wide three-quadrant azimuthal aperture. Iterative modeling of teleseismic broadband P waves and the deep-water tsunami recordings resolves the slip distribution. There are two large-slip patches with a southeastward rupture expansion at about 1.5km/s, with the second patch appearing to have ruptured with large slip at the trench. The event has relatively low short-period seismic wave energy release, but a typical overall moment-scaled total energy. The shallow rupture depth may be associated with the low level of short-period energy, and a near-total stress drop may account for a lack of underthrusting aftershocks among a highly productive aftershock sequence that included three events with Mw≥7.0.
All Science Journal Classification (ASJC) codes
- Earth-Surface Processes