For more than 20 years geophysicists and geologists have tended to classify the world's subduction zones in terms of their degree of coupling, or more specifically 'seismic coupling'. The Marianas slab is often seen as 'pulling back' from the island arc, thereby giving rise to active backarc spreading. Marianas-type subduction zones are very unlikely to generate great underthrusting (megathrust) earthquakes. After GPS measurements established that the Tonga island arc has by far the greatest rates of backarc spreading as well as plate convergence. Most geophysicists have concluded that Tonga, rather than the Marianas, is actually the least seismically-coupled subduction zone in the world. On 3 May 2006 a great (Mw = 7.9) shallow earthquake occurred near the Ha'apai island group in southern Tonga. The focal mechanism for its centroid moment tensor (CMT) almost exactly matches that expected for intraplate convergence along the shallow interplate thrust zone, i.e., a megathrust. If this interpretation is correct, this would be one of the most surprising earthquakes of the last twenty years, and it would require major rethinking on the mechanics of subduction as well as seismogenesis. However, all focal mechanisms are ambiguous, and the alternative explanation is that this event had a high angle fault plane, in which case it would constitute a great plate-tearing event. Shallow plate tearing events are rare, especially in slabs experiencing downdip compression, and no such event has been subject to careful study using GPS and broadband seismic array. It is vital to make near-field measurements quickly, while postseismic deformation remains vigorous. This small grant for exploratory research will allow a rapid survey of the rupture zone and its immediate surroundings, deployment of a broadband seismic array to study the aftershocks, survey GPS measurements to establish the sense of coseismic movements at previously established stations, and a combination of survey and continuous GPS measurements to monitor the postseismic deformation field produced by all great earthquakes. Also included is a rapid geomorphological survey of the region's coastlines to help constrain coseismic and postseismic vertical movements. No one has ever observed postseismic transients and aftershocks of a great and shallow plate tearing earthquake using modern seismic and GPS instruments. This project is a collaboration between three US institutions as well as partners in Tonga and the Pacific Rim countries. The work also could have significant implications for earthquake and tsunami hazards along Tonga.
