Arc segmentation and seismicity in the Solomon Islands arc, SW Pacific

This paper evaluates neotectonic segmentation in the Solomon Islands forearc, and considers how it relates to regional tectonic evolution and the extent of ruptures of large megathrust earthquakes. We first consider regional geomorphology and Quaternary vertical displacements, especially uplifted coral reef terraces. Then we consider geographic seismicity patterns, aftershock areas and vertical displacements for large earthquakes, focal mechanisms, and along-arc variations in seismic moment release to evaluate the relationship between neotectonically defined segments and seismicity. Notably, one major limitation of using seismicity to evaluate arc segmentation is the matter of accurately defining earthquake rupture zones. For example, shoreline uplifts associated with the 1 April 2007 M-w 8.1 Western Solomons earthquake indicate that the along-arc extent of rupture was about 50 km smaller than the aftershock area. Thus if we had relied on aftershocks alone to identify the 2007 rupture zone, as we do for most historical earthquakes, we would have missed the rupture's relationship to a major morphologic feature. In many cases, the imprecision of defining rupture zones without surface deformation data may be largely responsible for the poor mismatches to neotectonic boundaries. However, when a precise paleoseismic vertical deformation history is absent, aftershocks are often the best available tool for inferring rupture geometries. Altogether we identify 16 segments in the Solomon Islands. These comprise three major tectonic regimes or supersegments that correspond respectively to the forearc areas of Guadalcanal-Makira, the New Georgia island group, and Bougainville Islands. Subduction of the young and relatively shallow and buoyant Woodlark Basin and spreading system distinguishes the central New Georgia supersegment from the two neighboring supersegments. The physiographic expression of the San Cristobal trench is largely absent, but bathymetric mapping of the surface trace of the interplate thrust zone defines it adequately. The New Georgia supersegment has smaller arc segments, and more islands due to general late Quaternary forearc uplift very close to the trench where vertical displacement rates tend to be faster: prior to the 2007 earthquake it had much lower rates of seismic activity than the neighboring supersegments. Generally the mean along-arc lateral extent of Solomon arc segments is about 75 km, somewhat smaller than the segments reported in some other island arcs such as Japan (similar to 100-260 km), but larger than those of the Tonga (30-80 km) and Central New Hebrides arcs (30-110 km). These differences may be real but it may occur simply because the coral-friendly tropical environment of the South Pacific arcs, numerous emerged forearc islands, and high seismicity rates provide an unusually favorable situation for observing variations in vertical tectonic activity and thus for identifying segment boundaries. Over the past century seismic slip in the Solomons, as indicated by seismic moment release, has corresponded to about half the plate convergence rate; however, there are notable variations along the arc. Even with the 2007 earthquake, the long-term moment release rate in the New Georgia supersegrnent is relatively low, and this may indicate that large earthquakes are imminent. (C) 2011 Elsevier B.V. All rights reserved.