Geophys. J. Int. (2008) 174, 659–671 doi: 10.1111/j.1365-246X.2008.03838.x

Seismic moment release during slab rupture beneath the

Mike Sandiford School of Earth Sciences, University of Melbourne, Victoria 3010, . E-mail: [email protected]

Accepted 2008 April 30. Received 2008 April 30; in original form 2007 August 15

SUMMARY The highest intermediate depth moment release rates in occur in the slab beneath the largely submerged segment of the in the Banda Sea to the east of Roma, termed the Damar Zone. The most active, western-part of this zone is characterized by downdip extension, with moment release rates (∼1018 Nm yr–1 per 50 km strike length) implying the slab is stretching at ∼10−14 s−1 consistent with near complete slab decoupling across the 100–200 km depth range. Differential vertical stretching along the length of the Damar Zone is consistent with a slab rupture front at ∼100–200 km depth beneath Roma propagating eastwards at ∼100 km Myr–1. Complexities in the slab deformation field are revealed by a narrow zone of anomalous in-plane P-axis trends beneath Damar, where subhorizontal constriction suggests extreme stress concentrations ∼100 km ahead of the slab rupture front. Such stress concentrations may explain the anomalously deep ocean gateways in this region, in which case ongoing slab rupture may have played a key role in modulating the Indonesian throughflow in the Banda Sea over the last few million years. Key words: Seismicity and tectonics; Subduction zone processes; .

McCaffrey 1989b; Das 2004). This study focuses on the nature of 1 INTRODUCTION the deformation field at intermediate depths between and the The rupture of subducting slabs from the trailing surface plate that zone of much more intense activity beneath the island of Damar, must inevitably accompany continent-arc ‘collision’ is a crucial pro- further east. It builds on the previous studies (Cardwell & Isacks cess in the plate tectonic cycle (Davies & von Blackenburg 1995). 1978; McCaffrey et al. 1985; Das 2004; Milsom 2005) in utilizing GJI Seismology Consequently, numerous studies have attempted to elucidate the the now greatly expanded set of data now available through the CMT mechanisms of slab rupture using constraints derived from seis- catalogue. Seismic activity in this region has been previously inves- mic tomography (Spakman 1990), forward modelling (Yoshioka & tigated in several studies (Das 2004; Milsom 2005). Most notably, Wortel 1995; Wong et al. 1997; Gerya et al. 2004) and geological Das (2004) used a set of relocated events assembled in 1999 to de- observations (Meulen et al. 1998). Along with many others, these scribe the geometry of the slab system and reported relevant CMT studies have progressed our understanding of slab rupture processes. solutions for the relocated events. As of 2007 October, the CMT However, they have yet to be adequately tested using constraints catalogue comprised 226 intermediate depth (70–300 km) events derived from the deformation field in actively rupturing slabs. A between 127 and 131◦E along the southern limb (south of 6◦S) of crucial set of data relevant to the instantaneous deformation field the Banda slab in the vicinity of the island of Damar, providing a within deforming slabs is provided by focal mechanism solutions density data that allow much better resolution of deformation within and moment tensor data. With a few notable exceptions (Nothard the Banda slab, thereby leading to new insights into the mechanisms et al. 1996), past attempts to resolve the deformation field within of slab rupture. slabs have been hampered by data limitations. However, the growth of the Harvard CMT catalogue now make it timely to reassess con- 2 GEOLOGIC