A double seismic zone in New Britain and the morphology of the Solomon Plate at intermediate depths

We have studied the spatial configuration of earthquakes in the New Britain subduction zone by inverting regional and teleseismic P, PKP, and _pP arrival times using a hypocentroidal decomposition method. A previously unreported intermediate depth double seismic zone exists along a small, 70 km long segment of the slab. In this region, centroid moment tensor mechanisms suggest that the upper zone of seismicity shows horizontal, along‐strike compression and the lower zone shows downdip tension. This contrasts with most previously studied double seismic zones, which generally show downdip compression in the upper zone. We propose that along‐strike, rather than downdip compressional stresses exist in the upper seismicity plane due to membrane and lateral bending stresses resulting from the shape of the New Britain trench, which undergoes a 40° change in strike between 148° and 152° longitude. The double seismic zone is bounded by a tear in the slab which accommodates the majority of the change in strike that the slab must undergo. These results suggest that double seismic zones can show very rapid lateral variations controlled by the local tectonics, and that along‐strike membrane and bending stresses resulting from slab morphology can be significant in producing double seismic zones.