Intermediate-Depth Earthquakes in a Region of Continental Convergence: South Island, New Zealand

It is rare to find earthquakes with depths greater than 30 km in continent–continent collision zones because the mantle lithosphere is usually too hot to enable brittle failure. However, a handful of small, intermediate-depth earthquakes (30–97 km) have been recorded in the continental collision region in central South Island, New Zealand. The earthquakes are not associated with subduction but all lie within or on the margins of thickened crust or uppermost mantle seismic high-velocity anomalies. The largest of the earthquakes has M_L 4.0 corresponding to a rupture radius of between 100 and 800 m, providing bounds on the upper limit to the rupture length over which brittle failure is taking place in the deep brittle–plastic transition zone. The earthquake sources may be controlled by large shear strain gradients associated with viscous deformation processes in addition to depressed geotherms.