Microseismicity and P-wave tomography of the central Alpine Fault, New Zealand

We utilise seismic data from the central section of the Alpine Fault to locate earthquakes and image crustal structure in three dimensions. Tomography results from c. 6500 sources reveal the fault as either a southeast-dipping low-velocity zone or a marked velocity contrast in different parts of the study region. Where our model is best resolved, we interpret the Alpine Fault to be listric in nature, dipping steeply in the upper crust (50-60 degrees) and flattening to 25-30 degrees in the lower crust. The base of the seismogenic zone shallows from c. 15km beneath the footwall and Alpine Fault to c. 6km beneath the Southern Alps Main Divide, and then deepens to c. 15km by c. 10km further southeast. The shallow brittle-ductile transition overlies a broad low-velocity zone, which together likely result from the presence of fluids and elevated temperatures brought about by enhanced exhumation rate in this section of the Alpine Fault.