Low seismic-wave speeds and enhanced fluid pressure beneath the Southern Alps of New Zealand

A region of low seismic-wave speed is detected beneath the central Southern Alps of New Zealand on the basis of traveltime delays for both wide-angle reflections and P-waves from teleseismic events. Respective ray paths for t hese P-waves are mutually perpendicular, ruling out anisotropy as a cause o f the delays. The low-speed region measures about 25 km by 40 km, has a spe ed reduction of 6%-10%, and is largely above the downward projection of the Alpine fault. The most likely cause of the low-speed zone is high fluid pr essure due to excess water being released by prograde and strain-induced me tamorphism into the lower crust. Because enhanced fluid pressure reduces th e work required for deformation, the existence of the central Southern Alps low-speed zone implies that this part of the Australian-Pacific plate boun dary is relatively weak.