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 these P-waves are mutually perpendicular, ruling out anisotropy as a cause of the delays. The low-speed region measures about 25 km by 40 km, has a speed 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 pressure due to excess water being released by prograde and strain-induced metamorphism into the lower crust. Because enhanced fluid pressure reduces the work required for deformation, the existence of the central Southern Alps low-speed zone implies that this part of the Australian-Pacific plate boundary is relatively weak.