A geodetic study of the Alpine Fault through South Westland: using campaign GPS data to model slip rates on the Alpine Fault

Although the Alpine Fault has been studied extensively, there have been few geodetic studies in South Westland. We include a series of new geodetic measurements from sites across the Haast Pass and preliminary results from a recently established network, the Cascade array that extends from the Arawhata River to Lake McKerrow, a region that previously had few geodetic measurements. We compare the slip rates based on models that include both single and double faults, and consider Alpine Fault dips of 55° and near vertical. Our preferred solution models the Alpine Fault as an infinitely long fault, dipping at 55° with a second (proxy) fault to account for (inboard) distributed deformation. This gives results that are consistent with the Alpine Fault being a predominantly strike-slip fault with a slip rate of 30 ± 2 mm/yr and therefore demonstrates that the slip rate of the Alpine Fault is constant along strike. The locking depth for the fault in this region is c. 17 km. Assuming a near vertical dip angle results in unrealistic high slip rates.