Interseismic locking on a megathrust controls long term strain partitioning in a subduction zone

Abstract

<p>Along the Hikurangi Margin in New Zealand, the Pacific plate is obliquely (~45°) subducting beneath the Australian plate at ~40 mm/yr. The overlying deforming wedge has long term strain partitioning, with some trench-parallel plate motion focused on major dextral strike-slip faults in the back part of the deforming wedge. These faults pose a major seismic hazard, capable of rupturing as M6 to M8+ earthquakes.</p> <p>Here, I analyse GNSS measurements of the decadal interseismic strain rates in part of eastern North Island, where the northern Wellington Fault is the only major dextral strike-slip fault. The fault plane is subvertical, with a slip rate over the Holocene of 5 – 10 mm/yr. The interseismic deformation is well modelled by a simple 2-D elastic dislocation model in which the underlying subduction megathrust is fully locked at depths between 30 and 8 km. The creeping (or with repeated slow slip events) deeper and shallower parts of the megathrust slip at the full relative plate velocity. Importantly, the model contains <em>no</em> information about the slip rate and geometry of the major faults in the overlying deforming wedge, and it <em>cannot</em> predict these slip rates; the pattern of interseismic strain rates appears to be dictated solely by the locking on the megathrust. However, the strike-slip fault is precisely located where the interseismic shear strain rate is at its maximum, directly above the deeper locking line, and so locking on the megathrust appears to determine the pattern of long-term strain partitioning too. During a large magnitude earthquake on the northern Wellington Fault, the entire thickness of the wedge must rupture. The existence of several inactive parallel faults adjacent to the active trace suggests that the position of the megathrust locking line has moved over time.</p> <p>The paleoseismic record indicates that earthquakes on the Wellington Fault are independent of megathrust slip events. However, only about 30% of the component of trench-parallel relative plate motion is taken up on the northern Wellington Fault, so there must also be some oblique slip on the megathrust itself. The degree of partitioning is a consequence of the relative frequency of large magnitude earthquakes on the megathrust compared to those in the overlying deforming wedge, assuming constant stress drops for individual earthquakes. The interseismic megathrust locking must be a key factor here, because this controls the stress field at the point where a large magnitude earthquake is most likely to nucleate, and thus which faults will rupture.</p>