Locking on a megathrust as a cause of distributed faulting and fault-jumping earthquakes

Abstract

Seismicity on large faults is often characterized in terms of an independent recurrence time and magnitude distribution, which forms the basis for calculating future earthquake probabilities. The underlying assumption is that the driving mechanism for earthquakes on any particular fault is uniquely linked to that fault, determined by the rate of long-term creep on its deeper extension. However, our modelling of nearly 20 years of Global Positioning System data along the obliquely converging plate boundary in New Zealand shows that interseismic stress accumulation can be independent of the properties of the numerous crustal faults and controlled by locking on the megathrust. In this way, the interseismic driving mechanism for large crustal earthquakes is not linked directly to the individual major faults that rupture. This scenario predicts large-magnitude earthquakes with complex multifault ruptures in broad zones that ‘jump’ from fault to fault, following the contours of stress/strain loading. This can explain the November 2016 Mw7.8 Kaikoura earthquake that shattered the plate boundary in central New Zealand. Repeated episodes of this would create the observed complex array of active faults with the appearance of coherent slip. Our analysis opens up the possibility to use long-term Global Positioning System data to identify this type of earthquake behaviour.