Influence of a fault-segment junction on the rupturing behaviour of the Awatere Fault, and displacement-length scaling relationship for the Marlborough earthquake of 1848

Abstract

We present the results of an investigation into the neotectonic slip and paleoseismic fault rupture history of the central part of the Awatere Fault, a principal active strike slip fault within the Marlborough fault system in northeast South Island. This fault includes two geometric fault sections (the eastern and Molesworth sections) that differ by 10-15o in their strike and intersect one another across a complexly faulted zone (Mt Chisholm block) in the upper Awatere Valley. 

This fault junction is located in a remote part of the Awatere Valley near Molesworth Station, ~100 km to the southwest of the Awatere River mouth. The eastern section of the fault last ruptured during the Mw 7.5 Marlborough earthquake in 1848, an event which broke that entire section, and which also propagated beyond the fault junctions region and through Barefell Pass along a poorly expressed fault splay that strikes towards the Clarence Fault to the SW. Previous neotectonic and paleoseismic research had suggested that late Quaternary paleoearthquakes rupturing the eastern section of the Awatere fault were not synchronous with those rupturing the adjacent Molesworth section of that same fault, and that their junction has acted as a mechanical barrier to earthquake rupture propagation. Prior to this study, this hypothesis was not supported by systematic paleoseismic data. 

Focusing on the fault junction region, this EQC-funded project aimed to compile a new and detailed set of paleoseismic data relating to the location and magnitude of past earthquakes on either side of the fault junction, and neotectonic data constraining the rates of late Quaternary slip along both sections of the Awatere Fault.