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New Zealand’s South Westland Alpine Fault: What’s Down There and How Does It Make Earthquakes Stop?

Description: 

Approximately half of past major Alpine Fault earthquakes have arrested close to the transition from the listric Central Segment to the sub-vertical South Westland Segment. Here, the fault undergoes notable changes in geometry, slip rate, kinematics, bounding lithology, resolved stress, seismogenic behaviour and frictional properties, all of which have the potential to influence through-going rupture behaviour. Unravelling which factors contribute to observed conditional rupture arrest here – and therefore assessing likely future rupture behaviour – requires a combination of detailed observations about the fault’s physical state to inform realistic, physics-based earthquake rupture models.

Recent work to produce precise relocations of thousands of earthquakes and their focal mechanisms has illuminated the structure and microseismic behaviour of the segment boundary leading to improved quantification of the spatial variability in physical factors mentioned above. Here, we present these results and build on our observations to use those local earthquake phase-arrivals in 3D tomographic models of the segment boundary in unprecedented resolution. We present preliminary Vp, Vs and Vp/Vs models revealing large-scale changes in crustal structure across the segment boundary, high-velocity bodies associated with the Dun-Mountain ultramafic terrane and Livingstone Fault, and compare these with newly relocated seismicity to identify active structures. Our full suite of observations are then used to inform a series of 3D finite-element dynamic simulations of earthquakes along the fault. Iterative geometric and stress parameterisations allow us to assess the relative role different physical factors play in allowing ruptures to either propagate through, or terminate at the segment boundary. We find that successive on-fault stress changes from a series of earthquakes around the geometrical boundary can reproduce rupture arrest consistent with the paleoseismic record and surface slip values.

Session: Characteristics and Mechanics of Fault Zone Rupture Processes, from Micro to Macro Scales [Poster Session]

Type: Poster

Date: 5/2/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Emily

Student Presenter: No

Invited Presentation: 

Authors