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Dynamic Rupture Simulations on the Alpine Fault, New Zealand: Investigating the Role of Fault Geometry on Rupture Size and Behavior Over Multiple Earthquake Cycles

Description: 

The right-lateral transpressional Alpine Fault is the primary plate boundary fault on the South Island of New Zealand. At a broad scale, its onshore surface trace between Milford Sound in the southwest, and the branching Marlborough Fault System in the northeast consists of two planar sections connected by a major geometrical boundary at Martyr River. This boundary is characterized by both a dip change of as much as 40° over an along-strike length of only ~5 km. Several previous studies suggest that changes in dip along a strike-slip fault can affect rupture dynamics. It is therefore possible that this geometrical feature affects conditional earthquake segmentation behavior on the Alpine Fault, as documented by the extensive paleoseismic record.

We use the 3D finite element method to simulate multiple cycles of dynamic ruptures on the southwestern ~320 km of the Alpine Fault. We embed the faults in a 1D velocity structure and impose heterogeneous initial tractions computed using seismologically estimated local principal stress orientations and magnitudes computed using a critically-stressed crust model. We simulate the coseismic period using dynamic rupture simulations, then account for the interseismic period by incrementing shear stress based on a set time between events. For each dynamic rupture simulation, we compare the modeled rupture lengths and surface slip values to geologic and paleoseismic studies to ensure that we are producing physically-plausible simulations consistent with observations. We find that the dip change at the segment boundary is not inherently a barrier to rupture within single events, but that stress changes associated with rupture through this boundary in one earthquake can sometimes lead to segmentation in the next one. Our results suggest that rupture hazard on the Alpine Fault may depend both on the slip distribution of and the timing since the previous large earthquake. This also implies that both fault geometry in and of itself and long-term stress patterns resulting from that geometry are important considerations for hazard assessment on other geometrically-complex faults.

Session: Characteristics and Mechanics of Fault Zone Rupture Processes, from Micro to Macro Scales - II

Type: Oral

Date: 5/2/2024

Presentation Time: 10:30 AM (local time)

Presenting Author: Julian

Student Presenter: No

Invited Presentation: 

Authors