Spatial Heterogeneity in Microseismicity and Stress near Repeated Conditional Rupture Terminations on the Alpine Fault, New Zealand

Abstract:

The size of an earthquake and the character of the seismic radiation it produces are strongly controlled by the length of the fault that slips, yet this is rarely the full length of a fault’s surface exposure. Understanding the on-fault processes influencing eventual rupture length is an ongoing challenge in earthquake science. Extensive paleoseismic studies along New Zealand’s Alpine Fault have shown that large (M7+) earthquakes occur remarkable regularly, every 291+/-23 years, but typically rupture different combinations of fault segments. Specifically, ruptures often terminate at the South Westland and Central segment boundary, near Haast, and near the Central and North Westland segment boundary near Inchbonnie at the intersection with the Hope Fault. However, some ruptures propagate through these boundaries in multi-segment events, increasing the earthquake magnitude from M7+ to M7.8+.

To understand the transitional behaviour of these segment boundaries, and their role in conditionally halting through-going ruptures, we quantify spatial heterogeneity in factors which have elsewhere been proposed to influence rupture arrest, using the locations and physical properties of small earthquakes. Using micro-earthquake catalogues from new and existing dense seismic networks we map along-strike variations in seismogenesis including: seismogenic depth, on-fault seismic slip, triaxial stress field orientations, aseismic fault patches, fault geometry/structure and connectivity with intersecting faults. We present results of the geometry and mechanical state of these complex rupture barrier regions near Haast and Inchbonnie and discuss their contrasting behaviour with the more uniform central Alpine Fault segment in the context of large-scale rupture processes.