Seismic imaging across a rupture-limiting section boundary of the Alpine Fault, New Zealand

The Alpine Fault in Aotearoa New Zealand is a major plate boundary strike-slip fault, that has hosted great earthquakes in the past and is forecast to have a high chance of hosting an earthquake of magnitude 7 or greater in the next fifty years. The fault is thought to be segmented based on changes in fault geometry and kinematics, with sections rupturing both individually and in combination. We focus on an area where the fault is thought to transition between a vertical geometry to a dipping geometry, which has been a rupture boundary in past earthquakes. Specifically, surface mapping indicates that the fault changes from a near-vertical orientation in the South Westland Section, to dipping ~60-45° to the SE in the Central Section, but it is unclear how this change is accommodated in the subsurface.

 

We image the subsurface at this section boundary using both a temporary seismic array deployed along the fault and long-term seismic stations. We use teleseismic earthquakes to generate receiver functions which are sensitive to crustal structure below seismic stations. Using both velocity inversion and common conversion point stacking, we find there are distinct crustal structures on either side of the surface fault trace. Our results indicate that the southern vertical fault section may continue to the north past the segment boundary, and that both a vertical and dipping fault may co-exist beyond the segment boundary. This is consistent with microseismicity and previous tectonic studies, and has implications for fault rupture scenarios.