Crustal Structure Along the Central Alpine Fault from Receiver Function Analysis

In the South Island of New Zealand, the depth distribution of seismicity varies perpendicular and along-strike of the Southern Alps and the central Alpine Fault. This seismicity appears to be anti-correlated in depth with LFEs (low-frequency earthquakes) and tremor (Chamberlain et al., 2014). Specifically, the seismic-aseismic transition spatially varies between 7 and 15 km depth (Boese et al., 2012; Bourguignon et al., 2015), whereas the tremor and LFEs are 20-30 km deep. We investigate how the crustal structure relates to these different seismic phenomena to improve our understanding of the physical controls on deformation. Using the multi-taper spectral correlation approach of Park and Levin (2000), we computed P receiver functions for a number of temporary seismograph stations (SAMBA, ALFA, WIZARD) deployed for varied length of times between 2008 and 2015 and for permanent stations FOZ and FOZ of the GeoNet network. The data spans a 130 km long region along the Southern Alps. The receiver functions show a steeply southeast dipping Moho and increasingly strong crustal seismic anisotropy towards the Alpine Fault. We derive 1D Vp and Vp/Vs models below each selected station using a nonlinear approach (Sambridge, 1999) to account for the highly 3D crustal structure, and use these 1D models to map the seismic anisotropy and Moho below the Southern Alps.