Seismological Analysis of Contemporary and Future Alpine Fault Earthquakes Using the Southern Alps Long Skinny Array (SALSA)

Abstract The Alpine fault is a major transpressive plate boundary in southwestern New Zealand known on paleoseismological grounds to produce great (Mw∼8) earthquakes on <300 yr timescales. The fault is now late in its typical and well-determined interseismic period, having last ruptured in a great earthquake in 1717 C.E. Between October 2021 and November 2023, the Southern Alps Long Skinny Array (SALSA) consisting of 43 broadband and 4 short-period seismometers, was installed with ∼10 km interstation spacing along a 450 km stretch of the Alpine fault, between Maruia and Milford Sound/Piopiotahi. SALSA’s geometry was designed to enable the synthesis of virtual earthquakes representing slip at arbitrary points on the Alpine fault as part of research into the range of far-field ground motions that might result from geologically plausible rupture scenarios. Virtual earthquakes provide a means of evaluating ground motions from a much larger number of kinematic rupture scenarios than is computationally tractable using full-waveform simulations. In addition to providing closely spaced noise measurements, SALSA has yielded novel recordings of microseismicity and tremor along most of the onshore extent of the Alpine fault and will enable ongoing research into the present-day state of the fault and future rupture behavior. Here, we describe the rationale for deploying SALSA and present initial results and analysis that illustrate the dataset and indicate avenues for future research.