Reconstructing rupture dynamics of historical Alpine–Marlborough Fault earthquakes, Aotearoa–New Zealand

Forecasting seismic hazard on complex fault systems remains a global challenge, particularly where ruptures can cascade across structural transitions. Aotearoa–New Zealand’s (A–NZ) central transition zone exemplifies this, where the Alpine Fault (AF) and Marlborough Fault System (MFS) connect the Puysegur and Hikurangi subduction zones and pose a major seismic risk to A–NZ communities. The Alpine Fault is late in its interseismic cycle, with a 75% probability of rupture on its central segment within the next 50 years, and a high likelihood of this cascading into a Mw>8 multi-fault rupture onto the MFS. Understanding the behaviour of past earthquake sequences in this region is therefore a national priority to better estimate the extent and dynamics of future shaking. Instrumental records only span a fraction of an earthquake cycle, leaving critical gaps in recurrence patterns and rupture behaviour, which paleo-seismic archives can help to resolve.

We address this gap by integrating lake-sediment paleo-shaking records with calibrated ground-motion modelling and empirical source inversion. Using South Island lakes as binary seismometers, we reconstruct rupture scenarios for historical earthquakes in the central A–NZ transition zone. For each event, we define the probable fault planes and forward-model potential peak ground velocities at each lake site using a suite of ground-motion models that have been extensively tested and adopted in the New Zealand National Seismic Hazard Model. These modelled ground motions are then compared with age-dated mass-transport deposits, which record earthquake-induced shaking and allow calibration of the sequence and timing of events at each site. Finally, a source-inversion technique is used to identify rupture extents and magnitudes that satisfy both rupture-scaling constraints and the binary shaking evidence preserved in the sedimentary record.

In this presentation, we will demonstrate how our integrated approach constrains the magnitudes, rupture locations, and recurrence histories of eight historical earthquakes in central Aotearoa–New Zealand at unprecedented spatial and temporal resolution. The methodology reduces epistemic uncertainty associated with conventional intensity-based methods and is transferable to other complex fault systems, including subduction zones. Crucially, our research provides essential empirical inputs for time-dependent seismic hazard models in Aotearoa–New Zealand.