Abstract
Abstract Geological records of subduction earthquakes, essential for seismic and tsunami hazard assessment, are difficult to obtain at transitional plate boundaries, because upper-plate fault earthquake deformation can mask the subduction zone signal. Here, we examine unusual shell layers within a paleolagoon at Lake Grassmere, at the transition zone between the Hikurangi subduction zone and the Marlborough fault system. Based on biostratigraphic and sedimentological analyses, we interpret the shell layers as tsunami deposits. These are dated at 2145–1837 and 1505–1283 yr B.P., and the most likely source of these tsunamis was ruptures of the southern Hikurangi subduction interface. Identification of these two large earthquakes brings the total record of southern Hikurangi subduction earthquakes to four in the past 2000 yr. For the first time, it is possible to obtain a geologically constrained recurrence interval for the southern Hikurangi subduction zone. We calculate a recurrence interval of 500 yr (335–655 yr, 95% confidence interval) and a coefficient of variation of 0.27 (0.0–0.47, 95% confidence interval). The probability of a large subduction earthquake on the southern Hikurangi subduction zone is 26% within the next 50 yr. We find no consistent temporal relationship between subduction earthquakes and large earthquakes on upper-plate faults.
Highlights
At transitional zones between subduction and strike-slip plate boundaries, both fault systems can generate large earthquakes that produce similar geological signatures of vertical coastal deformation and tsunami (Clark et al, 2017)
Such interactions are important at populated plate boundary transition zones such as the northern San Andreas fault–Cascadia subduction zone and the Marlborough fault system (MFS)–Hikurangi subduction zone (HSZ) where clusters of large earthquakes could extend the duration of an earthquake and tsunami sequence, impacting how society prepares and recovers
At the southern HSZ, New Zealand, the possibility of a large crustal earthquake triggering a subduction earthquake came to prominence following the 2016 Mw 7.8 Kaikōura earthquake
Summary
At transitional zones between subduction and strike-slip plate boundaries, both fault systems can generate large earthquakes that produce similar geological signatures of vertical coastal deformation and tsunami (Clark et al, 2017). We present evidence for two paleotsunamis from Lake Grassmere that predate the earthquakes documented at M-W Lagoon, extending the record of subduction zone ruptures in the region to 2000 yr.
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