Evaluating earthquake hazard and risk using liquefied volcanic-ash layers in lakes

Abstract

Liquefied volcanic-ash or tephra layers, for which we have coined a new term, ‘tephra seismites’, are preserved in lake sediments in numerous 20,000-year-old lakes in the Hamilton Basin in northern North Island. Up to five tephra layers in cores taken from the lakes show signs of liquefaction - the phenomenon seen during the Christchurch earthquakes - that include disrupted tephra layers, voids, and dykes (injectites). These features show that one or more of four newly-discovered ‘hidden’ faults in Hamilton (see map) were responsible for previously unrecognised prehistoric earthquakes within the past 20,000 years (Kleyberg et al., 2015). Around 30 lakes are scattered amidst the faults, and hence cores are being taken from almost all of these to build up a picture of which fault(s) may have been active in that time. We anticipate that lakes closest to faults that have been active will show more tephra seismites than those farther from the faults.