New light through old windows – reaping the benefits of a palaeotsunami database.

Abstract

The New Zealand palaeotsunami database was first established in 2008 and has continued to grow in a somewhat ad hoc manner since then. While ostensibly a geological database, it has grown markedly through the addition of geomorphological, archaeological, anthropological and ecological data. The main tsunami research focus in New Zealand has tended to be along those coastlines either adjacent to a local subduction zone or exposed to events generated by similar distant sources; in this case - the eastern shores of New Zealand. Examining the database, we unexpectedly identified at least two large events within the past 700 years on the western shores. Event 1 occurred between 1470 and 1510AD and Event 2 between 1320 and 1450AD. Once the probable extent of these events was defined we were able to then propose a range of potential tsunamigenic sources including non-subduction fault ruptures and several slope failure mechanisms. While numerous sites with contemporaneously-aged evidence along the eastern shores point to significant probable subduction zone tsunamis, at least one anomalously high elevation data point stands out. The ~60 masl site at Korapuke Island fits comfortably within the timing of a probable 15th century palaeotsunami but evidence from neighbouring sites along 10s of kms of coastline range from only 0-10 masl. A subsequent reassessment of the site identified no unusual characteristics that might amplify runup and no local tsunamigenic source. However, a similar nearby site has high elevation (~45 m) deposits laid down in 1996 by a waterspout and it seems increasing likely that Korapuke Island may have the first reported palaeo-waterspout deposit. This adds an interesting wrinkle to the palaeotsunami-palaeostorm deposit debate, but also reveals the value of a palaeotsunami database. There are probably similar anomalies (and errors) in the database, but without the context of related data it is unlikely that they would be exposed to such scrutiny. © 2014, American Geophysical Unio