Report: TR04/33

Author: GeoEnvironmental Consultants in association with Dr Scott Nichol (University of Waikato)

Abstract

This report details the results of Stage One of a tsunami study of several key sites identified in an earlier reconnaissance of the Bay of Plenty. Five broad chronological periods of potential activity were identified. These are:

1. Post-Kaharoa (c. 500-600 yrs BP) - probably related to submarine volcanic activity.
2. Kaharoa (c. 750 yrs BP) - immediate post-eruption.
3. Pre-Kaharoa/Post-Taupo (c. 800-1800 yrs BP) – most probably associated with significant amounts of coastal subsidence, possibly as a result of large, subduction earthquakes to the east.
4. Taupo (c. 1850 yrs BP) - immediate post-eruption.
5. Pre-Taupo: (i) Waimahia (c. 3200 yrs BP)? (ii) Whakatane (c. 4800 yrs BP)? groundshaking (and local subsidence?) – the dating is currently somewhat arbitrary but these events relate to one or both of known sub-aerial volcanic eruptions and/or large, subduction earthquakes to the east, and/or local seismic sources.

A tentatively tsunami magnitude (high to low) hierarchy is suggested based on the limited data obtained to date: Event C, A, E(i), D/B, E(ii). The relative significance of the Taupo eruption (D) is difficult to estimate because of the sheer magnitude of the event and the resultant surfeit of tephra in the nearshore zone. However, from the perspective of the greater hazard, tsunami inundation in this instance seems to be less of a concern. There are currently too few data to be able to estimate magnitude and frequency with any reliability, but there would seem to be about six large (unknown magnitude at present) events in the last 5000 years or so. It is important to note though that the absence of visible deposits does not mean the absence of events. BUT the presence of high magnitude (>5m), low frequency events is a good indicator of the occurrence of many lower magnitude, higher frequency ones.

Three recommendations for possible additional work are highlighted as a result of the work to date:

• A detailed, site-specific analysis is needed of the Korapuke site.
• Other islands in the Mercury group such as Red Mercury, Ohinau, Motukoruenga, Double and Stanley should be visited.
• A detailed, high-resolution study of the Ohiwa Harbour, Jacobs Creek core is recommended. A Masters student project should be considered.

Tsunami Hazard for the Bay of Plenty and Eastern Coromandel Peninsula : Stage 1 [PDF, 649 KB]