Tsunami Hazard Posed to New Zealand by the Kermadec and Southern New Hebrides Subduction Margins: An Assessment Based on Plate Boundary Kinematics, Interseismic Coupling, and Historical Seismicity

Abstract

We assess the tsunami hazard posed to New Zealand by the Kermadec and southern New Hebrides subduction margins. Neither of these subduction zones has produced tsunami large enough to cause significant damage in New Zealand over the past 150 years of well-recorded history. However, as this time frame is short compared to the recurrence interval for major tsunamigenic earthquakes on many of the Earth’s subduction zones, it should not be assumed that what has been observed so far is representative of the long term. For each of these two subduction zones we present plate kinematic and fault-locking results from block modelling of earthquake slip vector data and GPS velocities. The results are used to estimate the current rates of strain accumulation on the plate interfaces where large tsunamigenic earthquakes typically occur. We also review data on the larger historical earthquakes that have occurred on these margins, as well as the Global CMT catalogue of events since 1976. Using this information we have developed a set of scenarios for large earthquakes which have been used as initial conditions for the COMCOT tsunami code to estimate the subsequent tsunami propagation in the southwest Pacific, and from these the potential impact on New Zealand has been evaluated. Our results demonstrate that there is a significant threat posed to the Northland and Coromandel regions of New Zealand should a large earthquake (M _w ≳8.5) occur on the southern or middle regions of the Kermadec Trench, and that a similarly large earthquake on the southern New Hebrides Trench has the potential to strongly impact on the far northern parts of New Zealand close to the southern end of the submarine Three Kings Ridge. We propose logic trees for the magnitude–frequency parameters of large earthquakes originating on each trench, which are intended to form the basis for future probabilistic studies.

Appendix: Logic Trees for Kermadec Trench and Southern New Hebrides Earthquake Sources

In order to quantitatively analyse the tsunami hazard to New Zealand from the Kermadec Trench and the southern New Hebrides, logic trees have been developed for the relevant earthquake source parameters. The logic trees represent the collective opinions of the scientists involved, after studying the available seismic and geodetic data.

A truncated Gutenberg–Richter distribution was chosen as the most suitable form for the statistical distribution of large earthquakes (M _w > 8) in these source regions, and a b-value of 1 was assumed, matching the global magnitude–frequency distribution.

The plate convergence rates for both sources are accurately known, making it possible to specify the source magnitude–frequency distributions using just the maximum-magnitude and the seismic coupling coefficient for each source.

For each source a range of maximum magnitudes was considered, up to the point where scaling rules suggest a single rupture would span the entire source region, and extending no lower than the largest historical events. A range of coupling coefficients, representing weak (0.3), moderate (0.6), and strong coupling (0.9) were selected. Weights were then assigned to each combination of maximum magnitude and coupling coefficient (Figs. 21, 22).

Fig. 21

Kermadec Trench logic tree

Fig. 22

Southern New Hebrides logic tree

For the Kermadec Trench the rate at which each combination of maximum magnitude and coupling coefficient produces earthquakes of magnitude >8 was plotted (Fig. 23). Historical data was judged to favour logic-tree branches that produced average intervals for M _w >8 in the 50–100 year range, and these branches were assigned the majority of weighting

Fig. 23

Average Recurrence Interval between earthquakes of M _w ≥8.0 on the Kermadec Trench plotted as a function of maximum magnitude and seismic coupling coefficient, assuming a truncated Gutenberg–Richter distribution and 72 mm/year convergence rate

Within the set of logic-tree branches that are consistent with the historical data we find branches with source parameters corresponding to the conventional interpretation that the Kermadec Trench has a relatively low maximum magnitude (<8.5–8.6; e.g. Berryman, 2005) and weak coupling, but we also find branches corresponding to an alternative interpretation where the Trench has a high maximum magnitude. In this latter view the reason that few earthquakes greater than M _w 8 have been historically observed is a consequence of the short observation time. In the logic-tree most weight was assigned to the established viewpoint, but some was also given to this alternative picture. It was felt that should the maximum magnitude be large then the seismic coupling would probably have to be relatively strong, as weak coupling would most likely represent small strongly-coupled asperities surrounded by large uncoupled regions, and this would be unlikely to allow large ruptures to develop. The weights for these branches were selected correspondingly. This picture of relatively strong coupling matches the observations from Raoul, and much more weight would be assigned to the high maximum magnitude and strong coupling combination if it was found that strong coupling exists at other trench locations.

For the southern New Hebrides, the combination of a short historical and instrumental record and a relatively short section of subduction zone, provides insufficient evidence with which to preferentially weight the maximum magnitude branches to the logic tree; in this situation we have selected a uniform distribution of weights between the largest historical event and the magnitude of a rupture spanning the full subduction zone. For the coupling coefficients we have placed greater weights on the low and medium strength coupling options in response to the elastic block modelling (Fig. 22).