Understanding non-elastic effects in accretionary wedges using the Kaikoura earthquake: Investigating a major potential tsunami hazarard

When an earthquake occurs, elastic energy that has been stored in rocks that have been undergoing gradual distributed deformation is released suddenly along one or more planar discontinuities known as faults. By definition faults are non-elastic, as they relieve stored elastic energy, localising damage. However, where the rocks on either side of the fault are not significantly damaged, horizontal and vertical coseismic surface movement determined from field observations, InSAR, or other techniques may be used to construct an elastic half-space model that predicts the direction and magnitude of slip at a full range of depths on the fault plane. However, in some cases these models are compromised by non-elastic behaviour of rock bodies, leading to inconsistent surface rupture results and likely significant errors in slip-at-depth estimates. Non-elastic effects in this sense may be caused by movements on subsidiary faults, folding of rock strata, change of shape of soft, deformable zones, and viscoelastic deformation of the ductile lower crust; these effects are witnessed most evidently by post-seismic movements, commonly observed in the days to months after large earthquakes, and also by aftershock earthquake events that are located off the main fault.

Improving our understanding of these effects is important, as the relationships between fault slip and earthquake magnitude forms a significant component of seismic hazard analysis. In this specific case, improving our model of slip along all faults involved in the 2016 Kaikoura earthquake will enhance models of crustal stress accumulations in this region, key to developing improved short and medium term earthquake forecasts and seismic hazard analysis. Further, developing an improved insight of the mechanisms that pertain in accretionary prisms located above subduction interfaces is highly significant for understanding tsunami generation, both in New Zealand and elsewhere.

We have an opportunity to take advantage of recent significant conceptual and processing developments in the field of 3-D strain modelling made at the Earth Observatory of Singapore (EOS), an Institute of Nanyang Technical University, Singapore. Combining geophysical and mathematical skills, the group headed by Asst. Prof. Sylvain Barbot has developed an approach to modelling complexity in crustal deformation that is significantly more computationally efficient than previous numerical approaches. This represents an unprecedented opportunity to take advantage of the very large and complex Kaikoura earthquake of 14th November 2016 to study in detail how accretionary wedges adjust their shapes as the position of tectonic plates, and the different components of the continental-ocean margin, evolve through time. The high quality data that we have collected as part of our on-going NERC urgency award (NE/P021425/1) makes this an attractive research collaboration for the EOS research team, while for us, it significantly extends the scope of our project, providing the opportunity to leverage our results to develop significant discoveries in the fundamental behaviour of fault movement, earthquake generation and landscape evolution. This will add significant value to the existing NERC-funded project, and form the basis for longer term research collaboration, whilst delivering important new information for seismic hazard analysis, helping to build resilience against natural hazards.

In the short term, during the time-period of the International Opportunities Fund (IOF) grant, the direct beneficiaries will be our NERC urgency project (NE/P021425/1) partners GNS Science (New Zealand), our industry partner Geospatial Research Ltd (GRL, UK) on that project, as well as the local rural landowners around the rupture on the northern extend of the South Island of New Zealand. GNS Science will benefit from a significantly enhanced analysis of existing observations beyond that performed to date, providing an improved understanding of the geological and fault structures at depth over a significant area. This analysis will also provide an opportunity to re-evaluate past earthquakes in the Marlborough Fault Zone (MFZ) and beyond, for example the historic ruptures of 1848 (estimated Mw of 7.8) on the Awatere fault and 1855 (estimated Mw of 8.2) on the Wairarapa fault. This MFZ is located close to New Zealand's capital city, Wellington (population 400,000), besides other significant population centres (Christchurch, Nelson, Blenheim), and improved understanding of the processes involved in MFZ earthquakes has significant potential implications for estimating seismic hazard. This event has likely increased stress on faults in and around Wellington, but understanding non-elastic effects is highly significant in estimating where and by how much.

In both New Zealand and beyond, significant hazard is posed by tsunamis generated either directly by sub-sea surface rupture during earthquakes, or by undersea slides triggered by seismic shaking. The presence of "pop-up" structures in the 2016 Kaikoura earthquake, at a range of scales, highlights the need for understanding their genesis. Where these are located underwater, they have the potential to represent significant tsunamigenic sources. Pop-ups may themselves be regarded as a form of non-elastic response; determination of the particular geometric and frictional properties of faults responsible for these structures is important in understanding how and where non-elastic effects away from the main faults present themselves, and the threat that they may represent for submerged accretionary prisms above subduction zones such as in SE Asia.

During our recent fieldwork, we have developed relationships with around a dozen rural landowners whose properties straddle faults involved in the 2016 event. Landowners' on-going observations of ground movements, fence ruptures, changes in water courses and other effects were valuable in planning our early research efforts for our urgency project, such as GNSS site selection. Landowners are also keen to gather all information regarding on-going deformation, in particular for vertical movements that have implications for the vulnerability to future flooding and modified drainage issues.

Longer Term and indirect impacts and benefits:

Development of these novel mathematical and modelling techniques, and their application to such a large and well-instrumented event, will widen impact by providing an approach for the improved interpretation of future events globally. Over the much longer term into the next decade, far beyond the timescale of this IOF project, the potential beneficiaries will be organisations charged with updating earthquake rupture forecasts and populations that these forecasts are designed to help protect. Updates to national scenarios will benefit populations with improved forecasts that are more likely to include the potential for these large complex earthquake rupture events, and by improving the preparedness for large earthquakes, resulting in reduced fatalities, injuries and property losses.