NSFGEO-NERC: Latest Pleistocene-Holocene incremental slip record of the Kekerengu-Jordan fault system, northern South Island, New Zealand

Mounting evidence suggests that the occurrence of large earthquakes on both single faults and more complex fault systems is not a random process. Increasing numbers of observations indicate earthquake clustering in time and space, with changes in the rates at which faults slip, and variations in the patterns of loading (which triggers earthquakes). Also seen are possibly coordinated waxing and waning of slip on mechanically complementary faults in regional fault systems, that is, one fault slipping for a period of time of up to several thousand years, but slip subsequently shifting to another nearby fault at a later time. Although a thorough understanding of both the causes and generality of these phenomena is of basic importance for fault mechanics, earthquake physics, and importantly, for more accurate assessments of seismic hazard, our ability to evaluate the importance of these behaviours has been severely limited by lack of available data. In particular, there are too few comprehensive observations of the timing of ancient earthquakes and measurements of changing fault slip rates over periods of many thousands of years to assess fully the collective behaviour of major plate-boundary fault systems in time and space. These major systems, which represent the locations where one tectonic plate slides past another one, are where many large and damaging earthquakes occur, and increasing our knowledge of how they work will significantly enhance our ability to forecast future earthquake probabilities, and to take steps to prepare for these events.

Our international team, including scientists from the UK, USA and New Zealand, will reconstruct the slip history of part of the Marlborough Fault System in South Island, New Zealand, over the last 15,000 years, and build a detailed record of ancient earthquake events over this time. We shall focus our efforts on the Kekerengu-Jordan Thrust System; these faults moved in the very large (magnitude 7.8) Kaikoura earthquake of the 14th November 2016, allowing us to relate this recent event that was recorded in significant detail to past earthquakes.

In the short term, during the time-period of this project, the direct beneficiaries will be our collaborators at GNS Science (New Zealand), as well as the local rural landowners around the research area near Clarence and Kekerengu, Marlborough District, on the South Island of New Zealand. GNS Science will benefit from a significantly enhanced analysis of past earthquake events, providing an improved understanding of earthquake behaviour over a significant area. This project will also provide an opportunity to re-evaluate the significance of past historic earthquakes in the Marlborough Fault System (MFS) and beyond, for example the Awatere fault rupture of 1848 (estimated Mw of 7.8) and the 1855 Wairarapa fault earthquake (estimated Mw of 8.2). Our project area and the MFS are located near Wellington, New Zealand's capital city with a population of 400,000; other nearby population centres include Christchurch, Blenheim and Kaikoura. Developing an improved understanding of the mechanisms involved in MFS earthquakes, and the whole plate boundary fault system, has significant potential implications for better estimation of seismic hazard.

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 observations of large scale "pop-up" structures which developed during the 2016 Kaikoura earthquake, besides smaller scale structures, highlights the need for understanding the regularity (or irregularity) of earthquakes on faults that are of particular concern as potential tsunami triggers.

During recent fieldwork in projects funded both by the NERC and the NSF, besides the activities of our New Zealand colleagues, we have developed relationships with rural landowners whose properties are close to faults involved in the 2016 Kaikoura earthquake. These landowners are keen to gather all information regarding earthquake risk, in particular vertical movements that have implications for the vulnerability to future flooding and modified drainage issues. Improving the understanding of how and when earthquakes occur on the fault system within the remit of this project, and studying in what ways these earthquakes vary in magnitude and recurrence interval, will be key to helping landowners plan as they develop their properties.

Longer Term and indirect impacts and benefits:

Development of a clear understanding of the timing of ancient earthquakes on the Kekeregu-Jordan Thrust System coupled to estimates for incremental slip at several key sites will provide a significantly enhanced baseline for estimating seismic hazard, beyond the historical records for the region. The potential beneficiaries will be organisations that provide earthquake rupture forecasts (primarily GNS, Wellington) and the populations that these forecasts are designed to help protect. Improved forecasts that include consideration of the large complex earthquake rupture events can enhance the preparedness for large earthquakes, resulting in reduced numbers of fatalities, severity of injuries and property damage.