Collaborative Research: Towards an Understanding of the Collective Behavior of Regional Fault Networks: The Marlborough Fault System, New Zealand

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The primary goal of this project is to determine how different faults in regional fault networks interact with one another to accommodate the relative motion of tectonic plates over time scales ranging from one to a few dozen earthquakes. The research team will focus on a particularly promising study area in northern South Island, New Zealand, where relative motions between the Pacific and Australian plates are partitioned amongst a set of four, parallel strike-slip faults known as the Marlborough fault system. Historical and paleo-earthquake data from the Marlborough fault system, which provides a useful analog for similar fault system elsewhere in the world (e.g., northern and southern California, Northwest Turkey, Hispaniola, parts of central and Southeast Asia, Iran-Pakistan), reveal tantalizing hints of complex earthquake occurrence, with possible temporal and spatial clustering of earthquakes that varies from cycle to cycle. But currently there are too few fault slip rate and paleo-earthquake age and displacement data to fully assess the collective spatial-temporal behavior of the Marlborough fault system. In order to document in detail how the four Marlborough faults share the tectonic plate motions, the research team will determine the rates of slip along each of these faults at a variety of time scales, ranging from a few to a few dozen earthquakes, as well as the ages and displacements of past earthquakes. Key to this effort will be the acquisition of about 300 square kilometers of high-resolution lidar digital topographic data from the four main Marlborough fault system faults. These data allow the efficiently mapping and measurement in unprecedented detail of fault offsets ranging from about 100 meters down to the smallest offsets that occurred in the most recent earthquakes. The Marlborough fault system is a particularly target-rich environment in this regard because many of the large fault-crossing rivers in the Marlborough region exhibit suites of river terrace edges that have been offset by variable amounts. Combining these offset features with age data from different geochronometers (radiocarbon and optically stimulated luminescence) will yield exceptionally detailed fault slip rates at a range of time scales from individual ruptures back though several dozen earthquakes. The researchers will also excavate trenches across the four faults to determine paleo-earthquake ages and displacements, allowing cross-correlation with the youngest slip rates. The resulting data, together with existing data and the results of ongoing studies by other groups, will allow documentation of the behavior of the Marlborough faults over a wide range of temporal and spatial scales, providing the information necessary for systematic comparison with the earthquake behavior of similar systems elsewhere in the world.The primary aim of this project is to advance understanding of the way regional networks of large faults store and release seismic energy, with a particular focus on determining the relative importance of so-called emergent phenomena such as clusters of large-magnitude earthquakes and periods of transiently elevated storage of seismic energy that may not be expected in the current understanding of earthquake physics and that are not accounted for in current seismic assessment strategies. The results will help the seismic hazard community to understand how regional fault networks distribute deformation in time and space - the keys to developing more accurate, next-generation seismic hazard assessment strategies, as well as the basis for future modeling efforts aimed at understanding the causes of such phenomena. This international effort will expand already strong scientific collaboration between the US researchers and their New Zealand collaborators, benefitting both groups by fostering increased interaction between the groups, both of which face similar seismic hazards in their respective countries. Specifically, in addition to working closely with seismic hazard planners at the US Geological Survey to ensure timely implementation of their results, the PIs are actively collaborating with colleagues at GNS Science, which is responsible for implementation of seismic hazard assessment in New Zealand, ensuring that the results of this project will be also incorporated into New Zealand's next-phase seismic hazard assessments.