Abstract - Paleoearthquakes at the Earth’s surface often produce variable recurrence intervals on individual faults. The origin of these variations and the extent to which they result from systematic processes is unresolved. To address this question we analyse displacements of geomorphic surfaces and of tephra horizons in 29 trenches excavated across normal faults in the Taupo Rift, New Zealand. Each trench typically yields information on 3-6 paleoearthquakes that ruptured the surface over the last 16-23 kyr. Recurrence intervals on 25 faults, which are distributed across the 15 km wide rift and record ca. 30-40% of the total extension, were constrained by geochemical fingerprinting and radiocarbon dating of up to 12 stratigraphic layers. Recurrence intervals range from ca. <1 ka to 15 ka, with the variability for individual faults often reaching an order of magnitude. We find that much of the variability in recurrence intervals on individual faults reflects a temporal and spatial clustering of paleoearthquakes on timescales of 5-10 kyr. The timing of periods of earthquake clustering are typically different for individual faults which, together with the uniform rate of total extension across the rift during the last 60 kyr, suggests that fluctuations in recurrence intervals were not driven by changes in the rift boundary conditions. Instead, we propose that the variable recurrence intervals arise principally due to fault interaction, which is achieved by faults intersection and deformation of the rock volumes between faults that do not intersect. All faults in the rift are components of a single kinematically coherent system for which the earthquake histories of each fault are interdependent. The largest faults in the system exert the greatest influence on the location and timing of paleoearthquakes and appear to be capable of stimulating or retarding paleoseismicity on smaller faults.
Abstract of talk given to:
International Union of Geodesy and Geophysics XXIV, Perugia, July 2007.