Late Quaternary paleoseismic history and surface rupture characteristics of the eastern Awatere strike-slip fault, New Zealand

The recurrence and magnitude of paleo-earthquakes on the eastern section of the dextral strike-slip Awatere fault (> 105 km long) are determined from stratigraphic evidence in a fault trench and measurement of fault-displaced geomorphic features. We identify at least six ground-rupturing events youn ger than 8330-8610 yr, the youngest being a historically recorded M-w simil ar to7.5 event in A.D. 1848 that ruptured the entire eastern section. The m aximum mean recurrence interval is similar to1.4 k.y., although repeat inte rvals range from 605 +/- 235 to 2500 +/- 600 yr. The smallest fault offsets on the easternmost 27 km of the onshore part of the fault indicate a mean strike-slip displacement of 4.9 m during the 1848 event. The second-smalles t offsets are approximately double the smallest displacements, suggesting t hat the last two events were ruptures of comparable dimensions (> 105 km) a nd magnitude. Because the Awatere fault terminates < 20 km northeast of the study area, the penultimate rupture probably extended inland, perhaps rupt uring the entire eastern section. Further evidence of characteristic earthq uake behavior is preserved in the fault trench, where a regular event subsi dence of similar to 50 cm per event is indicated. During the last two event s a slip maximum of 7 +/- 1 m occurred on a 5.5-km-long fault subsection co nsisting of adjacent restoring and releasing fault bends. These bends defin e a 1-km-wide side step in the fault that, although of short wavelength, ap parently strongly influences the amount of coseismic slip and moment releas e. New data on the age of late Quaternary alluvial surfaces in the Awatere Valley, when combined with observed horizontal displacements, indicate that strike slip has accrued on the Awatere fault at a constant rate of 6 mm/yr for at least the past 20 k.y.