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

摘要

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 ev-idence in a fault trench and measurement of fault-displaced geomorphic features. We identify at least six ground-rupturing events younger than 8330-8610 yr, the youngest being a historically recorded M_w approx 7.5 event in A.D. 1848 that ruptured the entire eastern section. The maximum mean recurrence interval is approx 1.4 k.y., although repeat intervals 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 dis-placement of 4.9 m during the 1848 event. The second-smallest offsets are approxi-mately double the smallest displacements, suggesting that the last two events were ruptures of comparable dimensions (>105 kin) and magnitude. Because the Awatere fault terminates <20 km northeast of the study area, the penultimate rupture prob-ably extended inland, perhaps rupturing the entire eastern section. Further evidence of characteristic earthquake behavior is preserved in the fault trench, where a reg-ular event subsidence of approx 50 cm per event is indicated. During the last two events a slip maximum of 7+-1 m occurred on a5.5-km-long fault subsection consisting of adjacent restoring and releasing fault bends.These bends define a 1-km-wide side step in the fault that,although of short wavelength,apparently strongly influences the amount of coseismic slip and moment release.New data on the age of late Qua-ternary 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.