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

Abstract

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 younger than 8330–8610 yr, the youngest being a historically recorded Mw ∼ 7.5 event in A.D. 1848 that ruptured the entire eastern section. The maximum mean recurrence interval is ∼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 displacement of 4.9 m during the 1848 event. The second-smallest offsets are approximately double the smallest displacements, suggesting that the last two events were ruptures of comparable dimensions (>105 km) and magnitude. Because the Awatere fault terminates <20 km northeast of the study area, the penultimate rupture probably extended inland, perhaps rupturing the entire eastern section. Further evidence of characteristic earthquake behavior is preserved in the fault trench, where a regular event subsidence of ∼50 cm per event is indicated. During the last two events a slip maximum of 7 ± 1 m occurred on a 5.5-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 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.