Prehistorical earthquakes on the Alpine fault, New Zealand

Studies of synchronous prehistorical landslides in rugged plate boundary mountain ranges can be used to estimate times of earthquakes, to locate the earthquake-generating fault zones, and to describe patterns of seismic shaking. Lichenometry can date regional rockfall events caused by large earthquakes in the Southern Alps of New Zealand with a 95% confidence level precision of less than or equal to 10 years. Spatial variation of rockfall abundance (an index of seismic shaking) is a function of the intensity and duration of seismic shaking and hillslope sensitivity to earthquakes, Four of the earthquakes of the past 1000 years appear to have caused exceptionally widespread regional rockfall events that were largest opposite the 350-km long Cook segment of the Alpine fault. Lichenometric age estimates (+/-10 years) of these Cook segment earthquakes at 1748, 1489, 1226, and 967 A.D. are supported by radiocarbon and weathering rind age estimates of times of coseismic geomorphic processes and by times of prehistorical forest-disturbance events. A mean earthquake recurrence interval of 260 years +/-15 years and similar patterns of seismic shaking imply constant rupture length and slip per event in accord with the characteristic earthquake model.