LATE HOLOCENE DISPLACEMENTS AND CHARACTERISTIC EARTHQUAKES ON THE HOPE RIVER SEGMENT OF THE HOPE FAULT, NEW-ZEALAND

On the Hope River segment of the Hope Fault, west of Hanmer Springs, New Zealand, a 36 m dextral offset of a dated river terrace riser indicates an average local late Holocene horizontal slip-rate of 10.5 +/- 0.5 m/kyr. A trench excavated across the fault in a nearby swamp revealed five silt layers within a 1.5 m thick column of peat. Radiocarbon dates and plant pollen indicate that the base of the swamp is approximately 700 years old, so peat has accumulated at an average rate of 2.35 +/- 0.6 mm/yr. The youngest silt layer was probably derived from a landslide triggered during an M 7-7.3 earthquake in 1888 AD, and the older silt layers are attributed to similar prehistoric (pre-1850 AD) earthquakes. Pollen from the lowest silt shows brief local dominance of lacebark (Hoheria cf. H. lyalli) during a phase of former beech (Nothofagus) forest. Sharp increases in the percentage of matagouri (Discaria toumatou) pollen follow the younger silt layers, which were deposited during a later period of fire-induced shrubland. Both lacebark and matagouri are vigorous colonisers of bared ground, and matagouri presently forms the dominant plant cover on fault scarps and landslides known to have been bared during the 1888 earthquake. We infer that this pattern of plant succession on bared sites has followed repeated surface rupture along this segment of the Hope Fault. The vertical spacing of the silt layers, and our calculated mean peat accumulation rate, indicate a recurrence interval for silt deposition of 81-200 years, and support a model proposing that the 1888 earthquake was a characteristic event for the Hope River segment of the Hope Fault.