A COMPARISON OF HISTORICAL AND PALEOSEISMICITY IN A NEWLY FORMED FAULT ZONE AND A MATURE FAULT ZONE, NORTH CANTERBURY, NEW-ZEALAND

The timing of large Holocene prehistoric earthquakes is determined by dated surface ruptures and landslides at the edge of the Australia-Pac ific plate boundary zone in North Canterbury, New Zealand. Collectivel y, these data indicate two large (M > 7) earthquakes during the last c irca 2500 years, within a newly formed zone of hybrid strike-slip and thrust faulting herein described as the Porter's Pass-to-Amberley Faul t Zone (PPAFZ). Two earlier events during the Holocene are also recogn ized, but the data prior to 2500 years are presumed to be incomplete. A return period of 1300-2000 years between large earthquakes in the PP AFZ is consistent with a late Holocene slip rate of 3-4 mm/yr if each displacement is in the range 4-8 m. Historical seismicity in the PPAFZ is characterized by frequent small and moderate magnitude earthquakes and a seismicity rate that is identical to a region surrounding the s tructurally mature Hope fault of the Marlborough Fault System farther north. This is despite an order-of-magnitude difference in slip rate b etween the respective fault zones and considerable differences in the recurrence rate of large earthquakes. The magnitude-frequency distribu tion in the Hope fault region is in accord with the characteristic ear thquake model, whereas the rate of large earthquakes in the PPAFZ is a pproximated (but over predicted) by the Gutenberg-Richter model. The c omparison of these two fault zones demonstrates the importance of the structural maturity of the fault zone in relation to seismicity rates inferred from recent, historical, and paleoseismic data.