HISTORY OF VERTICAL DISPLACEMENT OF KEREPEHI FAULT AT KOPOUATAI BOG, HAURAKI LOWLANDS, NEW-ZEALAND, SINCE C 10700 YEARS AGO

Thirteen tephra layers interbedded with peat, and a basal peat horizon, in four pairs of cores provide radiocarbondated reference horizons that indicate vertical displacement on the Kerepehi Fault at Kopouatai bog, Hauraki Lowlands. Progressive offset of the horizons with time shows that vertical fault movement, downthrown to the west, has been occurring for the past c. 10 700 radiocarbon years at an approximately uniform rate of c. 0.13 mm/yr. Step functions indicate faulting events (earthquakes) at c. 1400, c. 5600, c. 6800, and c. 9000 years ago, a mean recurrence interval of c. 2500 years. These findings support geophysical and geological evidence that the Hauraki Depression is an active rift, and show that active faulting occurs along the northern as well as southern extensions of the Kerepehi Fault. If such earthquakes occur randomly in time, and based on the return period of 2500 years, there are 2%, 18%, and 33% probabilities of a major earthquake affecting the Kerepehi Fault at Kopouatai bog in the next 50, 500, and 1000 years, respectively.