Constraints on the timing of the three most recent surface rupture events and recurrence interval for the Ohariu Fault: Trenching results from MacKays Crossing, Wellington, New Zealand

Abstract

A trench across the Ohariu Fault at MacKays Crossing revealed evidence and provided radiocarbon age constraints for three surface-rupture events in the last 5300 cal. yr BP. The two older events are dated for the first time at 5270-4410 and 4810-3260 cal. yr BP. The youngest event is correlated to the most recent event, dated elsewhere along the Ohariu Fault, at 1050-1000 cal. yr BP. Using statistical methods incorporating inter-event times, mean slip rate, and single-event displacement, and their uncertainties, a best estimate mean recurrence interval of 2200 yr is calculated for the Ohariu Fault, with minimum and maximum 68% and 95% confidence interval limits of 1300 and 3800 yr, and 800 and 7000 yr, respectively. Following the procedures outlined in the Ministry for the Environment Guidelines, these data place the Ohariu Fault in Fault-avoidance Recurrence Interval Class II (>2000 to 3500 yr), with medium to low confidence of classification.

Conditional probability of rupture of the Wairarapa and Ōhariu faults, New Zealand

“…The probability distribution rises to a maximum at the maximum of the 95% calibrated age range of the relevant bounding radiocarbon age (4870 yr ago; T1-5 of Litchfield et al 2006). The maximum bound of the age range of Event III (5330 yr ago; AD 2010 datum) is defined by the maximum of the 95% calibrated age range of the relevant bounding radiocarbon age (T1-6 of Litchfield et al 2006). From its maximum bound, the probability distribution rises to a maximum at 5110 yr ago (AD 2010 datum) based on the mid-point of the 95% calibrated age range of the relevant bounding radiocarbon age (T1-6 of Litchfield et al 2006).…”

“…The maximum bound of the age range of Event II (4870 yr ago; AD 2010 datum) is defined by the maximum of the 95% calibrated age range of the relevant bounding radiocarbon age (T1-5 of Litchfield et al 2006). From its maximum bound, the probability distribution rises to a maximum at 4470 yr ago (AD 2010 datum) based on the minimum of the 95% calibrated age range of the relevant bounding radiocarbon age (T1-5 of Litchfield et al 2006). We chose the minimum of the 95% calibrated age range of sample T1-5 as the point at which maximum probability is reached, as apposed to the mid-point of its age range, because sample T1-5 provides not only the maximum bounding age for Event II but also the minimum bounding age for Event III (see Litchfield et al 2006 for more detail).…”

“…4). The minimum bound of the age range of Event II (3320 yr ago; AD 2010 datum) is defined by the minimum of the 95% calibrated age range of the relevant bounding radiocarbon age (T1-8 of Litchfield et al 2006). The probability distribution rises to a maximum at the mid-point of the 95% calibrated age range of the relevant bounding radiocarbon age (3425 yr ago; T1-8 of Litchfield et al 2006).…”

“…The minimum bound of the age range of Event II (3320 yr ago; AD 2010 datum) is defined by the minimum of the 95% calibrated age range of the relevant bounding radiocarbon age (T1-8 of Litchfield et al 2006). The probability distribution rises to a maximum at the mid-point of the 95% calibrated age range of the relevant bounding radiocarbon age (3425 yr ago; T1-8 of Litchfield et al 2006). The maximum bound of the age range of Event II (4870 yr ago; AD 2010 datum) is defined by the maximum of the 95% calibrated age range of the relevant bounding radiocarbon age (T1-5 of Litchfield et al 2006).…”

“…The timing of Event III is constrained to have occurred between 4410 and 5270 cal yr BP (Litchfield et al 2006). In our calculations of conditional probability, we adopt a trapezoidal-shaped probability distribution for the timing of Event III (Fig.…”