Recent rupture of the Tararua section of the Wellington Fault and relationships to other fault sections and rupture segments

Technical Abstract

The slightly arcuate, concave to the SE, southern-most 75 km long section of the Wellington Fault was identified ten years ago as the Wellington-Hutt Valley segment (W-HV). Here, we tentatively define the next two rupture segments to the north, based on dated natural exposures along the slightly arcuate, concave to the NW, Tararua section of the Wellington Fault, and seven trench exposures on the relatively straight Pahiatua section further to the north.

At Totara Flats, along the Tararua section of the fault, a young aggradation event that inundated and killed a forest is dated at 1650-1450 AD. This is in the same age range as the best-estimate timing for the most recent rupture of the W-HK and suggests that aggradation at Totara Flats was probably directly linked with rupture of the W-HV.

Along the Pahiatua section, the timing of ruptures recorded in two adjacent trenches located 6km north of the Tararua Range is different from the rupture timing derived from the remaining five trenches located a further 16-20 km to the NE. Because single event dextral displacement along the Pahiatua section of the fault is consistently recognized to be about 4±1m, we expect such displacement to correspond to rupture lengths in excess of many tens-of-kilometres.

Therefore, we conclude that the trench sites on the southern part of the Pahiatua section, that exhibit a different rupture history to those nearer the Manawatu Gorge, must rupture with the Tararua section of the fault in order to attain a rupture length consistent with single-event displacement size. We refer to this segment as the Tararua-Putara rupture segment (T-P).The remaining part of the Pahiatua section, extending northeast to at least the Manawatu Gorge, is interpreted to belong to a different rupture segment which we refer to as the Woodville segment (W ), Average dextral slip rate estimates for the W-HV segment and the W segment are 6-7.6 mm/yr and 4.6-7.2 mm/yr, respectively. Single event dextral displacement is c. 4±1 m for both W-HV and W, and also for the northern part of T-P that is north of the Tararua Range. From these data, indicative average recurrence intervals for the W-HV and the W of c. 600 yrs and c. 800-1000 yrs, respectively, are obtained. Indicative elapsed time since the most recent rupture on each of the three segments is 400, 1150, and 250 yrs for W-HK T-P, and W, respectively. Of the three segments of the Wellington Fault discussed in this report, the Tararua-Putara segment is the most "likely" to rupture in the future, with the elapsed time at or probably exceeding the average recurrence interval. Earthquakes of approximately Mw 7.4-7.8 are expected on each of the segments based on the above rupture parameters, or larger, if two or more segment were to rupture together.

In the most recent cycle of events none of the fault rupture events so far identified on the Wellington Fault overlap in time with adjacent segments, indicating that rupture has not extended onto adjacent segments, and has not given rise to stress-triggered, along-strike, segment failure. However the most recent surface rupture on T-P overlaps in time with the penultimate surface rupture on W-HV to the south and W to the north. Therefore, the possibility exists that this may have been a much longer single rupture (implying larger earthquake magnitude) or that coulomb stress triggering may have played a role in a possible cascade of linked events in prior cycles.