Late Quaternary movement on the Wellington Fault in the Upper Hutt area, New Zealand

Abstract

Abstract The Wellington Fault is one of the major active dextral strike-slip faults of southern North Island. Quaternary stratigraphic studies and fault mapping along the southern part of the fault in Upper Hutt have provided some data on slip rates and recurrence intervals. Horizontal offsets of terrace risers and channels on terrace surfaces range from 3.7 m to 900 m. Increasing numbers of loess units, separated by paleosols, on successively older fluvial terraces in the Upper Hutt and Kaitoke Basins, provide a means of dating the terraces and, together with displacement measurements, a means of estimating horizontal slip rates on the Wellington Fault. At Emerald Hill, a terrace with an estimated age of 14 ± 4ka is dextrally offset by 104± 10m(after correcting for tread width differences of the terrace on either side of the fault). An older terrace estimated to be about 70 ± 5 ka is offset by 437 ± 20 m, and a 940 ± 40 m offset is associated with a terrace about 140 ± 10 ka. These displacement/age pairs indicate a dextral slip rate of 6.6 -0.6 +1.0 mm/yr. The rate of vertical movement on the fault is low, generally less than 10% of the horizontal rate. There has been a change in the upthrown side of the fault with time. Whereas earlier movement was predominantly upthrown to the northwest, since about 14 ka the fault has, in many places, been upthrown to the southeast. A low-level sequence of terraces at Te Marua is appreciably younger than 14 ka. The smallest recorded offset is 3.7 m dextral and 0.3 m vertical (upthrown to SE) of a channel on the second to lowest terrace. Successively higher terraces or channels are dextrally offset by 3.7–4.7, 7.4, and 18.0–19.0 m. These values suggest single-event dextral offsets of 3.4–4.7 m and that the fault has moved with this characteristic amount of movement on each of the past five occasions. Earthquakes of surface-wave magnitude 7.1–7.8 are likely with this size of single-event displacement, consistent also with a c.75 km long fault-rupture segment that is proposed to extend from south of the Wellington south coast to the Kaitoke Basin. This segment is here named the Wellington-Hutt Valley Segment. The average recurrence interval of fault movement can be estimated from the long-term dextral slip rate and the size of single-event displacements. Using the long-term dextral slip rate of 6.6 -0.6 +1.0 mm/yr and 3.4–4.7 single-event displacement, the average recurrence interval falls in the range 485–783 years, with a mean value of 634 years. Subsurface characteristics at natural exposures and trenches show the fault to be steeply dipping, with dominantly strike-slip movement. Although the Wellington Fault comprises a zone several hundred metres wide of crushing in bedrock greywacke, the strand of the fault that has moved in the past few tens of thousands of years is narrow, < 10 m wide, which coincides with surface deformation features. It is highly probable that the next movement on the fault will follow this pre-existing line of recent rupture.