Paleoseismicity of the Rotoitipakau Fault Zone, a complex normal fault in the Taupo Volcanic Zone, New Zealand

Abstract

Trenching of 6 of the 10 fault strands that comprise the 5 km long, 1 km wide Rotoitipakau Fault Zone, Taupo Volcanic Zone, New Zealand, shows that surface rupture has occurred at least eight times in the zone during the past 8500 yr. Single‐event displacements on each strand vary from a few decimetres to perhaps more than 2.5 m, and there may have been as much as 4.5–5.0 m of cumulative slip on several strands in a single event. Five airfall tephra whose ages span the past 8500 yr provide time lines within which cumulative slip rate has varied by approximately 10 times: from 1–2.5 mm/yr in the period from the AD 1886Tarawera Tephra to the present, and in the c. 4000 yr period from the 4.8 ka Whakatane Tephra to the 0.65 ka Kaharoa Tephra; to 11 mm/yr in the c. 500 yr period from 0.65 ka to AD 1886. All but one of the fault strands are downthrown to the southeast, suggesting that the fault zone is part of the western margin of the Whakatane Graben. This short fault zone may therefore be a splay of the Braemar Fault Zone, and the very large displacement: length ratios characteristic of fault rupture in this fault zone may be misleading if the fault connects to the southwest with the Braemar Fault. Alternatively, faults in this volcano‐tectonic province may exhibit different surface faulting characteristics than normal faults in nonvolcanic regions where most of the fault parameter scaling relationships have been developed. Large displacement: length ratios are a characteristic of fracturing above dike intrusions. Such fracturing is accompanied by well‐defined grabens and single‐episode (monogenetic) formation. In the Rotoitipakau Fault Zone, the lack of graben development, and polygenetic movement history, indicates the fault zone is a primary tectonic feature.