Whitemans Valley Fault and the earthquake-generating potential of Wellington's second order faults

Abstract

A NNE-SSW-tending linear scarp was trenched in Whitemans Valley, near Upper Hutt, and proved to be a fault scarp. Part of the c. 8-9 m high fault scarp trenched revealed twin reverse fault planes dipping at c. 45° to the west. Sediments displaced vertically by the fault planes include fan gravel and three loess/paleosol couplets. The fan gravel represents the youngest alluvial sedimentation at the site and at c. 80 thousand years (ka), it forms the oldest displaced unit seen in the trench. Chronology for the trench is based on coverbed stratigraphy, which consists of loess/paleosol couplets correlated with Porewa (c. 80-65 ka), Rata (c. 60-30 ka) and Ohakea (c. 24-14 ka) loesses. Possible dextral displacement of a stream channel suggests a dextrallateral component of slip along the fault, although there are no quantifiable data available.

With the exception of the topsoil, each unit in the trench has been vertically displaced by between 1.4 and 2.1 m (equivalent to dip-slip displacement of c. 2-3 m), which we interpret as evidence of a single rupture event. Younger units in the trench are apparently displaced more than older sediments, a fact best explained by assuming a strike-slip component (possibly with some SW dip on older sediments) and by variation in unit thickness. The rupture cuts and deforms a paleosol developed in the Ohakea Loess which is considered to be c. 10 ka or younger, but the topsoil is undeformed. Using the assumption that the c. 1.8 m vertical displacement is typical for a single rupture event, the c. 8-9 m high scarp represents 4 or 5 events. Rupture recurrence interval is estimated at 15-20 ka.

The fault can be mapped for a distance of c. 20 km between Witako Valley (SE Upper Hutt) and Wainuiomata. On the basis of the 2-3 m dip-slip displacement per event,  rupture on the Whitemans Valley Fault is thought to result in a M=7.04.3 earthquake.
This work outlines the rupture characteristics of a single "second order" fault in the Wellington region. It is conceivable that 10 similar "second order" faults in the Wellington region are active, suggesting that these faults may collectively contribute significantly to the region's seismic hazard.