Whitemans Valley Fault: A newly discovered active second order fault near Wellington, New Zealand—implications for regional seismic hazard

Abstract

A NNE-SSW-trending linear scarp, trenched in Whitemans Valley near Upper Hutt, proved to be a fault scarp upthrown to the west. The trench, in the steepest part of the 8-9 m high fault scarp, revealed two reverse fault planes dipping at c. 45° to the west. Sediments displaced by the faults include weathered fan gravel, three loess/paleosol couplets including the Porewan, Ratan and Ohakean loesses, Kawakawa Tephra (22.6 ka), and topsoil. The fan gravel represents the youngest alluvial sedimentation at the site and, at c. 80 000 yr (ka), it forms the oldest displaced unit seen in the trench. With the exception of the topsoil, each unit in the trench has been displaced vertically 1.4-2.1 m (equivalent to true dip-slip displacement of c. 3 m), which we interpret as a single-rupture displacement. A possible dextral displacement component is suggested by greater displacement of younger units in the trench than older units and possible dextral displacement of a stream channel. The rupture cuts and deforms a soil B horizon developed in the Ohakean loess, which is considered to be c. 10 ka or younger, but the topsoil is undeformed. Displacement on the fault strands in the trench is insufficient to account for the total vertical displacement represented by the 8-9 m scarp height, indicating the existence of a further fault strand(s) immediately to the west of the trench. The vertical displacement of c. 2 m observed in the trench is our best estimate of displacement for the most recent surface rupture earthquake, and total vertical displacement on the scarp may represent four or five events. Ifso, rupture recurrence interval is estimated at 15-20 000 yr. The c. 3 m, single event, dip-slip displacement is probably a minimum net single-event displacement. On the basis of the minimum net slip of c. 3 m per event, and the mappable length of c. 20 km, surface rupture of the Whitemans Valley Fault is thought to result in M 7.0 ± 0.3 earthquakes. The Whitemans Valley Fault is just one of conceivably 10 similar active "second order" faults in the Wellington region, suggesting that collectively these faults may make a noticeable contribution to the region's seismic hazard.

National Seismic Hazard Model for New Zealand: 2010 Update

“…Slip rates have been constrained for many faults (e.g., Marden and Neall, 1990;Van Dissen and Berryman, 1996;Begg and Van Dissen, 1998;Heron et al, 1998;Mouslopoulou et al, 2007;Little et al, 2009;Carne et al, 2011). Where paleoseismic data were absent, however, slip rates were assigned from (1) rates on faults along-strike where possible, particularly in the central to northern area, (2) relative geomorphic expression, and (3) inferred regional slip-rate budget (e.g., taking into account the decreasing margin-parallel component to the north).…”