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

ANNE-SSW-trending linear scarp, trenched in Whitemans Valley near Upper Hut t, proved to be a fault scarp upthrown to the west. The trench, in the stee pest part of the 8-9 m high fault scarp, revealed two reverse fault planes dipping at c. 45 degrees to the west. Sediments displaced by the faults inc lude weathered fan gravel, three loess/paleosol couplets including the Pore wan, 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 trenc h. With the exception of the topsoil, each unit in the trench has been displac ed 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 unit s in the trench than older units and possible dextral displacement of a str eam 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 tops oil 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 we st of the trench. The vertical displacement of c. 2 m observed in the trenc h 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. If so, rupture recurrence interval is estimated at 15-20 0 00 yr. The c. 3 m, single event, dip-slip displacement is probably a minimu m 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 though t to result in M 7.0 +/- 0.3 earthquakes. The Whitemans Valley Fault is jus t one of conceivably 10 similar active "second order" faults in the Welling ton region, suggesting that collectively these faults may make a noticeable contribution to the region's seismic hazard.