Paleoseismicity of the Ashley & Loburn faults, North Canterbury, New Zealand

Technical Abstract

A paleoseismic investigation was carried out in 1999 on the Ashley Fault, known from a 4.5km long, east-west scarp offsetting late quaternary fluvial surfaces north of the Ashley River. A second, parallel fault scarp was identified approximately 1.5 km to the north, named the Loburn Fault, projecting west in a convex arc to join the Ashley Fault at the confluence of the Ashley and Okuku Rivers.

Cycles of pleistocene aggradation, together with regional deformation, caused the migration of the local rivers, leaving former channels to be warped and displaced by structures propagating across them. Specifically, three former channels crossed the tilted and elevated wedge between the Ashley and Loburn Faults. These were progressively abandoned from east to west, Loburn 1 being occupied during Waimean to early Otiran times (Woodland-Windwhistle Formations ca 120-65 ka BP), Loburn 2 by late Otiran to early Aranuian (incised into Burnham Formation <18-22 ka BP) and Loburn 3 by mid Aranuian (grading to Springston Formation <10 ka BP). The Loburn is the older fault, causing uplift to a maximum scarp height of 28 m across the oldest channel, but only forming a 1 m high scarp, derived from two events, across the Loburn 2 channel and with no trace across Loburn 3. Conversely, the Ashley Fault appears to have been propagating eastward much later, generating a 1.5 m scarp across Loburn 3, but fading eastward on contemporaneous Springston surfaces. Propagation may be as young as latest Pleistocene, because a maximum scarp height of 5.5 m on the Burnham aggradation surface is only slightly more than the 5.0 m on the Loburn 2 channel incised into it.

Trench and scarp data suggest increments of between 0.2 to 0.5 m, throw per event along this scarp, plus a component of dextral slip. Four trenches yielded clear stratigraphic evidence of two or more events, but only one, across the 1.5 m scarp in the Loburn 3 channel, provided any radiocarbon datable material. In the trench clear evidence of one undated event during river occupation was followed by one post-abandonment offset, immediately preceding a dated peat at 4,785±255 years BP. Two further cycles of undeformed peat and silt may date scarp activity at 3,765±70 and 3,420±60 years BP, or merely reflect flooding. Higher degradation terrace offsets cumulating to 2.5m indicate one earlier event during Loburn 3 channel incision. Therefore three to five ground rupture average 0.5 m slip (Wells and Coppersmith, 1994) is indicative of an M6.6 earthquake, a conservative estimate for the following reasons. The throw is only a component of oblique net slip, maximum displacements are not to be expected close to the propagating end of a young fault, and the Ashley Fault is now known to be one segment of a fault zone which stretches for 40 km across the Canterbury Plains. Ruptures of this length generate M 7.0 or greater events. Uncertainties are compounded by the coincidence of the Ashley termination with the strike of the newly discovered northeast-southwest striking Springbank thrust, suggesting interlinked activity and possibly extending the potential seismic source generated by the combined system much closer to Christchurch city.