Investigating the timing and intensity of past earthquake activity on the southern Alpine Fault using soft-sediment deformation structures

Abstract

Extending along the West Coast from offshore Caswell Sound to the Martyr River, the southern Alpine Fault has the potential of producing earthquakes of Mw 7 or greater. Past studies have led to the development of a paleoseismology record spanning the last 8,000 years into the Holocene. However, no pre- Holocene earthquake records currently exist and it would be good to expand this record into the Pleistocene to understand the behaviour of the Alpine Fault over longer time scales.

The Cascade River is located in south Westland and follows part of the main trace of the southern Alpine Fault. Outcrops of proglacial silts in this valley are the focus of this study, containing horizons of folded silts, bound above and below by undeformed sediment. Horizons of deformed sediment are seismites formed by earthquake triggered slumping and the resulting seiche waves, which cause shear and folding of laminated silts.

Evidence of earthquake shaking was also observed in the ARM/κ, magnetic susceptibility, declination, and inclination of DRM in paleomagnetic analysis of these silts. These anomalies are hypothesised to result from redeposition and realignment of magnetic grains, block rotations, and increasing clastic sediment supply during earthquake events.

Radiocarbon dating results in average calibrated ages of ∼16,340 cal yr BP and ∼16,230 cal yr BP in the Cascade River and McKay Creek outcrops respectively. Based on these ages and sedimentological evidence, rhythmites are interpreted to be varves, used to aid in determination of the timing of these events. Three earthquake events are identified and their maximum ages are calculated to be 15,980 ± 66, 16,320 ± 88, and 16,650 ± 16 cal yr BP.

Based on the morphology of these horizons they were likely formed by shaking intensities of Mw 7 - 8. The average recurrence interval is calculated to be 370 ± 77 years, which is consistent with the Holocene recurrence interval of 329 ± 68 years determined by Berryman et al. (2012). It can be concluded based on this, that there has been no significant change in recurrence interval between Alpine Fault events of the late Pleistocene to the most recent events of the Holocene.