The 1968 Inangahua, New Zealand, and 1994 Northridge, California, earthquakes: Implications for northwest Nelson

Abstract

The 1968 Inangahua earthquake ruptured a west‐dipping, largely blind reverse fault, here called the Rotokohu Fault, that is close to the east‐dipping Lyell Fault at the surface. Most surface rupture was secondary. The Lyell Fault was reactivated west‐side‐up in 1968, in the opposite sense to its long‐term displacement. The west‐dipping Inangahua Fault was also reactivated in the opposite sense to its long‐term offset of west‐side‐up; this rupture may have been related to folding. The southern boundary of secondary surface rupture is a “cross‐basin structure”, a lateral ramp connecting the blind Rotokohu Fault to a blind fault at the western margin of the Grey‐Inangahua Depression. Increase of Coulomb stress at the lateral ramp in 1968 would shorten the time before failure of the continuation of the Rotokohu Fault south of the lateral ramp. In its proximity to another fault with opposite dip, the 1968 earthquake resembles the 1994 Northridge, California, earthquake, in which the blind source fault was beneath another active fault dipping in the opposite direction, uplifting its hanging wall and footwall. The blind, thick‐skinned reverse fault on which the 1968 earthquake occurred is marked at the surface by a monocline with evidence of late Quaternary folding and secondary flexural‐slip faulting. Shortening rates in northwest Nelson are higher in the Murchison Basin than they are farther west, suggesting that the high seismicity from the White Creek Fault to Westport may not be representative of long‐term behaviour and earthquake potential.