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

The 1968 Inangahua earthquake ruptured a west-dipping, largely blind revers e 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 F ault was reactivated west-side-up in 1968, in the opposite sense to its lon g-term displacement. The west-dipping Inangahua Fault was also reactivated in the opposite sense to its longterm offset of west-side-up; this rupture may have been related to folding. The southern boundary of secondary surfac e 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-Inangahu a Depression. Increase of Coulomb stress at the lateral ramp in 1968 would shorten the time before failure of the continuation of the Rotokohu Fault s outh of the lateral ramp. In its proximity to another fault with opposite d ip, the 1968 earthquake resembles the 1994 Northridge, California, earthqua ke, in which the blind source fault was beneath another active fault dippin g in the opposite direction, uplifting its hanging wall and footwall. The b lind, thick-skinned reverse fault on which the 1968 earthquake occurred is marked at the surface by a monocline with evidence of late Quaternary foldi ng and secondary flexural-slip faulting. Shortening rates in northwest Nels on 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.