REVERSE FAULT EARTHQUAKE PAIRS: SAN FERNANDO VALLEY 1971 AND 1994 AND NELSON, NEW ZEALAND 1929 AND 1968

The 1994 Northridge earthquake ruptured a blind reverse fault beneath and across strike from another reverse fault dipping in the opposite direction on which the 1971 Sylmar earthquake occurred. The blind Northridge fault was not recognized before the 1994 earthquake, but uplift of the footwall of the overlying Santa Susana fault during the earthquake led to evidence of long-term uplift of that fault, which occurs within the Santa Susana Mountains rather than at a range front. The 1994 earthquake occurred on both sides of the lateral ramp that marked the western termination of the 1971 earthquake, but the western edge of main moment release was close to the Gillibrand Canyon lateral ramp farther west. Aftershocks were recorded west of this lateral ramp to the western end of the Santa Susana fault at the surface. The 1968 Inangahua, New Zealand, earthquake (M 7.2) ruptured the west-dipping, largely blind Rotokohu fault that is close to the east-dipping Lyell fault at the surface. The earthquake was accompanied by secondary surface rupture, uplift of the footwall of the Lyell fault, and reactivation of the Lyell fault in the opposite sense from its long-term separation. The southern boundary of secondary surface rupture is a lateral ramp connecting the blind Rotokohu fault to a blind fault at the western margin of the Grey-Inangahua Depression. South of the lateral ramp, the blind fault is at the Paparoa Range front, with late Cenozoic gravels preserved in the footwall, whereas to the north, the fault is in the mountains, and gravels are not preserved. Farther east and across strike, the 1929 Murchison earthquake (M 7.7) produced surface rupture on the east-dipping White Creek fault, which has the same sense of displacement as the Lyell fault to the west. The unanswered question in both earthquake pairs: why did the second earthquake occur across strike rather than along strike from the first?