--> --> Abstract: Reverse Fault Earthquake Pairs: San Fernando Valley 1971 and 1994 and Nelson, New Zealand 1929 and 1968, by R. S. Yeats; #90904 (2001)
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Reverse Previous HitFaultNext Hit Earthquake Pairs: San Fernando Valley 1971 and 1994 and Nelson, New Zealand 1929 and 1968

R. S. Yeats
Geosciences, Oregon State Univ, Corvallis, OR

The 1994 Northridge earthquake ruptured a blind reverse Previous HitfaultNext Hit beneath and across strike from another reverse Previous HitfaultNext Hit dipping in the opposite direction on which the 1971 Sylmar earthquake occurred. The blind Northridge Previous HitfaultNext Hit was not recognized before the 1994 earthquake, but uplift of the footwall of the overlying Santa Susana Previous HitfaultNext Hit during the earthquake led to evidence of long-term uplift of that Previous HitfaultNext Hit, 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 Previous HitfaultNext Hit at the surface. The 1968 Inangahua, New Zealand, earthquake (M 7.2) ruptured the west-dipping, largely blind Rotokohu Previous HitfaultNext Hit that is close to the east-dipping Lyell Previous HitfaultNext Hit at the surface. The earthquake was accompanied by secondary surface rupture, uplift of the footwall of the Lyell Previous HitfaultNext Hit, and reactivation of the Lyell Previous HitfaultNext Hit in the opposite sense from its long-term separation. The southern boundary of secondary surface rupture is a lateral ramp connecting the blind Rotokohu Previous HitfaultNext Hit to a blind Previous HitfaultNext Hit at the western margin of the Grey-Inangahua Depression. South of the lateral ramp, the blind Previous HitfaultNext Hit is at the Paparoa Range front, with late Cenozoic gravels preserved in the footwall, whereas to the north, the Previous HitfaultNext Hit 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 Previous HitfaultNext Hit, which has the same sense of displacement as the Lyell Previous HitfaultTop 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?

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California