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