Schematic summary of research findings showing the sequence of slip
behavior. Credit: UC Riverside
Preparation and good timing enabled Gareth Funning and a team of
researchers to collect a unique data set following the 2014 South Napa
earthquake that showed different parts of the fault, sometimes only a few
kilometers apart, moved at different speeds and at different times.
Aided by GPS measurements made just weeks before the earthquake and data
from a new radar satellite, the team found post-earthquake fault movement,
known as afterslip, was concentrated in areas of loosely packed sediment. Areas
where the fault passed through bedrock tended to slip more during the actual
earthquake.
Sections of Highway 12, which runs through the earthquake zone, were broken
during the initial 6.0 magnitude earthquake and were further damaged in the
coming days due to afterslip. In some areas the afterslip damage exceeded the
initial damage from the earthquake.
"No one has seen variability in afterslip like we saw," said
Funning, an associate professor of earth sciences at the University of
California, Riverside. "This helps us address a big question: Can we use geology
as a proxy for fault behavior? Our findings suggest there is a relationship
between those two things."
The findings could have significant implications for earthquake hazard
models, and also for planning earthquake response. If geological information
can give a guide to the likely extent of future earthquakes, better forecasts
of earthquake damage will be possible. And if areas likely to experience afterslip can be
identified in advance, it can be taken into account when building or repairing
infrastructure that crosses those faults
California, in particular the Hayward and Calaveras
Faults, which run along the east side of the San Francisco Bay, seems more
susceptible to afterslip than other earthquake-prone regions throughout the
world, Funning said.http://phys.org/news/2016-07-post-earthquake-fault-movement.html
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