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Parkfield, California: A Brief Explanation
Parkfield has been the site of moderate earthquakes (M about 6) about every
20 years. Based on the
historic record and other evidence, the U.S. Geological Survey had predicted
that a "Parkfield" earthquake would occur before the end of 1993. Although the
expected earthquake has yet to occur, Parkfield remains the most likely knowns
site in California for a moderate earthquake and is densely instrumented in
anticipation of that event. Our goal is to study the complete earthquake cycle
using many instruments to record the deformation prior, during, and after this
earthquake. These observations will guide both theoretical and laboratory studies
on the causes of earthquakes.
Crustal deformation monitoring at Parkfield
Geodesy
A two-color Electronic Distance Meter (EDM) measures the changes in distances
on several baselines near Parkfield at a precision of about 0.5 mm. These changes
reflect fault slip on the San Andreas and strain accumulation.
GPS (Global Positioning System) measures displacements at several points near
Parkfield with a precision of 2 to 3 mm in the horizontal and about 10 mm in the
vertical.
Volumetric Strainmeters
These instruments measure volumetric strain changes in the Earth near active faults.
Over short time periods they can detect changes of 1 part-per-billion (1 inch
in 16,000 miles).
Three-component Borehole Strainmeters
These instruments measure strain changes on 3 components in the Earth near active
faults. Over short time periods they can detect changes of 10 parts-per-billion
(1 inch in 1600 miles).
Creepmeters
Creepmeters continuously monitor fault movement within a few meters of fault zones
to characterize the rate and nature of fault slip. They can detect changes of
about 0.1 millimeters.
Magnetometers
These instruments measure local changes in the earth's magnetic field near active
faults. Over short time periods they can detect changes of as small as 1/50,000th
of the earth's magnetic field.
Water Level
Fluid pressure transducers in well boreholes measure changes in the height of
the ground water table associated with each well-aquifer system. Over short time
periods they can detect water level changes of a few millimeters.
Comments on this page or inquiries about the data referenced should be directed
to John Langbein (langbein@usgs.gov),
Malcolm Johnston (mal@usgs.gov), or Will Prescott
(wprescott@usgs.gov).
Last updated Jan 5, 2000