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Parkfield Magnetometer Network
Figure. This map shows the locations of magnetometers that are installed near Parkfield, California.
The magnetometers measure absolute magnetic field simultaneously at all sites.
To descriminate against magnetic fields disturbances in the ionosphere and magnetoshpere,
and to isolate local magnetic fields, differences between adjacent sites are determined.
Magnetic fields are determined to a precision of 0.2 nT (nanoTesla).
The locations of these sites are shown in relation to the San Andreas
fault and the location of the 1966 M6 Parkfield earthquake.
The purpose of the experiment is to detect magnetic perturbations
that result from changes in the mean state of crustal stress since
both magnetic remanence and magnetic
susceptibility have a stress sensitivity
of about 0.0001 per bar. For a 10 bar stress change, this translates
into a local field change of about 1 nT if the magnetization is 0.1 A/m.
All data are transmitted by satellite to the USGS headquarters in Menlo Park, Ca.
Data samples are taken every 10 minutes.
Plots of long-term and recent data showing the changes in magnetic field between adjacent sites are shown as follows:
Data for the past week
- Plot Differenced Data
Caution, these data are generated automatically and have not been checked
for errors.
Data for the past month
- Plot Differenced
Data Caution, these data are generated automatically and have not been checked
for errors.
Data for past year
- Plot Differenced
Data Caution, these data are generated automatically and have not been checked
for errors.
Data reduction
Processing of magnetic field data involves the following steps
after storage on the lowfreq computer in Menlo Park:
1) conversion of raw, 10 minute magnetic field data to
nanoteslas.
2) simple differencing of the data from adjacent stations.
3) windowing of differenced data to remove spurious data points.
4) removal of windowed points from the converted 10 minute
data set.
5) generation of hour averages from the 10 minute differenced data.
6) smoothing the hourly differenced data with a 3-day window.
7) generation of smoothed daily averages.
In addition to the daily processing, corrections and/or
deletions to these data are made due to telemetry and instrument
malfunctions, field visits, equipment changes, and any other explainable
changes.
8) The daily data were detrended using least-squares
and the 10 year data plots are smoothed with a 11 day
running mean. These detrended data (1985-1995) can be found on
"thebeach" at /mag/PARK (binary files).
For more details or to obtain the raw data, contact:
- Bob Mueller (mueller@thecove.wr.usgs.gov)
- Malcolm Johnston (mal@usgs.gov)
