Note: The two-color EDM at Parkfield has been retired after 20 years of service. The last set of measurements
were made in the spring of 2005. Monitoring the deformation at Parkfield is now being done with
GPS. For continuity with the older, EDM measurements, some of GPS measurements can be overlayed
against the EDM data; these are shown in the section: Combination EDM and GPS line-length changes from 1984.
Otherwise, the GPS data can be seen at Parkfield GPS monitoring page.
Click on the map to get a seperate copy
The map shows the locations of baselines that are measured near Parkfield, California, using a two-color laser distance measuring instrument (EDM).
The
two-color EDM measures distances to a precision of 0.3 to 1.0 mm for ranges
between 1 and 9 km.
The locations of these baselines are shown in relation to the San Andreas
fault and the location of the 1966 M6 Parkfield earthquake.
Plots of the changes in length of these frequently measured baselines can be shown. These baselines,
which
use the central station at CARR as a common end point, have been
measured approximately 3 times each
week since mid-1984.
The baselines
are color coded to indicate the relative stability of the end point monuments.
RED indicates that the reflector end of the baseline is unstable in that
it responds to a combination of local soil disturbances due to rainfall and
the tectonic signal. BLUE indicates that the end of the baseline responds
primarily to tectonic signals and these baselines are relatively free of
contamination due to local soil disturbances. GREEN indicates that the baseline
responds to local soil disturbances to a degree between RED and BLUE.
The observations of distance changes are contaminated by 3 sources;
1) short-term instrument instability, 2) localized movement of the central monument
at CARR, and 3) localized movement at the remote reflector sites. The degree
of localized monument instability at the remote sites is shown with the
colors, but there is no attempt to adjust for these non-tectonic displacements.
Since the other 2 instabilities are present in all data, estimates of these
parameters are calculated and their effects are removed from the data.
Instrument instability appears to be a length proportional error which is
best revealed with comparisons of distance changes between the Parkfield
two-color EDM and a second two-color EDM. The instrument
corrections shown in the plots were computed by: 1) for each baseline, removing
the mean value, secular rate, and seasonal fluctuations; then 2) for
each day where a sufficient number of baselines were measured, computing
the average, residual line-length normalized to its length, which is termed
the "instrument" adjustment"; and 3) adjusting
the data by removing the effect of the "instrument adjustment" and adding
back the secular rate, seasonal fluctuations and mean value. In the plots,
the "Instrument adjustment" shown is for a 5 km long baseline.
The central monument at CARR is also unstable. Since distance measurements
are made over the entire 360 degree arc, it is possible to track the position of
the central monument with time. However, since some of the distant reflector
monuments are noisy, too, only the distance changes from the most stable
monuments are used to track the position of the central monument. Once the
position of the central monument with time is computed, the data from all
baselines are adjusted for the movement of the central monument.
Click
here for the raw data
Index
Location of geodetic network at Parkfield
Plotting the data
Data since mid-1984 (NO LONGER UPDATED)
Data for past year (NO LONGER UPDATED)
Combination EDM and GPS line-length changes from 1984
--- NEW!
(back to index)
Description of data reduction
Raw Data
For more details, contact:
