The old web page can be found here. As of May 30, 2008, I have discontinued updating this page.
This page contains links to plots of strain data from the PBO borehole strainmeter network. The plots presented below explore rate changes in these data. Specifically, a period of data is selected for the previous 100 days, previous 40 days, and previous 10 days; a rate is fit to the all of the data in those time intervals and, for the last 10% of the time series (10 days, 4 days, and 1 day) a rate change is also fit. The estimated rate change is compared with its standard error. The plots are updated once per day through automated scripts.
The "stack plots" in the table below show the changes in strain in two columns; the first being the tensor strain changes in geographic coordinates; and the second column being the instrumental strain changes in the local coordinates of the instrument. For instrument strain, the strainmeter consists of 4 extensometers; three arrayed at 120 degree angles from each other and a forth at 90 degrees with one of the three extensometers. If one of the extensometers is labeled the 1-axis and the extensometer at 90-degrees is labeled 2, then it is possible to identify two different shear components; the differential shear, E11 - E22, and the shear strain, 2E12. With the four extensometers, it is possible to combine the extensometers together to obtain the strains measured by the instrument. The two traces that show each component of the instrument strain represents two different combinations of extensometers for each of the strains.
| Region | Stack Plots | ||
|---|---|---|---|
| last 100 days | last 40 days | last 10 days | |
| Olympic Peninsula | 100 days | 40 days | 10 days |
| Southern Oregon | 100 days | 40 days | 10 days |
| Southern Calif | 100 days | 40 days | 10 days |
This new page represents consolidation of recent work with processing USGS borehole strainmeter data and a newer (better?) method to calibrate the PBO strainmeters. Nearly all of the scripts (programs) used to process and update these PBO data are also used to process the USGS strainmeter data. Hopefully, similarity of the scripts applied to the different data sets will result in better "up-keep" of the results presented here. Finally, both the raw and "cleaned" data should be obtained from UNAVCO.
The solid triangles on the map below mark the locations of the strainmeter being monitored here. Open and smaller triangles show many (but not all) of the PBO strainmeters that I am not analyzing for a variety of reason.
Each borehole strainmeter consists of 4 gauges that measure extension in four different directions. The regional strain measured by each gauge is primarily affected by the presence of the borehole, and the material used to cement the strainmeter into the borehole (about 100 to 200 meters depth). There are a number of secondary contributions that may affect the measurement, too.
Since the Earth Tide is a well known repeating source of strain, I have used both the M2 (12.42 hour period) and the O1 (25.82 hour period) to calibrate these strainmeters in terms of the dilatation (Eee + Enn), and two shear components, 2*Een and Eee - Enn. Principally, I have varied the parameter that discribes the orientation of strainmeter and the two parameters that characterize the isotropic coupling of the borehole strainmeter to the surrounding rock. In addition, I have allowed small variations of the gain of each strainmeter gauge (20%) and another 20% variation in the presumed isotropic coupling. More discussion on calibration can be found in a paper Hart et al. In addition, at nearly of all of the strainmeters (with the exception of those located in S, Calif.), I found that coupling of the vertical strain into the horizontal extensions was required to calibrate these strainmeters.
