I need some help. Can someone set me straight on reading BSM (borehole strainmeter) data? Please!
I've been looking at PBO data
for over a year, and this has me puzzled because of the large "units"
The BSM in question is B934
, in Leggett, CA. You can see its location and channel orientations
in this 2-page
Here are 2 images I grabbed from a few days ago (yes, I'm too lazy to update them, lol), showing a large increase in compression on channels 2 & 3.
The 1st shows 7 days, the 2nd, the past 30.:
(upward/postitive slopes are extensional, downward/negative slopes show compression)
If you click the B934 link above, and scroll down to the "all data" (since installation) you see a total of 5,245,000 counts.
The last 30 days show 513,900 counts.
So, almost 10% of the total compressive strain has accumulated in only the last month, out of a total of 64 months since installation. That seems
significant to me.
Unfortunately, only 1 other of the regional BSM's has data available, meter B045
, and it
doesn't show the same increase; probably because it is located North and East of the Mendocino Triple Junction, whereas B934 is South and
East---different stress regimes.
The 2 days that show the large increases, 15 January & 26 January do not coincide with any large, moderate, or even small quakes of interest in the
area. Borehole strainmeters are sensitive to pore pressures and barometric pressures, but the closest rainfall was on the 11th/12th & and 28th/29th,
as shown here
This is getting long-winded, but one more page to share.
If you look at the processed data
for the past 30-days, the strain scale used is nearly 10
Strain is dimensionless, but is talked about as if it had "units". Most strainmeters can pickup sensitivities of 0.1 nanostrain (equal to 1 part
per 10 billion---pretty sensitive).
Evelyn Roeloffs, USGS, states that, "10 microstrain is a ballpark value for the coseismic strain within 5 km of a M7 earthquake".
source, page 9
Um, that's big, especially for this location.
So......was this large increase in compression from the S-SSW direction from asesimic slip? It looks to have occurred over several hours in each
An instrument malfunction?
Or am I completely misreading this data--which is entirely possible.
Set me straight people.