Charles, I have seen such arrangements for exact repositioning in kinematic mounts for telescope objective instruments and some differential GPS monuments. But I was being a bit hyperbolic in suggesting that a quake would slide an LP seis (but speaking from experience in noting that > I < might shove it off the glass plates). We had an Mb 8.6 at Adak in 1986, 30 km from the main vault that indicated almost half of g on the seismoscope; the LP seismometers slid a couple of mm, as indicated by tracks in the dust on the glass plates, and remained functional but tilted off center by several mm. Regarding the carpet discussion: I would not follow the advice that an LP horizontal with sufficient sensitivity can be operated on top of a carpet, especially a horizontal with its very high tilt sensitivity. By "sufficient sensitivity" I mean the ability to routinely record the 6-second microseism background with at least a 10:1 signal to noise ratio. These typically run from 2 to 10 (during a storm) microns peak-to-peak. Magnitude formulas want the wave amplitude entered in milli-microns (nanometers), so smaller events have to be read over the mean of the microseisms. It is not a matter of spectral sensitivity, since all the seismic waves of interest are much larger than the instrument (a 1 second surface wave is 2+ km long). But the P-wave of a M 5.7 near Mexico city is only a few times larger in amplitude than the microseisms at St. Louis, and wouldn't be seen on a less sensitive system. If I don't see the microseisms I try to find out what went wrong. Seismologists have not gone to all the effort of piers, vaults, proximity to bedrock, etc, because they have too much money (fat chance), but to reduce the noise of the instrument environment. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>