Can you please take me off your address list? Thank you ---------- >From: S-T Morrissey>To: psn-l@.............. >Subject: 6-second microseisms >Date: Mon, Aug 30, 1999, 9:57 PM > >Re microseisms from hurricane Dennis: > >Here at St. Louis the level of the 6-second microseisms as recorded >from the STM8 broadband in the basement had increased by a factor of 10 over >the previous week. THe peak level from the VBB was about 20 millivolts. >With a broadband sensitivity of 5.29 mv/micron/second, this indicates >a peak velocity of 3.4 microns/second. At a period T of 6 seconds, >the angular frequency is w =2*pi/T, or 1.05*sec^-1. The velocity is >divided by w to give a peak displacement of 3.6 microns. > >Here are some notes I have previously posted: > >Re: 6-second microseisms and hurricanes: > >An interesting phenomenon in the eastern USA is that when a large >wind storm due to a deep barometric low occurs off the east coast, >particularly off of New England, the wave energy pounding on the >continental shelf propagates through the lithosphere under the >eastern USA as a 6-second surface wave, at about half the period of >the oceanic wave. This is also true of any storm off the east >coast, except that the period will be different, varying from 4 to >10 seconds. Storms off the southwest coast cause a longer storm wave, >but the predominant energy is still from 5 to 8 seconds. > >This storm microseism noise is the major reason that the original >global seismograph stations were configured as two separate instruments: >a SP or short-period, peaking in gain at about 1 second, at 50k to 100k, >and the LP, or long-period, peaking at 15 to 30 seconds, with magnifications >from 1.5k to 6k. During a microseism storm, even the LP record could be >a scribbled mess; usually the storm peak would only last for a several >hours, so we wouldn't interrupt the recording. > >With the advent of wide dynamic range digital recording and VBB response >instruments, we deal with the microseisms in later digital processing. >However, for a visible monitor record on a drum recorder, we usually >use a "twin-T" notch filter to reduce the microseisms by 40db (1/100). >The twin-T is passive and easy to make with 3 resistors and 4 capacitors. >Even with the filter, a strong storm can fill the record with noise, which >is easily seen as 10 waves per minute. > >A historical note on tracking hurricanes before the use of >satellites and aircraft: small seismograph arrays were used. >At our old station FLO outside St. Louis, two additional small >vaults were built 1/4 mile west and north of the main vault. >THen intermediate period (10 second) seismometers were used, >connected to 6-second galvanometers for photo recording with a >peak response at 6-seconds. >THe relative phase of the microseisms crossing the L shaped >array provided a vector pointing toward the origin if the waves. >It was much more accurate than 3-component particle motion analysis. >With several such stations (one was at Spring Hill, AL, another at >Rochester, NY) providing a pointer, the location of the hurricane >could be estimated, even when hundreds of km from shore. > >There are some examples of the spectral noise from microseisms on >my web page featuring figures and data; there is even an example of >a storm peak from a hurricane last year. >http://www.eas.slu.edu/People/STMorrissey/index.html > >Regards, >Sean-Thomas > >_____________________________________________________________________ > >Public Seismic Network Mailing List (PSN-L) > >To leave this list email listserver@.............. with the body of the >message: leave PSN-L _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>