PSN-L Email List Message

Subject: Re: 6-second microseisms
From: "Mark & Lori" mlpg@.................
Date: Tue, 31 Aug 1999 10:33:01 -0700


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)


[ Top ] [ Back ] [ Home Page ]

Larry Cochrane <cochrane@..............>