## PSN-L Email List Message

Subject: Re: Earthquake strength
From: =?ISO-8859-1?Q?J=F3n_Fr=EDmann?= jonfr@.........
Date: Thu, 10 Jul 2008 01:59:35 +0000

```Hi

Local earthquakes have higher frequancy compnment then telesesmic
earthquakes (more then 1500 km away).  Due to that fact a 30 second
sensor won't detect local earthquakes well or not at all. For optimal
dection of local and regional earthquakes you need a sensor that is 1Hz,
but geophones up to 4,5Hz also work quite well. But are not as senstive
to smaller earthquakes, by best dection is ML0,5 (unchecked size) at 18
km.

You might be missing local earthquakes in you area by just using a 30
second instrument.

Regards.
J=F3n Fr=EDmann.

On Wed, 2008-07-09 at 18:34 -0700, Barry Lotz wrote:
> Steiner
> S-T Morrissey posted a discussion about this to psn on 13 nov 1999. I
> plotted a log-log sheet
> with "sensor count" on the y axis and distance from sensor on the x
> axis and drew magnitude lines (m =3D3.0,3.5,4.0, etc) . I then plotted
> individual points for each quake I recorded on this graph. I got
> pretty good correlation.
> Regards
> Barry
>=20
> John Lahr  wrote:
>         Steiner,
>        =20
>         You could use the equation for Ms magnitude to compute the
>         expected
>         surface wave amplitude at your location. The equation is given
>         in this
>         http://jclahr.com/science/psn/magnitude_files/ML_magnitude.xls
>         under Sheet 2.
>        =20
>         This will just be an rough estimate of the amplitude you may
>         see
>         because the radiation pattern of the earthquake will also
>         effect the
>         amplitude.
>        =20
>         Cheers,
>         John
>        =20
>         At 02:08 AM 7/8/2008, you wrote:
>         >For my web page I automatically retreave current earthquake
>         data from
>         >USGS and NORSAR in order to make a table of earthquakes which
>         my
>         >instrument should be able to detect. I then calculate arrival
>         times
>         >for the P and S waves and add that to my plot as well.
>         >
>         >Quakes that satisfy the equation sqrt(10)^(m+2.5) > d, where
>         m is the
>         >magnitude and d is the distance in km, are currently in my
>         list, and
>         >that limit seems roughly to correspond to what I can detect.
>         >
>         >That is, I'm assuming that the strength of an earthquake
>         drops by the
>         >square of the distance. Is that a valid assumptation for
>         earthquakes?
>         >
>         >--
>         >Steinar Midtskogen
>         >http://voksenlia.net/met/seismometer/
>        =20
>        =20
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