Hi Chris.

Then you have a broadband seismometer, rather then a low period one. A
small local earthquake can have a frequancy up to 22Hz, if it is a small
one (0.0 up to 3.5). If it is a bigger earthquake, it has higher
frequancy responce. I see that in my data all the time.=20

Steinar Midtskogen

Earthquake detection also depends on what type of rock layer your sensor
is on. Old crust carries earthquakes better then a new one. In my area I
can record earthquakes better that are north of me then earthquakes that
are south of me.

This is because of the curst, it is broken south of me and also contains
a lot of magma in areas.=20

Did you record the ML3.6 earthquake in Belgium today ?

http://www.emsc-csem.org/index.php?page=3Dcurrent&sub=3Ddetail&id=3D92228

Regards.
J=F3n Fr=EDmann.


On Thu, 2008-07-10 at 00:00 -0400, ChrisAtUpw@....... wrote:
> In a message dated 2008/07/10, jonfr@......... writes:
>=20
> > Local earthquakes have higher frequency component 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.=20
>=20
>=20
> Hi Jon,
>=20
>        Sorry, but this is not correct. Our 20 second Lehmans detect
> ALL frequencies  from 0.05 Hz up to the limit of the low pass filter,
> 5 Hz in our case. If you had a 10 Hz filter fitted, this would be your
> limit. The response is flat with velocity between these limits.
>=20
> For optimal=20
> > 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.
>=20
>=20
>        Local quakes have components up to about 10 Hz, but may show
> only small amplitude surface waves at long periods. You can extend the
> range of a 4.5 Hz geophone down to about 0.5 Hz electronically. It
> will then respond to all quakes with amplitudes above the local noise
> levels.
>=20
>        Geophone responses are usually limited by local environmental
> noise, not by electronic noise. They pick up everything that can be
> detected within their and the amplifier's response range! Have a look
> at the FFT plot of a local quake on your geophones?
>=20
>        Regards,
>=20
>        Chris Chapman=20

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