PSN-L Email List Message

Subject: Re: Earthquake strength
From: ChrisAtUpw@.......
Date: Thu, 10 Jul 2008 00:00:55 EDT


In a message dated 2008/07/10, jonfr@......... writes:

> 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. 

Hi Jon,

       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.

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.

       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.

       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?

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/07/10, jonfr@......... writes:

Local earthquakes have higher f= requency 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.


Hi Jon,

       Sorry, but this is not correct. Our 20=20= second Lehmans detect ALL frequencies  from 0.05 Hz up to the limit of=20= the low pass filter, 5 Hz in our case. If you had a 10 Hz filter fitted, thi= s would be your limit. The response is flat with velocity between these limi= ts.

For optimal
dection of local an= d 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.<= /BLOCKQUOTE>

       Local quakes have components up to abou= t 10 Hz, but may show only small amplitude surface waves at long periods. Yo= u can extend the range of a 4.5 Hz geophone down to about 0.5 Hz electronica= lly. It will then respond to all quakes with amplitudes above the local nois= e levels.

       Geophone responses are usually limited=20= by local environmental noise, not by electronic noise. They pick up everythi= ng that can be detected within their and the amplifier's response range! Hav= e a look at the FFT plot of a local quake on your geophones?

       Regards,

       Chris Chapman

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