PSN-L Email List Message

Subject: Re: Okhotsk quake
From: Bob Hancock carpediem1@.........
Date: Mon, 24 Nov 2008 12:27:29 -0700

Steiner -

There is a USGS web site that computes phase arrival times.  Beware  
that the surface waves times may not be accurate, but the body waves  
should be within a second or two.  At the top in the header before the  
list of phase arrival times, it lists the peak amplitude of 20 second  
surface waves.  You can compare that with what your instrument is  
capable of recording.  A geophone works in a higher frequency range  
than surface waves.   What you see primarily with a geophone is the  
initial P waves and other higher frequency waves.  See link to USGS  

The focal mechanism of an earthquakes, the ray path of the waves to  
your station, and the sensitivity of your instrument will determine  
what waves are visible at your station.  Using this event, if you pick  
a selection of other stations and look at the waves that are visible,  
you will see significant variations.  To read more about focal  
mechanisms see the following link:

There is no clear cut answer to your question about an earthquake that  
is 7000 km showing up on a 4.5 Hz geophone.  The answer is depend upon  
several factors including the sensitivity of your geophone,  
orientation of the instrument (unless it is 3 channel with proper  
orientation), and the depth, magnitude, and focal mechanism of the  
event.    P wave recording should not be a problem assuming you're on  
a lobe with peak amplitude.  If you are in a null area you will not  
record the event.  Following is a link for focal mechanisms:

Bob Hancock

On Nov 24, 2008, at 10:52 AM, Steinar Midtskogen wrote:

> Bob,
>> diminished.  For comparison, I would suggest that you download this
>> event through the Wilber II option on the IRIS web site at:
> A useful resource, thanks.  I downloaded the SAC files from the
> Kongsberg station, excellent data.  They also had a very strong peak
> at about 1.1 - 1.2 Hz for the first three minutes or so of the P wave,
> like I saw on my geophone.  I didn't expect that a geophone could
> record quakes that distant.
>>> Will a quake nearly 7000 km away every show up on a 4.5 Hz
>>> geophone?
> Typo, I meant "ever".
> -- 
> Steinar
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