PSN-L Email List Message

Subject: Re: Local vs distance events
From: John or Jan Lahr JohnJan@........
Date: Mon, 23 Oct 2006 08:45:28 -0700

At 07:48 AM 10/23/2006, you wrote:
>Hi,  I was asked to explain the different between Sensors  developed 
>for local events vs those developed for long distance events.
>Is there a summary posted that explains this in terms easy to 
>understand?   This is for a group of children and adults.
>Thanks, Ted

Hi Ted,

Local earthquakes are rich in all frequencies.  The best frequencies 
for picking sharp arrival times for P, S and other phases are the 
high frequencies, above 1 Hz.

As the waves from an earthquake travel away from the source, the 
higher frequencies are attenuated more quickly than the low 
frequencies.  At a great distance most of the energy is at 
frequencies lower than 1 Hz.  In particular, surface waves with 
frequencies of 1/20 Hz (20 second period) usually are the most 
prominent signal for very distant events (call teleseisms).

The other factor for consideration is the frequency of the "noise" in 
the earth.  There is a peak in this noise at a period of  4 to 5 
seconds.  This is due to what are called higher-mode surface waves 
from non-earthquake sources such as wind and ocean storms.   Look a 
the graph on this site to see a plot of the background noise:

In the days of paper seismograms, two different systems were 
generally used: a short-period system for frequencies higher than 1 
Hz and a long period system with peak response near 1/15 Hz (15 
period).  See: 

With the advent of digital recording, many new systems are "broad 
band," meaning that they are sensitive to a wide range of 
frequencies.  In order to view data from one of these systems it is 
often best to use a digital filter to create either a short period or 
a long period seismogram.  This graph shows the growth of broad-band 
stations and the decline of the older worldwide photograph-paper 
systems from the WWSSN ( World-Wide Standard Seismograph 

The response of the broad-band station at Corvallis, Oregon (COR) is 
shown on this page:  .
On this page   one can toggle 
between the broad-band seismogram and one that is filtered to become 
either a short period or a long period seismogram.  Very distant 
events are usually not even visible on the short-period seismogram, 
where as a local or regional event will be seen best on the 
short-period record.

Hope this helps!  I've included a lot of details so that you will 
have to background necessary to develop a simpler explanation.


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