Good question.

It is mostly about how many quakes you are likely to see.  Even 
though there are many quakes every day in California, not many are 
large enough or close enough to see above the seismic background 
noise, which is much higher at the higher frequencies.  You might 
only see one quake every few days.  And to some degree the nearer 
quakes aren't as 'interesting', rarely showing much in the way of 
distinct phases--just a single, fairly short, pop.

With a good longer-period instrument and a good location, you may see 
several distant quakes per day lasting for many minutes and sometimes 
showing multiple phases.  Some of these will be the ones you hear 
about in the news.

The best of all worlds is a broadband instrument covering both high 
and low frequencies, which can be tuned to see either type of quake 
simply by changing the WinSDR filter and gain settings.

see:   http://bnordgren.org/seismo/gif_images.htm

I suspect the reason that most on-line helicorder traces are filtered 
to long periods (excessively filtered in my opinion) is to avoid 
confusing the earthquake traces with local noise, for folks who 
aren't familiar with what they are seeing.  The people who are really 
studying the local quakes get the raw data files and would seldom, if 
ever, look at the filtered on-line traces.

Brett

At 10:32 AM 10/15/2010, you wrote:

>>It is about this geophone vs lehman or other long period instruments. In
>>California you have many local quakes. These quakes have frequencies
>>higher than long distance quakes. What is the logic of (just/or 
>>popularity of) long
>>period instruments out there? What precipitated this trend? The 1-5 
>>Hz units should be better on local events -- which you have a lot.
>There are many California quakes and there is the activity in 
>Washington State and Yellowstone but you never see local seismic 
>detectors being posted with periods of 1-4.5 Hz -- except possibly 
>tchannel.  Shouldn't these units help differentiate human noises 
>like quarry activity from smaller local earthquake activity?
>




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