PSN-L Email List Message
Subject: Re: Vertical vs. Horz.
From: Brett Nordgren brett3nt@.............
Date: Tue, 22 Dec 2009 17:34:20 -0500
At 07:13 AM 12/22/2009 -0700, you wrote:
>Merry Christmas Folks, I hope you all enjoy holidays.
The same to you and to everyone on the list.
> I may have asked this question before, but need to ask it again.
>I have recorded more earthquakes on vertical sensors then horizontal
>sensors. Several different approaches for both, and a variety of
>periods, from 1 to 25 seconds.
>My next sensor will be a vertical. Before I begin I wanted to ask the
>I know the 1.5 second vertical, will see the P and if the event is large
>enough, it will see other phases.
>Picture a free hanging spring with a mass.
>My theory is the P wave stimulates the spring from an angle, a bit like
>its coming from your feet to your head.
>An example would be if the earthquake occurred 180 degrees from the
>sensor, the result would be directly from your feet to your head.
>1 I know this is an over simplification, but does the P arrives at an
>angle (bottom to top) and this influences the spring? Yes, I have seen
>different explanations, wave simulator etc. but these did not answer the
>question. If this is not the case can someone explain how the vertical
>spring is influenced.
What you suggest I believe is true, that P waves may have some vertical
component, though there might possibly be another contribution related to
something that Randall mentioned. We frequently see P waves on the
verticals, sometimes quite strongly, which might in part relate to the fact
that when something gets squeezed in one direction, it will expand somewhat
to the sides. This ratio of the sideways expansion to the original amount
of compression is called Poisson's Ratio, which for many materials is about
1/3. As an horizontal P wave expands and compresses the earth, it may
possibly be causing additional vertical motion due to that effect, which
you should see on the vertical (though I don't know what Poisson's ratio
might be for dirt). Like most earth motion descriptions, it is probably a
rather complex combination of those and some other effects.
>The obvious advantage of a vertical is they see many earthquakes, the
>disavantage they are limited and also see a lot of other things which are
>not earthquakes. All of my verticals have been 1 to 2.5 seconds. This
>range has never seemed to be and advantage or disadvantage. The 2.5
>second seemed to work no better then the 1 second. I think this is
>because the sensor is just picking up the P (mainly) anyhow.
>However on a big event it does show the P, the S and some other
>phases. A typical vertical sensor recording will have a trace of 20
>mins vs. a 3 hours trace on the Lehman.
>2. If I could achieve a long period vertical, what would be the ideal
>target, if there is one? An example the period target for a Lehman style
>is 20 seconds.
From what I have seen, much of the surface-wave energy of large teleseisms
is in the region of 18 to 20 seconds, and sometimes longer. 20 seconds
will see a lot, while 40 or 50 seconds can often see somewhat more.
> Before I being, I wanted to understand, if the longer period (vertical)
> would be any benefited. I know I could create a vertical with 3
> seconds. I don't know if I could create one with 6 seconds. If there
> is no advantage to the longer period in the Z axis, then it would serve
> no purpose.
One problem is that there is so much microseism noise in the six-second
region and also around 12 seconds. I generally use a 0.08Hz (12.5 second)
4 or 6 pole low pass filter to cut out most of that noise, while allowing
through much of the big-quake frequencies. An instrument that can only go
down to 6 seconds will unfortunately be quite good at displaying the noise
while missing most teleseism surface waves.
> One's location has an influence on which sensor type works for
> them. I enjoy having both vertical and horz, shorter and longer
> periods. In this case, I want to target a longer period vertical if the
> longer period would be better.
The only problem is that the movement of a seismometer in response to an
error force, such as from small temperature changes, etc. increases as
1/Frequency^2 It is way harder to make a stable instrument for 50
seconds than for 10 or 20 or 6. Long period verticals almost certainly
need to use a feedback design if they are going to be sensitive enough to
see distant quakes while at the same time, insensitive enough to
temperature and other variations to not wander off to maximum output.
Watch our wiggles
or watch some very very good wiggles
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