## PSN-L Email List Message

Subject: ARRIVAL TIMES: Quiet Before The Storm
From: rsparks rsparks@..........
Date: Fri, 04 Sep 2009 06:41:48 -0700

```The quiet period before the arrival of a quake may be an illusion.

Our seismic detectors always detect something, and that something is always a sine wave.  (We will ignore that the background may contain waves of several frequencies)  Now when the energy from a quake arrives, it will add to the energy already coming from the background sine waves.  As a result, at the beginning of the quake signal, you are beginning the process of adding two waves together.

Now consider that the new second wave can begin at any time in the life of the background wave.  If the two waves are of the same frequency, it is possible for the two waves to be of exactly the same frequency and phase, so that the sum of the waves would appear to be a smooth increase of amplitude, all at one frequency.  The two waves must be exactly synchronized for this to occur but it is done all the time in the power industry when a new generator is connected to the grid.

More likely the quake wave will join the background wave out of phase.  This will make it difficult to tell exactly when the new wave begins because the background wave is unlikely to be zero amplitude.  We must remove the background wave to learn exactly the time of arrival of the new wave.

Any quiet period before the second wave arrival may be an illusion resulting from arrival occurring during a peak and seemingly level time period for the background wave.  The inverse could also happen where the arrival coincided with the peak rate of change of the background wave, with the result that the second wave appears to arrive EARLIER than true arrive time (an illusion).

Reasonable?

Roger

Subject: ARRIVAL TIMES: Quiet Before The Storm
From:    "Geoffrey"
Date:    Wed, 2 Sep 2009 17:50:11 -0700

I have noticed in some Earthquakes you seem to notice
a short increase in quietness just before the impulse arrives.

Is this a real phenomena or simply a coincidence ?

Does this mean the real arrival time is
the beginning of the noticeable quiet ?

WHY THE QUIET ??
What physics would possibly muffle the noise ?

arrival of Possibly eP instead of simply P ?

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