PSN-L Email List Message

Subject: Re: seismograph item?
From: meredithlamb meredithlamb@.............
Date: Fri, 06 Dec 2002 08:53:20 -0700

Hi Doug and all,

Thanks Doug for the info.  Can see where they are as
you describe; the sensors are especially clearer on the SMA-2


Doug Crice wrote:

> The SMA-1 looks complete to me.  The sensors are those odd devices on
> the left in the photograph.  The accelerometer in an SMA-1 is just a
> damped mass with a small mirror attached. In the presence of
> acceleration, the mirror rotates and moves the spot of light on the
> film. The film is moving, and thus a seismograph (or more properly, an
> accelerograph) is drawn on the film. An event marker put time ticks on
> the film at 100 ms intervals.
> It is a 3-component system recording on 100 mm film in a canister. The
> canister can be removed and opened in a dark space for developing. The
> unit is triggered into activity by a separate sensor (also built into
> the unit) and keeps recording for the duration of the event (or until it
> runs out of film).
> The sensitivity is on the order of +/- 1 g. And of course its purpose is
> to record vibrations from strong earthquakes. I remember that quite a
> stir was created when an SMA-1 recorded an acceleration of 1.01 g during
> the Paicoma Dam earthquake in Southern California. Up until then, it was
> believed that the maximum possible acceleration in any earthquake was
> about 1/4 g.
> The SMA-2 was the next generation and replaced the film with magnetic
> tape recording. Kinemetrics figured out how to get an electrical signal
> from the damping current in those optical sensors and record it on tape
> (analog FM).
> Thousands of these units were installed around the world to study ground
> spectral amplification and structural response in earthquakes. Every
> high rise in the LA area got three units: one in the basement, one in
> the center, and one in the roof to look at how the building reacted to
> the earthquake. They were wired together for synchronized timing so you
> see how the roof zigged when the basement zagged.
> The cases are rigid cast aluminum, air and water tight, and are worth
> the price for the case alone, but it would be a shame to dismantle these
> classic instruments.
> Doug Crice
> Geostuff/GeoRadar Inc.
> 12996 Somerset Drive                phone 1-530-274-4445
> Grass Valley,  CA  95945  USA    fax 1-530-274-4446


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