I've built a prototype horizontal-pendulum seismic sensor using a design that
Brady Romberg, a School of Mines student, came up with.  This still needs
a lot of work, but I thought the mechanical design was quite interesting.

I've posted a seismogram and some photos here:
http://jclahr.com/science/psn/romberg/

The weakness in this design is the vertical rod and it's attachment to the
horizontal plate.  I think flexure in the rod and/or attachment is what limits
the period to about 2 seconds.  Perhaps the mass needs to hang from a V-
shaped support rather than a single rod.

Cheers,
John

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