Ian,
    Since you asked about period extension to increase sensitivity, let me explain something very few seem to understand.  The sensitivity of a simple pendulum is just like that of every other seismometer; i.e., you can show that it is proportional to the square of the period.  This can be proven from
the equations of motion of the various mechanical oscillators in general, but for the pendulum, understanding is trivial.
    As anybody would expect from 'horse-sense', the longer the 'quasi-rigid' pendulum, the greater the sensitivity of the instrument if the sensor is placed at the bottom.  Because the period of the pendulum is given by 2 pi times the square root of the ratio of length to earth field (little g), one
sees immediately then, from this well known expression, that the sensitivity is proportional to the square of the period.
   So then--to increase your sensitivity, hang as long a rod as you can find, consistent with your house size, and then place your sensor at the bottom.  There are many possible sensor types to go with this incredibly cheap but probably very effective earthquake detector.   They could be moire' pattern
types with white light (incrediby simple) to function as a tsunami detector.  Or they might be greatly, greatly sensitive by means of the Ronchi approach, who made optical testing famous by means of a coarse grating (much better than the classic knife edge test).  The Ronchi ruling works with white
light!  Or the sensor might be capacitive in nature like my SDC array.  The list of possibilities goes on and on, thus my interest in the serendipty that is likely to come out of the listserve.
   Randall