Magnitude of a fault and fault length

The experiment that my students did was to relate the magnitude of an earthquake to the fault length.  The students did this by attaching bungees to an 8ft board and an 8 inch board and measuring how far they had to pull the bungee before the boards would move.  They did 3 trials each. Even though they had 2 bungees attached to the long board it still took a significantly greater amount of force to move the long board than the short board.  This proves that the stresses involved in moving a long fault are greater than moving a short fault line.  Example:  The San Andreas fault is approximately 800 miles long, and therefore it can produce earthquakes up to 9.0 in magnitude, whereas a fault only a few miles long can only produce a 2 or 3 magnitude.  The students realized that since they are living on a fault that is very short (~ 1/4 mile), purchasing earthquake insurance is not really cost effective.  Earthquake insurance has a $50,000 deductible! The students had to trouble shoot how to pull the bungees equally on the long board and came up with using a broom handle.