Use a way stronger compression
The deflate compression is the algorithm used for zip archives. It is loss less and features some more tweaking options in order to get smaller results.
With the usage of quite heavy compression options, we can reduce the usual amount of memory of shakemap geotiff files to need only around 60% of the original size using the lzw algorithm.
Of cause, read and write times will increase. But as the size of a single shakemap is relativly small (our problem with the size is the pure amount that we have for the earthquake explorer), it should not be totally bad to work with those files later.