Based on this USGS information I need to ensure a sufficiently high sampling rate to ensure the data collected is useful. Most earthquake waves appear to have a frequency of < 20Hz so taking Nyquist in account I need to be sampling at least at 40Hz.
In a tight loop doing nothing but communicating with the accelerometer I read X,Y & Z acceleration values then averaged the results over 10 lots of 10000 readings.
MMA7660FC – 6 bit acceleration value in 1 byte for each axis 650 RPS.
ADXL345 – 10 bit acceleration value in 2bytes for each axis 435 RPS
The devices also have configurable acquisition rates & in some cases FIFOs for queuing readings which I need to look at some more