It is shown from numerical experiments that the average slip (and seismic moment) due to the fracture of multiple asperities on a rectangular fault is independent of the distribution of the asperities on the fault for all practical purposes, as long as the average stress drop on the fault is the same for the different distributions. Hence, using the relations between average slip and average stress drop for a smooth fault to estimate the stress drop for an earthquake which is due to the failure of a few large asperities gives a sufficiently good estimate of the stress drop. Relations between the average slip and average stress drop for simple rectangular faults of different length to width ratios are determined numerically. Both of the cases when the fault slip direction is parallel to the fault length and when it is parallel to the fault width are considered. The 1985 Mexican earthquake is discussed briefly as an example of this kind of earthquake model.