How much shaking could one expect from an earthquake?
The amount of shaking from an earthquake depends on the size of the earthquake, how close one is to the earthquake (also if it is deep or shallow), what the surface conditions are at the surface (if one is on solid rock, the shaking will be much less than if one were on a valley of sediments, which could act like a bowl full of jelly), and the competence of the structure one is in. For this reason, the modified Mercalli scale was developed to measure the amount of shaking, It is based on human perception and the amount of damage one may expect from a certain amount of shaking—the figure below is an example of a typical “shake map” put out by the USGS after a “felt” event. Also shown below is the modified Mercalli scale, to give one an idea of how the scale reflects different amounts of shaking. The figure is an example of a magnitude 3.7 event at the Geysers Geothermal Field and the amount of shaking observed, by distance. In general, the bigger the event, the more shaking there is, if the distance is constant. Note, however, that the shaking is also dependent on the distance from the earthquake event, as well as the amount of energy in the earthquake. In general, the amount of shaking decreases as 1/distance from the event, regardless of how big the event is.