As implied before, a mainshock is simply the largest earthquake in a
sequence of earthquakes. It is usually significantly greater in size
than the second largest earthquake in the same sequence. In terms
of magnitude, a numerical measure of the "size" of earthquakes
(often abbreviated as a capital M),
a mainshock is usually at least one-half of a whole unit of
magnitude larger than its largest aftershock (or foreshock, a term
we'll get to shortly). For example, if the mainshock of a sequence is
of magnitude 6.5, you would generally not expect to see any aftershocks
larger than magnitude 6.0 in that same sequence. Most aftershocks
will be much smaller -- several units of magnitude less -- than the
mainshock. An exception to this rule can be found in earthquake
swarms, sequences of earthquakes that do not have a clearly
defined mainshock.
Swarms are a special case we will look at much later in this section.
It may help to illustrate the relations, as well as the distinctions, among these different classes of earthquakes (foreshocks, mainshocks, and aftershocks) by studying an example of seismicity in southern California. The chart on the left is a list of all earthquakes in southern California, greater than magnitude 3.0, that occurred on April 22, 1992. The time, magnitude, and location (in latitude and longitude) of these events is given on chart. These three measures are all we need to differentiate foreshocks, mainshocks, and aftershocks.
As you can see, one large earthquake on the list far exceeds the others
in magnitude. This is the Joshua Tree earthquake, magnitude 6.1. Note
its location. Now, look at all of the events that follow it. All these
events are smaller in magnitude, but have roughly the same location as
the Joshua Tree earthquake. Thus, we can call these events
aftershocks of the M 6.1 Joshua Tree mainshock.
Notice how the seismicity rate increases dramatically after the
Joshua Tree earthquake -- so much so that this list had to be cut short!
This is the same sort of jump in seismicity rate you should
have seen in the example of March 1998 from
Activity #5: Seismicity Rates.
