An earthquake swarm involves a local area receiving a series of some number of earthquakes, experienced within a relatively short length of time. Although there is no standard time period for this measurement, the United States Geological Survey (USGS) mentions that the entire event may take place during anywhere from days, to weeks, or even to months. An earthquake swarm is a common event to occur prior to the eruption of a volcano.
During February, 2008, over 500 combined earthquakes and aftershocks hit nearby Mexicali, in Mexico. This is one example of an earthquake swarm taking place. Over 1000 small magnitude earthquakes were considered to be part of that earthquake storm from only the period of February through April. In this case, the highest magnitude measured was only 4.7.
More recently, an earthquake swarm occurred at Yellowstone National Park, during December 2008 through January 2009. According to Dr. Jacob Lowenstern of the USGS, earthquake swarms at Yellowstone are common, however this was the largest one there since 1985. Overall, around 900 earthquakes were recorded near the Yellowstone Lake area from December 26th through January 8th. The largest was a 3.9 magnitude earthquake. In contrast, the 1985 swarm lasted for about 3 months, with the largest magnitude earthquake reaching 4.9.
Earthquake swarms differ from a single earthquake followed by aftershocks. Where the additional events are considered aftershocks, there will be a particular earthquake that stands out as the main one. In an earthquake swarm, there is no obvious main earthquake, but rather a sequence of similar ones.
An earthquake swarm is also different than an earthquake storm, which is a proposed theory on earthquakes. In the theory, it is considered that one earthquake triggers a sequence of additional large magnitude earthquakes, which are all located in the same tectonic plate. This would happen as stress is transferred along the fault. The theory also differs from aftershocks in that the earthquakes in the "storm" occur years apart from each other.
During an earthquake, remember safety guidelines. These are not comprehensive, but are a few beginning safety tips:
If you are inside, stay there, and try to move underneath a strong desk or interior wall.
Avoid taking cover near or below heavy objects, sharp objects, hanging objects, large mirrors, fireplaces, etc.
If you are cooking, turn off the stove or oven and then look for cover.
If you are outside, move to an open area away from possible falling objects, and in particular away from buildings, trees, and power lines.
If you are driving, smoothly slow the vehicle to a stop at the side of the road, and stay in your car. Avoid stopping below or near bridges, overpasses, trees, power lines, large signs, etc.