Earthquake Aftershocks

Since the traumatizing major (magnitude 7) earthquake that struck near Port-au-Prince, Haiti, on January 12, 2010, the residents of that Caribbean country have experienced the fright of a sequence of aftershocks—the smaller earthquakes or tremors that follow a major earthquake.

The Relationship Between Earthquakes and Aftershocks

Large, rigid portions of the Earth’s surface, known as tectonic plates, are always in motion. When these plates get stuck at the edge, or fault, shock waves occur that are felt as an earthquake. As the tectonic plates adjust after the earthquake, more shock waves are felt on the Earth’s surface—this is an aftershock.

Aftershock Patterns

Insights about the behavior of aftershocks have been derived from the study of their patterns. Studies have shown that with time, aftershocks decrease in frequency and in magnitude. Nonetheless, just as earthquakes are unpredictable, so are aftershocks, and the uncertainty as to when they will take place adds to their danger, especially in locations in which structures have not been built to withstand the violent shaking.

After an earthquake significant enough to do damage, several aftershocks occur during the first hour after the main shock, and then their number decreases in inverse relationship to the passage of time: thus, the second day has about one-half the number of aftershocks; the fifth day has one-fifth the number, and so forth.

Earthquakes meet the definition of aftershocks if they happen in a region at a greater rate than before the big earthquake. The amount of time that a sequence of aftershocks occurs is based on how big the main shock was. Aftershocks can take place weeks or decades after the main shock, and according to the U.S. Geological Survey (USGS), the sequence of aftershocks in Haiti will continue for months, if not years.

Aftershocks are, on average, 1.2 less in magnitude—or the amount of energy released—than the big earthquake. That means aftershocks pack more than 10 times less energy than the main shock. Another generalization that has been drawn from studies of data is that the number of lower-magnitude aftershocks far outnumber the higher-magnitude ones in the entire sequence of aftershocks after the big earthquake.

The Destructiveness of Aftershocks

Although not nearly as strong as the main shock, aftershocks can not only do additional physical damage after an already-devastating earthquake, but they can also increase chaos and fear in an area such as Haiti that has suffered tremendous loss of life and shelter. And, sadly, the psychological aftershocks to the people touched by the horror and devastation will last a lifetime.