Geology And Geophysics

An Overview on Earthquake Aftershocks



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Earthquake ! ! ! The ground suddenly moves, readjusts, resettles, and then, later on, moves again…all a part of the planet’s shifting plate tectonics. An earthquake is a terrifying and destructive tragedy, but, aftershocks, the shocks that reoccur many times afterward, can be far more dangerous than the original earthquake.

An earthquake has three phases: foreshocks, the main shock and aftershocks. They occur in patterns, but, sometimes, it’s difficult to immediately determine which one is which. Foreshocks give seismologists warning that an earthquake, the main shock, can possibly happen anytime in the near future.

After most of the rumbling is over, the largest shock to have occurred is called the main shock. Aftershocks continue after the main quake for days, months, years, decades or even longer. They can be very dangerous.

Fortunately, the process of aftershock readjustment is an almost predictable science. Individually, the occurrence of aftershocks can’t be exactly foretold, so it’s unknown when they’ll happen or how severe they’ll be, however, groups of aftershocks do follow patterns.

There has been some success in predicting a range of expected occurrences and magnitudes. Science continues to study earthquakes in the hope of learning more about them.

Aftershocks occur above fault planes in the earth. Sometimes an earthquake can even start another aftershock on a nearby fault. Discerning between the two events can be difficult. As a general rule, aftershocks reside within the fault plain of the main shock, and they occur within a certain distance of the fault.

Coincidentally, that distance is also the length of fault itself. These aftershock patterns can be used to determine the size of the faulted area that was affected by movement. This assessment usually works, except for larger magnitude earthquakes, which have many aftershocks that can extend their effects far away into other zones.

Over time, all aftershocks decrease in magnitude and occurrence. They’re no longer considered to be aftershocks when they can’t be distinguished from the normal earthquake level for that area. A place like California is known for earthquakes, so the normal background value is quite large.

It’s considered to take about 10 years for aftershocks to die down in the California San Andreas Fault. In New Madrid, Missouri, aftershocks are still being felt from one of the largest series of earthquakes that ever happened in the United States – they occurred in 1811-1812, almost 200 years ago.

After a devastating earthquake, people are far too busy dealing with the devastation of the event. Maybe that’s why aftershocks can sometimes take people by surprise. Aftershocks can be more dangerous than the original earthquake. Buildings and structures have already been damaged, often in unseen ways, and may unexpectedly collapse upon people.

Many federal, state and local agencies have made great strides in educating the public about the dangers of earthquakes.

Earthquakes suddenly shift the ground, with a great rumbling, and for some time afterward, aftershocks occur as the ground adjusts to its new position. Earthquakes happen without much warning, but their aftershock patterns can be generally predicted to provide some measure of protection. That’s why the monitoring and studying of earthquakes is critical for savings lives and property.

Useful references:

Seismic Design of Building Structures, Michael Lindberg and Kurt McMullin, 9th edition.

Seismic Design Review Workbook, Stephen Hiner, 2009.

http://earthquake.usgs.gov/earthquakes/step/explain.php, What are Aftershocks, Foreshocks and Earthquake Clusters, Earthquake Hazards Program, U.S. Geological Survey.

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