Aftershocks that follow an earthquake instill as much fear and panic as the first round of violent tremors. Scientists and emergency rescuers face the difficult task of trying to predict whether the earth will rattle again after a disaster. Between 70 and 75 earthquakes strike across the globe every year when the planet's underground tectonic plates shift, according to the U.S. Marine Corps. Subsequent rumblings happen within hours or even months after the initial episode, leaving geologists to ponder what can happen after an aftershock.
The earth's underground plates have moved over, around and under each other for millions of years. This unseen activity goes awry when plates collide, causing pressure to rupture and the earth's crust to shake. Later, more stress builds on the fault line where the ground ruptured, touching off reverberations called aftershocks. In the aftermath of an earthquake aftershock, pressure keeps mounting and touching off additional shocks, the Marine Corps reports.
Scientists immediately review data following an aftershock to determine whether more ground-shaking is on its way. A ravaged area is prone to multiple aftershocks during the first hour of an earthquake. The risk of subsequent fallout declines over time but more underground eruptions can occur over the next several months. Researchers look at the length of the fault that caused the first rumblings because aftershocks generally happen at a distance no greater than the length of the fault that touched off the disaster, the U.S. Geological Survey reports.
One of scientists' toughest tasks is classifying each episode as a foreshock, main shock or aftershock, according to the U.S. Geological Survey. Researchers typically assume the first round of shaking is a main episode. They later might realize it was a warning called a foreshock. Or, they determine the first violent tremors were the main shock and everything that followed was the aftershock. Each classification depends upon the magnitude of each event, such as the 7.0 Haitian earthquake in January 2010 followed eight days later by a 6.1 aftershock.
Rescuers take on the daunting job following aftershocks of searching for earthquake victims and survivors among rubble that has been weakened at least twice. Aftershocks are lower in catastrophic magnitude but often catch already devastated communities by surprise and force the collapse of structures that were damaged in the first disaster, according to the Federal Emergency Management Agency. Earthquake fatalities seldom occur because of the ground movement but rather because of falling debris or buildings.
- U.S. Marine Corps: Earthquakes: NEOs and Disaster Preparation
- U.S. Geological Survey: What are Aftershocks, Foreshocks and Earthquake Clusters?
- Federal Emergency Management Agency: Are You Ready? Earthquakes
- "Live Science"; USGS: Today's Haiti Earthquake Was An Aftershock; Jeanna Bryner; January 2010
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