Aftershocks are smaller earthquakes that follow a larger seismic event, known as the mainshock, occurring in the same general area. They occur within the mainshock’s rupture zone or nearby, generally within one to two fault lengths away from the mainshock fault. These subsequent tremors are a common part of an earthquake sequence, as the Earth’s crust continues to adjust after a significant rupture. Understanding why aftershocks occur and their characteristics is important for communities living in earthquake-prone regions. While often smaller, they can still pose considerable risk and add to the initial damage.
Understanding Aftershocks
Distinguishing aftershocks from foreshocks and mainshocks is based on their sequence and relative magnitude. Foreshocks are smaller earthquakes that precede a larger one in the same location, though an earthquake can only be identified as a foreshock after a larger event occurs. The mainshock is simply the largest earthquake in a seismic sequence. Aftershocks are almost always smaller in magnitude than the mainshock they follow. While aftershocks are typically smaller, a later event initially thought to be an aftershock could be reclassified as a new mainshock if it turns out to be larger than the preceding one.
The Mechanics Behind Aftershocks
Aftershocks result from the redistribution of stress within the Earth’s crust following a major earthquake. When a mainshock occurs, the sudden slip along a fault releases energy, but it also alters the stress field in the surrounding rock. This change can increase pressure on nearby fault segments or previously unstressed areas, making them more susceptible to rupture.
The mainshock does not uniformly release all accumulated stress across a fault system; instead, it shifts stress to adjacent areas. These newly stressed areas then experience their own smaller ruptures as they seek to find a new state of equilibrium. This process can involve adjustments along the primary fault plane or trigger movement on other nearby, interconnected faults.
Duration and Magnitude of Aftershocks
The frequency and magnitude of aftershocks typically decrease over time following a mainshock. Aftershocks are most frequent and strongest immediately after the main event, gradually becoming less common and weaker as days, weeks, and months pass. For example, the rate of aftershocks can decrease significantly, with about ten times as many occurring on the first day compared to the tenth day.
The duration of an aftershock sequence can vary widely, ranging from a few weeks to several months, or even years, depending on the magnitude of the mainshock. Larger mainshocks tend to produce more numerous and longer-lasting aftershock sequences. While aftershocks are generally smaller than the mainshock, the largest aftershock is typically about 1.1 to 1.2 magnitude units smaller than the main event.
Staying Safe During Aftershocks
Preparation for aftershocks is important, as they can occur minutes, days, or even months after the initial earthquake. If you feel an aftershock, immediately practice “Drop, Cover, and Hold On”. This involves dropping to your hands and knees, taking cover under sturdy furniture, and holding on until the shaking stops. Also stay away from windows and heavy objects that could fall.
After the shaking subsides, exercise caution, especially around damaged buildings. Do not enter structures that appear unstable, and be aware of potential hazards like falling debris, broken glass, or downed power lines. Have an emergency plan in place and monitor local news for safety instructions and updates from authorities.