Aftershocks are a series of smaller tremors that follow an initial earthquake and can be just as damaging as the main shock in some cases.
These can often be unpredictable and, due to high frequency (for example, 6,000 aftershocks occurred with the 2018 Anchorage earthquake), pose a recurrent threat to people and property.
In fact, contrary to popular belief that these quakes only last a week or so, smaller aftershocks can actually last for centuries.
Understanding key features of aftershocks—such as the magnitude, time, or location—can help in the estimation of potential risk, and help people better prepare for them.
Here’s everything you need to know about aftershocks.
The Magnitude (M) of the Aftershock
The magnitude of the aftershock is predictable to a certain degree, as opposed to its predecessor.
As per the empirical Bath’s law the largest aftershock will be almost 1.1 to 1.2 Richter smaller than the main shock. This means that an M 7.0 earthquake will result in aftershocks of M 5.8 or 5.9 on the Richter scale.
Such observations have assisted seismologists in differentiating between aftershocks and main shocks.
Considering that aftershocks are never as large or larger than the first shock, tremors felt after the original earthquake that are bigger in magnitude than the first one will not be classified as aftershocks.
Another benefit of this law is that it prepares people for additional tremors effectively – now that they know the magnitude of the pending seismic shift, they can take measures to deal with them.
The Timing of the Aftershock
Wouldn’t it be great if you knew when the next earthquake tremor might hit you?
Well, thanks to a Japanese seismologist, and a law named after him, we can.
The empirical Omori’s law suggests that as the seismic stress is released after the initial shock, and the fault begins to recover, there is less tectonic activity as time passes. But this “decay of activity,” as it is called, is not uniform.
While the temporal decay is more rapid in the beginning and you feel a larger number of aftershocks right after, the frequency is bound to decrease with time. This is primarily because the rate of decay slows down, increasing the time between consecutive tremors.
Using the formula: N(t) ∝ (c+t)-p seismologists can calculate the pattern of seismic decay, using the number of aftershocks (N), time since the main shock (t) and two negligible constants (c and p).
This pattern can then be used to determine when to expect the next tectonic tremor.
The Location of the Aftershock
While there’s no mathematical equation to calculate the location and no law that defines it, empirical observations have helped develop a rupture-area pattern.
A study done after the Sumatra-Andaman earthquake mapped the aftershocks between 1300 and 1600 KMs of area. They concluded that the majority of the seismic activity was seen along the edges of the area affected by the initial shock.
This makes sense if you think about it.
Consider the initial fault line: even though it ruptured to release the stress on the tectonic plates, some of the residual pressure might be left at the edges of the destroyed area – where the earthquake stopped. It can be expected of these regions to show further seismic activity to let go of that stress.
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