What is an earthquake swarm

What is an earthquake swarm? When does it occur? What happens?

Following an earthquake, we often hear mention of an earthquake swarm. But what is it exactly?

An earthquake swarm is a phenomenon whereby a sequence of usually minor earthquake tremors occur within a short period of time (over the space of a few months) and all within a certain area.

Earthquake swarms featuring strong tremors usually account for just a small percentage. In these cases, we usually see tremors increasing in frequency and strength. These tremors are known as warning tremors. Destructive earthquake tremors are almost always preceded by weaker tremors. However, it is not a given that there will be a strong quake. In many cases, the tremors observed are actually weak and continuous. This is regarded as a preferable phenomenon as faults get a chance to release energy gradually. Then there are cases where the main shock is alternated with weak tremors.

One example is the earthquake that occurred in Italy’s Abruzzi region.

The Abruzzi earthquake, which occurred on the 6th of April in 2009, was preceded by an earthquake swarm that started about 6 months earlier (on the 14th of December 2008 with a magnitude of 1.8 on the Richter scale).

Most notably, seismographs recorded two warning tremors on the 30th of March with a magnitude of 4.2 on the Richter scale and on the 5th of April towards 10 in the evening (a few hours before the destructive shock at 3.32 am) of roughly the same strength as the one recorded in late March.

To sum up, during the months when an earthquake swarm is occurring, it is always prudent to be on alert and take the relevant precautions.