Earthquake swarm

An earthquake swarm is a series of smaller quakes that happen close together in time and space. In seismology, these quakes occur in a local area over a short period, which can be days, months, or even years. Unlike normal quakes where one big shake — called the main shock — is followed by smaller ones known as aftershocks, an earthquake swarm has no clear main quake. All the quakes in a swarm are roughly the same size, making it different from the usual pattern after a big earthquake. These swarms can be important for scientists to study because they help us understand how the Earth moves and changes.

History and generalities

The Ore Mountains (Erzgebirge), which form the border between the Czech Republic and Germany, often have earthquake swarms. In 1899, a scientist named Josef Knett studied small quakes and called them “swarm earthquakes” because they looked like a busy group of bees on a map.

One famous swarm happened near Matsushiro, close to Tokyo. From 1965 to 1967, about a million small quakes occurred there. Scientists could feel a quake every couple of minutes at one point! These quakes were linked to movement deep underground. Earthquake swarms are often seen near volcanoes or where water moves through rocks. They can happen in many places, not just where tectonic plates meet. Even though these quakes are usually small, they can still be worrisome because we can't always predict them.

Examples

Earthquake swarms are many small quakes that happen in one place over a short time. In a normal quake, one big quake happens first, followed by smaller ones. In a swarm, no single quake is bigger than the rest.

Here are some notable examples from around the world:

Asia

India

Since November 2018, an earthquake swarm has been happening in Dahanu, Maharashtra. Ten to twenty quakes are felt each day, and they are usually smaller than magnitude 3.5.

Philippines

In April to August 2017, Batangas province had a swarm with four quakes between magnitudes 5.5 and 6.3. Another swarm happened on Panay Island in October 2020, with quakes from magnitudes 2.5 to 4.5.

Europe

Iceland

A large swarm started on Iceland’s Reykjanes Peninsula in October 2023, with over 20,000 quakes recorded. The largest was magnitude 5.2.

Czech Republic / Germany

The area where the Czech Republic and Germany meet has had earthquake swarms since the late 1800s. Big swarms happened in 1908, 1985–1986, 2000, and 2008.

France

The Ubaye Valley in the French Alps is known for its swarms. A big swarm occurred in La Condamine-Châtelard between 2003 and 2004. Another happened from 2012 to 2014.

Central America

El Salvador

In April 2017, Antiguo Cuscatlán near San Salvador had almost 500 quakes in two days. The quakes were between magnitudes 1.5 and 5.1.

North America

United States

Nevada had about 1,000 quakes between February and November 2008. The strongest was magnitude 4.9. The Yellowstone Caldera in Wyoming has had many swarms since the late 1900s.