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, have long been known for frequent earthquake swarms. In 1899, a scientist named Josef Knett studied a series of small quakes and created the term “swarm earthquake” because the points where the quakes happened 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 happened in this area. Scientists could feel a quake every couple of minutes at one point! These quakes were linked to movement deep underground, maybe caused by a big quake the year before. Earthquake swarms are often seen near volcanoes or in places where water moves through rocks underground. They can happen in many places, not just near where tectonic plates meet. Even though these quakes are usually small, they can still be worrisome because we can't always predict when they will stop or if a bigger one might happen.

Examples

Earthquake swarms are sequences of many small quakes happening in a local area over a short period. Unlike normal earthquakes, where one big quake (called a main shock) is followed by smaller ones (aftershocks), in a swarm no single quake stands out as the main one.

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 daily, usually smaller than magnitude 3.5.

Philippines

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

Europe

Iceland

A large swarm began on Iceland's Reykjanes Peninsula in October 2023, with over 20,000 quakes recorded. The largest reached magnitude 5.2, and the activity was linked to movement beneath the Earth's surface.

Czech Republic / Germany

The border area between the Czech Republic and Germany has had recurring swarms since the late 1800s, with notable activity in 1908, 1985–1986, 2000, and 2008.

France

The Ubaye Valley in the French Alps is known for its swarms. A big swarm happened in La Condamine-Châtelard between 2003 and 2004, with over 16,000 quakes. Another occurred from 2012 to 2014.

Central America

El Salvador

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

North America

United States

Nevada had about 1,000 quakes between February and November 2008, with the strongest at magnitude 4.9. The Yellowstone Caldera in Wyoming has had many swarms since the late 1900s, with over 3,000 quakes recorded in 1985 alone.