An Earthquake swarm is a sequence of mostly small earthquakes with no identifiable mainshock. Swarms are usually short-lived, but they can continue for days, weeks, or sometimes even months. They often recur at the same locations. Swarms are observed in volcanic environments, hydrothermal systems, and other active geothermal areas.
Occurrences of Swarms across the world:
- India: Since 11 November 2018, an earthquake swarm has been observed in the region of Dahanu, Maharashtra, an otherwise aseismic area. Ten to twenty quakes are felt daily, with magnitudes usually smaller than 3.5 (maximum magnitude 4.1 in February 2019).
- Philippines: An earthquake swarm occurred from early April 2017 to mid August 2017 in the Philippine province of Batangas.
- Europe:
- Czechia/Germany: The western Bohemia/Vogtland region is the border area between Czechia and Germany where earthquake swarms were first studied at the end of the 19th century. Swarm activity is recurrent there, sometimes with large maximum magnitudes,
- France: Ubaye earthquake swarms In Alpes-de-Haute-Provence, the Ubaye Valley is the most active seismic zone in the French Alps. Swarm activity in an area where usually only a few lowmagnitude events occur every year.
- Central America
- El Salvador: In April 2017, the Salvadoran municipality of Antiguo Cuscatlán, a suburb of San Salvador, experienced a sequence of close to 500 earthquakes within 2 days.
- Northern America
- United States: Between February and November 2008, Nevada experienced a swarm of 1,000 lowmagnitude quakes generally referred to as the 2008 Reno earthquakes The Yellowstone Caldera, a supervolcano in NW Wyoming, has experienced several strong earthquake swarms since the end of the 20th century.
- Atlantic Ocean: In El Hierro, the smallest and farthest south and west of the Canary Islands, hundreds of small earthquakes were recorded from July 2011 until October 2011.
Impact of earthquake swarms:
- Each earthquake within the swarm redistributes stress, which may in turn influence the subsequent swarm evolution, especially if the crust is in a critical state.
- Slider-block models have shown that earthquake swarms can result from a self-organized critical stress field without any external pore pressure source which can cause damage.
- Earthquake swarm activity also shares some common features with tectonic earthquake clusters, in particular embedded aftershock sequences which point to an important role for stress triggering collapse of structures.
- The potential for destruction from these events varies widely. Some cause considerable amount of damage but others are relatively harmless.
- Dhanau swarm caused casualties and damage to structures.
- Low intensity swarms which cause just shaking, maybe a cause of residents of inhabitants of the area. Especially in areas with reservoirs.
- They can be witnessed even in areas with no documented seismic activity in the recent past as was seen in Rhone Valley region.
Differences between an earthquake and earthquake swarm:
| Earthquake | Earthquake swarm | |
| Main Shock | Definite main shock | Not present |
| After shock | Generally occur after the mains shock | No after shocks |
| Occurrence Duration | One main shock but followed by Aftershocks, which become less frequent with time, although they can continue for days, weeks, months, or even years for a very large mainshock. | Usually short-lived, but they can continue for days, weeks, or sometimes even months. |
| Cause | Sudden release of energy in the Earth’s lithosphere that creates seismic waves. | Hydro-seismicity due to water percolation as well as seismic activity. |
| Frequency of occurrence | Regularly | Rare |
| Reoccurrence | Can happen at varied time intervals. | They can reoccur frequently. |
| Magnitude | Low to high | Low |
| Intensity | Low to high | Low |
| Destruction to life and property | Very high | Relatively less |
