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: In the border region between Czechia and Germany, specifically in western Bohemia and Vogtland, scientists began studying earthquake swarms back in the late 19th century. This area experiences recurrent seismic activity, sometimes with significant magnitudes.
France: In Alpes-de-Haute-Provence, a picturesque region in the French Alps, the Ubaye Valley stands out as the most active seismic zone. Surprisingly, this area, known for its tranquility, experiences earthquake swarms, where typically only a few low-magnitude events occur each 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
- Earthquakes in a swarm can shift stress around, influencing how the swarm develops, especially if the Earth’s crust is in a delicate state.
- According to slider-block models, earthquake swarms can emerge due to a naturally organized stress field, without an external pressure source, potentially leading to damage.
- Similarities exist between earthquake swarm activity and tectonic earthquake clusters, including embedded aftershock sequences, indicating stress plays a crucial role in triggering structural collapses.
- The level of destruction from earthquake swarms varies; some cause significant damage, while others are relatively harmless.
- The Dhanau swarm resulted in casualties and structural damage, highlighting the potential for harm in certain events.
- Low-intensity swarms, causing only shaking, may still concern residents, especially in areas with reservoirs.
- Earthquake swarms can occur even in regions without recent seismic activity, as observed in the Rhone Valley region.
Differences between an Earthquake and Earthquake Swarm
| Earthquake | Earthquake swarm | |
|---|---|---|
| Destruction to life and property | Very high | Relatively less |
| Intensity | Low to high | Low |
| Magnitude | Low to high | Low |
| 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. |
Read also: Measuring of Earthquakes
