Earthquake Swarms

Earthquake swarm of January 2019, near Carúpano, Venezuela, west of Trinidad.

Earthquake swarms are a series of earthquakes within a local area, occurring within a relatively short period of time. These swarms can occur over the length of hours, days, weeks and even months. They are different from earthquakes that result in aftershocks as no single earthquake can be identified to be a clear main shock in the sequence.

Graphical representation of an earthquake swarm and a typical earthquake sequence. It should be noted that, in a typical sequence, foreshocks do not occur in every case. Credit: ETH Zurich

Across the world, earthquake swarms occur, with hundreds to thousands of earthquakes occurring across several months to years that could be directly attributed to a swarm.

The two most dominant hypotheses, explaining the occurrence of earthquake swarms, are the influence of magma and groundwater injection.

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Magma (Volcanic) Earthquake Swarms

In the Eastern Caribbean, where volcanic islands exist, most of the earthquake swarms that do occur are typically volcano-tectonic in nature. Volcano-tectonic earthquakes are earthquakes induced by movement of magma or fluid underground. This movement can cause pressure changes, causing surrounding rock to break or move and trigger minor earthquakes.

The most recent case of this type of swarm is on the island of Guadeloupe, in the area of La Soufrière. Over the course of 12:19AM January 19th 2019 through 6:51AM January 23rd 2019, the Volcanolgical and Seismological Observatory of Guadeloupe recorded 501 seismic events, all at a depth less than 2.5 kilometers and less than magnitude 1.0.

Fracking & Waste Water Injection Earthquake Swarms

In cases where hydraulic fracturing, also called fracking, or wastewater disposal occurs underground, induced earthquakes can occur. In places like the United Kingdom and Oklahoma in the United States, wastewater disposal and (sometimes) fracking have induced earthquake swarms. The cause of these earthquakes, according to the United States Geological Survey are as follows:

“Earth’s crust is pervasively fractured at depth by faults. These faults can sustain high stresses without slipping because natural “tectonic” stress and the weight of the overlying rock pushes the opposing fault blocks together, increasing the frictional resistance to fault slip. The injected wastewater counteracts the frictional forces on faults and, in effect, “pries them apart”, thereby facilitating earthquake slip.”

Earthquake Swarms In T&T

In the Trinidad and Tobago region, neither of the above causes of swarms occur. Within the seismic zones, there is little to no subsurface waste water disposal, nor are there magmatic volcanoes triggering earthquake swarms. The prevailing theory for regions such as ours, is that these quakes are related to stress release on smaller faults, within a fault system.

Over time, as the South American Plate and the Caribbean Plate moves past one another, stress builds up across these major fault systems, which include smaller perpendicular faults. When stress reaches a critical point, these small earthquakes begin to occur. Since these faults are fairly small compared to the larger regional faults, such as the El Pilar fault and the Central Range fault, these earthquakes tend to be minor to micro sized (<M3.0). However, these quakes can occasionally occur at magnitudes greater than M4.0 and even M5.0.

These seismic swarms are regularly detected by seismic monitoring agencies such as the U.W.I. Seismic Research Centre for Trinidad and Tobago, as well as the remainder of the Eastern Caribbean and the Venezuelan Foundation for Seismological Research for Venezuela.

Most swarms can only be detected by seismographs, as the magnitudes are generally too small to be felt. At times, there may be a larger quake within this swarm that can cause some alarm, but usually no damage or injury occur.

Notable swarms include the East Trinidad swarm of 2004, the Southwest Tobago swarm of 1997 and the another Southwest Tobago swarm of 2016-2017.

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Does a swarm mean the ‘big one’ is coming?

No. Although large earthquakes are sometimes preceded by smaller associated earthquake swarms (called foreshocks), swarms of seismic activity do not necessarily indicate that a large earthquake is to follow. It is important to reiterate that earthquakes cannot be predicted.

However, tiny earthquakes increase earthquake risk because they increase the overall earthquake frequency on a given fault. If these swarms of quakes are occurring on smaller faults, that run perpendicular to the main fault (i.e. the El Pilar Fault, Central Range Fault etc.), these smaller earthquakes increase the likelihood (probability) of a larger earthquake occurring on the main fault. It is impossible to predict whether this larger quake will occur.

Would an increased number of small earthquakes reduce the chance of a larger earthquake occurring?

Seismologists have observed that for every magnitude 6 earthquake there are about 10 of magnitude 5, 100 of magnitude 4, 1,000 of magnitude 3, and so forth as the events get smaller and smaller. This sounds like a lot of small earthquakes, but there are never enough small ones to eliminate the occasional large event.

It would take 32 magnitude 5’s, 1000 magnitude 4’s, OR 32,000 magnitude 3’s to equal the energy of one magnitude 6 event. So, even though we always record many more small events than large ones, there are far too few to eliminate the need for the occasional large earthquake. (Source: USGS)

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