At 6:44 AM Thursday 28th January 2021, a (preliminary) minor Magnitude 3.0 (Md or Mt) earthquake occurred 2.47 KM WNW of San Juan, Trinidad and Tobago, 7.60 KM WNW of Tunapuna, Trinidad and Tobago and 9,60 KM E of Port of Spain, Trinidad and Tobago. This event was located at 61.433°W and 10.666°N, at a very shallow depth of 7.42 Kilometers.
Less than two hours later, at 8:22 AM, another (preliminary) minor quake struck just south of the initial quake, at magnitude 2.7 (Md or Mt) located at 61.427°W and 10.618°N. The later quake occurred at 4.71 KM SE of San Juan, Trinidad and Tobago, 7.36 KM WSW of Tunapuna, Trinidad and Tobago and 11.2 KM ESE of Port of Spain, Trinidad and Tobago.
This information (above) is preliminary from the U.W.I. Seismic Research Centre, the authority for seismic and volcanological information in the Eastern Caribbean. This information may change when additional data is processed by a seismologist.
Both events have been reported felt across Northern and Central Trinidad with one or two short jolts. Some have reported hearing a rumble before the shaking. You can submit felt reports to the University of the West Indies Seismic Research Centre.
According to the UWI SRC, “These events are very shallow and are on land so that’s why they are being felt although they are below the 3.8 magnitude threshold (the magnitude at which most earthquakes are felt by humans).”
There is no tsunami threat.
There are four conditions necessary for an earthquake to cause a tsunami:
- The earthquake must occur beneath the ocean or cause material to slide in the ocean.
- The earthquake must be strong, at least magnitude 6.5.
- The earthquake must rupture the Earth’s surface and it must occur at shallow depth – less than 70 KM below the surface of the Earth.
- The earthquake must cause vertical movement of the seafloor (up to several meters).
None of these conditions occurred.
Note that across the globe, different seismic monitoring agencies use different methods, or several methods, for processing quake parameters. Each method has its limitations and will likely produce different results within the ranges of the uncertainty of that data. This is generally accepted within the scientific community.
Based on felt reports and other seismic solutions, this quake occurred in seismic zone 6, on-land Trinidad.
This quake occurred along one of the many strike-slip faults at the base of the Northern Range, just north of the El Pilar Fault.
Trinidad is a highly faulted area, with several fault systems running across the island – all due to compensation of the lateral movement of the Caribbean and South American plates. There are several major fault systems that run across, on land, Trinidad including the El Pilar Fault system, the Central Range Fault, Northern Range Fault, Darien Ridge, Los Bajos Fault and the Arima Fault. Earthquakes on-land across Trinidad are typically less than 50 kilometers.
There was some variance in this typical depth due to an earthquake swarm in the Toco area in 2001. There have also been some moderate magnitude earthquakes occurring at a deeper depth.
On December 2nd 2004, events with a magnitude 5.5 and 5.1 occurred in the central north-east of Trinidad. Fault plane solutions suggest a normal motion with a component of right-lateral strike-slip. The location of these earthquakes and the corresponding focal mechanisms coincides with the Northern Range faults dipping southward mapped by Algar & Pindell (1993) beneath the Caroni Swamp Area.
Earthquakes *cannot* be predicted – meaning the precise time, date, magnitude, depth, etc. cannot be known ahead of time based on current research and technology.
Generally, across the Eastern Caribbean, a seismically active area, earthquakes of this magnitude, up to M8.0 and greater, are possible and this statement has been repeated by seismologists at the U.W.I. Seismic Research Centre for decades.
Each year, over 2,200 seismic events are recorded in the Eastern Caribbean. On average, the Eastern Caribbean has seen a pattern of major (M7.0-M7.9) quakes every 20 to 30 years. That pattern has stayed true. The last major (M7.0-7.9) quake occurred north of Martinique in 2007.
Historical patterns indicate great quakes (M8.0+) on the Richter Scale have occurred every century or so in the region. The probability of another event at that level is high since the last >M8.0 earthquake occurred in 1843.
Now is the time to create or go over your earthquake preparedness plan and know what to do during, before and after an earthquake. See here for more details.
Why did I hear the earthquake?
The rumbling you hear before the shaking occurs is the P-Wave, a quake’s first seismic wave creating a sound wave.
The speed of sound in air is 330 meters per second. The speed of seismic waves in the earth can not be described by a single number because it depends heavily on the material through which the waves are passing.
It is well known that sound is created in air when an oscillating object is in contact with it. The sound really is oscillations in pressure, displacement and speed of the air, in concordance with the object creating them.
Now, the seismic waves themselves include oscillations of the surface of the earth which is in contact with the air. Therefore, they cause oscillations in the air. However, the frequency of these oscillations is so low that we hear the least part of them as sound. Still, the frequency of the fastest waves, the P-waves, may be more than 30 Hertz and thus be audible as sound. The sound induced by seismic waves in the air have been compared to the boom in underground stations when a train is approaching.
We normally sense P-waves as an earthquake to a lesser degree than for instance the S-waves following them. If an earthquake has not been very strong or we are reasonably far away from its center we will not at all sense the P-waves as an earthquake but only hear the sound induced by them in the air. Still, we may feel the S-waves quite clearly and then other waves which often arrive later, especially at a distance.
Why did I feel two shakes?
An earthquake releases a number of seismic waves, which cause different types of shaking.
The compressive P-waves – essentially sound passing through rock – are the fastest seismic waves, and arrive first. You may feel an up-and-down jolt.
The sideways-shaking S-waves (following the green lines) arrive some seconds later, traveling a little over half the speed of the P-waves; the delay is a direct indication of the distance to the quake. Both of these are body-waves, that pass directly through the earth’s crust.
Following the S-waves are various kinds of surface-waves – Love waves and Rayleigh waves – that travel only at the earth’s surface. Surface waves are smaller for deep earthquakes, which have less interaction with the surface. For shallow earthquakes – less than roughly 60 km deep – the surface waves are stronger, and may last several minutes; these carry most of the energy of the quake and cause the most severe damage.
For shallow quakes like this morning’s, people would have felt the first jolt of the P-waves and these the side-to-side shaking of the S-waves.