Faults & Earthquakes

A fault is (generally) a flat fracture or break in a section of rock. These can be relatively small, which you are able to see in outcrops of rock to thousands of kilometers long, usually along plate boundaries. Faults could be as long as hundreds of kilometers, such as the El Pilar Fault that runs across Trinidad and Northeastern Venezuela to as small as less than a millimeter. It is important to note that faults generally are not one single fracture within a rock. Hence, geologists and geophysicists typically refer to fault zones.

An earthquake occurs when there is a release of stress within a layer of rock, usually at a zone of existing weakness within a rock. Earthquakes don’t necessarily happen on existing faults, but once an earthquake takes place a fault will exist in the rock at that location.

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Normal Faults
Strike-Slip Faults
Reverse & Thrust Faults

Generally, there are three different types of faults: normal, strike-slip and reverse. It is common to see a combination of these three types in reality, though purely normal, reverse and strike-slip faults do exist.

Normal Faults

In areas where rock is being pulled apart or extended, normal faults generally form. At a normal fault, rock drops on one side of the fault, relative to the other side of the fault. The dropped section of rock is called the footwall and the other side of rock, that is relatively higher, is called the hanging wall.

 A photo of the Los Iros Landslide, which occurred after the August 21st, 2018 M6.9 Earthquake. Here, extension, as a result of the landslide, “pulled apart” the blocks of land to show clear normal faulting and fault scarps.
A photo of the Los Iros Landslide, which occurred after the August 21st, 2018 M6.9 Earthquake. Here, extension, as a result of the landslide, “pulled apart” the blocks of land to show clear normal faulting and fault scarps. Photos: Los Iros Landslide: Trinidad and Tobago Weather Center; Diagram of Normal Faulting: Physical Geology (Brooks/Cole-Thompson, 2006).

Normal faults occur across much of Trinidad and Tobago, typically appearing near regional (large) strike-slip faults, such as in the Gulf of Paria. Large normal faults also exist off the Eastern and Northern Coasts of Trinidad.

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Strike-Slip

Strike-slip faults are when one area of rock slides past another laterally. These types of faults can be further subdivided into left-lateral (sinistral) or right-lateral (dextral) strike slip faulting.

Left-lateral (sinistral) strike-slip faulting occurring at the Los Iros Landslide site, displacing the road several meters laterally and vertically from the original position. Photos: Xavier Moonan, SanAndreasFault.org Note that the lithosphere is the crust, although it may seem like a separate entity in the diagram
Left-lateral (sinistral) strike-slip faulting occurring at the Los Iros Landslide site, displacing the road several meters laterally and vertically from the original position. Photos: Xavier Moonan, SanAndreasFault.org Note that the lithosphere is the crust, although it may seem like a separate entity in the diagram.

Several large faults across Trinidad are strike-slip in nature, including the El Pilar Fault which runs south of the Northern Range across Trinidad and into the Northwestern Paria Peninsula of Venezuela. Other notable strike-slip faults are the Central Range Fault and the Los Bajos Fault.

Reverse & Thrust Faults

Reverse faults tend to occur in areas where the crust is being compressed together, such as in convergent plate boundaries (below). This type of fault is essentially the opposite of a normal fault. Instead of an extensional force acting on a slab of rock, a compressive force acts instead. This causes the stressed slab of rock to fracture and resulting in one area of rock rising above the other area of rock along a fault plane.

Diagram showing the mechanics of a reverse fault. Credit: U.S. National Park Service
Diagram showing the mechanics of a reverse fault. Credit: U.S. National Park Service

Reverse faults can also be subdivided into two sub-types. Thrust, where the dip of the fault is 45 degrees or less, and overthrust, where the dip is less than 15 degrees. Reverse faults are responsible for some of the most damaging and powerful earthquakes to date. These earthquakes are dubbed megathrust earthquakes.

Since 1900, all earthquakes of magnitude 9.0 and greater have been megathrust earthquakes. No other type of known terrestrial source of tectonic activity has produced earthquakes of this scale.

Reverse faults are associated with earthquakes of magnitude 8.0 or more generally. Strike-slip faults can produce major earthquakes up to approximately magnitude 8.0. Earthquakes associated with normal faults tend to reach an upper limit at approximately magnitude 7.0.

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In real life, faulting is not as clear cut as theory makes it out to be. Usually, faults are not solely up and down or side to side movement. There is usually a combination of fault movements occurring simultaneously. Trinidad and Tobago is a perfect example of this, where all three major fault movements – strike-slip, reverse and normal – occur simultaneously or in tandem.

It is important to note that across Trinidad and Tobago, the longer faults (i.e. the faults that has the potential to produce the largest earthquakes) at shallow to intermediate levels, are strike-slip in nature. Hence, the largest possible earthquake in the immediate vicinity of Trinidad and Tobago usually near its maximum at magnitude 8.0. However, reverse faulting occurs just north and east of Trinidad and Tobago. Worst-case modelling shows that quakes up to magnitude 8.5 are possible and as a region, we should be prepared since it is not a case of if this quake occurs, but when.

Related: Earthquake Safety

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