Super-rare but not impossible: Strong earthquakes within stable continental areas often come as a surprise but they can happen almost everywhere.
Shift happens. This term, mainly used by seismologists and other earthquake scientists, is as short as it is perfect to describe the global distribution of seismic activity. Yes, there are plates, faults and volcanic areas that are strongly influencing earthquake activity and seismic hazards, leading to the division into earthquake-prone and "earthquake-free" areas in public perception. And yes, there are areas which are often shaken by large quakes. Japan, Chile, Indonesia, just to name a few. And yes, there are areas where no living human has ever experienced ground-shaking. Parts of Scandinavia, Oceania and western Africa can be named here. But now and then we get these surprising events where people are asking with shaking head: “How can there be such a large earthquake in Guyana?”
Guyana, located in the North of the South American Continent, is not known as a typical earthquake area. However, Venezuela and the Caribbean subduction zone are not too far away so that areas along the northern coast, the capital Georgetown for example, now and then experience a minor shake from distant earthquakes. But the more you the south you go, the lower becomes the earthquake frequency. Southern Guyana, sparsely populated, mostly by indigenous people, has not experienced any large earthquake in times of human recordings.
Until January 31st, 2021.
The Magnitude 5.7 earthquake has hit the south-western province Upper Takutu-Upper Essequibo, an area slightly larger than Croatia and slightly smaller than West Virginia with a population density lower than Alaska. Luckily. Quakes of this size, especially in not prone areas, prove a significant threat. But this quake seems to have only limited affects with a couple of severely damaged homes in Guyana, ground cracks, small landslides, and also limited non-structural damage in nearby areas of Brazil. However, for the local population, this was probably the first earthquake most of the people have ever experienced. The first and the strongest.
If we have a look at the earthquake history of Guyana, it becomes clear why Guyana is not considered as an earthquake-prone area: Only a couple of events have ever been recorded onshore, a few more offshore. The strongest of these events were in the mid-M4 range, one offshore near the northern tip of Guyana and two others in Upper Takutu-Upper Essequibo. Many more earthquakes were detected further to the northwest along the Caribbean arc and in Venezuela, including some events that were felt in Guyana. But considering only the local seismicity, Uppter Takutu-Upper Essequibo seems to be the country’s most active earthquake region.
Guyana’s low seismic activity can be explained due to its position within a so-called stable craton: An area of old continental litosphere that has not underwent significant tectonic processes like volcanism or rifting. Therefore, only a limited number of small faults is present, mostly very old ones and due to the absence of nearby active plate boundaries, the tectonic stresses are rather low. In case of Guyana, the craton is called “Amazonia” and consists of rocks that are older than one billion years.
It is not too surprising that the area around today's M5.6 Guyana earthquake does not produce many earthquakes, since it is an old craton (map from Casquet et al., 2012) pic.twitter.com/rJxvRzavpv— Jascha Polet (@CPPGeophysics) January 31, 2021
With no large and active tectonic features nearby, only small faults and no present volcanism, no local sources for tectonic stress build-up exists. However, due to the movement of tectonic plates, stresses are present everywhere to a certain degree. But usually these stresses exist in a stable condition. This means that the pressure within the rock is too low to exceed the friction within the zones of weakness (mostly faults). But this stable state can be critically. We see this for example in areas with induced seismicity by gas extraction or enhanced geothermal stresses: Minor changes in pore pressure or vertical stresses are enough to cause seismic activity along these old faults. But not only human activities can lead to imbalance. Over many thousands of years, the forces from tectonic processes, even if their origin is hundreds of miles away, have a minor impact on the state of stress. Distant earthquakes, minor changes in plate motion and heterogeneities within mantle and crust slowly but steadily increase the level of stress. If this stress is not released by other tectonic changes, the stable will become imbalanced. Faults that have never ruptured before in human history suddenly become instable. A strong earthquake happens.
This is how the Guyana quake came to be. Over a timespan that exceeds all human recordings the seismic energy for this event was created. Smaller earthquakes that happened now and then were the sign that this area is not as stable as one might think.
What does this mean for other regions with a low seismic activity?
Shift happens. Everywhere. Even if you have never experienced an earthquake before in your area, nobody can promise that you won’t in the future. Stress is present everywhere. Tectonic faults, even if old and small, can one day become active again if the circumstances are right. The Earth and its tectonic processes are billions of years old. Mankind has experienced a few thousand year with even less perpetuated in writings. If areas like Japan, Chile and Indonesia have large areas every few years, people know that they have to expect. If large earthquakes happen only every few hundred years like in parts of Central Europe or the Eastern US, people might know that these things might happen, but they are usually less prepared and more vulnerable as preparation is seen as less important. But if large quakes happen only once every 10,000 years, people in those areas might not even know that there is any hazard, they have no preparation and are often deadly surprised.
Guyana was a lucky escape. The low population density significantly reduced the impact. But the next super-rare earthquake will come, and it will come as a surprise.