Tectonic Plates and Tectonic Waves Cause Earthquakes

Earthquakes usually occur due to the sudden breaking of underground rocks and the consequent rapid movement along a fault. The sudden release of energy that built up in the process causes seismic waves that make the ground shake. It is well known that the movement of the tectonic plates can cause earthquakes, but we should not neglect the role that the tectonic waves play in it. Let’s first have a look at the theory behind tectonic plates and then move on to tectonic waves.

According to the USGS, a tectonic plate is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere¹.

We know seven major plates: African, Antarctic, Eurasian, Indo-Australian, North American, Pacific, and South American. They are all marked on the map below. You can see that their size can vary a lot (the Pacific and Antarctic Plates are among the largest), from a few hundred to thousands of kilometers.

Picture 1: Tectonic plates around the world.

Their thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere1. Due to the latter, the depth of an earthquake determines how much it will be felt on the surface and, therefore, how much damage it will cause. Earthquakes occur in the Earth’s crust (i.e., upper mantle), which ranges from the Earth's surface to about 800 km deep (about 500 miles). The USGS also reports that the strength of earthquake shaking diminishes with increasing distance from the earthquake's source. This leads us to the conclusion that the strength of shaking at the surface from an earthquake that occurs at 500 km deep is considerably less than if the same earthquake had occurred at 20 km depth². If an earthquake occurs at a depth of, for example, 300 km, not many people will pay attention to it because the energy released is not strong enough to impact the surface.

Tectonic plates probably developed very early in the earth's 4.6-billion-year history, and they have been drifting about on the surface ever since like slow-moving bumper cars repeatedly clustering together and then separating¹.

It is important to keep in mind that the tectonic plates are constantly moving. Their movement has formed large mountain ranges like the Himalayas and the Andes. Or, more accurately, they are still forming them today. The Himalayan Mountain range and Tibetan plateau have formed as a result of the collision between the Indian Plate and Eurasian Plate, which began 50 million years ago and continues today. Whereas the Andes were formed by tectonic activity whereby the earth is uplifted as one Nazca plate (oceanic crust) subducts under the South American plate (continental crust).

Earthquakes occur along the fault lines, i.e., cracks in Earth's crust where tectonic plates meet, more specifically where plates are subducting, spreading, slipping, or colliding. When this happens, the tectonic plates that meet can get stuck and the pressure starts to build up until it breaks loose. The earthquake releases energy in waves that travel through the earth's crust and cause shaking. The magnitude of an earthquake depends on how much pressure has built up.

Based on the knowledge of tectonic plates we can forecast or “predict” earthquakes. However, predicting or forecasting earthquakes based solely on the movement of tectonic plates is often not a reliable enough source. This is when we start talking about the tectonic waves. Why are they important?

Because they are the real triggers of earthquakes in the earth’s crust; or, said differently, tectonic waves are where earthquakes are. Thousands of tectonic waves travel through the earth’s crust triggering earthquakes in unstable tectonic zones.

The best everyday analogy with tectonic waves are dominoes. When one domino falls, all subsequent dominoes will also fall, leading to a “wave” of collapsing dominoes. In the earth’s crust, the “dominoes” are tectonic faults and blocks of rock, and the domino effect in the crust leads to the so-called tectonic waves or also strain waves. These waves have all possible velocities up to 6000 m/s. There exist very slow tectonic waves that are related to large earthquakes, and ultrafast tectonic waves on a daily basis.

Based on a careful analysis of the distribution of past global earthquakes, Quantectum’s Operational Center can calculate the current and future positions of tectonic waves. When these waves pass through unstable fault systems and plate-tectonic boundaries, they can cause large earthquakes. Therefore, tectonic waves define endangered regions where dangerous seismic states can occur.

Sources:
1) USGS. What is a Tectonic Plate? Accessed on 19-Jul-2022. Available at: https://pubs.usgs.gov/gip/dynamic/tectonic.html
2) USGS. At What Depth do Earthquakes Occur? Accessed on 19-Jul-2022. Available at: https://www.usgs.gov/faqs/what-depth-do-earthquakes-occur-what-significance-depth