Earthquakes: Understanding the forces beneath the Earth’s surface

Earthquakes are among the most dramatic and destructive natural phenomena. They occur when energy stored in the Earth’s crust is suddenly released. This energy release results from the movement of tectonic plates, leading to seismic waves that shake the ground. Here’s an in-depth look at what causes earthquakes, how they are measured, and recent significant seismic events.

What causes earthquakes?

Earthquakes occur due to the sudden release of energy stored in the Earth’s crust. This release is typically triggered by the movement of tectonic plates along faults. These movements can be categorized into three main types of interactions:

Convergent (destructive) boundaries: When two tectonic plates collide, one plate is often forced beneath the other in a process known as subduction. This process creates immense pressure and friction, leading to powerful earthquakes. A prime example of this is the subduction of the Pacific Plate beneath the North American Plate along the west coast of North America​ (USGS).

Diagram of a convergent boundary. Image by domdomegg/

Diagram of a convergent boundary. Image by domdomegg

Divergent (constructive) boundaries: At divergent boundaries, tectonic plates move away from each other. This separation allows magma from the mantle to rise and create new crust. Earthquakes at these boundaries are generally less intense but are still significant. An example is the Mid-Atlantic Ridge, where the Eurasian Plate and the North American Plate are moving apart (USGS).

Diagram of a convergent boundary. Image by domdomegg/

Diagram of a convergent boundary. Image by domdomegg

Transform boundaries (faults): At transform boundaries, plates slide horizontally past each other. The San Andreas Fault in California is a notable example. The friction here causes stress to build up until it is released in an earthquake (USGS).

Diagram of a transform boundary. Image by domdomegg/

Diagram of a transform boundary. Image by domdomegg

Measuring earthquakes

To quantify the size and impact of earthquakes, scientists use a variety of scales and instruments.

Seismographs: Instruments that combine a seismometer, a timing device, and a recording device to detect and record ground motion caused by seismic waves. Analyzing seismograph data helps determine an earthquake’s location, depth, and magnitude​.

Richter scale: Developed in 1935 by Charles F. Richter, this logarithmic scale measures the magnitude of an earthquake based on the amplitude of seismic waves. Each whole number increase on the Richter scale corresponds to a tenfold increase in measured amplitude and about 31.6 times more energy release, meaning a magnitude 7.0 earthquake would be massively more impactful than a 6.0.

Moment magnitude scale (Mw): This scale provides a more accurate measure of an earthquake’s total energy release and is widely used today, especially for large, distant, or deep earthquakes.

Modified Mercalli intensity (MMI) scale: Unlike the Richter and moment magnitude scales, which measure the energy released by an earthquake, the MMI scale measures the intensity of shaking and its effects on people, structures, and the Earth’s surface. This scale ranges from I (not felt) to XII (total destruction). The MMI Scale is qualitative and is based on observed effects rather than instrumental readings.

European macroseismic scale (EMS): Similar to the MMI scale, the EMS measures the intensity of an earthquake based on its observed effects. The EMS is used primarily in Europe and ranges from I (not felt) to XII (completely devastating). It takes into account factors such as building damage, ground effects, and human perception of the shaking​.

Interesting earthquake data

Frequency of earthquakes: Approximately 500,000 detectable earthquakes occur worldwide each year. Of these, about 100,000 can be felt, and around 100 cause significant damage​ (USGS)​. This means that, on average, over 1,300 earthquakes occur daily, most of which are too small to be felt by humans​ (USGS)​​​.

The largest recorded earthquake: The most powerful earthquake ever recorded was the Great Chilean Earthquake on May 22, 1960, with a magnitude of 9.5. It caused extensive damage and triggered a tsunami that affected coastal areas across the Pacific Ocean.

Deepest earthquakes: While most earthquakes occur at depths of less than 70 kilometers (43 miles), some can occur at much greater depths. These deep-focus earthquakes are typically associated with subduction zones. The deepest recorded earthquake occurred in 2015 at a depth of 700 kilometers (435 miles) in the Bonin Islands, Japan.

Manmade earthquakes: Human activity can sometimes cause detectable seismic activity, a phenomenon known as induced seismicity. These “manmade earthquakes” tend to be minor, but not without exceptions. For example, a 1952 Mw 5.7 earthquake in Oklahoma is suspected to have been caused by deep injection of wastewater by the oil industry.

Recent significant earthquakes

2023 Turkey-Syria earthquake: On February 6, 2023, an Mw 7.8 earthquake struck southern Turkey and northern Syria. This event caused massive destruction, leading to over 50,000 deaths and extensive damage to infrastructure. The economic losses were substantial, estimated at $148.8 billion in Turkey and $14.8 billion in Syria. The international community responded with a large-scale rescue operation involving over 141,000 individuals from 94 countries​.

2023 Marrakesh-Safi earthquake: On September 8, 2023, an Mw 6.9 earthquake struck the Marrakesh-Safi region in Morocco. It was the most powerful earthquake recorded in Morocco’s history, resulting in the deaths of nearly 2,960 people and significant damage to historical landmarks.

2023 Herat earthquakes: In October 2023, a series of earthquakes measuring Mww 6.3 each struck the Herat Province in Afghanistan. These events caused substantial damage, with the World Health Organization estimating around 1,482 fatalities and thousands more injured​.

2024 Noto earthquake: On January 1, 2024, an Mw 7.5 earthquake struck the Ishikawa region of Japan, causing severe shaking and damage. This earthquake resulted in 245 deaths and significant damage to buildings and infrastructure​.

Why understanding earthquakes matters

Understanding earthquakes is crucial for several reasons:

Insight into Earth’s interior: Earthquakes provide valuable information about the structure and composition of the Earth’s interior. By analyzing seismic waves, scientists can infer details about the Earth’s mantle and core.

Preparedness and mitigation: By studying seismic activity, scientists can identify areas at high risk for earthquakes and develop building codes and safety measures to minimize damage and casualties.

Early warning systems: While predicting earthquakes in advance remains beyond current scientific capabilities, early warning systems can provide crucial seconds to minutes of warning before significant shaking begins. These systems use a network of seismographs to detect the initial P-waves and send warnings to affected areas. However, these systems cannot predict the exact time, location, and intensity of future earthquakes. The Earth has numerous fault lines and spreading zones where shaking could potentially begin, making precise predictions challenging. Additionally, earthquakes often trigger a cascade of secondary natural disasters, such as landslides, dam breaks, and tsunamis, which contribute to their overall destructive potential​.

Earthquakes are a natural consequence of the dynamic processes occurring within the Earth’s crust. While they can be devastating, our growing understanding of their causes and characteristics enables us to better prepare for and mitigate their impacts. Through continued research and technological advancements, we can improve our ability to predict and respond to these powerful events, enhancing safety and resilience in earthquake-prone regions around the world.

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