Hey Lykkers! Have you ever wondered what causes earthquakes? Earthquakes have been a constant threat since the beginning of human history, and China has documented seismic activity for over 2,000 years!

Despite scientists' best efforts, the exact cause of earthquakes still remains a bit of a mystery. But through over a century of research, we've developed several theories, including one of the most well-known models: the elastic rebound theory.

This theory suggests that large-scale shifts in the Earth's surface rocks are the primary cause of powerful earthquakes.

How Earthquakes Really Happen

At the core of earthquake theory is the idea that the Earth's deep structural forces cause the Earth's outer layer to deform. These large-scale deformations are what lead to earthquakes. Along geological faults, the sudden movement of rocks causes seismic waves to radiate, triggering the earthquake. In laboratory experiments, scientists apply pressure to rocks, which causes them to break or fracture in various ways. Sometimes, a fracture splits the rock into pieces, and these pieces can slide past one another, which is known as brittle failure. Other times, the rocks don't suddenly slip but slowly grind together along an inclined fault plane, which doesn't release energy as quickly as brittle failure does.

The Role of Faults in Earthquakes

In nature, large cracks in the Earth's surface are called geological faults. Some of these faults are incredibly long, stretching for miles across the landscape. Just like the rocks in the lab experiments, these faults can either slowly slide past each other or suddenly break, releasing energy in the form of an earthquake. When the fault breaks suddenly, the rocks on either side move in opposite directions.

The Elastic Rebound Theory: The Key to Understanding Earthquakes

The most widely accepted physical model of how earthquakes occur stems from the research of American engineer Reid, who studied the 1906 San Andreas Earthquake. Reid discovered that between 1851 and 1906, the western side of the fault had shifted 3.2 meters north-northeast. After comparing this data with measurements taken after the earthquake, Reid concluded that significant horizontal shear occurred along the fault both before and after the earthquake.

Scientists now believe that earthquakes are caused by the elastic rebound of deformed rocks along geological faults. This means that when rocks suddenly slide, they release the built-up strain energy and snap back to their previous, undisturbed positions. The longer and wider the deformed area, the more energy is released, and the stronger the earthquake. Vertical strain is also common and can create fault scarps—steep cliffs that can reach several meters high, extending for dozens or even hundreds of kilometers.

How Rock Mechanics Help Us Understand Earthquakes

In laboratory experiments, scientists study how rocks behave under pressure and heat to better understand the strain changes that occur within Earth's rocks before an earthquake. In these experiments, water-saturated rock samples are compressed under high temperature and pressure, simulating the slow deformation of the Earth's crust due to tectonic forces. As rocks near fault lines undergo microfracturing, water slowly diffuses into the cracks and pores, leading to further weakening of the fault. This makes the fault more likely to slip, triggering an earthquake.

Earthquake Precursors: Foreshocks and Aftershocks

By studying the process of microfracturing near major faults, scientists are gaining a better understanding of foreshocks and aftershocks. Foreshocks are smaller earthquakes that occur along faults before the main earthquake. These smaller tremors result from strain and microfracturing along the fault, but the main rupture has yet to occur. The limited movement during foreshocks slightly alters the stress patterns in the area. Eventually, the movement of water and the distribution of microcracks lead to a larger rupture—this is when the main earthquake occurs. Following the main rupture, frictional heat along the fault alters the physical conditions of the fault zone, making it more likely for additional fractures to occur. These secondary fractures are called aftershocks, which can continue for days or even weeks after the main event.

The Ongoing Cycle of Strain and Energy Release

After a large earthquake, the strain energy in the region decreases, but the process doesn't stop there. Over time, new strain begins to accumulate as the Earth's tectonic forces continue to push and pull on the fault. This leads to further deformations and the potential for more earthquakes in the future.

Understanding how earthquakes work is key to preparing for and protecting ourselves from these natural disasters. By studying the behavior of rocks, faults, and the energy stored within the Earth, we are getting closer to predicting earthquakes and understanding their full impact. Stay curious and keep learning, Lykkers! The more we know, the safer we can be in the face of nature's power.