12.7 - Earthquakes

Earthquakes are natural phenomena caused by the sudden release of energy in the Earth's crust, resulting in seismic waves. These waves can cause significant shaking of the Earth's surface, which can lead to destruction and loss of life. They mainly occur along fault lines, where tectonic plates meet and move relative to each other.

Earthquakes are often caused by the movement of tectonic plates beneath the Earth's surface. The Earth's outer shell, or lithosphere, is broken into several large and small tectonic plates. When these plates move, they either collide, pull apart, or slide past each other. This movement creates stress and pressure at the plate boundaries. When the stress exceeds the strength of the rocks along the fault line, it causes the rocks to break and move suddenly. This release of energy results in an earthquake.

The energy released during an earthquake travels in the form of seismic waves. These waves move through the Earth and can be detected by instruments called seismographs. Seismic waves are classified into two types: body waves and surface waves.

• Body Waves: These waves travel through the interior of the Earth and are further divided into two types:
  • P-Waves (Primary Waves): These are compressional waves that travel the fastest and are the first to be detected by seismographs. They move the ground in the direction of wave propagation.
  • S-Waves (Secondary Waves): These waves are slower than P-waves and move the ground perpendicular to the direction of wave propagation. S-waves cause more damage compared to P-waves.
• Surface Waves: These waves travel along the Earth's surface and typically cause the most damage. They are slower than body waves and move in circular or elliptical motions.

Earthquakes are measured using two main scales:

• Richter Scale: This scale measures the magnitude of an earthquake, i.e., the amount of energy released. The Richter scale is logarithmic, meaning that each whole number increase on the scale represents a tenfold increase in amplitude and approximately 32 times more energy release.
• Moment Magnitude Scale (Mw): This scale is used to measure the size of larger earthquakes, especially those with a magnitude greater than 7. It is based on the seismic moment, which considers the fault length, fault slip, and the material properties of the fault zone.

The magnitude of an earthquake can be represented using the formula for the Moment Magnitude Scale:

$ M_w = \frac{2}{3} \cdot \log_{10}(M_0) - 10.7 $

Where:

• $ M_w $: Moment magnitude of the earthquake
• $ M_0 $: Seismic moment, which depends on the fault length, fault slip, and the material properties of the fault

The impact of an earthquake can be devastating. It can result in the collapse of buildings, bridges, roads, and infrastructure, causing significant economic and human losses. Additionally, earthquakes can lead to secondary effects like tsunamis, landslides, and fires, which may further complicate rescue and relief efforts.

Seismic activity is monitored worldwide by organizations like the United States Geological Survey (USGS) and other national and regional seismic monitoring networks. Early warning systems have been developed in certain regions to provide advance alerts, helping to mitigate the damage caused by earthquakes.