# What are the main features of earthquakes?

Cambridge iGCSE Geography > The Natural Environment > Earthquakes and Volcanoes > What are the main features of earthquakes?

# What are the main features of earthquakes?

## What are earthquakes?

An earthquake represents a sudden and sometimes violent, movement of the Earth’s crust. The Earth’s crust is set into motion due to a rapid release of energy. On average, our planet is subjected to nearly 1000 earthquakes every day. Thanks to technological advancements and increased global awareness of natural disasters, information about earthquakes is disseminated more rapidly.

## Measuring Earthquakes

The magnitude of an earthquake is commonly quantified using the Richter Scale. Developed in 1935 by Charles F. Richter, the Richter Scale measures the amplitude of seismic waves produced by an earthquake. It’s a logarithmic scale, meaning that each whole number increase on the scale represents a tenfold increase in measured amplitude and roughly 31.6 times more energy release. However, the Richter Scale has limitations, especially for huge earthquakes or those more than 600 kilometres from the seismograph.

The Moment Magnitude Scale (Mw) was introduced to overcome the limitations of the Richter Scale in the late 20th century. It provides a more accurate and consistent measurement of an earthquake’s size. Unlike the Richter Scale, which relies on the amplitude of seismic waves, the Moment Magnitude Scale is based on the earthquake’s seismic moment. This seismic moment is calculated using the rupture area along the fault, the average amount of slip, and the rocks’ rigidity.

According to historical records dating back to 1900, the world can expect around 16 major earthquakes yearly. These include 15 events in the magnitude seven range and at least one that reaches or exceeds magnitude 8.0.

## The Pacific Ring of Fire

An astonishing 90% of the world’s earthquakes occur along the Pacific Ring of Fire. This zone forms a horseshoe shape, extending 40,000 km around the edge of the Pacific Ocean, making it the world’s most earthquake-prone region.

## What are the causes of earthquakes?

Earthquakes can occur due to the movement of magma within volcanoes or along the edges of tectonic plates, where they interact. The friction between tectonic plates can cause them to become stuck, building pressure over time. When this pressure is released, it results in an earthquake.

## Focus and Epicentre

• Focus: The point where tectonic plates slip or rupture within the Earth’s crust is known as the focus. Deep-focus earthquakes are often found in subduction zones, while shallow-focus earthquakes occur where plates move apart or slide past each other. Shallow earthquakes tend to be more destructive, as the energy released reaches the surface more quickly.
• Epicentre: The point on the Earth’s surface directly above the focus. Seismic waves radiate from the focus, causing the strongest sensations at the epicentre, with intensity diminishing farther away. Most severe damage typically occurs close to the epicentre.

## Effects of Earthquakes

Earthquakes result in two main effects on the Earth’s surface: shaking and slipping of the crust. Movement at the surface can exceed 10 meters in the largest events, and underwater slips may even trigger tsunamis.

## Recording Earthquakes

The strength of an earthquake is captured using a seismograph, with the results displayed on a seismogram. In addition to the moment magnitude and Richter scales, the Mercalli scale may also be used to gauge the physical damage caused by an earthquake, ranking it in Roman numerals.

## Aftershocks

After a significant earthquake, minor tremors, known as aftershocks, may occur. These are typically the result of the crust adjusting to the initial shock.

## Summary

• Earthquakes are sudden movements of the Earth’s crust caused by a rapid release of energy. Around 1000 occur daily worldwide.

• The Richter Scale, developed in 1935, measures an earthquake’s amplitude but has limitations. The Moment Magnitude Scale (Mw) provides a more accurate assessment.

• Historical records show an average of 16 major earthquakes per year, including 15 in the magnitude seven range and at least one exceeding 8.0.

• The Pacific Ring of Fire is a horseshoe-shaped zone around the Pacific Ocean that accounts for 90% of the world’s earthquakes, making it the most earthquake-prone region.

• Earthquakes are triggered by magma movement in volcanoes or friction between tectonic plates causing them to slip or rupture.

• The focus is the rupture point within the Earth’s crust, and the epicentre is the surface point directly above. Shallow-focus earthquakes are often more destructive.

• Earthquakes can cause shaking and slipping of the Earth’s surface, and movements exceeding 10 meters can trigger tsunamis.

• Seismographs record earthquake strength and the Mercalli scale measures physical damage. Aftershocks are minor tremors following significant earthquakes resulting from the crust’s adjustment.