Lesson Explainer: Earthquakes Geology

In this explainer, we will learn how to explain how earthquakes happen, describe their effects, and compare between different types of earthquakes’ seismic waves and between an earthquake’s intensity and magnitude.

Millions of earthquakes happen all over the world every year, from huge catastrophic events to small unnoticeable ground tremors.

An earthquake is the vibrations resulting from a sudden release of energy from within Earth’s crust. It results in waves propagating out from the earthquake’s origin, vibrating the rocks it travels through. These vibrations then travel as waves through Earth.

Earthquakes have occurred throughout geologic time. However, some more recent examples of earthquakes include the 2011 Japan earthquake, the 2004 Indian Ocean earthquake, and the 1992 Cairo earthquake.

The 2011 earthquake of Japan struck the northeast coast of Japan’s largest island, Honshu. A tsunami was generated that caused a great disaster and many people were killed.

The 1992 Cairo earthquake mainly affected the city of Cairo and northeastern Egypt. This earthquake destroyed many houses and buildings. Approximately 600 people died, mainly due to collapsing houses and schools.

The 2004 Indian Ocean earthquake occurred offshore Sumatra and is one of the deadliest natural disasters in history. It generated a tsunami that affected many Asian countries and killed tens of thousands of people. The scale of the damage in Indonesia can be seen in Figure 1.

Earthquakes can vary in size, location, and type. They are classified based on their depth of origin and cause. The three types of earthquakes that we will discuss in this explainer are tectonic earthquakes, volcanic earthquakes, and plutonic earthquakes.

Tectonic earthquakes are the most common type of earthquakes. They can occur along faults and along plate boundaries where tectonic plates meet. They occur when there is a sudden dislocation of rocks in the crust or upper mantle, resulting in vibrations in the rock. This sudden dislocation of rocks can result in the rocks fracturing and faulting.

At a plate boundary, the two plates grind together. If they get stuck, the pressure builds up. When the pressure between the two plates becomes too great, they break loose. This causes the rocks to fail and to suddenly fracture or slip. The built-up energy is released as kinetic energy that travels as vibrations through Earth’s crust, as demonstrated in Figure 2.

A volcanic earthquake is an earthquake that is associated with volcanic activity. This usually involves a volcanic eruption or magma moving within the crust.

One scenario where moving magma commonly causes earthquakes is at a convergent plate boundary. This is a boundary between two plates that are moving toward each other and one is often subducted beneath the other.

Plates that are subducted are subject to high temperatures, melting the rocks and turning them into magma. This magma is less dense than the surrounding rocks, so it rises upward. It then accumulates in magma chambers in the subsurface. These chambers feed magma up into volcanoes at the surface. The movement of magma in and out of these chambers leaves voids that are infilled by collapsing rock. The movement of the collapsing rock results in fracturing around the chamber, generating volcanic earthquakes.

The movement of magma within the volcano also can cause earthquakes. This can lead to a volcanic eruption and a powerful ejection of lava, ash, and gases from the volcano. The power of these events can also trigger volcanic earthquakes.

Figure 3 shows how volcanic earthquakes can occur in Earth’s crust.

Because these types of earthquakes are restricted to volcanic areas, they occur locally and on a smaller geographical scale compared to tectonic earthquakes. For example, a swarm of more than 140 small earthquakes were recorded at the summit of the Kilauea volcano in Hawaii in August 2021. These earthquakes occurred due to magma moving within the volcanic island but did not result in any eruptions on land.

We will now focus on terminology associated with earthquakes. The first term that we will discuss is an earthquake’s focus. This is the point within Earth’s crust where an earthquake originates.

Earthquakes with a focus at a depth greater than 300 km are known as plutonic earthquakes. They mostly originate in the upper mantle due to the subduction of oceanic plates at convergent plate boundaries. Because they occur at such great depths, their impact at the surface is minimal, as seen in Figure 4.

Plutonic earthquakes do not occur deeper than 700 km. This is because below this point, rocks are not brittle enough to fracture.

Directly above the focus of an earthquake is its epicenter. An earthquake’s epicenter is the point at Earth’s surface where the earthquake’s effects are first felt. The strength and effects of an earthquake decrease with distance from the epicenter, which can be seen in Figure 5.

When an earthquake occurs, it releases waves of energy. These waves are called seismic waves.

There are two types of seismic waves produced by earthquakes, including surface waves and body waves.

Surface waves are the most destructive type of seismic waves. They are the waves that result in surface vibrations, infrastructure destruction, and landslides. They are confined to the upper layers of Earth’s crust, meaning that they are the most easily detected waves. They are complex waves as seen in Figure 6.

Surface waves are the slowest type of seismic wave. This means that they are the last waves to be detected during an earthquake.

The other type of seismic waves that we will look at is body waves. Unlike surface waves, these waves can travel through the internal structure of Earth. Body waves can be classified into two groups known as primary waves and secondary waves.

Primary waves (P-waves) are the fastest seismic waves and are the first waves to be detected during an earthquake. They are longitudinal waves. This means that they are compressional, pushing rock particles back and forth, as seen in Figure 7. Primary waves can travel through all mediums: solids, liquids, and gases.

Secondary waves (S-waves) are slower than primary waves. They are transverse waves. This means that they travel with an oscillatory motion, pushing rock particles up and down, as seen in Figure 8. Secondary waves can only travel through solids.

Seismic waves can be used to study the internal structure of Earth. This can be done by studying their properties and what kind of mediums they can travel through.

Primary seismic waves can travel through solids and liquids, whereas secondary seismic waves can only travel through solids.

Using this knowledge, geologists have been able to determine which parts of the internal structure of Earth are liquid and which parts are solid, depending on which seismic waves can travel through them.

For example, secondary waves cannot pass through the outer core, because it is liquid. Primary waves, however, can travel through the outer core because they can travel through liquids.

Seismic waves can be detected at seismographic stations. These stations have measuring equipment that responds to ground movement and noises. When an earthquake occurs, a seismographic station will detect the vibrations caused by the earthquake and also the strength of the vibrations.

The epicenter of an earthquake can be determined from the results of multiple seismographic stations.

For example, Figure 9 shows the location of three seismographic stations labeled A, B, and C. Each one can detect seismic waves within a certain radius and determine their velocity and relative arrival times. This allows us to draw a radius around the seismographic stations. From these results, a point can be determined by finding the intersection of these radii. This point represents the epicenter of the earthquake.

An earthquake’s strength can be measured in a variety of ways; the two most common methods are intensity and magnitude.

Earthquake intensity is a measurement of the surface damage resulting from an earthquake. It is most commonly measured with the Mercalli scale as seen in Figure 10. The Mercalli scale is a 12-point scale, represented by Roman numerals, devised in 1902, and remodified in 1931. One is the lowest value, which reflects very weak earthquakes that are not felt by humans. Twelve is the highest value, which reflects very strong earthquakes that cause mass destruction.

Earthquake magnitude measures the amount of energy released when an earthquake occurs. It is measured on the Richter scale, which is shown in Figure 11. This scale was developed in 1935 by Charles Richter. On average, detected earthquakes range from 0 to 9; however, earthquakes have been recorded higher than 9 on the Richter scale. An earthquake in Chile in 1960 registered approximately 9.4 to 9.6 on the Richter scale and it is the most powerful earthquake ever recorded.

Figure 12 shows a comparison between the Mercalli and Richter scales, showing how the magnitude and intensity of an earthquake correspond. The higher the earthquake’s magnitude, the greater its intensity and the more damage it causes.

Let’s summarize what we have learned so far.