Earthquakes: Causes, Waves, and Measurement Scales

When the ground beneath our feet suddenly shakes, buildings sway, and objects fall from shelves, we’re experiencing one of nature’s most powerful forces. Earthquakes have shaped our planet’s landscape for billions of years and continue to affect millions of people worldwide. Understanding how earthquakes occur, how energy travels through the Earth, and how we measure these events is crucial for disaster preparedness and management.

How earthquakes occur
Understanding focus and epicenter
Tectonic and volcanic earthquakes
Types of earthquake waves
Primary waves (P waves)
Secondary waves (S waves)
Surface waves
Measuring earthquakes
The Richter scale and magnitude
The Modified Mercalli Intensity Scale
Why both scales matter

How earthquakes occur

Earthquakes happen when two blocks of Earth suddenly slip past each other. The Earth’s outer layer isn’t one solid piece-it’s broken into massive sections called tectonic plates that constantly move, slide, and collide with each other. These movements occur along fractures in the Earth’s crust known as faults.

When tectonic plates move past each other, their rough edges get stuck while the rest of the plate keeps moving. Pressure builds up over time, sometimes for years or even decades. Eventually, the force becomes too great, and the edges suddenly unstick and slip. This rapid movement releases enormous amounts of stored energy, creating the shaking we feel as an earthquake.

Understanding focus and epicenter

Two key terms help us locate where an earthquake begins. The focus (also called hypocenter) is the point inside the Earth’s crust where the earthquake actually starts-where the rocks first break and slip. This can be anywhere from a few kilometers to hundreds of kilometers below the surface.

The epicenter is the point on the Earth’s surface directly above the focus. When you hear news reports about earthquake locations, they’re usually referring to the epicenter. This is typically where the strongest shaking occurs, though the actual damage pattern depends on many factors including soil type, building construction, and distance from the fault.

Tectonic and volcanic earthquakes

Most earthquakes are tectonic earthquakes, caused by sudden movement along faults and plate boundaries. These occur at the edges where plates collide, pull apart, or slide past each other. The famous San Andreas Fault in California is one example where plates slide horizontally past each other, generating frequent earthquakes.

Volcanic earthquakes happen when magma moves beneath active volcanoes. As molten rock pushes through cracks and chambers underground, it creates pressure changes and rock fractures that generate seismic waves. These earthquakes are typically smaller than tectonic ones but serve as important warning signs of potential volcanic eruptions.

Types of earthquake waves

When an earthquake occurs, energy radiates outward from the fault in all directions as seismic waves, similar to ripples spreading across a pond when you drop a stone. Scientists classify these waves into two main categories: body waves that travel through the Earth’s interior, and surface waves that move along the Earth’s outer layer.

Primary waves (P waves)

P waves, or primary waves, are the fastest seismic waves and the first to arrive at seismic recording stations. They’re compression waves that push and pull rock particles back and forth in the same direction the wave is traveling. Think of how sound waves travel through air-P waves work similarly but move through solid rock, liquids, and gases.

These waves can travel at speeds of 5,000 meters per second through granite, which is why they reach monitoring stations first. P waves cause the ground to compress and expand, creating a back-and-forth motion. While they’re fast, they typically cause less damage than other wave types because their amplitude is relatively small.

Secondary waves (S waves)

S waves, or secondary waves, are shear waves that shake the ground perpendicular to the direction of travel. Unlike P waves, S waves move particles up and down or side to side at right angles to the wave’s path. This shearing motion is more destructive to buildings and structures.

S waves travel about 60% the speed of P waves, which is why they arrive at seismograph stations after P waves. A critical characteristic of S waves is that they cannot travel through liquids or gases. This property has helped scientists determine that Earth’s outer core is liquid-S waves cannot pass through it.

Surface waves

Surface waves travel along the Earth’s surface rather than through its interior. Although they’re the slowest of the three wave types, they can be much larger in amplitude and are often the most destructive. There are two main types of surface waves.

Love waves cause horizontal motion that moves the surface from side to side perpendicular to the wave’s direction. This snake-like movement can cause tremendous damage to building foundations. Rayleigh waves create a rolling motion similar to ocean waves, producing both vertical and horizontal ground movement. Some people have reported actually seeing the ground roll during strong earthquakes.

Measuring earthquakes

Scientists use two fundamentally different approaches to describe earthquake size: magnitude scales that measure the energy released, and intensity scales that describe the effects at specific locations.

The Richter scale and magnitude

The Richter scale, developed by Charles Richter in the 1930s, was the first widely-used magnitude scale. It measured earthquake size based on the amplitude of seismic waves recorded on seismographs. The scale is logarithmic, meaning each whole number increase represents a tenfold increase in wave amplitude and about 32 times more energy released.

For example, a magnitude 6.0 earthquake releases 32 times more energy than a magnitude 5.0 earthquake, and 1,024 times more than a magnitude 4.0 earthquake. Today, scientists primarily use the moment magnitude scale, which works better for large earthquakes and is applicable globally. It measures the total energy released based on fault size and slip distance.

The magnitude scale has no upper limit. The largest recorded earthquake was a magnitude 9.5 in Chile in 1960, though theoretically larger earthquakes are possible. Each earthquake has only one magnitude, regardless of where it’s measured.

The Modified Mercalli Intensity Scale

While magnitude tells us about an earthquake’s total energy, intensity describes the amount of shaking and damage at a particular location. The Modified Mercalli Intensity Scale, developed by Giuseppe Mercalli and later modified in 1931, uses observations from witnesses and damage assessments to assign intensity values.

The MMI scale ranges from I (not felt) to X (extreme destruction). Lower numbers describe how people felt the earthquake, while higher numbers are based on observed structural damage. For instance, intensity III means felt indoors by many people but causing no damage, while intensity VIII involves considerable damage to ordinary buildings with partial collapse.

Unlike magnitude, a single earthquake produces many different intensity values depending on distance from the epicenter, local soil conditions, and building construction quality. The intensity is usually highest near the epicenter and decreases with distance, though local geology can create pockets of stronger shaking far from the source.

Why both scales matter

Both measurement approaches serve important purposes. Magnitude scales are more scientifically objective and allow comparison between earthquakes worldwide, while intensity scales provide practical information about actual impacts on communities. For disaster management, knowing both the magnitude and the intensity distribution helps emergency responders allocate resources effectively and understand which areas need immediate attention.

What do you think? How might understanding the difference between earthquake magnitude and intensity change the way communities prepare for seismic hazards? What role does knowledge of seismic wave types play in designing earthquake-resistant buildings?

References

PDF 📄

Table of Contents

How earthquakes occur 🔗

Understanding focus and epicenter 🔗

Tectonic and volcanic earthquakes 🔗

Types of earthquake waves 🔗

Primary waves (P waves) 🔗

Secondary waves (S waves) 🔗

Surface waves 🔗

Measuring earthquakes 🔗

The Richter scale and magnitude 🔗

The Modified Mercalli Intensity Scale 🔗

Why both scales matter 🔗