Seismology is the study of earthquakes and the waves they generate. The word seismology comes from the Greek word seismos meaning “shaking” or “earthquake”, something the ancient civilizations in Greece and Italy were all too familiar with. This region lies near a tectonic plate boundary, making earthquakes a frequent occurrence.
People once believed that the ground shaking was a sign of divine anger. Today, science has revealed the tectonic forces behind earthquakes, but they remain difficult to predict, and their sudden, powerful shaking can still be shocking and destructive. So why do earthquakes happen? How can they move the ground so dramatically? And why can’t we predict exactly when one will strike?
Although the rocks beneath your feet may look boring and still, the earth is actually in motion, constantly shifting around trying to accommodate the movement of plate tectonics on the surface and convection of material in the mantle. This creates stresses throughout the crust, where tightly packed rocks are pushed or pulled in different directions causing them to move or deform. When the stress becomes too great, it is suddenly released along faults or plate boundaries. This release of energy sends out seismic waves, which are what we feel during an earthquake.
Types of Seismic Waves
When an earthquake happens, it generates two main types of seismic waves: body waves and surface waves. Body waves travel through the body of the Earth, making it deep into the interior, while surface waves travel around the Earth’s on the surface.
P-waves (Primary waves) are compressional waves that move particles back and forth parallel to the direction the wave is traveling. This type of particle motion is also known as longitudinal. They are the fastest type of seismic wave and can travel through solids, liquids, and gases.
S-waves (Secondary waves) are shear waves that move particles side to side, perpendicular to the direction of wave travel. This type of particle motion is also known as transverse. These waves are slower than P-waves and can only move through solids.
To visualize a P-wave, imagine pushing and pulling on a slinky. The coils compress and expand in the same direction you’re pushing and pulling, which is the direction the wave is traveling. For an S-wave, imagine holding one end of a rope and shaking it up and down while your friend holds the other end. The wave travels along the rope toward your friend, but the rope itself moves up and down, while the wave moves horizontal. Because S-waves require a solid medium to transfer this side-to-side motion, they can’t travel through liquids.
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The other type of wave from earthquakes, surface waves, travel along the Earth’s surface and are usually responsible for the most intense shaking and damage during an earthquake. There are two main types:
Rayleigh waves move in an elliptical, rolling motion, both vertically and horizontally
Love waves move the ground side-to-side in a horizontal motion, perpendicular to the wave’s path.
Because surface waves stay near the surface and interact with structures directly, they cause much of the destruction we associate with major earthquakes. All waves lose energy as they travel, so body waves traveling through the interior of the earth lose their energy before reaching the surface. Surface waves on the other hand, travel along the surface, and the vertical rolling motion of Rayleigh waves can lift and drop the ground causing mass destruction.
The Seismic Wave’s Journey
Seismic waves aren’t just signals of destruction, they’re messengers, carrying valuable information about the earthquake source and the materials they pass through on their journey.
The shape and timing of the waves tell us key details about the origin of the earthquake, known as the source mechanism. For example:
The relative arrival times of P-waves and S-waves at different stations (seismometers) allow us to triangulate the location of the earthquake on a map. Since we generally know the speed of these waves, the difference in arrival time of the P and S-wave tells us how far away the source is.
The time difference between the first arriving P-waves at different stations also helps determine the depth of the rupture (also known as hypocenter).
The first motion direction of the P-wave, whether it pushes upward or pulls downward, can reveal how the fault moved: up/down (thrust), side-to-side (strike-slip), or something in between.
The duration and frequency content of the waves tell us how large and complex the rupture was. Longer, lower-frequency signals usually mean a bigger rupture zone or slower slip, while sharp, high-frequency signals might mean a smaller, abrupt break.
Just as light bends through glass or water, seismic waves change speed and direction as they travel through the Earth. These changes are the key for scientists to see inside the Earth without drilling.
If a wave slows down, it likely passed through soft sediment or partially melted rock.
If a wave speeds up, it likely passed through hard consolidated rock with tightly packed crystals.
If a wave disappears, like an S-wave that fails to arrive, it may have encountered a liquid layer, like the outer core.
If a wave bends or splits, it may indicate the presence of a boundary, like the crust-mantle interface (the Moho) or a subducted slab.
Can We Predict Earthquakes?
This is one of the most common questions in seismology, and one of the hardest to answer.
While scientists have developed an understanding of why earthquakes happen and how seismic waves behave, we still cannot reliably predict the exact time, location, and magnitude of a future earthquake. That’s because earthquakes are the result of complex interactions deep underground, where stress accumulates silently and rupture happens suddenly with no detectable warning. The stress along a fault may build slowly and be released in one big rupture or in many smaller ones. There are no consistent precursor signals (like foreshocks or gas emissions) that appear before every quake, and when they do occur, they vary from one fault system to another. As a result, reliable, short-term prediction remains scientifically out of reach.
Even though we can’t predict earthquakes precisely, we can still prepare, using probabilistic forecasting and early warning. Probabilistic forecasting is used to calculate the probability of a rupture occurring over long timescales (years to decades) based on historical earthquake records, fault slip rates (how fast the plates are moving), and simulations of stress buildup and release. For example, a fault might have a 1-in-100 chance of producing a magnitude 7+ earthquake within the next 30 years. These probabilities are used in building codes and hazard maps to inform risk-based decisions. They’re only probabilities, but they help communities understand the long-term threat and plan accordingly. Earthquake early warning systems, like ShakeAlert, give people a few seconds to a minute of warning after an earthquake has started but before strong shaking arrives. Seismometers near the epicenter detect the first-arriving P-waves, which are fast but usually non-destructive. Then the warning system rapidly estimates the earthquake’s location and size. Alerts are sent out to people before the slower and more damaging Rayleigh and Love waves arrive. These alerts give people just enough time to shut down machinery, pause surgeries, and drop, cover, and hold on.