How earthquakes happen, how they are measured, and why the ground shakes harder in some places than others.
What is an earthquake?
An earthquake is the release of stress from the Earth's tectonic plates. The zone where two tectonic plates come together is called a fault. Prior to an earthquake, tectonic forces result in a gradual buildup of strain energy stored on either side of the fault. When the local stresses along the fault become too large, the fault slips suddenly, or ruptures, and releases the stored strain energy. This rupture on the fault plane is called the focus, and the projection of this point on the ground surface is called the epicenter.
When a rupture occurs along a fault, the strain energy stored on either side of the fault is released in the form of seismic waves and heat. These seismic waves propagate away from the ruptured fault zone and through the geologic layers of rock and soil. The process of seismic wave propagation causes the ground to shake.
What types of earthquakes are there?
The most common earthquake is a shallow event where two tectonic plates slide past one another. Deeper earthquakes usually occur when one plate dives under another plate. Other earthquakes can also occur as a result of volcanic activity, collapses of the ground, and man-made explosions.
What types of seismic waves are generated?
Three types of waves are created when energy is released in an earthquake.
P waves
The primary wave is the fastest and can move through both liquid and solid rock. Like sound waves, P waves are compressional, meaning they compress and expand matter as they move through it.
S waves
Secondary waves follow directly behind the P waves, travel at right angles to the direction of motion, and move only through solid matter. They matter more than P waves because they are usually larger and produce both vertical and horizontal motion at the ground surface.
Surface waves
The slowest of the three. Surface waves move close to or on the outside surface of the ground, rather than through the Earth's interior as P and S waves do.
How are earthquakes measured?
Geologists use seismographs to record surface and body waves. When motion is recorded, a seismogram is created, which shows how big the waves were and how long they lasted. Using records from several seismograph stations, the epicenter and focus can be located through triangulation. From there, an earthquake can be quantified in three ways.
What geological factors affect ground shaking?
Several factors affect the intensity of ground shaking at a site: the magnitude of the earthquake, the distance to the epicenter or focus, and the soil conditions underfoot. In general, ground shaking at sites within 5 kilometers of the fault rupture is about twice as strong as shaking felt 10 to 15 kilometers away. Shaking near faults can also generate pulses that impose large displacement demands on structures. Significant deposits of soft soil can amplify seismic waves and increase displacement demands on buildings, and soft, saturated soils carry the potential for liquefaction.
How do earthquakes interact with buildings?
Ground shaking causes vibratory motions at the base of a structure, and the structure actively responds to those motions. Damage occurs when the displacements imposed on the structure push the building beyond its elastic state. How bad the damage gets depends on the type of inelastic deformation that occurs.
Ductile: like bending a straw
Structures that deform in a ductile manner bend without breaking apart. They are sturdy and have the ability to protect lives.
Brittle: like snapping a twig
Structures that deform in a brittle manner can fail suddenly and collapse, which is how earthquakes cause human casualties.
Where to learn more
For more in-depth information about earthquakes, see our Links page. To find out what you can do at home, see How to Fix My Home and Earthquake Preparedness.