What Are the Causes of Vibrations During an Earthquake?

Earthquakes result from the stored energy and sudden release of tectonic plates in the Earth's crust. Volcanoes sometimes produce earthquakes. When two opposing tectonic plates converge at a major fault area and the pressure becomes too great, they slip over or under each other with a sudden and violent breakage. The breakage can happen near or under the Earth's surface and stretch for hundreds or thousands of miles. The breaking force travels through the crust or bedrock, causing shaking and vibration. Earthquake vibrations involve shock waves, soil condition and other factors.

Initial Breakage

• The first triggering effect of vibrations in an a margin or fault-type earthquake starts with the sudden release of energy when two tectonic plates snap apart. If the earthquake happens in a submarine environment, it displaces great volumes of water, as well as transmits energy into the sea floor bedrock. Volcanic explosions start vibrations with a primary concussion wave that radiates out from its source. The process of all earthquake or volcanic-induced vibrations begins with pressure shock waves radiating outward from the "focus," or exact origin.

P-Waves

• P-waves, or primary waves (first waves), share identical characteristics to sound waves. They are short wavelength, high-frequency waves that travel in a longitudinal direction. They can travel through liquids and solids. When the ground is pulled backward and forward, it crushes solid material together and pulls it apart, which causes small ground movements in the form of P-wave vibrations. P-waves can become reflected or refracted, which means they can change their wave pattern. They can sometimes evolve into S-waves.

S-Waves

• S-Waves, called secondary waves, arrive after the P-waves because they travel more slowly. S-waves have short wavelength and high frequencies similar to P-waves, yet they do not travel in a longitudinal direction, but move transversely through material. S-waves can not travel through a liquid. S-waves leave evidence on the Earth's surface in the form of twisted fencing, curved railroad tracks and building walls that have displaced sides. S-waves turn objects and particles 90 degrees to the direction of the wave.

L-Waves

• L-waves, called surface waves, have long wavelengths and low frequencies. They originate near the epicenter of the earthquake and can only move through the crust at its outer perimeter. Since they reside near the surface, they produce the most damage on buildings and other structures. They exhibit motions that resemble the waves in the ocean, only the ground motion becomes circular, forcing it to rise and fall. Combined with fires, tsunamis and landslides, the damaging characteristics of L-waves accounts for most of the property damage and loss of life in earthquakes.

Soil Types

• The soil type over which structures have been built affect their ability to withstand vibrations, or succumb to excessive shaking and fail. The slower S-waves represent the most dangerous shock wave to the soils and cause the heaviest and most prolonged vibrations in loose materials. Such materials include mud, silts, gravel, sand and artificial fill. These types of materials contain loosely compacted structures, with little or no adhesion to surrounding rock. Mesozoic bedrock, which dates from 64 million to 245 million years, and some volcanics and Franciscan bedrock have better resistance to shaking and vibration. With the looser compacted soils, the vibrations will be more noticeable and last longer.