8: Crustal Deformation and Earthquakes
- Page ID
- 28263
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Crustal deformation occurs when applied forces exceed the internal strength of rocks, physically changing their shapes. These forces are called stress, and the physical changes they create are called strain. Forces involved in tectonic processes as well as gravity and igneous pluton emplacement produce strains in rocks that include folds, fractures, and faults. When rock experiences large amounts of stress and breaks, energy is released in the form of seismic waves, commonly known as an earthquake.
- 8.1: Stress and Strain
- Stress is the force exerted per unit area and strain is the physical change that results in response to that force. When the applied stress is greater than the internal strength of rock, strain results in the form of deformation of the rock caused by the stress. Strain in rocks can be represented as a change in rock volume and/or rock shape, as well as fracturing the rock. There are three types of stress: tensional, compressional, and shear.
- 8.2: Deformation
- When rocks are stressed, the resulting strain can be elastic (temporary change), ductile (bending), or brittle (breaking). This change is generally called deformation. The type of deformation a rock undergoes depends on pore pressure, strain rate, rock strength, temperature, stress intensity, time, and confining pressure.
- 8.3: Geological Maps
- Geologic maps are two dimensional (2D) representations of geologic formations and structures at the Earth’s surface. Geologists use geologic maps to represent where geologic formations, faults, folds, and inclined rock units are.
- 8.4: Folds
- Geologic folds are layers of rock that are bent by ductile deformation. Folds are most commonly formed by compressional forces at depth, where hotter temperatures and higher confining pressures allow ductile deformation to occur.
- 8.5: Faults
- Faults are the places in the crust where brittle deformation occurs as two blocks of rocks move relative to one another. Faults normally occur closer to the surface where the rock is cooler and tends to break, though they can occur much deeper under the right conditions.
- 8.6: Earthquake Essentials
- Earthquakes are felt at the surface of the Earth when energy is released by blocks of rock sliding past each other, i.e. faulting has occurred. Seismic energy thus released travels through the Earth in the form of seismic waves. Most earthquakes occur along active plate boundaries. Intraplate earthquakes (not along plate boundaries) occur and are still poorly understood.
- 8.7: Measuring Earthquakes
- People feel approximately 1 million earthquakes a year, usually when they are close to the source and the earthquake is large enough to feel. Large earthquakes (>7.0) are extremely rare. The United States Geological Survey (USGS) Earthquakes Hazards Program real-time map shows the location and magnitude of recent earthquakes around the world.
- 8.8: Earthquake Risk
- Earthquake magnitude is an absolute value that measures pure energy release. Intensity, however, i.e. how much the ground shakes, is determined by several factors. In general, the larger the magnitude, the stronger the shaking and the longer the shaking will last. The severity of shaking is influenced by the location of the observer relative to the epicenter, the direction of rupture propagation, and the path of greatest rupture.
Thumbnail: Epicenter of the 2010 Chile earthquake with a collapsed building in Concepción. (CC-SA-BY; Claudio Núñez).