4.6: Chapter Summary

4.1 Stress and Strain

• Stress is a force distributed over an area. Rocks can experience compression, tension, and shear stresses.
• Strain is a rock's response to stress. A rock can either deform elastically, plastically, or it can fracture (brittle deformation). Generally, higher temperatures yield more plastic behavior and rocks are generally more plastic when they are wet. If stress is applied quickly, rocks are more likely to break.
• Deformation can be measured using strike and dip. Strike is a compass direction that is parallel to the plane being measured and a horizontal surface. Dip is an angle between 0 (horizontal) and 90 (vertical), measured perpendicular to strike. These measurements are taken using a special compass with an inclinometer.

4.2 Folding

• When rocks deform in a ductile fashion, typically at higher temperatures deep in the crust, they may form folds.
• Anticlines are A-shaped folds with oldest rocks at the center, Synclines are U-shaped folds with the youngest rocks at the center. If the ages of the rocks are unknown, then they are called antiforms and synforms respectively.
• Anticlines and synclines can have a variety of orientations: being upright, recumbent, or plunging (or anywhere in between!). They can also have a variety of interlimb angles from isoclinal (parallel or near parallel) to gentle.
• Domes are folds that resemble and upside down bowl, with their limbs dipping away in all directions. Basins are bowl-shaped folds.

4.3 Jointing and Faulting

• Joints are fractures in rocks that have not been displaced. In other words, the two sides of the fracture have not moved relative to one another. Exfoliation joints and columnar joints frequently form in igneous rocks.
• Joints are often formed in tension, but they can also be formed in compression.
• Faults are any fractures that have been displaced. There are dip-slip, strike-slip, and oblique-slip faults.
• Dip-slip faults include normal faults, wherein the hanging wall moves down relative to the footwall; reverse faults wherein the hanging wall moves up relative to the footwall, and thrust faults which are low-angle (<30 degree) reverse faults. Typically, normal faults form where the crust is experiencing extension, and thrust and reverse faults form where the crust is undergoing shortening.
• Strike slip faults are faults which move parallel to their strike. They can be right-lateral (dextral) or left-lateral (sinistral).
• Faults can also have components of both dip-slip and strike-slip. We call these oblique-slip faults.

4.4 Earthquakes

• Earthquakes are the energy released when a fault ruptures.
• Stress is stored within the Earth and is released during an earthquake.
• Foreshocks are smaller earthquakes that precede a larger earthquake, and aftershocks are smaller earthquakes that follow a larger earthquake.
• Body waves move through the Earth. P-waves occur when the particle motion of a wave is in the same direction as the wave motion. S-waves are when the particle motion is perpendicular to the wave motion. P-waves cause up-and-down motion, while S-waves cause side-to-side motion.
• Surface waves move on surfaces or interfaces and include Love and Rayleigh waves. Love waves travel side-to-side on an interface or surface and cause side-to-side motion. Rayleigh waves travel with circular motion like an ocean wave and are experienced as a rolling motion.
• A seismogram is the record of the arrival of earthquake waves. Seismic waves arrive in an order - P-waves, S-waves, and then surface waves.
• Earthquakes begin at a hypocenter inside Earth. The epicenter is the point at the surface of Earth directly above the hypocenter.
• Earthquakes are located by the triangulation of the distances from 3 or more seismic stations.

4.5 Measuring Earthquakes

• Seismometers are machines or sensors that measure earthquake waves. Seismographs are recording devices that have largely been replaced by computers.
• The record of the arrival of seismic waves is a seismogram.
• Seismic networks are arrays of seismometers within an area. The density or magnitude of completeness of a network determines how small of an earthquake can be reliably recorded.
• Intensity is a measure of the damage done by an earthquake. The most commonly used intensity scale is the Mercalli Scale.
• Magnitude measures the energy released by an earthquake. Earthquakes have one magnitude, but magnitudes can be calculated many different ways and not all magnitude scales calculate the same values. Magnitude scales are based on the ground motion of an earthquake as measured on a seismogram rather than on damage.
• The most common magnitude scale reported and used today is moment-magnitude (M_W). Historically, Richter magnitude (M_L) was used and reported. M_L and M_W are the same for earthquakes less than M5. For earthquakes greater than M5, M_W is more accurate.
• Magnitude scales are logarithmic. Each increase of 1-unit means 10 times more ground motion and 32 times more energy.
• Essentially all of California is likely to experience an earthquake with an intensity level of Mercalli VI within the next 100 years.
• ShakeAlert is the earthquake early warning system on the west coast of the United States.