Earthquakes and Geology
Owen Borville
January 30, 2021
Learning, Geology, Science
An earthquake is a sudden ground motion or vibration of the Earth, produced by a rapid release of stored energy. Earthquakes result from a sudden slip on a fault and the resulting ground shaking and radiated seismic energy caused by the slip. Earthquakes can also result from motion caused by volcanic activity or by other sudden stress changes in the Earth.
Seismology is the study of earthquakes and the structure of the Earth using naturally and artificially generated seismic waves. A seismograph or seismometer is an instrument that can detect and record seismic waves produced by an earthquake or explosion.
Acceleration (seismic) is the increase in velocity of ground shaking during an earthquake. The peak acceleration is the largest acceleration recorded by a particular station during an earthquake. An accelerograph is an instrument that records the acceleration of the ground during an earthquake (also commonly called an accelerometer).
The brittle-ductile transition zone is the depth in the crust of the Earth where the crust changes from being brittle (or tending to break) above, to being ductile (or tending to bend) below. Most earthquakes occur in the brittle portion of the crust above the brittle-ductile boundary.
Seismic waves are elastic waves generated by an impulse such as an earthquake or an explosion. Seismic waves may travel along or near the Earth's surface (as with Rayleigh and Love waves) or through the Earth's interior (as with P and S waves). Body waves are seismic waves that move through the interior of the Earth, as opposed to surface waves that travel near the Earth's surface.
P-waves (primary wave or compressional wave) are seismic body waves that travel through the Earth and vibrate parallel to the direction of their movement. P-waves are the fastest waves to reach a seismograph. P-waves travel through solid, liquid, and gas and are refracted or bent when passing through liquid. Refraction of p-waves occurs at the Earth’s mantle-core boundary.
S-waves (shear wave or secondary wave) are seismic body waves that travel through the Earth and shake the ground perpendicular to the direction the waves are moving. S-waves have a shearing motion that makes them slower than p-waves. S-waves can pass through solid rock but not gas or liquid.
Surface waves are seismic waves that move over the surface of the Earth and are slower than p-waves or s-waves. Two principal types of surface waves are Love and Rayleigh waves and these are generated during earthquakes. A Love wave (L-wave) is a surface wave having a horizontal motion that is transverse or perpendicular to the direction the wave is traveling. A Rayleigh wave is a seismic surface wave causing the ground to shake in an elliptical motion with no transverse, or perpendicular motion.
The earthquake epicenter is the point on the Earth’s surface located directly above the focus of an earthquake. The focus (or hypocenter) is the location inside the Earth where an earthquake occurs. Rock ruptures at this spot and seismic waves radiate outward in all directions. An earthquake can have a shallow or deep focus.
Intensity is one method used to measure the strength of an earthquake. Intensity measures the effects of an earthquake on buildings and the reactions of people. The Modified Mercalli Intensity Scale measures the intensity of an earthquake as a number (written as a Roman numeral) describing the severity of an earthquake in terms of its effects on the Earth’s surface and on humans and their building structures. Many intensities exist for an earthquake and depend on where a person is located, unlike the magnitude, which is one number for each earthquake.
Magnitude is the most commonly used measure of earthquake size and describes the total amount of energy released during an earthquake, allowing geologists to compare earthquakes occurring in different parts of the world. The Richter scale measures the magnitude of an earthquake and is determined from the logarithm of the amplitude of waves recorded by seismographs. Magnitude is expressed in whole numbers and decimal fractions. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude.
The moment magnitude is a physical quantity proportional to the slip on a fault multiplied by the area of the fault surface that slips. The moment magnitude can be estimated from seismograms and from geodetic measurements. The moment magnitude is related to the total energy released in an earthquake. The moment magnitude provides an estimate of earthquake size that is valid over the complete range of magnitudes. The seismic moment is a measure of the size of an earthquake based on the area of fault rupture, the average amount of slip, and the force that was required to overcome the friction sticking the rocks together that were offset by faulting. Seismic moment can also be calculated from the amplitude spectra of seismic waves.
An earthquake hazard is anything associated with an earthquake that may affect the normal activities of people. This includes surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, tsunamis, and seiches.
A tsunami is a large ocean wave commonly caused by an earthquake occurring underneath the ocean. Tsunamis occur when the ocean floor shifts suddenly as a result of an earthquake, displacing a vast volume of the overlying water. Tsunamis can reach 30 meters in height or more. Major submarine slides and exploding volcanic islands can also cause tsunamis.
Ground failure is a general reference to landslides, liquefaction, lateral spreads, and any other consequence of shaking caused by earthquakes that affects the stability of the ground. Liquefaction is a process by which water-saturated sediment temporarily loses strength and acts as a fluid. Liquefaction is common during earthquakes. Lateral spreads or flows refer to landslides that commonly form on gentle slopes and have rapid fluid-like flow movement.
Seismic seiches are standing waves set up on enclosed bodies of water such as rivers, reservoirs, ponds, and lakes when seismic waves from an earthquake pass through the area. They are in direct contrast to tsunamis which are giant sea waves created by the sudden uplift of the sea floor.
A major factor in earthquake damage is the type of soil or rock in which the buildings are built. Buildings built on soft sediment will be much more vulnerable to damage than buildings built on hard rock. The type of building construction also affects the potential damage. Reinforced concrete and brick buildings are stronger and more earthquake resistant than non-reinforced buildings. Buildings constructed with softer materials such as clay or mud are likely to receive the most damage.
Earthquakes occur primarily near tectonic plate boundaries, faults, and intra-plate faults. Convergent plate boundaries usually produce the strongest earthquakes.
The Circum-Pacific Belt, which circles the edge of the Pacific tectonic plate, produces the largest number of earthquakes on Earth. This belt includes the western edge of North and South America, the Aleutian Islands of Alaska, Japan, The Philippines, Micronesia, Tonga, New Zealand, and the South Pacific Ocean. The second largest region of earthquakes extends from Mediterranean Europe to the Himalaya Mountains and into Indonesia. Intra-plate earthquakes occur between plate boundaries and are less common and usually smaller in intensity than earthquakes occurring along plate boundaries. Intra-plate earthquakes are commonly caused by inactive faults which become active.
An active fault is a fault that is likely to cause another earthquake sometime in the future. Faults are commonly considered to be active if they have moved one or more times in the recent years. An aftershock is an earthquake which occurs after the original or first earthquake and can occur several weeks or months after the original earthquake. Foreshocks are relatively smaller earthquakes that precede the largest earthquake in a series, which is termed the mainshock.
Owen Borville
January 30, 2021
Learning, Geology, Science
An earthquake is a sudden ground motion or vibration of the Earth, produced by a rapid release of stored energy. Earthquakes result from a sudden slip on a fault and the resulting ground shaking and radiated seismic energy caused by the slip. Earthquakes can also result from motion caused by volcanic activity or by other sudden stress changes in the Earth.
Seismology is the study of earthquakes and the structure of the Earth using naturally and artificially generated seismic waves. A seismograph or seismometer is an instrument that can detect and record seismic waves produced by an earthquake or explosion.
Acceleration (seismic) is the increase in velocity of ground shaking during an earthquake. The peak acceleration is the largest acceleration recorded by a particular station during an earthquake. An accelerograph is an instrument that records the acceleration of the ground during an earthquake (also commonly called an accelerometer).
The brittle-ductile transition zone is the depth in the crust of the Earth where the crust changes from being brittle (or tending to break) above, to being ductile (or tending to bend) below. Most earthquakes occur in the brittle portion of the crust above the brittle-ductile boundary.
Seismic waves are elastic waves generated by an impulse such as an earthquake or an explosion. Seismic waves may travel along or near the Earth's surface (as with Rayleigh and Love waves) or through the Earth's interior (as with P and S waves). Body waves are seismic waves that move through the interior of the Earth, as opposed to surface waves that travel near the Earth's surface.
P-waves (primary wave or compressional wave) are seismic body waves that travel through the Earth and vibrate parallel to the direction of their movement. P-waves are the fastest waves to reach a seismograph. P-waves travel through solid, liquid, and gas and are refracted or bent when passing through liquid. Refraction of p-waves occurs at the Earth’s mantle-core boundary.
S-waves (shear wave or secondary wave) are seismic body waves that travel through the Earth and shake the ground perpendicular to the direction the waves are moving. S-waves have a shearing motion that makes them slower than p-waves. S-waves can pass through solid rock but not gas or liquid.
Surface waves are seismic waves that move over the surface of the Earth and are slower than p-waves or s-waves. Two principal types of surface waves are Love and Rayleigh waves and these are generated during earthquakes. A Love wave (L-wave) is a surface wave having a horizontal motion that is transverse or perpendicular to the direction the wave is traveling. A Rayleigh wave is a seismic surface wave causing the ground to shake in an elliptical motion with no transverse, or perpendicular motion.
The earthquake epicenter is the point on the Earth’s surface located directly above the focus of an earthquake. The focus (or hypocenter) is the location inside the Earth where an earthquake occurs. Rock ruptures at this spot and seismic waves radiate outward in all directions. An earthquake can have a shallow or deep focus.
Intensity is one method used to measure the strength of an earthquake. Intensity measures the effects of an earthquake on buildings and the reactions of people. The Modified Mercalli Intensity Scale measures the intensity of an earthquake as a number (written as a Roman numeral) describing the severity of an earthquake in terms of its effects on the Earth’s surface and on humans and their building structures. Many intensities exist for an earthquake and depend on where a person is located, unlike the magnitude, which is one number for each earthquake.
Magnitude is the most commonly used measure of earthquake size and describes the total amount of energy released during an earthquake, allowing geologists to compare earthquakes occurring in different parts of the world. The Richter scale measures the magnitude of an earthquake and is determined from the logarithm of the amplitude of waves recorded by seismographs. Magnitude is expressed in whole numbers and decimal fractions. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude.
The moment magnitude is a physical quantity proportional to the slip on a fault multiplied by the area of the fault surface that slips. The moment magnitude can be estimated from seismograms and from geodetic measurements. The moment magnitude is related to the total energy released in an earthquake. The moment magnitude provides an estimate of earthquake size that is valid over the complete range of magnitudes. The seismic moment is a measure of the size of an earthquake based on the area of fault rupture, the average amount of slip, and the force that was required to overcome the friction sticking the rocks together that were offset by faulting. Seismic moment can also be calculated from the amplitude spectra of seismic waves.
An earthquake hazard is anything associated with an earthquake that may affect the normal activities of people. This includes surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, tsunamis, and seiches.
A tsunami is a large ocean wave commonly caused by an earthquake occurring underneath the ocean. Tsunamis occur when the ocean floor shifts suddenly as a result of an earthquake, displacing a vast volume of the overlying water. Tsunamis can reach 30 meters in height or more. Major submarine slides and exploding volcanic islands can also cause tsunamis.
Ground failure is a general reference to landslides, liquefaction, lateral spreads, and any other consequence of shaking caused by earthquakes that affects the stability of the ground. Liquefaction is a process by which water-saturated sediment temporarily loses strength and acts as a fluid. Liquefaction is common during earthquakes. Lateral spreads or flows refer to landslides that commonly form on gentle slopes and have rapid fluid-like flow movement.
Seismic seiches are standing waves set up on enclosed bodies of water such as rivers, reservoirs, ponds, and lakes when seismic waves from an earthquake pass through the area. They are in direct contrast to tsunamis which are giant sea waves created by the sudden uplift of the sea floor.
A major factor in earthquake damage is the type of soil or rock in which the buildings are built. Buildings built on soft sediment will be much more vulnerable to damage than buildings built on hard rock. The type of building construction also affects the potential damage. Reinforced concrete and brick buildings are stronger and more earthquake resistant than non-reinforced buildings. Buildings constructed with softer materials such as clay or mud are likely to receive the most damage.
Earthquakes occur primarily near tectonic plate boundaries, faults, and intra-plate faults. Convergent plate boundaries usually produce the strongest earthquakes.
The Circum-Pacific Belt, which circles the edge of the Pacific tectonic plate, produces the largest number of earthquakes on Earth. This belt includes the western edge of North and South America, the Aleutian Islands of Alaska, Japan, The Philippines, Micronesia, Tonga, New Zealand, and the South Pacific Ocean. The second largest region of earthquakes extends from Mediterranean Europe to the Himalaya Mountains and into Indonesia. Intra-plate earthquakes occur between plate boundaries and are less common and usually smaller in intensity than earthquakes occurring along plate boundaries. Intra-plate earthquakes are commonly caused by inactive faults which become active.
An active fault is a fault that is likely to cause another earthquake sometime in the future. Faults are commonly considered to be active if they have moved one or more times in the recent years. An aftershock is an earthquake which occurs after the original or first earthquake and can occur several weeks or months after the original earthquake. Foreshocks are relatively smaller earthquakes that precede the largest earthquake in a series, which is termed the mainshock.
