4.2: Earthquakes

Types of Faults

Faults are the places in the crust where brittle deformation occurs as two blocks of rocks move relative to one another. There are three major fault types: normal, reverse, and strike-slip. Normal and reverse faults display vertical, also known as dip-slip, motion. Dip-slip motion consists of relative up and down movement along a dipping fault between two blocks, the hanging wall and the footwall. In a dip-slip system, the footwall is below the faultplane, and the hanging-wall is above the fault plane. An excellent way to remember this is to imagine a mine tunnel through a fault; the hanging wall would be where a miner would hang a lantern, and the footwall would be at the miner’s feet. Faults are more prevalent near and related to plate boundaries but can occur in plate interiors as well. Faults can show evidence of movement along the fault plane. Slickensides are polished, often grooved surfaces along the fault plane created by friction during the movement. A joint or fracture is a plane of breakage in a rock that does not show movement or offset. Joints can result from many processes, such as cooling, depressurizing, or folding. Joint systems may be regional affecting many square miles. (9 Crustal Deformation and Earthquakes – An Introduction to Geology, n.d.)

“Moab Fault” by Andrew Wilson” is licensed under Public Domain.

Normal faults move by a vertical motion where the hanging-wall moves downward relative to the footwall along the dip of the fault. Tensional forces create normal faults in the crust. Normal faults and tensional forces are commonly caused at divergent plate boundaries and where tensional stresses are stretching the crust. An example of a normal fault is the Wasatch Fault along the Wasatch Range.

Grabens, horsts, and half-grabens are all blocks of crust or rock bounded by normal faults. Grabens drop down relative to adjacent blocks and create valleys. Horsts go up relative to adjacent down-dropped blocks and become areas of high topography. Where together, horsts and grabens create a symmetrical pattern of valleys surrounded by normal faults on both sides and mountains. Half-grabens are a one-sided version of a horst and graben, where blocks are tilted by a normal fault on one side, creating an asymmetrical valley-mountain arrangement. The mountain-valleys of the Basin and Range Province of Western Utah and Nevada consist of a series of full and half-grabens from the Salt Lake Valley to the Sierra Nevada Mountains. When the dip of a normal fault decreases with depth (i.e., the fault becomes more horizontal as it goes deeper), the fault is listric. Extreme versions of listric faulting occur when substantial amounts of extension occur along very low-angle normal faults, known as detachment faults. The normal faults of the Basin and Range appear to be-come detachment faults at depth. (9 Crustal Deformation and Earthquakes – An Introduction to Geology, n.d.)

“Horst and Graben” by the United States Geologic Survey is licensed under Public Domain.

Reverse faults, caused by compressional forces, are when the hanging wall moves up relative to the footwall. A thrust fault is a reverse fault where the fault plane has a low dip angle (generally less than 45 degrees). Thrust faults bring older rocks on top of younger rocks and can cause repetition of rock units in the stratigraphic record. Convergent plate boundaries with subduction zones create a particular type of “reverse” fault called a megathrust fault. Megathrust faults cause the most significant magnitude earthquakes and commonly cause tsunamis.

“Reverse Faults” by the United States Geologic Survey is licensed under Public Domain.

Strike-slip faults have a side to side motion. In the pure strike-slip motion, crustal blocks on either side of the fault do not move up or down relative to each other. There is left-lateral, called sinistral, and right-lateral, called dextral, strike-slip motion. In left-lateral or sinistral strike-slip motion, the opposite block moves left relative to the block that the observer is standing on. In right-lateral or dextral strike-slip motion, the opposite block moves right relative to the observer’s block. Strike-slip faults are most associated with transform boundaries and are prevalent in fracture zones adjacent to mid-ocean ridges.

Bends in strike-slip faults can create areas where the sliding blocks create compression or tension. Tensional stresses will create transtensional features with normal faults and basins like California’s Salton Sea, and compressional stresses will create transpressional features with reverse faults and small-scale mountain building, like California’s San Gabriel Mountains. The faults that play off transpression or transtension features are known as flower structures.

An example of a right-lateral strike-slip fault is the San Andreas Fault, which denotes a transform boundary between the North American and Pacific plates. An example of a left-lateral strike-slip fault is the Dead Sea fault in Jordan and Israel. (9 Crustal Deformation and Earthquakes – An Introduction to Geology, n.d.)

“Strike-slip Fault” is licensed under Creative Commons Attribution 4.0 International.

Causes of Earthquakes

People feel approximately 1 million earthquakes a year. Few are noticed far from the source. Even fewer are significant earthquakes. Earthquakes are usually felt only when they are greater than a magnitude 2.5 or greater. The USGS Earthquakes Hazards Program has a real-time map showing the most recent earthquakes. Most earthquakes occur along active plate boundaries. Intraplate earthquakes (not along plate boundaries) are still poorly understood. (9 Crustal Deformation and Earthquakes – An Introduction to Geology, n.d.)

Earthquake energy is known as seismic energy, and it travels through the earth in the form of seismic waves. To understand some of the basics of earthquakes and how they are measured, consider some of the fundamental properties of waves. Waves describe a motion that repeats itself in a medium such as rock or unconsolidated sediments. The magnitude refers to the height, called amplitude, of a wave. Wavelength is the distance between two successive peaks of the wave. The number of repetitions of the motion over time, called cycles per time, is the frequency. The inverse of frequency, which is the amount of time for a wave to travel one wavelength, is the period. When multiple waves combine, they can interfere with each other. When the waves are in sync with each other, they will have constructive interference, where the influence of one wave will add to and magnify the other. If the waves are out of sync with each other, they will have destructive interference. If two waves have the same amplitude and frequency and are ½ wavelength out of sync, the destructive interference between them can eliminate each wave.

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The elastic rebound theory explains the release of seismic energy. When the rock is strained to the point that it undergoes brittle deformation, built-up elastic energy is released during dis-placement, which radiates away as seismic waves. When the brittle deformation occurs, it creates an offset between the fault blocks at a starting point called the focus. This offset propagates along the surface of rupture, which is known as the fault plane.

The fault blocks of persistent faults like the Wasatch Fault of Utah are locked together by friction. Over hundreds to thousands of years, stress builds up along the fault. Eventually, stress along the fault overcomes the frictional resistance, and slip initiates as the rocks break. The deformed rocks “snap back” toward their original position in a process called elastic rebound. Bending of the rocks near the fault may reflect this buildup of stress, and in earth-quake-prone areas like California, strain gauges that measure this bending are set up in an attempt to understand more about predicting an earthquake. In some locations where the fault is not locked, seismic stress causes a continuous movement along the fault called fault creep, where displacement occurs gradually. Fault creep occurs along some parts of the San Andreas Fault.

The release of seismic energy occurs in a series of steps. After a seismic energy release, energy begins to build again during a period of inactivity along the fault. The accumulated elastic strain may produce small earthquakes (on or near the main fault). These are called fore-shocks and can occur hours or days before a massive earthquake, but they may not occur at all. The main release of energy occurs during the major earthquake, known as the mainshock. Aftershocks may then occur to adjust strain that built up from the movement of the fault. They generally decrease over time. (9 Crustal Deformation and Earthquakes – An Introduction to Geology, n.d.)

Earthquake Zones

Nearly 95 percent of all earthquakes occur along with one of the three types of tectonic plate boundaries, but earthquakes do occur along all three types of plate boundaries. About 80 percent of all earthquakes strike around the Pacific Ocean basin because it is lined with convergent and transform boundaries. Called the Ring of Fire, this is also the location of most volcanoes around the planet. About 15 percent take place in the Mediterranean-Asiatic Belt, where convergence is causing the Indian Plate to run into the Eurasian Plate, creating the largest mountain ranges in the world. The remaining 5 percent are scattered around other plate boundaries or are intraplate earthquakes. (Reading: The Nature of Earthquakes | Geology, n.d.)

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Transform Plate Boundaries

Transform plate boundaries occur when two tectonic plates grind parallel to each other rather than colliding or subducting. Deadly earthquakes occur at transform plate boundaries, creating strike-slip faults because they tend to have shallow focuses where the rupture occurs. The faults along the San Andreas Fault zone produce around 10,000 earthquakes a year. Most are tiny, but occasionally one is massive. In the San Francisco Bay Area, the Hay-ward Fault was the site of a magnitude 7.0 earthquake in 1868. The 1906 quake on the San Andreas Fault had a magnitude estimated at 7.9.

During the 1989 World Series, a magnitude 7.1 earthquake struck Loma Prieta, near Santa Cruz, California, killing 63 people, injuring 3,756, and cost $6 billion. A few years later, in Northridge, California, a magnitude 6.7 earthquake killed 72 people, injured 12,000 people, and caused $12.5 billion in damage. This earthquake occurred on an unknown fault because it was a blind thrust fault near Los Angeles, California.

Although California is prone to many natural hazards, including volcanic eruptions at Mt. Shasta or Mt. Lassen, and landslides on coastal cliffs, the natural hazard the state is linked with is earthquakes. New Zealand also has strike-slip earthquakes, about 20,000 a year, but only a small percentage of those are large enough to be felt. A 6.3 quake in Christchurch in February 2011 killed about 180 people. (Reading: The Nature of Earthquakes | Geology, n.d.)

Convergent Plate Boundaries

Earthquakes at convergent plate boundaries mark the motions of the subducting lithosphere as it plunges through the mantle, creating reverse and thrust faults. Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin. The Philippine Plate and the Pacific Plate subduct beneath Japan, creating a chain of volcanoes and produces as many as 1,500 earthquakes annually.

In March 2011, an enormous 9.0 earthquake struck off Sendai in northeastern Japan. This quake, called the 2011 Tōhoku earthquake, was the most powerful ever to strike Japan and one of the top five known worldwide. Damage from the earthquake was over-shadowed by the tsunami it generated, which wiped out coastal cities and towns. Two months after the earthquake, about 25,000 people died or were missing, and 125,000 buildings were damaged or destroyed. Aftershocks, some as large as major earthquakes, have continued to rock the region. A map of aftershocks is seen here. Recently, the New York Times created an interactive website of the Japan earthquake and tsunami.

The Pacific Northwest of the United States is at risk from a potentially massive earthquake that could strike any time. Subduction of the Juan de Fuca plate beneath North America produces active volcanoes, but large earthquakes only hit every 300 to 600 years. The last was in 1700, with an estimated magnitude of around 9.0. The elastic rebound theory, as applied to subduction zones, can be viewed here.

Massive earthquakes are the hallmark of the thrust faulting and folding when two continental plates converge. The 2001 Gujarat earthquake in India was responsible for about 20,000 deaths, and many more people became injured or homeless. In Understanding Earthquakes: From Research to Resilience, scientists try to understand the mechanisms that cause earthquakes and tsunamis and how society can deal with them. (Reading: The Nature of Earthquakes | Geology, n.d.)

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Divergent Plate Boundaries

Many earthquakes occur where tectonic plates are moving apart or where a tectonic plate is tearing itself apart. Earthquakes at mid-ocean ridges are small and shallow because the plates are young, thin, and hot. On land where continents split apart, earthquakes are larger and stronger. A classic example of normal faulting along divergent boundaries is the Wasatch Front in Utah and the entire Basin and Range through Nevada.

“Wasatch Fault” by the United States Geologic Survey is licensed under Public Domain.

Intraplate Boundaries

Intraplate earthquakes are the result of stresses caused by plate motions acting in solid slabs of the lithosphere. In 1812, a magnitude 7.5 earthquake struck near New Madrid, Missouri. The earthquake was strongly felt over approximately 50,000 square miles and altered the course of the Mississippi River. Because very few people lived there at the time, only 20 people died. Many more people live there today. A similar earthquake today would undoubtedly kill many people and cause a great deal of property damage. (Reading: The Nature of Earthquakes | Geology, n.d.)