Topics

 

  1. Terminology

 

  1. Structure of the Earth

 

  1. Faults and Fault Zones, Plate Tectonics

 

  1. Earthquakes/History

 

  1. Anatomy of an Earthquake

 

  1. Detecting Earthquakes, Seismographs, and Seismograms

 

  1. Computing Earthquake Locations

 


Terminology

 

1.       Aftershock – An earthquake which follows a larger earthquake or main shock and originates I or near the rupture zone of the larger earthquake.  Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time.

 

  1. Amplitude – The maximum height of a wave crest or depth of a trough.

 

  1. Array – An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver.

 

  1. Arrival – The appearance of seismic energy on a seismic record.

 

  1. Arrival Time – The time at which a particular wave phase arrives at a detector.

 

  1. Aseismic – Not associated with an earthquake, as in aseismic slip.  Also used to indicate an area with no record of earthquakes; an aseismic zone.

 

  1. Body Wave – A seismic wave that can travel through the interior of the earth.  P-waves and S-waves are body waves.

 

  1. Central Angle – An angle with the vertex at the center of the Earth, with one ray passing through the hypocenter (and also the epicenter) and the other ray passing through the recording station.

 

  1. Consolidated – Tightly packed.  Composed of particles that are not easily separated.

 

  1. Core – The innermost layers of the Earth.  The inner core is solid and has a radius of about 1300 kilometers.  (The radius of the Earth is about 6371 kilometers.)  The outer core is fluid and is about 2300 kilometers thick.  S-waves cannot travel through the outer core.

 

  1. Continental Drift – The theory, first advanced by Alfred Wegener, that Earth’s continents were originally one land mass.  Pieces of the land mass split off and migrated to form the continents.

 

  1. Crust – The thin out layer of the Earth’s surface, averaging about 10 kilometers thick under the oceans and up to about 50 kilometers thick on the continents.  This is the only layer of the Earth that humans have actually seen.

 

  1. Earthquake – Shaking of the Earth caused by a sudden movement of rock beneath its surface.

 

  1. Earthquake Swarm – A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.  Frequently associated with imminent volcanic eruptions.

 

  1. Elastic Wave – A wave that is propagated by some kind of elastic deformation, that is, a change in shape that disappears when the forces are removed.  A seismic wave is a type of elastic wave.

 

  1. Epicenter – That point on the Earth’s surface directly above the hypocenter of an earthquake.

 

  1. Fault – A weak point in the Earth’s crust and upper mantle where the rock layers have ruptured and slipped.  Faults are caused by earthquakes, and earthquakes are likely to reoccur on pre-existing faults.

 

  1. First Arrival – The first recorded signal attributed to seismic wave travel from a source.

 

  1. Focus – That point within the Earth from which originates the first motion of an earthquake and its elastic waves.

 

  1. Foreshock – A small tremor that commonly precedes a larger earthquake or main shock by seconds to weeks and that originates in or near the rupture zone of the larger earthquake.

 

  1. Great Earthquake – An earthquake having a magnitude of 8 or greater on the Richter scale.

 

  1. Hazard – A risk.  An object or situation that has the possibility of injury or damage.

 

  1. Hypocenter – The calculated location of the focus of an earthquake.

 

  1. Intensity – A measure of the effects of an earthquake at a particular place on humans, structures and (or) the land itself.  The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the pint and the local geology at that point.

 

  1. Isoseismal Line – A line connecting points on the Earth’s surface at which earthquake intensity is the same.  It is usually a closed curve around the epicenter.

 

  1. Landslide – An abrupt movement of soil and bedrock downhill in response to gravity.  Landslides can be triggered by an earthquake or other natural causes.  Undersea landslides can cause tsunamis.

 

  1. Latitude – The location of a point north or south of the equator.  Latitude is shown on a map or globe as east-west lines parallel to the equator.

 

  1. Leaking Mode – A surface seismic wave which is imperfectly tapped so that is energy leaks or escapes across a layer boundary causing some attenuation, or loss of energy.

 

  1. Liquefaction – The process in which a solid (soil) takes on the characteristics of a liquid as a result of an increase in pore pressure and a reduction in stress.  In other words, solid ground turns to jelly.

 

  1. Lg Wave – A surface wave which travels through the continental crust.

 

  1. Longitude – The location of a point east or west of the prime meridian.  Longitude is shown on a map or globe as north-south lines left and right of the prime meridian, which passes through Greenwich, England.

 

  1. Love Wave – A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation (travel).  It is named after A. E. H. Love, the English mathematician who discovered it.

 

  1. Low-velocity Zone – Any layer in the Earth in which seismic wave velocities are lower than in the layers above and below.

 

  1. Magnitude – A measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations.  This is a logarithmic value originally defined by Charles Richter (1935).  An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released.  In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake – or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake!  There is no beginning nor end to this scale.  However, rock mechanics seems to preclude earthquakes smaller than about –1 or larger than about 9.5.  A magnitude –1.0 event release about 900 times less energy than a magnitude 1.0 quake.  Except in special circumstances, earthquakes below magnitude 2.5 are not generally felt by humans.

 

  1. Major Earthquake – An earthquake having a magnitude of 7 to 7.99 on the Richter scale.

 

  1. Mantle – The layer of rock that lies between the crust and the outer core of the Earth.  It is approximately 2900 kilometers thick and is the largest of the Earth’s major layers.

 

  1. Microearthquake – An earthquake having a magnitude of 2 or less on the Richter scale.

 

  1. Microseism – A more or less continuous motion in the Earth that is unrelated to an earthquake and that has a period of 1.0 to 9.0 seconds.  It is caused by a variety of natural and artificial agents.

 

  1. Modified Mercalli Scale – Mercalli intensity scale modified for North American conditions.  A scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, that is designated by Roman numerals.  It does not have mathematical basis; instead it is an arbitrary ranking based on observed effects.

 

  1. Mohorovicic Discontinuity (The Moho) – The boundary surface or sharp seismic-velocity discontinuity (pronounced Mo-ho-ro-vi-chich) that separates the Earth’s crust from the underlying mantle.  Named for Andrija Mohorovicic, the Croatian seismologist who first suggested its existence.

 

  1. P-Wave – Primary, longitudinal, irrotational, push, pressure, dilatational, compressional, or push-pull wave.  P-waves are the fastest body waves and arrive at stations before the S-waves, or secondary waves.  The waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave.  P-waves can travel through all layers of the Earth.  P-waves are generally felt by humans as a bang or thump.

 

  1. Paleomagnetism – The natural magnetic traces that reveal the intensity and direction of Earth’s magnetic field in the geologic past.  Also, the study of these magnetic traces.

 

  1. Paleoseismology – The study of ancient (prehistoric) earthquakes.

 

  1. Period – The time between two successive wave crests.

 

  1. Phase – The onset of a displacement or oscillation on a seismogram indicating the arrival of a different type of seismic wave.

 

  1. Plate – One of the huge sections which make up the Earth’s crust.  The plates are continuously moving.

 

  1. Plate Boundary – The place where two or more plates in the Earth’s crust meet.

 

  1. Plate Tectonics – The theory that the Earth’s crust and upper mantle (the lithosphere) is broken into a number of more or less rigid, but constantly moving, segments or plates.

 

  1. Rayleigh Wave – A type of surface wave having a retrograde, elliptical motion at the Earth’s surface, similar to the waves caused when a stone is dropped into a pond.  These are the slowest, but often the largest and most destructive, of the wave types caused by an earthquake.  They are usually felt as a rolling or rocking motion and in the case of major earthquakes, can be seen as the approach.  Named after Lord Rayleigh, the English physicist who predicted its existence.

 

  1. Recurrence Interval – The approximate length of time between earthquakes in a specific seismically active area.

 

  1. Reflect – To bounce back from a surface.

 

  1. Refract – To bend or change direction.

 

  1. Richter Scale – The system used to measure the strength of an earthquake.  Developed by Charles Richter in 1935 as a means of categorizing local earthquakes.  It is a collection of mathematical formulas; it is not a physical device.

 

  1. Rupture Zone – The area of the Earth through which faulting occurred during an earthquake.  For very small earthquakes, this zone could be the size of a pinhead, but in the case of a great earthquake, the rupture zone may extend several hundred kilometers in length and tens of kilometers in width.

 

  1. S-Wave – Shear, secondary, rotational, tangential, equivoluminal, distortional, transverse, or shake wave.  These waves carry energy through the Earth in very complex patterns of transverse (crosswise) waves.  These waves move more slowly than P-waves, but in an earthquake they are usually bigger.  S-waves cannot travel through the outer core because these waves cannot exist in fluids, such as air, water, or molten rock.

 

  1. Seiche – A free or standing wave oscillation of the surface of water in an enclosed basin that is initiated by local atmospheric changes, tidal currents, or earthquakes.  Similar to water sloshing in a bathtub.

 

  1. Seismic Belt – An elongated earthquake zone, for example, circum-Pacific, Mediterranean, Rocky Mountain.  About 60% of the world’s earthquakes occur in the circum-Pacific seismic belt.

 

  1. Seismic Constant – In building codes dealing with earthquake hazards, an arbitrarily-set acceleration value (in units of gravity) that a building must withstand.

 

  1. Seismicity – Earthquake activity.

 

  1. Seismic – Of or having to do with earthquakes.

 

  1. Seismic Sea Wave – A tsunami generated by an undersea earthquake.

 

  1. Seismic Zone – A region in which earthquakes are known to occur.

 

  1. Seismogram – A written record of an earthquake, recorded by a seismograph.

 

  1. Seismograph – An instrument that records the motions of the Earth, especially earthquakes.

 

  1. Seismograph Station – A site at which one or more seismographs are set up and routinely monitored.

 

  1. Seismologist – A scientist who studies earthquakes.

 

  1. Seismometry – The instrumental aspects of seismology.

 

  1. Signal-to-Noise Ratio – The comparison between the amplitude of the seismic signal and the amplitude of noise caused by seismic unrest and (or) the seismic instruments.

 

  1. Spread – The layout of seismometer or geophone groups from which data from a single shot (the explosive charge) are recorded simultaneously.

 

  1. Spreading Center – An elongated region where two plates are being pulled away from each other.  New crust is formed as molten rock is forced upward into the gap.  Examples of spreading centers include the Mid-Atlantic Ridge and the East African Rift.

 

  1. Subduction – The process in which one lithospheric plate collides with and is forced down under another plate and drawn back into the Earth’s mantle.

 

  1. Subduction Zone – An elongated region along which a plate descends relative to another plate, for example, the descent of the Nazca plate beneath the South American plate along the Peru-Chile Trench.

 

  1. Surface of the Earth – The value given is the depth below the surface of the mean spheroid.  The mean spheroid is a uniform approximation to the true shape of the Earth.  No adjustment is made to the depth due to any differences between the true Earth and the mean spheroid.  For example, the minimum depth that will be given is 0 kilometers, even though a quake directly under Mount Everest (elevation 8848 meters) could legitimately have a depth of –6 kilometers and still be 2 kilometers underground.  On the other hand, a depth of 10 kilometers would actually be more than 1 kilometer above the ocean floor of Challenger Deep (elevation –11,033 meters) in the Marianas Trench of the Pacific Ocean.

 

  1. Surface Waves – Waves that move over the surface of the Earth.  Rayleigh waves and Love waves are surface waves.

 

  1. Teleseism – An earthquake that is distant (usually more than 20 degrees) from the recording station.

 

  1. Tidal Wave – A term that seismologist hate.  The correct word for the big waves people often call “tidal waves” is tsunami.  True “tidal waves” – or waves caused by tides – are the ordinary waves people see on the ocean.

 

  1. Travel Time – The time required for a wave train to travel from its source to a point of observation.

 

  1. Tsunami – One or a series of huge sea waves caused by earthquakes or other large-scale disturbance of the ocean floor.  (Referred to incorrectly by many as a tidal wave, but these waves have nothing to do with tides.)  The word tsunami is Japanese, meaning “harbor wave.”

 

  1. Unconsolidated – Loosely arranged, not cemented together, so particles separate easily.

 

  1. UTC – Coordinated Universal Time.  The time scale based on the atomic second but correct every now and again to keep it in approximate sync with the earth’s rotation.  The corrections show up as the leap seconds put into UTC – usually on New Year’s Eave.  In the most common usage, the terms GMT and UTC are identical.

Structure of the Earth

 

The size of the Earth -- about 12,750 kilometers (km) in diameter-was known by the ancient Greeks, but it was not until the turn of the 20th century that scientists determined that our planet is made up of three main layers: crust, mantle, and core. This layered structure can be compared to that of a boiled egg. The crust, the outermost layer, is rigid and very thin compared with the other two. Beneath the oceans, the crust varies little in thickness, generally extending only to about 5 km. The thickness of the crust beneath continents is much more variable but averages about 30 km; under large mountain ranges, such as the Alps or the Sierra Nevada,

however, the base of the crust can be as deep as 100 km. Like the shell of an egg, the Earth's crust is brittle and can break.

 

Cutaway views showing the internal structure of the Earth. Below: This view drawn to scale demonstrates that the Earth's crust literally is only skin deep.  Below right: A view not drawn to scale to show the Earth's three main layers (crust, mantle, and core) in more detail (see text).

 

 

 

 

Below the crust is the mantle, a dense, hot layer of semi-solid rock approximately 2,900 km thick. The mantle, which contains more iron, magnesium, and calcium than the crust, is hotter and denser because temperature and pressure inside the Earth increase with depth. As a comparison, the mantle might be thought of as the white of a boiled egg. At the center of the Earth lies the core, which is nearly twice as dense as the mantle because its composition is metallic (iron-nickel alloy) rather than stony. Unlike the yolk of an egg, however, the Earth's core is actually made up of two distinct parts: a 2,200 km-thick liquid outer core and a 1,250 km-thick solid inner core. As the Earth rotates, the liquid outer core spins, creating the Earth's magnetic field.

 

Not surprisingly, the Earth's internal structure influences plate tectonics. The upper part of the mantle is cooler and more rigid than the deep mantle; in many ways, it behaves like the overlying crust. Together they form a rigid layer of rock called the lithosphere (from lithos, Greek for stone). The lithosphere tends to be thinnest under the oceans and in volcanically active continental areas, such as the Western United States. Averaging at least 80 km in thickness over much of the Earth, the lithosphere has been broken up into the moving plates that contain the world's continents and oceans. Scientists believe that below the lithosphere is a relatively

narrow, mobile zone in the mantle called the asthenosphere (from asthenes, Greek for weak). This zone is composed of hot, semi-solid material, which can soften and flow after being subjected to high temperature and pressure over geologic time. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere.

 

Plate Tectonics, Faults, and Fault Zones

 

Anyone who has ever stared at a map or globe must have noticed how well the eastern coast of South America and the western coast of Africa would fit together much like two pieces in a puzzle.  Were they once together or is this just a coincidence?  If they were once together, why are they now an ocean apart? 

 

The concept that the two continents were once joined is not a very old concept.  Only recently in scientific history have scientists come to realize that these continents, as well as others, were once very different than they are today.  The theory of continental drift was the concept of German scientist Alfred Wegener in 1912.  Not only did Wegener notice the puzzle “fit” of several continents, he was able to make the connection by other means – fossils.

 

Plate tectonics refers to the study of the formation and movement of large pieces of rigid, solid land masses that move or “float” across the surface of the earth.  The Earth’s surface is divided into a dozen major plates and several minor plates.  Some of the plates are moving toward each other, and some are moving apart.  For example, the South American plate and the African plate are moving apart as are the North American plate and the Eurasian plate.

 

 

Other plates, however, are not spreading apart.  Since the Earth is finite in size, plates that are spreading apart, must be colliding with another plate on its opposite side.  Some plates slide past each other instead of colliding.  For example, in California, the Pacific plate and North American plate are sliding past each other.  However, in South America, the South American plate is moving westward directly colliding with the Nazca plate.  In this case, the westward moving South American plate is riding over the Nazca plate, which is in turn forced downward.  The leading edge of the South American plate is marked all along the South American continent by the Andes Mountains.  The trailing edge of the Nazca plate is marked by a deep underwater trench where it is being forced downward by the advancing South American plate.  An example of this action can be viewed on the right hand side of the diagram below.

 

 

Where plates collide, the result is usually a mountain range formed along the leading edge of the advancing plate; whereas, a trench forms where the second plate is being forced downward.  Where plates spread apart, a feature known as a “ridge” is formed.  A ridge can extend for nearly the length of the earth from pole to pole.

 

One of the best known ridges is the Mid-Atlantic Ridge.  The southern half of this ridge is bounded by the South American plate to the west and the African plate to the east.  Heated material (magma) due to convection currents below the boundary rises to the surface causing the sea floor to spread outward to the east and west.  This is the primary engine for the basis of continental drift.

 

Plate boundaries can be classified into the categories of diverging boundaries, sliding boundaries, converging boundaries collision, and converging boundaries subduction.  Diverging boundaries, as previously discussed, are plate boundaries that are spreading apart.

 

In some cases, plate boundaries are sliding past each other.  The North American plate and the Pacific plate is a prime example of a sliding boundary.  It is denoted by a break or crack in the Earth’s crust known as a fault.  The southwestern part of California and the Pacific plate are moving to the northwest with respect to the rest of the United States and the North American plate. 

 

Contrary to popular belief, California will not fall into the Pacific.  Instead, San Diego and Los Angeles will continue to move ever so slowly to the northwest toward the San Francisco/Oakland area.  Perhaps millions of years from now, these cities will be cross-town neighbors!

 

When plates converge, two possible results can occur.  One result, the collision boundary, causes the two plates to merge and is usually forced upward to form mountain ranges.  An example is the collision between the Indian subcontinent with Asia.  The result is the Himalayan Mountains.  Other similar collisions caused the Ural Mountains to form 300 million years ago when Europe collided with Siberia.  The Appalachian Mountains formed when the North American plate collided with northern Africa.  Of course, these continents are now spreading apart.

 

The other result of converging plates is a subduction boundary.  A subduction boundary is formed when one plate plunges down under another overriding plate.  The plunging plate is said to be subducting beneath the overriding plate.  The result of subduction is usually a deep-sea trench.  The collision of South America with the Nazca plate is an example of subduction.

 

The three basic types of faults are determined by how a plate is interacting with its neighbor.  When plates are pulling apart, a fault known as a normal fault is formed.  A normal fault (extensional) occurs when the rocks on one side of the fault plane drop down with respect to the rocks on the other side.  The Mid-Atlantic Ridge is an example of this type of faulting.

 

When plates converge, a reverse fault (compressional) is typically found.  Reverse faults result from the stresses that push toward each other.  Reverse faults are important in mountain building because they allow the crust to be shortened as the plates collide.

 

The third type of fault is the strike-slip fault (transform).  The San Andreas Fault in California is an example of this type of fault.  It is formed when two plates slide horizontally past each other.

                                                          

As the interactions of plates form different types of faults, so do they produce different types of earthquakes. In 1969, Muawia Barazangi and James Dorman published the locations of all earthquakes which occurred from 1961 to 1967. Most of the earthquakes are confined to narrow belts and these belts define the boundaries of the plates. The interiors of the plates themselves are largely free of large earthquakes, that is, they are aseismic. There are notable exceptions to this. An obvious one is the 1811-1812 earthquakes at New Madrid, Missouri, and another is the 1886 earthquake at Charleston, South Carolina. As yet there is no satisfactory plate tectonic explanation for these isolated events; consequently, we will have to find alternative mechanisms.

 

Plate tectonics confirms that there are four types of seismic zones. The first follows the line of mid-ocean ridges. Activity is low, and it occurs at very shallow depths. The point is that the lithosphere is very thin and weak at these boundaries, so the strain cannot build up enough to cause large earthquakes. Associated with this type of seismicity is the volcanic activity along the axis of the ridges (for example, Iceland, Azores, Tristan da Cunha).

 

The second type of earthquake associated with plate tectonics is the shallow-focus event unaccompanied by volcanic activity. The San Andreas fault is a good example of this, so is the Anatolian fault in Northern Turkey. In these faults, two mature plates are scraping by one another. The friction between the plates can be so great that very large strains can build up before they are periodically relieved by large earthquakes. Nevertheless, activity does not always occur along the entire length of the fault during any one earthquake. For instance, the 1906 San Francisco event was caused by breakage only along the northern end of the San Andreas

fault. 

 

The third type of earthquake is related to the collision of oceanic and continental plates. One plate is thrust or subducted under the other plate so that a deep ocean trench is produced. In the Philippines, ocean trenches are associated with curved volcanic island arcs on the landward plate, for example the Java trench. Along the Peru - Chile trench, the Pacific plate is being subducted under the South American plate which responds by crumpling to form the Andes. This type of earthquake can be shallow, intermediate, or deep, according to its location on the down-going lithospheric slab. Such inclined planes of earthquakes are know as Benioff zones.

 

The fourth type of seismic zone occurs along the boundaries of continental plates. Typical of this is the broad swath of seismicity from Burma to the Mediterranean, crossing the Himalayas, Iran, Turkey, to Gibraltar. Within this zone, shallow earthquakes are associated with high mountain ranges where intense compression is taking place. Intermediate- and deep-focus earthquakes also occur and are known in the Himalayas and in the Caucasus. The interiors of continental plates are very complex, much more so than island arcs. For instance, we do not yet know the full relationship of the Alps or the East African rift system to the broad picture of

plate tectonics.

 

How can plate tectonics help in earthquake prediction? We have seen that earthquakes occur at the following three kinds of plate boundary: ocean ridges where the plates are pulled apart, margins where the plates scrape past one another, and margins where one plate is thrust under the other. Thus, we can predict the general regions on the Earth's surface where we can expect large earthquakes in the future. We know that each year about 140 earthquakes of magnitude 6 or greater will occur within this area which is 10 percent of the Earth's surface.

 

But on a worldwide basis we cannot say with much accuracy when these events will occur. The reason is that the processes in plate tectonics have been going on for millions of years. Averaged over this interval, plate motions amount to a several millimeters per year. But at any instant in geologic time, for example, the year 1977, we do not know exactly where we are in the worldwide cycle of strain buildup and strain release. Only by monitoring the stress and strain in small areas, for instance, the San Andreas fault, in great detail can we hope to predict when renewed activity in that part of the place tectonics arena is likely to take place.

 

In summary, plate tectonics is a blunt, but, nevertheless, strong tool in earthquake prediction. It tells us where 90 percent of the Earth's major earthquakes are likely to occur. It cannot tell us much about exactly when they will occur. For that, we must study in detail the plate boundaries themselves. Perhaps the most important role of plate tectonics is that it is a guide to the use of finer techniques for earthquake prediction.

 

 

History of Earthquakes

 

The scientific study of earthquakes is comparatively new. Until the 18th century, few factual descriptions of earthquakes were recorded, and the natural cause of earthquakes was little understood. Those who did look for natural causes often reached conclusions that seem fanciful today; one popular theory was that earthquakes were caused by air rushing out of caverns deep in the Earth's interior.

 

The earliest earthquake for which we have descriptive information occurred in China in 1177 B.C. The Chinese earthquake catalog describes several dozen large earthquakes in China during the next few thousand years. Earthquakes in Europe are mentioned as early as 580 B.C., but the earliest for which we have some descriptive information occurred in the mid-16th century. The earliest known earthquakes in the Americas were in Mexico in the late 14th century and in Peru in 1471, but descriptions of the effects were not well documented. By the 17th century, descriptions of the effects of earthquakes were being published around the world - although these accounts were often exaggerated or distorted.

 

The most widely felt earthquakes in the recorded history of North America were a series that occurred in 1811-1812 near New Madrid, Missouri. A great earthquake, whose magnitude is estimated to be about 8, occurred on the morning of December 16, 1811. Another great earthquake occurred on January 23, 1812, and a third, the strongest yet, on February 7, 1812. Aftershocks were nearly continuous between these great earthquakes and continued for months

afterwards. These earthquakes were felt by people as far away as Boston and Denver. Because the most intense effects were in a sparsely populated region, the destruction of human life and property was slight. If just one of these enormous earthquakes occurred in the same area today, millions of people and buildings and other structures worth billions of dollars would be affected.

 

The San Francisco earthquakes of 1906 was one of the most destructive in the recorded history of North America - the earthquake and the fire that followed killed nearly 700 people and left the city in ruins.  The great 1906 San Francisco earthquake and fire destroyed most of the city and left 250,00 people homeless.

 

The Alaska earthquake of March 27, 1964, was of greater magnitude than the San Francisco earthquake; it released perhaps twice as much energy and was felt over an area of almost 500,000 square miles.  The ground motion near the epicenter was so violent that the tops of some trees were snapped off. One hundred and fourteen people (some as far away as California) died as a result of this earthquake, but loss of life and property would have been far greater had Alaska been more densely populated.

 

Anatomy of an Earthquake

 

We have looked at plate tectonics, types of faults, and the history of earthquakes.  But what is an earthquake and what is associated with them?

As previously discussed, earthquakes are the direct result of continental plates sliding past each other.  If you have ever taken two rocks and rubbed them together, you have no doubt noticed they do not smoothly slide past each other.  Instead, it is a rough jerky type of motion.  Solid rock has high friction properties.

 

Large tectonic plates behave in a similar fashion.  They do not move smoothly past each other.  Instead, they “lock up” building up tremendous amounts of stress.  When the stress becomes too high, a portion of the plates “snap” past each other releasing copious amounts of energy in the form of an earthquake.

 

When an earthquake fault ruptures, it causes two types of deformation:  static and dynamic.  Static deformation is the permanent displacement of the ground due to the event.  The earthquake cycle progresses from a fault that is not under stress, to a stressed fault as the plate tectonic motions driving the fault slowly proceed, to rupture during an earthquake and a newly-relaxed but deformed state.

Typically, someone will build a straight reference line such as a road, railroad, pole line, or fence line across the fault while it is in the pre-rupture stressed state.  After the earthquake, the formerly straight line is distorted into a shape having increasing displacement near the fault, a process known as elastic rebound.

 

The second type of deformation, dynamic motions, are essentially sound waves radiated from the earthquake as it ruptures.  While most of the plate-tectonic energy driving fault ruptures is taken up by static deformation, up to 10% may dissipate immediately in the form of seismic waves.

 

The mechanical properties of the rocks that seismic waves travel through quicklyorganize the waves into two types.  Compressional waves, also known as primary or P waves, travel fastest, at speeds between 1.5 and 8 kilometers per second in the Earth’s crust.  Shear waves, also known as S waves travel more slowly, usually at 60% to 70% of the speed of P waves.

 

P waves shake the ground in the direction they are propagating, while S waves shake perpendicularly or transverse to the direction of propagation.

 

Although wave speeds vary by a factor of ten or more in the Earth, the ratio between the average speeds of a P wave and of its following S wave is quite constant.  This fact enables seismologists to simply time the delay between the arrival of the P wave and the arrival of the S wave to get a quick and reasonably accurate estimate of the distance of the earthquake from the observation station.  Just multiply the S-minus-P (S-P) time, in seconds, by the factor 8km/s to get the approximate distance in kilometers.

 

The dynamic, transient seismic waves from any substantial earthquake will propagate all around and entirely through the Earth.  Given a sensitive enough detector, it is possible to record the seismic waves from even minor events occurring anywhere in the world an any other location on the globe.  Nuclear test-ban treaties in effect today rely on our ability to detect a nuclear explosion anywhere equivalent to an earthquake as small as Richter Magnitude 3.5.

 

An earthquake generates a series of waves that penetrate the entire Earth and travel at and through its surface.  Each wave has a characteristic time:  each has its own move of travel.  They are quite complex, but a few basic facts will explain how they travel through the Earth and how an earthquake’s epicenter can be determined from seismograph records.

 

There are four basic types of seismic waves; two preliminary body waves that travel through the Earth and two that travel only at the surface (L waves). Combinations, reflections, and diffractions produce an infinity of other types, but body waves are the main interest in this discussion.

 

Body waves are composed of two principal types; the P (primary) wave, comparable to sound waves, which compresses and dilates the rock as it travels forward through the Earth; and the S (secondary) wave, which shakes the rock sideways as it advances at barely more than half the P-wave speed.

 

Figure 2. Travel-time curves with idealized seismograms (earthquake records superimposed).

 

The P wave is designated the primary preliminary wave because it is the first to arrive at a seismic station after an earthquake. It travels at a speed usually less than 6 kilometers per second in the Earth's crust and jumps to 13 kilometers per second through the core.

 

The S wave is the secondary preliminary wave to be recorded. It follows paths through the Earth quite similar to those of the P-wave paths, except that no consistent evidence has yet been found that the S wave penetrates the Earth's core.

 

The lines labeled P, S, and L in the curves shown on figure 2 represent the travel time required for each phase at distances of 0 to 1300 kilometers from the earthquake's epicenter. They mark the points on the record at which these waves first arrive at the station.

 

The simplest method of locating an earthquake on a globe is to find the time interval between the P- and S-wave arrivals at several seismograph stations. The distance to the earthquake from each station is then determined from standard travel-time tables and travel-time curves. Great-circle arcs are drawn on the globe

using the distance of the earthquake to the station as a radius. All the arcs should intersect at a common point - the epicenter.

 

Another method of locating an earthquake is to use the P-wave arrival-time minus origin-time (P - O) interval instead of distance. This method is more common because the time can be taken directly from surface focus travel-time tables assuming an origin of 00 hours. This method, however, requires that travel-time tables be available for various depths of focus. For locating a deep shock, one 700 kilometers deep, for example, travel-time tables and travel-time curves for that depth have to be used to calculate the origin time and distances.

 

Other wave types can be generated inside the Earth by P and S waves, as shown in figure 3 (following page). As many as five different wave groups or phases can emerge when a P or S wave encounters a discontinuity or interface within the Earth.

 


Figure 3. Propagation paths of combinations of P, S, and L waves from an earthquake focus.

 

Detecting Earthquakes, Seismographs, and Seismograms

 

Seismographs are based on the principal of inertia and the differential motion between a free mass (which tends to remain at rest) and a supporting structure anchored in the ground (which moves with the vibrating Earth).  Seismographs can be constructed to record both horizontal and vertical motion.  The most common seismograph is the horizontal seismograph.

 

The seismograph consists of a large aluminum plate with a support along the rear of the plate which holds the pendulum.  A horizontal pendulum rests against the bottom of a screw on the rear support.  The pendulum is supported by guitar string.  Near the end of the pendulum is a lead weight used for mass as well as a coil of copper wire.

 

The pendulum is carefully balanced and has a naturally occurring period of around 16 seconds.  Paddles dipped in oil prevent the pendulum from swinging excessively (damping).  Two large magnets are positioned so that the coil fits between them with a clearance of around ¼” on either side.

 

Since the base is in contact with the “Earth” and the pendulum is suspended by a string, any lateral motion in the Earth moves the base and the magnets leaving the pendulum to remain in place.  The displacement between the coil at the end of the pendulum and the magnets generates very small electrical currents which in turn is interpreted by a computer as seismic waves.

 

 

More sophisticated seismographs, such as those at the United States Geological Survey, record the information on drums as well as on computers.  The seismograph can measure earthquakes within the continental United States as small as magnitude 4.5 should the quake occur along the west coast.  Smaller, closer quakes can be measured such as those along the New Madrid fault zone in southeastern Missouri and northeast Arkansas; however, no such events have occurred since the seismograph has gone online.

 

Quakes of 5.5 or higher can usually be detected in the western hemisphere with larger quakes of 6.5 or higher being detected anywhere in the world.  Recorded earthquakes will be entered into the Public Seismic Network’s database and displayed on the internet along with other postings from around the world.