About Seismic


What Is Seismology?

Seismology is the science of studying earthquakes and related phenomena. In Greek, Seismos = Shaking & logos = Science. A Seismologist is a scientist who studies earthquakes and seismic waves.


What is an Earthquake?

Earthquake is a sudden movement or vibration of a part of the earths top layers caused by sudden release of energy stored as elastic strain in the underlying rocks. This energy reaches us as series of vibrations travelling through the body of earth and are called as seismic waves .


Types of Seismic Waves

There are two types of seismic waves. They are:

Body waves - travel through the earth's interior.

Surface waves - can only move along the surface of the planet

Body Waves

P Waves

P WavesThe first kind of body wave is the P wave or primary wave. The particle motion of P waves is parallel to the direction of propagation of the wave. This is the fastest of seismic waves. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock as it moves through; just like sound waves push and pull the air. Have you ever heard a big clap of thunder and heard the windows rattle at the same time? The windows rattle because the sound waves were pushing and pulling on the window glass much like P waves push and pull on rock. Sometimes animals can hear the P waves of an earthquake. Usually we only feel the bump and rattle of these waves.

S Waves

S Waves The second kind of body wave is the S wave or secondary wave. The particle motion of the wave is perpendicular to the direction of propagation of the wave. An S wave is slower than the P wave and can only travel through solid rock. This wave moves rock up and down, or side-to-side.The arrow shows the direction that the wave is moving.

Surface Waves

Love Waves

LoveWavesnamed after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side.The arrow shows the direction that the wave is moving.

 

Rayleigh Waves

Rayleigh Waves The other kind of surface wave is the Rayleigh wave, named for John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.

Where Do Earthquakes Happen?

Earthquakes occur all the time all over the world, both along plate edges and along faults.

Along Plate Edges

Most earthquakes occur along the edge of the oceanic and continental plates. The earth's crust (the outer layer of the planet) is made up of several pieces, called plates. The plates under the oceans are called oceanic plates and the rest are continental plates. The plates are moved around by the motion of a deeper part of the earth (the mantle) that lies underneath the crust. These plates are always bumping into each other, pulling away from each other, or past each other. The plates usually move at about the same speed that your fingernails grow. Earthquakes usually occur where two plates are running into each other or sliding past each other.

A map of the world's plates (From Shedlock & Pakiser, 1997)

Along Faults

Earthquakes can also occur far from the plate boundaries, along faults. Faults are cracks in the earth where sections of a plate (or two plates) are moving in different directions. Faults are caused by all that bumping and sliding the plates do. They are more common near the edges of the plates.

Types of Faults

Types of Faults Normal faults are the cracks where one block of rock is sliding downward and away from another block of rock. These faults usually occur in areas where a plate is very slowly splitting apart or where two plates are pulling away from each other.
Strike-slip faults are the cracks between two plates that are sliding past each other. We can find these kinds of faults along the mid-oceanic ridges. The San Andreas fault is a strike-slip fault. It's the most famous California fault and has caused a lot of powerful earthquakes.
Reverse faults are cracks formed where one plate is pushing into another plate. They also occur where a plate is folding up because it's being compressed by another plate pushing against it. At these faults, one block of rock is sliding underneath another block or one block is being pushed up over the other.





Seismic Deformation

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.

Seismic Deformation 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.


Seismic Waves

Seismic Waves 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 quickly organize 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 secondary or 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 8 km/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 at 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.

How are earthquakes measured?

Earthquakes are measured by their magnitude which is a number that characterizes the relative size of an earthquake. Magnitude is based on measurement of the maximum motion recorded by a seismograph. Several scales have been defined but the most commonly used are:

  1. Local Magnitude Ml
  2. Body Wave Magnitude Mb
  3. Surface Wave Magnitude Ms
  4. Moment Magnitude Mw

Seismographs and Seismograms

Seismograph and Seismogram Sensitive seismographs are the principal tool of scientists who study earthquakes. Thousands of seismograph stations are in operation throughout the world, and instruments have been transported to the Moon, Mars, and Venus. Fundamentally, a seismograph is a simple pendulum. When the ground shakes, the base and frame of the instrument move with it, but intertia keeps the pendulum bob in place. It will then appear to move, relative to the shaking ground. As it moves it records the pendulum displacements as they change with time, tracing out a record called a seismogram.

GraphOn this example it is obvious that seismic waves take more time to arrive at stations that are farther away. The average velocity of the wave is just the slope of the line connecting arrivals, or the change in distance divided by the change in time. Variations in such slopes reveal variations in the seismic velocities of rocks. Note the secondary S-wave arrivals that have larger amplitudes than the first P waves, and connect at a smaller slope.

While the actual frequencies of seismic waves are below the range of human hearing, it is possible to speed up a recorded seismogram to hear it. You can click on this earthquake recording to hear a seismogram from the 1992 Landers earthquake in southern California, recorded near Mammoth Lakes in an active volcanic caldera by the USGS. The original record, 800 seconds long, has been speeded up 80 times so that you hear it all within 10 seconds.

75 kb u-law 149 kb WAV 75 kb Quicktime

The clicks at the beginning of the recording are the sharp, high-frequency P waves, followed by the rushing sound of the drawn-out, lower-frequency S waves. This recording is also interesting because of the small, local earthquakes within the Mammoth caldera that sound like gunshots. The passage of the S wave from the magnitude 7.2 Landers event through the caldera actually triggered a sequence of small earthquakes there. The triggered earthquakes are similar to a burst of creaks and pops you hear from your house frame after a strong blast of wind. Landers triggered earthquakes up to magnitude 5.5 throughout eastern California and Nevada, and in calderas as far away as Yellowstone.

Listen to more earthquakes

Locating Earthquakes

Locating Earthquakes The pricipal use of seismograph networks is to locate earthquakes. Although it is possible to infer a general location for an event from the records of a single station, it is most accurate to use three or more stations. Locating the source of any earthquake is important, of course, in assessing the damage that the event may have caused, and in relating the earthquake to its geologic setting.


Given a single seismic station, the seismogram records will yield a measurement of the S-P time, and thus the distance between the station and the event. Multiply the seconds of S-P time by 8 km/s for the kilometers of distance. Drawing a circle on a map around the station's location, with a radius equal to the distance, shows all possible locations for the event. With the S-P time from a second station, the circle around that station will narrow the possible locations down to two points. It is only with a third station's S-P time that you can draw a third circle that should identify which of the two previous possible points is the real one.