What are earthquakes?

Posted: September 30, 2012 by phaedrap1 in Uncategorized

With the increase in earthquake frequency, many people have become interested in what causes quakes to happen and what the different categories of data mean.  Unfortunately, a lot of the information available does not provide the basic information in a way a layman or beginner can easily understand.  This article aims to do just that.  To begin here are some important terms:

Body waves – A seismic wave that travels through the Earth rather than across its surface.

Deep-focus – Earthquakes occurring within the subducting oceanic plates as they move beneath the continental plates. Appearing along fault lines, these are earthquakes with focus much deeper within the earth. A deep focus earthquake occurs when two tectonic plates slide towards one another followed by subduction.   Deep-focus earthquakes occur at greater focal depths of 300 – 700 km.

Epicenter – the point on the earths surface directly above the hypocenter

Fault – A break in the rocks that make up the Earth’s crust, along which rocks on either side have moved past each other.

Hypocenter – Also called the Focus.  The place in the earth where the earthquake rupture starts

Love waves – A Love wave is confined to the surface of the crust, Love waves produce entirely horizontal motion. It’s the fastest surface wave and moves the ground from side-to-side.  The Love wave was named after A.E.H. Love, a British mathematician.

Primary (P) Waves – Also called Compression waves. This is the fastest kind of seismic wave, and, consequently, the first to ‘arrive’ at a seismic station. 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 it moves through just like sound waves push and pull the air.

Rayleigh waves – 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. The Rayleigh wave is named for John William Strutt, Lord Rayleigh.

Secondary (S) waves – The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium. It is this property of S waves that led seismologists to conclude that the Earth’s outer core is a liquid. S waves move rock particles up and down, or side-to-side–depending on the direction that the wave is traveling in.

Shallow-focus – Earthquakes are crustal earthquakes, caused by faults and movements of the continental plates. The focus of these are nearer the surface of the earth.  Shallow-focus earthquakes occur at depths less than 70 km.

Subduction – Happen at the place where two tectonic plates meet and one slides under the other.

Surface waves – Traveling only through the crust, surface waves are of a lower frequency than body waves, and are easily distinguished on a seismogram as a result. Though they arrive after body waves, it is surface waves that are almost enitrely responsible for the damage and destruction associated with earthquakes. This damage and the strength, of the surface waves are reduced in deeper earthquakes.

Tectonic Plates – Also called lithospheric plates are massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plate size can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest.

An earthquake is a shaking of the ground caused by the sudden breaking and moving of tectonic plates in the earth’s rocky outermost crust.  When an earthquake occurs, energy is released and radiates in waves along the fault.  All quakes are either shallow or deep-focus.   Shallow-focus earthquakes are usually larger and therefore more damaging than deep-focus earthquakes.  Earthquakes occur on both land and under the ocean.  These quakes under the ocean can cause tsunamis.

Faults are created when the earth’s crust becomes stressed and slips suddenly.   Movement of the tectonic plates is usually very slow, pressure within the earth’s crust builds up as they slip and collide with one another. One type of fault is called a normal fault, this happens when rock that is hanging over the slanted fault plane slips downwards, another fault is the reverse or thrust fault, this occurs when the rock pushes upward.  Vertical displacement happen when one side of the fault moves upward above the other side and can be created by both normal and reverse faults.  Finally, the strike-slip fault is a horizontal fault where the areas of rock slide past one another. These occur in areas where there is a transform plate boundary. The San Andreas fault in California is an example of a strike-slip fault.
When the rocks along a fault move suddenly, vibrations occur and waves send energy through the earth. The waves which create the most damage to the earth are called Sheering waves because they move the ground back and forth.
All of the rock in the path of a Primary (P) wave is compressed and expanded alternately.  P waves are also the fastest earthquake wave, in strong rocks, they can travel at speeds of up to four miles per secnd.  Once the P waves have begun radiating out from the focus of the rupturing fault, Secondary (S) waves follow. The S waves shear or twist the rocks by moving up and down or side to side, perpendicular to the direction the waves are traveling rather than compress it. S waves don’t travel as fast as P waves at their fastest only moving at about two miles per second.  The Love wave  is the fastest surface wave and moves the ground from side-to-side. Confined to the surface of the crust, Love waves produce entirely horizontal motion. 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.
The magnitude or size, how long an earthquake lasts, and how much shaking actually occurs are factors that determine how much damage an earthquake causes.  Large quakes can easily crack the ground, cause buildings to collapse and create tsunamis that can cause devastating damage.  The Ritcher scale, developed by Charles Ritcher, an American seismologist in 1925, was the first seismic magnitude scale widely used.  The height or amplitude of the surface waves are measured  by the Ritcher scale, which is a logarithmic scale, every successive unit of magnitude represents ten times the amount of the previous number because earthquake waves can cause ground displacement between less than a millimeter and several meters.  This was addressed by the use of the logmaritic recorded wave heights, so that a quake of magnitude three on the Ritcher scale is ten times more than magnitude two and one hundred times more than magnitude one.
Although the Ritcher scale is still used, seismologists now prefer to use a new magnitude scale called the moment magnitude scale.  The moment magnitude is calculated by multiplying the area the quake occurred in and the amount of displacement that occurred in the slip.  Seismologists believe that the moment magnitude scale is more reliable for measuring earthquakes, especially those that above a seven on other scales  because other scales only take into account part of the seismic waves, and the moment magnitude measures the total size of the earthquake.

In the last few years we have seen earthquakes occur which have created very destructive tsunamis.  In 2004 there was a 9.1 earthquake in the Indian Ocean, the largest strike-slip quake ever recorded,  which spawned a tsunami that killed over 230,000 people and in 2011 there was a 9.0 that triggered a 7 metre high tsunami which has created the worse nuclear disaster in history. Just recently scientists have discovered that two earthquakes measuring  8.6 and 8.3 magnitude close to the Sumatran subduction zone have caused the earth’s crust to break up along a new plate boundary.  With all these events happening staying alert to what is going on is imperative.

These are only some of the basic concepts that apply to earthquakes.  There is much more information available.  If interested more can be found at the following sites:

http://www.earthquake.usgs.gov/earthquakes/
http://www.ceri.memphis.edu/
http://www.emsc-csem.org/Earthquake/

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