Earthquakes

The Interior of the Earth
The earth is divided into 3 main layers, namely the crust, the mantle and the core. Although the core and mantle may be about equal in thickness, the core actually forms only 15 percent of the Earth's volume, whereas the mantle occupies 84 percent. The crust makes up the remaining 1 percent.

Image from http://oceansjsu.com/105d/exped_commotion/5.html

Theory of Plate Tectonics
Plate is a large, rigid slab of lithospheric rock. Tectonics comes from the Greek word "to build". Together, “Plate Tectonics” refers to how the Earth's surface is built of plates. The theory of plate tectonics states that the Earth's outermost layer is fragmented into a dozen or more large and small plates (lithosphere) that are moving relative to one another as they ride atop hot, yielding material (asthenosphere).

The theory explains the how and why behind mountains, volcanoes, and earthquakes, as well as how, long ago, similar animals could have lived at the same time on what are now widely separated continents.

Earth's lithosphere presently is divided into eight large plates with about two dozen smaller ones that are drifting above the mantle at the rate of 5 to 10 centimeters per year. The eight large plates are the African, Antarctic, Eurasian, Indian-Australian, Nazca, North American, Pacific, and South American plates. A few of the smaller plates are the Anatolian, Arabian, Caribbean, Cocos, Philippine, and Somali plates.

Left: Map of tectonic plates. Right: Location of earthquake epicenters 1963-1998. The location of earthquakes coincides remarkably with the plate boundaries, providing evidence supporting the theory of plate tectonics. (Images from USGS.)

Why do Earthquakes occur?
The plate’s boundaries where plates meet or collide are marked by concentrations of earthquakes and volcanoes. Usually one plate will bend beneath another plate at the boundary. As the lithospheric plates are generally solid and rigid, tremendous stress, strain and tension build up in the areas around the boundary of the plates as the plates bend, producing stress and strain cracks on the plates. These are known as fault zones. When stress and strain on certain parts of the plates exceed the threshold that can be sustained by the elasticity of the rocks in that area, rupture occurs, releasing strain energy which is called an earthquake. This energy is transmitted through the plates in the form of seismic waves, heat and sound. The amount of energy released will determine the magnitude or strength of the earthquake.

Although earthquakes generally occur in areas around and near the plate boundaries, intra-plate earthquakes are possible as well. Faults do develop within the plate itself.

Epicentre of an Earthquake

An epicentre is the point on the Earth's surface directly above the hypocentre of an earthquake. The hypocentre (focus) is the calculated location of the point within the Earth where the earthquake originates. An earthquake is a rupture that occurs along the length of a fault and is not easily quantified into a point.

from http://openlearn.open.ac.uk/file.php

Foreshock, Mainshock and Aftershock
Foreshock and aftershock are relative terms. Foreshocks are earthquakes which precede larger earthquakes in the same location. Aftershocks are smaller earthquakes which occur in the same general area following a larger event or "mainshock". Generally, aftershocks represent minor readjustments along the fault that has ruptured during the main shock. The frequency of these aftershocks gradually decreases with time.

Study of Earthquakes

Seismology is the study of earthquakes and seismic waves that move through and around the earth. A seismometer is an instrument that records the shaking of the earth's surface caused by seismic waves. The first such instrument was invented in 132 A.D. by the Chinese astronomer and mathematician Chang Heng (picture on the right).  Modern instruments use electronics to accurately measure earthquake signals. One such instrument is the Force Balance Accelerometer. In a Force Balance Accelerometer, an inertial mass is held nearly motionless relative to a frame by an electronic negative feedback loop. The motion of the inertial mass relative to the frame is measured, and the feedback loop applies a magnetic or electrostatic force to keep the mass as motionless as possible. The voltage needed to produce this force is the output of the seismometer and is recorded digitally. Professional seismic observatories usually have instruments measuring three axes: north-south, east-west, and up-down.

Chang Heng invented the first seismoscope called the dragon jar

Types of Seismic Waves
Seismic waves are the waves of energy caused by the sudden rupture of rock within the earth or an explosion.

There are several types of seismic waves. The two main types of waves are body waves and surface waves. Body waves can travel through the earth's interior, but surface waves radiate along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves.

Body Waves
Body waves arrive at a seismic station after the occurrence of an earthquake before the surface waves and are of higher frequencies. Body waves are the P and S waves and these have periods of around 0.01 seconds to 50 seconds.  The P wave or primary wave is the fastest kind of seismic wave, and, is the first to 'arrive' at a seismic station. The P wave can move through solid rock and fluids. It pushes and pulls the rock it moves through and hence is also known as compressional wave. Subjected to a P wave, particles move in the same direction that the wave is moving in. An S wave or secondary wave is usually 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--perpendicular to the direction that the wave is traveling in.

Surface Waves
Travelling only through the crust, surface waves are of a lower frequency than body waves. Surface waves such as the Love and Rayleigh waves have periods of around 10 to 350 seconds. It is the surface waves that are almost entirely responsible for the damage and destruction associated with earthquakes. This damage and the strength of the surface waves are reduced in deeper earthquakes. A Love wave moves the ground from side-to-side produce only horizontal motion. A Rayleigh wave rolls along the ground and 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 Rayleigh waves, which can be much larger than the other waves especially in the vicinity of the earthquake.

Magnitude of Earthquakes
There are different types of magnitude scales used to measure the magnitude of an earthquake. Each scale measures different characteristics of the seismic waves.

Richter Scale (Ml)
In most earthquake media reports, the value of the different magnitude scales is confused with the Richter magnitude scale. The Richter scale was invented by Charles F. Richter in 1934. The Richter magnitude is calculated from the amplitude of the largest seismic wave recorded for the earthquake, no matter what type of wave was the strongest. Although Richter originally proposed this way of measuring an earthquake's magnitude, the magnitude is obtained from the measurement of the peak amplitude of ground motion from a Wood-Anderson seismometer and the difference of arrival times between the P and S waves. The Richter scale (Ml) has a period of measurement of 0.1s and 1.0s. It can only be used to measure earthquakes less than 500km away. That is why the Richter scale is sometimes called the Local Scale.

There are now other magnitude scales available for seismic studies:

Body-wave Magnitude (Mb)
This scale uses the first 5 seconds of arrival P-waves. It measures waves within the frequency range of the P and S waves (body waves).

Surface-wave Magnitude (Ms)
This scale is based on maximum amplitude Rayleigh wave (from 20s period). This frequency range is within the surface wave range of the seismic waves.

Moment Magnitude (Mw)
Seismologists have more recently developed a standard magnitude scale that is completely independent of the type of instrument. It is called the moment magnitude, and it comes from the seismic moment, Mo. Moment Magnitude is not based on instrumental recordings of a quake, but describes something physical about an earthquake. Seismic moment depends on the rigidity of the rocks (or the resistance of the rocks to shearing), the area over which the rupture has taken place and the slip distance. Seismic moment is measured in dyne-centimetres, but can be conveniently converted into a magnitude scale, Mw.

Moment Magnitude is also a measure of total energy released by an earthquake. Of these scales, Mw is most widely used in the scientific seismology community to determine the magnitude of an earthquake.

In general, the following applies to all magnitude scales:

	Magnitude Scale Effect on areas close to the epicentre 1.0 - 4.0 (Weak earthquake) Minor or no damage 5.0-6.0 ( Moderate earthquake) Moderate to severe damage 6.0 and above (Strong earthquake) Severe to widespread severe destruction

Magnitudes Provided by Seismic Monitoring May Differ
Magnitude computations may differ amongst the different seismic monitoring centres due to the various factors, such as the different algorithm used for computations, the different sensors used for detection, the different magnitude scales adopted by the different centres etc. The magnitude can be revised a few times within a few hours of the earthquake event, upon availability of data from seismic stations. Re-computation of earthquake magnitude is commonly practiced by other seismic centres.

The Earthquake Intensity Scale
Seismologists use a separate method to estimate the effects of an earthquake, called its intensity. Intensity should not be confused with magnitude. Although each earthquake has a single magnitude value, its effects will vary from place to place, and there will be many different intensity estimates.

A seismic intensity scale measures or rates the effects of an earthquake at different sites and is commonly used to describe the severity of earthquake effects.

Some things that affect the amount of damage that occurs are:

• Structural design- Depends on local building and construction codes and regulations.
  
  
• Distance from the earthquake- Seismic waves attenuate rapidly with distance, so the energy of earthquakes also attenuate rapidly with distance.
  
  
• Type of surface material (rock or soft soil) the buildings rest on- Ground motion is affected by the nature of the rocks and formation. Solid rock usually shakes less than soft soil.

• I-III: Slight ground tremors - tremors are generally not perceptible
  
  
• IV-VI: tremors felt by many, but with minor structural damages
  
  
• VII-VIII: moderate structural damages
  
  
• Greater than IX is related to major damages and may cause widespread destruction
  
  

	The Modified Mercalli Intensity Scale Scale Description of Effects I Usually not felt. II Felt by few people especially those living on the upper floors of buildings. III Felt noticeably by people indoors especially those living on the upper floors of building. Some people can also feel a bit disorientated by slight swaying, especially those living on high floors. Some furniture slightly disturbed. IV Felt indoors by many, outdoors by few during the day. At night, some awakened. Furniture, windows, doors, hanging lights disturbed. V Felt by many people both indoors and outdoors. Some unstable objects may fall off. VI Felt by all. Furniture significantly disturbed. Minor damage. VII Minor to moderate damage to buildings. VIII Moderate damages to buildings especially those of poor construction. Furniture overturned. IX Moderate to major damage to buildings and structures. X Major to severe damage to buildings and structures. XI Widespread severe destruction. XII Total destruction. No building and structure is left standing.

Myths about Earthquakes
“Are we experiencing more earthquakes?”
Although it may seem that we are having more earthquakes, earthquakes of magnitude 7.0 or greater have remained fairly constant over time. In the last twenty years, there is an increase in the number of earthquakes we have been able to detect each year. One contributing factor is the tremendous increase in the number of seismograph stations in the world – between 1931 and today, the number of stations increased from about 350 to more than 8000. The enhanced monitoring capabilites meant that we are now able to locate earthquakes more rapidly and even locate the many small earthquakes which were undetected in earlier years.

Improvements in global communications and the increased interest in the environment and natural disasters meant that the public now learns about more earthquakes, not just more frequently, but also more rapidly. 

“Can we forecast earthquake?”
We are not able to forecast an earthquake - the exact time and place where an earthquake will occur. However through historical and past data, seismic agencies around the world are able to compile statistical estimates and probabilities of an earthquake of certain magnitude occurring within a period of time over a certain fault or area. These forecasts are very general, covering a long period and a large area.

The challenges in forecasting are numerous. The most important of which is our limited understanding and knowledge of the Earth. Till today, exactly what drives plate tectonics is not known. The earth's surface and interior are highly dynamic, plates and plate boundaries change gradually with each day with each new earthquake, redistributing strain and stress, creating and destroying faults, etc. We continue to face problems with sparse data available to seismologists and geologists. Despite an increasing number of seismic monitoring stations and better communications, the number of observation stations remains insufficient. In addition, the advent of modern seismometer after the 1880s limited the temporal span of available data for analysis. All these limit our ability to forecast.

“Can we prevent earthquakes?”
We are unable to prevent an earthquake. It is a force of nature and the power released is incredibly large. To illustrate, we can compare the energy released by explosive TNT and the energy released by a moderate earthquake. A moderate earthquake of magnitude 5.5 can produce energy of about 1020 ergs. The same amount of energy can only be matched by detonating 80,000 tonnes of TNT. That is a lot of energy. (Information from http://www.seismo.unr.edu/ftp/pub/louie/class/100/magnitude.html)

This begs the question of whether we can trigger enough smaller earthquakes to prevent a large event. It would take 32 magnitude 5 earthquakes, 1000 magnitude 4 earthquakes, 32,000 magnitude 3 earthquakes to equal the energy released by a single magnitude 6 event. Although seismic monitoring networks always record many more small events than large ones, there are never enough to eliminate the occurrences of the occasional large earthquake.

To artificially trigger earthquakes when our understanding of the dynamics of the earth is so limited may prove to be a dangerous pursuit, especially when many high seismicity zones are so near populated areas – one might trigger a damaging earthquake.
Although we cannot prevent an earthquake, we can take mitigation actions to reduce loss of life and property damage.

1. Design buildings that conform to the seismic zoning of the area
   
   
2. Public education on what to do during an earthquake

Impact of Earthquakes on Singapore
Singapore is located about 400 km from the nearest known earthquake source. To date, there have been no records of earthquakes occurring in Singapore but weak tremors are occasionally felt from distant earthquakes in Sumatra. The tremors are weak and are felt by a few people living on high floors over some areas in Singapore.

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