BOOK OF SCIENCE NOTES
GEOLOGY
PART G - SEISMOLOGY (The Study of Earthquakes)
1. Earthquakes
(a) Causes
(b) Breaking and Bending of Rocks
(c) New Words
(i) Focus
(ii) Epicentre
(iii) Isoseismal Lines
(d) Occurrence of Earthquakes
(e) Tsunamis
 
2. Seismic Waves
(a) Types of Waves
(b) P Waves
(c) S Waves
(d) L Waves
(e) Shadow Zone
 
3. Detection of Earthquake Waves
  		4. Measurement of the Intensity of Earthquakes
(a) Modified Mercalli Scale
(b) The Richter Scale
 
5. Damage by Earthquakes
(a) Types of Damage
(b) Faults
 
6. Structure of Earth
(a) The Study of P and S Waves
(b) The Crust
(c) The Mantle
(d) The Core
(e) Some Measurements of the layers
(f) Some Other Measurements about Earth
 
7. The Plate Theory
 
1. Earthquakes
(a) Causes
Rocks will bend, stretch or compact, when under strain. However, there is a limit to how much strain the rock will take, depending on the type of rock. and its depth under the surface.
When the limit is exceeded, the rock will break, and move along a plane of weakness. At the same time, their potential energy of strain or distortion, will be released suddenly. This sudden release of energy produces shock waves, which are transmitted throughout the planet in several ways - called P. S, and L waves. The effect of these shock waves becomes less with distance from the origin, because the whole released energy is radiated in all directions, and is gradually transformed to heat energy.
Such releases of energy happen thousands of time every year, on Earth, but most of the shocks can only be detected by very sensitive Instruments. Occasionally, there is a large release of energy - large enough for one of the waves to be detected by simple observation. Shock waves which cause little or no damage, are called tremors, while those which cause considerable damage are called earthquakes.
The fact that these releases of energy are so frequent in so many parts of the world, Indicate that mountain building processes are in progress at the present time.
(b) Breaking and Bending of Rocks
When rocks fracture and move, the resulting arrangement of rock strata, is called a fault. (See PART F for fuller details)
Distorted rocks can remain distorted, without actually breaking - they stay bent, and we say that the rock strata is folded.
(c) New Words
(i) Focus The focus of an earthquake (large or small) is the location in the crust where the rock displacement occurs. Most such displacements occur at considerable depths under Earth's surface - mostly between 8 and 35 kilometres. We use the phrase "depth of origin".
(ii) Epicentre The epicentre of an earthquake (large or small) is the position on Earth's surface immediately above the focus of that earthquake.
(iii) Isoseismal Lines Lines can be drawn on a map, around an epicentre, joining places at which the intensity of the shock waves, if the same. These lines are not circles - the Intensity decreases with distance from the epicentre, but does so unevenly, depending on the rocks through which the shock waves are travelling. Such a map is called an isosiesmal map.
(d) Occurrence of Earthquakes
Earthquakes are more likely to occur in certain parts of the world. These parts are called earthquake zones. A world map showing these earthquake zones, is interestingly similar to a world map showing volcanic zones.
Clearly, there is an association between earthquakes and volcanoes. Earthquakes (rarely major ones) are associated with volcanoes, but most earthquakes (especially major ones) have no direct connection with volcanoes, even though they may happen in volcanic regions.
(e) Tsunamis
When the focus of a major earthquake is under the ocean, the ocean floor is greatly disrupted, and this causes the movement of a lot of water. This movement is popularly called "a tidal wave", but such a name is completely wrong - it has nothing to do with tides. The correct name is a tsunami.
It is a wave in the water of very long wavelength (perhaps up to 100 kilometres), considerable displacement of water from its normal level, i.e. considerable amplitude (perhaps to 30 metres), and travelling at speeds such as 100 kilometres per hour. Such tsunamis travel for thousands of kilometres across oceans, but they decrease in size as they travel away from the epicentre of the earthquake.
If you were on a ship in the open ocean, you would not notice this tsunami - the ship would slowly and easily ride over it. However the displacement of such huge amounts of water can have disasterous effects when the ocean water is restricted in its movement by the presence of land.
2. Seismic Waves
(a) Types of Waves
Earthquakes cause three types of vibrations, and these vibrations travel away from the focus of the earthquake in all directions.
(i) P Waves, also known as Primary waves, or Push/Pull waves.
(ii) S Waves, also known and Secondary waves, or Shear waves.
(iii) L Waves, also known as Long waves, or surface waves.
(b) P Waves
These are compression waves, which travel in straight lines in any one material, but are refracted and reflected by changes in the material in which they travel. They travel in all materials, and travel faster than the other two types of earthquake waves (between 8 & 13 kilometres per second). They are the least destructive of earthquake waves, but they do travel very long distances.
(c) S Waves
These are transverse waves, which also travel in straight lines in any one solid material, but are refracted and reflected by changes in the solid material in which they travel. They cannot travel in liquids, and in solids they travel at a speed which is between the speeds of the other waves (between 3 & 8 kilometres per second). They also do little or no damage, but they do travel very long distances through solid rock.
(d) L Waves
These are also transverse waves which are set up when P and S waves strike a sharp boundary between materials, especially at Earth's surface. Hence they occur only in the shallow depths of Earth's crust. They are the slowest of the three types of earthquake wave (between 3 & 4 kilometres per second). They are more violent waves and do most of the
damage. Fortunately, they do not travel the long distances travelled by P & S waves.
(e) Shadow Zone
S waves do not get to the opposite sides of the planet, from a particular earthquake. They cut out suddenly at 105°C from the epicentre. P waves also cut out here. but suddenly reappear at 143° from the epicentre.
Thus for any particular earthquake, there is a shadow zone around Earth when no waves arrive. It is "in shadow" relative to that earthquake. Such measurements enable fairly exact measurements to be made about the layered structure of Earth - see para. 6, below.
3. Detection of Earthquake Waves
When a heavy mass is suspended freely, it will not shake when its surroundings shake, and so the amount of shaking of the ground, can be marked.
Such instruments are called seismographs or seismometers and the record they make of the earthquake wave is in the form of zig-zag marks on graph paper. Such a record is called a seismogram.
When a series of earthquake waves is received from a particular earthquake, at three different seismic measuring stations, the location and focus of the earthquake can be determined.
4. Measurement of the Intensity of Earthquakes
(a) Modified Mercalli Scale
This is a series of stages, from 1 to 12, describing what is observed at those stages. It Is a measure of the effects of an earthquake, not so much of the intensity of the earthquake, although the two are connected.
(b) The Richter Scale
This is a mathematical logarithmic scale which indicates the amount of energy released at the focus. This logarithmic scale has a base of about 250. so an earthquake which registers 7 on the Richter Scale liberates 250 times more energy than one which registers 6 on this scale.
Realise that this scale does not Indicate any effects of the earthquake - this depends on how far away the epicentre may be.
5. Damage by Earthquakes
(a) Types of Damage
Even major earthquakes do little damage in open and natural country - there is little there to damage. However, where there are man-made structures, very much damage can be done to such structures.
The commonest damage is fallen buildings, and this also Includes broken gas mains and broken electrical cables, and also broken dam walls.
The earthquake itself causes few fatalities amongst people, but the damage caused by the earthquake - falling buildings, fire (gas & electrically caused), and flooding waters - can cause a large number of fatalities amongst people.
It is the L waves which do most of the damage, and fortunately they do not travel far from the epicentre. P and S waves, even from tremors, can travel through to the other side of the planet (with the proviso that S waves cannot travel through liquid).
(b) Faults
A fault is a visible result of an earthquake, and a fault can change the shape of the land. Where rocks have broken in a fault, the rock strata is then weak along the line of the fault, and further movement is likely along this line of weakness. The total displacement of many movements along such a line can be many hundreds of metres. See PART F for more details.
6. Structure of Earth
(a) The Study of P and S Waves
An examination of the speeds of P and S waves from earthquakes all over the world, show that this planet has a layered structure. Careful study of the times of arrival at a seismic station, and their strengths on arrival, indicates the amount of reflection and refraction at boundaries between layers, and from this information, the location of the marked changes in the material of Earth, can be determined.
(b) The Crust
This is part of Earth with which we are most familiar, especially the uppermost parts of the crust. It can be as thin as 5 kilometres under the deeper parts of oceans, and up to 35 kilometres thick under some of the continents.
It consists of two minor layers, called the sial and the sima. The sial makes up most of the continents, dominating elements being Si and Al. The sima extends completely around the planet, being under both continents and oceans, the dominating elements being Si and Mg.
The junction between the crust and the mantle is known as the Mohorovicic Discontinuity (moho, for short), named after the scientist who discovered it in 1909.
(c) The Mantle
This is more solid than liquid, but the top parts do flow extremely slowly - the speed of growth of your finger nails is much faster. There Is a vague boundary at the depth of 1000 kilometres, so some scientists refer to an upper mantle and a lower mantle.
(d) The Core
This must be liquid because S waves will not travel through it. There are internal boundaries at depths of 4800 and 5100 kilometres, so indicating that the core can be divided into upper, middle and inner cores. There is reason to suggest that the inner core may be solid.
(e) Some Measurements of the layers
Layer 
Average Thickness 
Volume of Earth 
Mass of Earth 
Crust  
30km  
2%  
1%  
Mantle  
2900km  
82%  
67%  
Core  
3440km  
16%  
32%  

(f) Some Other Measurements about Earth
Total Volume 1.08 x 1021 cubic metres
Total Mass 5.98 x 1024 kilograms
Average radius 6,370 kilometres
Total Surface Area 8 x l06 hectares (land plus sea)
Average Density 5.5gms/cc

Note: The average density of rocks of Earth's crust is 2.5 gms/cc, so the mantle and core must have a density of about 5.6 gms/cc. It is believed that the mantle is very rich in iron and the core is even richer in iron.
7. The Plate Theory
Earth's crust is made up of six main plates, which are moving about the surface of Earth being dragged along by the slow-moving currents in the upper mantle - see para. 6(c) above.
Strain at the edges of these plates, caused by collisions and scraping, account for most earthquakes and zones of mountain building, e.g. in New Zealand, in Peru, the Himalayas, etc.
The discovery of these plates and their movement has resulted in a tremendous development in geological knowledge in very recent years. See PART E for fuller details.

(c) 1987 R.H.Smith