Cross section of the Earth
The earth is composed of a number of layers with different properties, see the figure below. Roughly, the earth can be divided into three layers: the core, the mantle and the crust. The core is the inner part of the earth and can be divided into the solid inner core and the liquid outer core. Similarly, the mantle can be divided into the upper and the lower mantle. The crust is the outermost part of earth. It is just 10-80 km thick and as such resembles in proportion the peel of an apple. The crust is composed of a number of plates, called continental plates, that are moving relative to each other. Most earthquakes occur as a result of these plate motions.
Cross section of the Earth
The map shows the earth at night. Densely populated areas are visible due to the enhanced light intensity. The green lines show plate boundaries where we have the largest earthquake activity. Earthquakes are shown as orange dots, volcanoes as red triangles. Future catastrophes will occur in places where earthquakes strike in densely populated areas.
Stresses, which are built up in rocks due to for example plate motions, are released when the rocks can no longer withstand the stress. The result is a sudden displacement along a plane called a fault. This results in an earthquake, which is a sudden release of energy moving through the crust as seismic waves. The waves are felt as ground shaking at the surface.
A fault
The focus (hypocenter) of an earthquake is the point on the fault plane where the rupture initiates. The epicentre is the point at the surface directly above the focus. The fault plane is defined from its direction at the surface, its slope and the direction of the slip. The slip is unevenly distributed depending on the roughness of the fault plane. Most of the energy is released in areas where the fault plane is rough. Such areas are called asperities.
Fault plane
There are three main types of plate boundaries, depending on whether the plates are moving away from each other (divergent plate boundaries), towards each other (convergent plate boundaries) or sliding past each other (transcurrent plate boundaries).
Different kinds of plate boundaries
There are three main types of faulting: normal, reverse and strike-slip. A combination of two of these results in oblique-slip faulting, which is quite common. The fault types are dependent upon tectonic stresses in rocks, which are mainly controlled by the regional stress field in connection with the plate motions. In the following, the three types of faulting are described and illustrated.
Normal faulting occurs in connection with an earthquake where rocks on both sides of the fault plane move away from each other, and one of the blocks is displaced downwards in relation to the other. On divergent plate boundaries, two plates move away from each other and new material rises from the mantle. Earthquakes will usually occur on normal faults, and volcanoes are often seen in connection with divergent plate boundaries. One example of a divergent plate boundary is the mid-Atlantic ridge, where e.g. Iceland is affected by earthquakes and volcanoes.
Normal fault
The picture shows a normal fault in connection with a destructive earthquake in November 1999, close to the city of D�zce, Turkey.
Reverse fault ruptures occur in connection with earthquakes where the rocks are pressed against each other and one of the blocks is displaced upwards along the fault plane. On convergent plate boundaries, two plates collide. One plate moves down underneath the other and is pushed down into the mantle where it melts and dissolves. Earthquakes occurring in these regions usually have reverse mechanisms. Examples of convergent plate boundaries can be found in Alaska, Himalaya, Japan, Taiwan and western South America. It is along such so-called �subduction zones� the largest proportion of the earthquake activity occur globally
Reverse fault
The picture shows a reverse fault rupture in connection with the destructive Chi Chi earthquake in Taiwan in 1999. The small picture shows close-up picture. Strike-slip faulting occurs in connection with an earthquake where the rocks on both sides of the fault plane are displaced in an opposite horizontal direction. On a transcurrent plate boundary, plates are sliding past each other. This results in strike-slip faults. Examples of transcurrent plate boundaries are seen in California and in Turkey.
Strike-slip fault
The picture shows a strike-slip fault rupture, which displaces the railway line.
The pictures are taken in connection with the earthquake in Izmit, Turkey, on August 17, 1999.
There are four main types of seismic waves. P waves (primary waves) have particle motion in the same direction as the wave propagates. S waves (secondary waves) have particle motion at a right angle to the direction of propagation. P and S waves are also called body waves because they propagate though the earth�s interior. Surface waves (Love and Rayleigh waves), on the other hand, propagate only along the surface of earth. Love waves (named after A.E.H. Love (1863-1940)) have a particle motion at a right angle to the direction of propagation. Rayleigh waves (named after Lord Rayleigh (1842-1919)) have a retrograde particle motion, meaning that the particle motion is circular, opposite the direction of propagation.
Different types of seismic waves
The traditional way to measure the magnitude of an earthquake is by using the Richter scale. The Richter magnitude is based on the amplitude of ground motion as it is registered on seismographs, and the distance to the earthquake. Charles Richter introduced the Richter scale in California in1935, where he assigned a magnitude of 3 to an earthquake at 100 km's distance, causing 1 mm amplitude of the ground motion as recorded on his special equipment (Wood-Anderson seismograph).
The magnitude is determined from the maximum amplitude as seen on the seismogram. In order to calculate the magnitude, the amplitude is corrected for the distance, as shown on the figure.
Magnitude scales like the Richter scale are logarithmic, which means that one unit increase corresponds to a 10 times increase in ground motion and ca. 32 times increase in the energy released by the earthquake. The Richter scale is still used by seismologists because of its popularity. However, seismologists today prefer to use another magnitude based on seismic moment. Seismic moment is determined from the size of the fault plane, the amount of slip, and the roughness of the fault plane.
| 2 | Rarely felt by humans |
| 2.5 | Energy similar to a moderate lightning bolt |
| 3.5 | Energy similar to a strong lightning bolt |
| 4 | Felt by humans, possibilities of damage |
| 5 | Energy similar to an average tornado |
| 6 | Energy similar to the Hiroshima atomic bomb |
| 7 | Damaging earthquake, often causing loss of life. Capable of creating a tsunami. |
| 8 | Energy similar to the world�s largest nuclear explosion |
| 9 | Catastrophic earthquake causing great damage |
The table shows how often earthquakes with different magnitudes occur around the world on average.
| Description | Magnitude | Average number pr year |
|---|---|---|
| Catastrophic | 8 and higher | 1 |
| Very strong | 7-7.9 | 18 |
| Strong | 6-6.9 | 120 |
| Moderate | 5-5.9 | 800 |
| Weak | 4-4.9 | 6200 |
| Small | 3-3.9 | 49000 |
| Very small | Less than 3 | Magnitude 2-3: ca 365000 Magnitude 1-2: ca 3000000 |
Earthquake activity in Norway and surrounding offshore areas is connected to the geological structures. The plate boundary on the mid-Atlantic ridge is one of the most important elements. In addition, earthquakes occur along graben structures (normal fault systems) in the North Sea and along the continental margin. On land, the largest activity occurs in Sunnhordland and Nordland.
Earthquake epicenters 1980-2004
With the exception of the arctic areas around the mid-Atlantic ridge, Norway is situated far from plate boundaries, and one may ask why there are earthquakes here. The answer is that stresses build up in the crust due to other mechanisms. There are mainly four mechanisms causing stress build-up in the area around Norway. �Ridge-push� is associated with the divergent plate boundary in the North Atlantic and is considered an important source of regional stresses along the Norwegian coast and on land. Secondly, the continental margin plays an important role in the processes of stress build-up. Melting of ice and the following uplift after the latest ice age causes vertical stresses along the coast. Finally, vertical forces due to sediment loading on the sea bottom add to the stresses in the region.
Mechanism for stress build-up around the Norwegian Sea
On October 23rd, 1904, an earthquake in the Oslofjord made the front page of Aftenposten. The earthquake struck Oslo in the middle of church-time and had a magnitude of 5.4. The earthquake caused panic in several places, in addition to significant damage to buildings.
Bildet viser et avisutklipp fra 24. oktober 1904.
This map shows earthquake hazard in Europe prepared by the European Seismological Commission. The strong red colours indicate increased hazard of strong shaking caused by future earthquakes. In Norway and surrounding areas, the risk of strong shaking is relatively small. The largest risk of strong earthquakes in Europe is in the Mediterranean area in countries like Greece, Turkey, Italy and Spain. These countries are situated close to plate boundaries and have large fault systems capable of generating large earthquakes.
High earthquake risk is a function of high earthquake hazard combined with high vulnerability. In other words, earthquake risk becomes higher when the location of epicentres for large earthquakes coincides with densely populated areas. One such recent example is in Turkey, where large destructive earthquakes frequently occur. On August 17th, 1999, a large earthquake occurred in Izmit in Turkey. The earthquake caused severe damage along the 150 km long fault and resulted in 19 000 casualties. Following this earthquake there is now an increased earthquake hazard in the Marmara Sea where a future large earthquake is expected to have catastrophic consequences in Istanbul, a city of almost 12 million inhabitants. In order to be as well prepared as possible for a future earthquake, scientists are working on the possible ground motion caused by such an earthquake. The calculations are based on assumptions of the magnitude (M=7.5) and the properties of the crust and the fault.
The pictures show the buildings that collapsed in connection with the catastrophic earthquake in Izmit, Turkey in 1999. Much of the damage was the result of bad construction practices.
Tsunami is a Japanese expression, which means �harbour wave�. This is a large wave generated by sudden changes in the sea bottom due to for example an earthquake or a landslide. The change initiates the motion of enormous water masses. At the open sea, these waves can reach velocities of 800 km/h, but they are not felt by ships due to their long wavelength and shallow height. The water usually pulls back before the waves reach land. The velocity decreases and the waves increase dramatically in height, but the waves are still relatively long when they hit land, and they are therefore capable of reaching far inland. When the waves pull back they drag along all loose material towards the sea. Usually two-three waves reach the coast in this way with several minutes inbetween.
How a tsunami is generated
On December 26th, 2004, a catastrophic earthquake (M=9) occurred northwest of Sumatra in Indonesia. The epicentre of the earthquake was below the sea and a large tsunami was generated, causing severe damage in large areas around the Indian Ocean.
This picture, which is taken from the satellite Jason 1 (NOAA), shows the tsunami wave two hours after the earthquake. The colours show changes in the sea surface level, making wave propagation visible.
Tsunami warning has proven to be a useful tool in avoiding catastrophes. A sensor measuring the water pressure is placed on the sea bottom and sends a signal to a buoy, which transmits the signal to land via satellite. Changes in water pressure indicate a passing tsunami wave. An essential requirement to generate a tsunami is that a large earthquake occurs on the sea bottom. This is registered at seismic stations on land, determining location and magnitude. A tsunami warning system then confirms if a tsunami wave is travelling towards land. Such a tsunami warning system is installed in the Pacific Ocean.
The figure shows schematically the different
components of a tsunami warning system based on
a
sea-bottom sensor and satellite communication. Such a
system is installed in the Pacific Ocean.
The figure shows schematically the different components of a tsunami warning system based on a sea-bottom sensor and satellite communication. Such a system is installed in the Pacific Ocean.
The pictures show the buoy, which enables satellite communication.