The Why? of earthquakes

The layers of the earth's crust

The ground, which is below our feet is the uppermost part of the earth's crust. This crust is 10 kilometres thick under the sea, and 40 kilometres thick under the continents. Below the earth's crust you can find stones with variable hardness, thickness and state of matter, the so-called mantle. Below 2900 kilometres is the external core of the Earth, then the inner core.

Reasons for earthquakes

Earthquakes classified by in the manner of genesis

  • 85% - Structural trembles (Pangea).
  • These quakes rise by the resolving of elastic tensions in the form of elastic energy, after the crust's breaking. These quakes only burst out in crust, which is in solid state. These are the most devastating.
  • 7% - Quakes in connection with volcanic activity (A piece of the energy of erupting volcanos forming into elastic waves).
  • 3% - Sinking in - collapsing quakes (Collapsing magma chambers - implosion, caving in of cavern). Artifical trembles (nuclear, mine and other explosions).
  • 5% - Disturbance of earth:
    • Shaking of machines, trembles made by vehicles;
    • Vibration by atmospherical occurrences.

The eruption of earthquakes is influenced by inner tension condition and the external factors, which cause burden change: the erosion, climate change - e.g. the thaw of mass ice -, artificially increased tension, e.g.: shaping of water depots, changing of air pressure, attraction of celestial bodies etc.

Movement types of earth's crust

Different parts of the earth's crust are approaching, departing, pressing together, breaking, getting crushed by the effects of tectonic forces. These movements are accomplished very slowly and chang the Earth's surface. Probably 200 million years ago dry land was one contiguous solid mass, which was surrounded by one confluent ocean. The continents of today were got separated from this solid mass (The Pangea).

The major lithosphere platforms

The platforms which are between 70 kilometres and 150 kilometres structurally include the massive crust and the upper part of the mantle. (The deeper part of the mantle till approximately 700-750 kilometres depth is the asthenosphere). These two structural member are mechanically unified, they move together. The biggers "carrying" the oceans and the continents - according to that possible to be oceanic, terrestial (continental) and "complex" platforms. They are moving continuously and/or episodically according to each another and to Earth's rotation axis.

The movement appears in the "swimming" of continents (continental drift), in the expanding of seating of ocean (rent-valley-system), otherwise in the genesis of oceanic crust and in the annihilation of it in the regions of oceanic graben. On the basis of laws of the sphere geometric the shifting of platforms on the Earth's surface is rotation, which is clearly describable with the angle of rotation and with the pole of rotation. It is clear that grand geological occurrences of today (earthquakes, volcanism, changing of levels etc.) are also significantly attached to the edges of plates.

Expanding of the ocean's seating
Expanding of the ocean's seating

Matter of mantle (magma), which presses throught the rift of massive crust laterally boil to the edge of receding lithospherical plates, and fills the void in, which arises because of the dilatation. These crust edges are growing (divergens) borders. The process is episodical, which is followed by periodical volcanic and seismic incidents. The earthquakes rise in 0-30 kilometres depth, because massive crust is thin. The mid-ocean ridges and the rent-valley-systems (the common borderline with the growing plate-edge) are the scenes of genesis of the oceanic crusts and the oceans. The opening of Africa along the East-African fracture-system is part of a new ocean's genesis. As a result of this, the separated block (East Africa) will be a new continent in the hollow of Indian Ocean, in one million years time.



The shoving of an oceanic plate under a continental plate
Diving (Subduction)

In the course of plates collision the old, refrigerated, rigid oceanic plate dives under the thicker continental plate, with this it originates the deepest-seated regions of the Earth, the oceanic grabens. The medium and deep centred earthquakes are rising in diving, mechanically rigid oceanic plates in the depth of 700-750 kilometres. The so-called Benioff zones are miscellaneous tilting planes, which can be traced out by the centres of the quakes. Deeper down, the matter of the crust dissolves into the matter of the mantle. The friction heat, enlargement gaps etc., which are induced in the course of this process lead to active volcanic activity. These island-bend structures, which are described by treble consist of deep-sea-trough, active volcanic chains and seismic activity, the scenes of the particular destruction of the earth's crust and the origin of the continental crust (e.g. the island-bends of the Pacific - the Cirkum-Pacific area).



The collision of continental (mainland) plates /Mountain formation/
The collision of continental plates

Durable subduction is doesn't form, because the thickness difference is larger between the continental crust and the mantle. The mountain-chains are form in the shocking zones by the turning up of the stone-masses. As a result of the bearer plate of India and the Eurasian plate's collosion, the enormous masses of Himalaya's mountains have been formed.



The moving of the fracture systems
Fracture systems

Mechanical tensions are accumulate in the plates in the course of the motions. The energy of these is partially discharged through the earthquakes and appears in the breaking of the matter of the crust. These refractional zones can be many thousands of kilometres in lenght, the parts, which are separated by the breakings, could shift - compared to each other - several times through each earthquake (seismic reflection).
The most illustrious is the Californian St. Andreas Fault System. Through the earthquake in 1906, they registered 6 metres displacement along the fault line, which was devastated San Francisco.



The plate tectonics

The global- or plate tectonics makes an attempt at unified explanation of the large structural occurrences, which form the earth's crustal - formation of mountain ranges, the oceanic graben, formation of volcanic and seismic zones, the movement of parts of the world etc. -. Although some basic ideas of hypothesis under discussion (e.g.: Expectation of Africa's and South America's late concatenation by the similar shape of coasts.) date back to the last century, the development of this theory was in the sixties. The correctness of plate tectonics' suppositions and conclusions since then was verified by thousands of proofs, firstly in connection with the consequences. There are only unexplained questions referring to producing and preserver forces of enormous proportioned movements.

In plate tectonics' opinion the Earth's surface is divisible in to six big (first-class platforms) and approximately twenty smaller, so-called second-class lithosperical-platforms at the moment.

The plates of the Earth (Click to enlarge)


The anatomy of the earthquakes

Explanation in the glossary Explanation in the glossary Faults in the world Waves caused by earthquakes The structure of earthquakes

Most earthquakes is occur along the contact line of two stone plates, the fault. Because of the crustal plates' dislocation the stones deform, and then when the increased tension suddenly evolves they are broken. Where - deeply under the surface of the earth - the stones break, there is the hypocentre of the quake. The effect of evolving force shock waves, so-called seismic waves start from this point, which propagate in the interior of the earth and along its surface. The waves shake the earth's surface. The quake is the strongest in the epicentre, which is the point of the surface, which is exactly above the hypocentre.



Places of earthquakes

The dispersion of earthquakes is not equable on the Earth's surface, decisive majority of them are occurring in properly confinable narrow zones. Generally, we distinguish three primary seismic zones. The first, most important is around the Pacific Ocean, so-called Circum-Pacific zone, where approximately 70 percent of earthquakes arise. The second as it is called the Alp-Himalaya zone, which is connected to the Circum-Pacific zone at Indonesia from the Mediterranean Sea's area, through Turkey, Central Asia and China. In this zone 15 percent of world's earthquakes arise. The third zone is procreated by the mid-ocean ridge, which ones are responsible for 7 percent of the earthquakes. The remaining approximately 8 percent could virtually occur anywhere, but each of the big, destructive quakes could in connection with the above-mentioned three zones.

The earthquakes' geographical division like this, is explained by the plate tectonics' theory. According to that, the enormous tectonic plates, which cover the Earth are moving compared to each other at a speed of a few centrimetre, yearly. This way, the cause of earthquakes is, that the accumulated tension is releasing in consequence of the movement along their border. Each one of the mentioned main seismic zones coincide with the borders of tectonic plates. Smaller accumulation of tension also could be within every single stone plates, for this reason medium or smaller quakes could arise farther from the plates' border too.

Seismic zones

You can observe, when the radio or the papers give account about larger earthquakes, there are systematically returning places. Devastating quakes often occur for example: in China, Chile, Peru, Guatemala, Japan, Italy, ex-Yugoslavia, Romania and along Indonesian islands.

We can get interesting picture about the geographical division of the quakes, if we determine every significant earthquake's epicentre for a long time and we represent these on a map. It is noticable, that the most of the earthquakes are occurring along narrow zones, which zones form a continuous network all around the world; with this they circumscribe extensive seismically calm areas. The network of seismically active zones are connected to such typical superficial shapes, like the deep-sea-trough, the mid-ocean ridges, the big, young mountain chains, or the areas of active volcanic activities. According to the observations, there are four sort of seismic zones, on the basis of partially the occurring quakes' mechanism and partially the superficial shapes in connection with them.

The first type of the earthquakes are come forward along very narrow zones and in most cases combined with active basalt-volcanism. These quakes have exclusively small focal depth, which ones have not more than a few 10 kilometres focal depth. These kinds of earthquakes mainly occur along the ridge-line of the huge risings below the oceans - the so-called oceanic ridges -, in the line of the East African network of trenches, or for example on Iceland.

The second main type of the earthquakes similarly show up along narrow zones, and these are also quakes with small focal depth, but active volcanic activity can't be experienced at all on these areas and in contrary to the previous type, only the overloading of the shearing tension is causing the quakes.
Very good examples of this type are the St. Andreas Fault in California and the Antalonian Fault in North Turkey. There is measurable surface movement at both of them in parallel with the fault line. These kinds of quakes are also occurring along the transform faults, which are slashing the ridge-lines of oceanic ridges.

The third type of the seismic zones is closely connected to the deep-sea-troughs' area; generally island-arches with active volcanic activity are connected to these (mainly on the east coast of Pacific Ocean). In these areas, quakes with all levels of focal depth are occur and the big-sized quakes are frequent. If we picture the quakes with the depth of medium and deep focal depth, we will observe, that quakes only occur deeper than 100 kilometres at deep-sea-troughs' areas and at single parts of the Alp-Himalayan zone. It is interesting to note, that the hypocentres of the quakes are in thin regular plate-alike zones, which are stooping below the continents at the deep-sea-troughs, namely they are settling along the Benioff zones.

The fourth group of the earthquakes are made of quakes, which expand from the west part of North-Africa through the Mediterranean Sea to China. Here the epicentres take place dispersedly in wide zones, the big part of the quakes are occurring in small depth, but rarely quakes with medium and deep focal depth are also occurring. In these regions, often occur extremely large-sized earthquakes causing enormous devastations, for example the recently occurred quakes in Algeria, Italy, ex-Yugoslavia, Romania, Iran and China.

Faults


St. Andreas Fault (Click for picture) The outline of the Californian St. Andreas Fault (Click to enlarge) /Click to 'St. Andreas Fault' to get a picture of it/

Californian faults

California lies at the border of two enormous plate, on the Pacific and on the North American. These shifting plates formed the fault network on the area of the state (red lines) in the earth's crust. The most famous is the St. Andreas Fault with its 1100 kilometres length. San Francisco established along this fault.




The meeting plates of Japan

Faults of Kobe

The earthquakes are frequent in Japan, because the country lies at the meeting of four enormous tectonic plate. From these plates, the Pacific plate moves the fastest, with the speed of 10 cm, yearly. It is diving under the Eurasian plate. The earthquake of Kobe arose, when the Eurasian plate wasn't able to resist the huge accumulating tension, which arose by the Pacific plate's pushing. The hypocentre of the earthquake was at southeast of 20-30 kilometres from Kobe, along the Nojuma-fault line.




The "timed bomb" of Los Angeles

The view of Los Angeles

Nobody knew the Puente Hills Fault - which is a seismic fracture below Los Angeles in the depth of 3-15 kilometres - till 1999, when an American seismologist from South California examined it and suddenly realized with dismay, that what kind of dangers are in it.
Generally the geologists are digging a trench across the fault line, to examine, how far it is shifted in the course of the earthquake. But the Puente Hills is a fake fault. The formations like this never get to the surface, so it is very hard to track their history. However the geologists eluded this in the case of Puente Hills and they examined the sediment layers for significant foldings produced by preceding earthquakes. The results opened the door to explore the seismical history of this fault line below Los Angeles. This analysis showed, that the fault line caused 4 catastrophic earthquake in the past 11 000 year, which ones' strength were stronger than the 7th division of Richter scale. The time of the most recent was 2000 - 3000 year ago. The good news is, that the devastating earthquakes are occuring only in a couple of thousand years. On the other hand, the bad news is, that those are infernally huge earthquakes.

The focal-mechanism of earthquakes

Segment of focal sphere

The definitive part of the earthquakes has tectonic origin, namely the triggering cause of the occuring of quakes is the accumulating mechanical tension in the earth's crust. As a result of tension the effect of appearing forces is, that the stone block is brake, or along the already existing fault, the adjoining two plate shifting in relation to each other. The focal mechanism of the earthquake shows, that what kind of process passed by in the hypocentre at the quake, how is the fault plane took its position, what was the direction of the shifting and also could conclude to the main attributes of the tenseness area, which caused the quake.


On the upper-left illustration you can see the segment of the focal sphere (a fictive sphere around the hypocentre). On the A lower-right illustration you can see the covered distance of the wave in the interior of the earth from the focus to the spectator.


The way of the wave

There are several methods of the focal-mechanism solution. The most known, most popular method is uses the direction of the P-waves' first deviation for the solving. On the earthquake measuring stations, the initial direction of the ground movement caused by the P-waves depends on the engaged location of the station according to the fault plane. If a fair amount of station were succeeded with the measuring of the direction of the P-waves' first deviation, then at the best it is possible to conclude: the dominating directions of the tension in the hypocentre, the position of the fault plate as well as the occurred direction of dislocation along the plane.

See also in the section named "Focal-mechanisms of the Hungarian earthquakes", demonstrated with an example.


The levels of focal depth

According to the focal depth, the earthquakes are enumerated to 3 groups of focus:

  • shallow - (0-30 kilometres) /along mid-oceanic ridges/
  • medium - (30-300 kilometres) /in the environment of high mountains, at deep-sea-troughs and below the island arches/
  • deep (300-750 kilometres).
The elastic energies significantly discharge in shallow-focused quakes, because the viscosity and the occurrence of solid state is decreasing towards the Earth's depth. The surface-near - originating in a few kilometres depth - quakes are rare too, because the shearing strength of this zone's stones (also because the frequency of sedimentary rocks) are smaller than deeper.

The times of the earthquakes

The occurrences of the earthquakes around the world

According to the observations, the occuring of the earthquakes is not completely random, but it shows a certain regularity. The decisive part of the earthquakes is concentrated to same thin strip-like regions of the Earth. These seismically active zones are surrounded by enormous calm (so-called aseismic) territories. Although, according to the experinces, there are smaller (M<6 Richter-sized) quakes, which could happen anywhere and anytime on the Earth, but the frequency of these are greatly little in the aseismic countries.

The commonness of the earthquakes

According to the inquiry of the earthquakes' temporal occurences, the various sized quakes do not happen with the same frequency. The table is summarizes, how many quakes rise yearly of the different quakes on the Earth. It is noticeable, that the small-sized quakes are rising by hundreds daily, but they are only give the unimportant part of the total energy. The yearly discharging 1018J total energy's significant part is evolving along one or two larger earthquake.

Energy
[J]
No. of quakes
per year
Total energy
[J]
1017 - 101811018
1015 - 1017101017
1013 - 10151001016
1011 - 101310001015
109 - 1011100001014
107 - 1091000001013

"Slow earthquakes" (Northwest America)

When two crustal plates slip over each other, or one of them slips into the other, tensions are accumulate along the junction line. If these tensions suddenly dissolve in the breaking of stones, earthquakes rise. With the help of the GPS place-locating system, the scientists were recently discovered an other kind of tension dissolving. It happens, that the accumulated tensions "softly" dissolve, namely not with sudden breaking, but with some easy to shape deformation. At this time, there is not any "jerking", namely no earthquake is rising, even the seismographs do not get the sense of it. It can be only detected by the help of the most accurate localization system, the GPS, if the gently moving crustal plates are suddenly start moving faster. The seismologists named this manifestation slow earthquake.
In the diving zone, which formed the American Cascade-mountains, the subsequent analysis of the measurement data shows, that in the latter ten years, since the GPS measurements are going on, these slow earthquakes are periodically rising, and one started on 7th February 2002. The seismologists are examining the connections between these slow earthquakes and the devastating earthquakes.