Real Time Seismicity in Greece for the last 24 hours (Automatic Alerts): click here
Since many people ask us about the chance of having an earthquake on Crete, we here will try to explain the geological situation. The questions came after the earthquakes in Turkey and Athens/ Greece, August and September and November 1999.
First of all we
like to make clear that in the past decades there have been NO earthquakes
on Crete that have caused serious damage. The last big earthquake in
1953 nevertheless caused major damage across Greece, and triggered big
social changes due to emigration following loss of life and property
etc. It is one reason why, in some areas, there are no old houses left.
Largest earthquakes by magnitude
But earthquakes do happen sometimes on Crete."As been mentioned Crete is an 'active' area with small earthquakes every year. The last significant earthquake was about 5 years ago , 6,3R as I remember. I was standing on my patio at the time and suddenly instead of standing on concrete I felt that I had jumped onto a bed. I had water in bowls (for the cats) and this started sloshing from side to side, spilling out of the bowls. A little later I visited an office on the 5th floor of a block in Chania where there was a free standing iron decorative stand against a wall and I could see marks on the wall where it had swayed from side to side about 10cm or so. However, there were no reports of severe structural damage, just odd bricks and tiles in the road from the tops of old buildings, and a few old garden walls down. I remember at the time feeling that there was less damage than happened a few years previously in England when there was a hurricane.
One must remember that the Richter scale is based upon energy released and this is not always a good indication of 'effect'. An structural engineer friend said that the Turkish eathquake was very long in duration as these things go although the R value was not too high. The earthquake in Athens was only 5.9 but it was very fast moving which exagerated its impact.
The building regulations in Greece are very strict with respect to Earthquake resistance: Accoring to my friend amongst the most stringent in the world. To obtain planning permission you must supply a complete structural 'static' analysis of the foundations and skeleton to show that it will withstand earthquakes. All factors are specified in the plans, the sizes of columns etc, the steel and Grade of concrete are all specified. The problem is that the building must be constructed according to the design and with good building practices. For instance the owner, or the builder may decide to save money by perhaps using less steel or by using a lower strength grade of cement, or may not excavate enough to ensure solid foundations. The builder may add water to the specified mix of concrete to make it easier to work, he may not place the steel correctly within the concrete formwork, he may not 'cure' the cement properly (keeping it wet during the setting time, perhaps 5 days). These things should be checked by the engineer responsible for the building, but ... I believe that in Athens the Architects/Engineers and builders responsible for the construction of the buildings that collapsed are currently being sought. Perhaps we shall learn more.
Of course quite a number of people decide to build without obtaining planning permission, but that's another story. Another problem, particularly with commercial buildings, is that the owners may decide to modify them when business need change, perhaps without consulting an Engineer to question the impact of the changes. With respect to the steel often seen protruding from the tops of buildings, as I think someone else has said, the Greeks are very family oritented, when they build they do often plan for further stories for their children eventually. If they can afford it they build beforehand and rent-out the upper floors, always intending their children to eventually live there. Well built old houses often resist earthquakes but in a completely different way than new buildings. Problems arise when they are not properly maintained or are 'renovated' in a 'modern' way using large quantities of concrete which tends to work against their intrinsic resistance modes."
Most of them aren't even noticed, some are strong, but since they happen far below sea level, no damage is done to the surface, apart from maybe an old building partially collapsing. Since some decades the Greek government has issued severe earthquake building regulations, which are served everywhere. All buildings should be made out of armoured concrete, to survive a 'normal' earthquake of 7 on Richter's scale.
Many people on Crete have worked hard to save money for building a house or hotel etc. and it is in everybodies interest to follow these regulations.
To look in the historical library of Earthquakes, click here.
The biggest earthquake on Crete was the explosion of the volcano on Thera (Santorini) in the middle of the 15th century B.C.:
The Thera blast - twice that of Krakatau and 40 times that of Mount
St. Helens-destroyed the island's civilization, and through tidal waves,
destroyed Minoan cities on surrounding shores, especially on Crete.
Beneath five distinct layers of pumice and ash lies evidence of this
cultural and spiritual center of Minoan culture: pastures then cultivated
with crops and orchards, each field divided by rock walls and shelters;
uncultivated uplands with wild animals; gentle streams and valleys incised
into soft volcanic rock; multistoried buildings with internal plumbing;
and country villas scattered across the landscape." - Dr. Floyd McCoy,
in Ground Truth, Earthwatch Research Report ".
To read the 'full story', click here.
Let us look at the geological map:
The map is almost self-explanatory. South of the Islet of Gavdos, following the South coast of Crete, is 2400 meters deep in the Libyan Sea the break line or trench of Ptolemeus.
We can't remember the source of this image
There are three types of plate boundaries on the surface of the earth: divergent, convergent and transform. Divergent boundaries are two plates which are moving away from each other leaving room for material from the mantle to seep into the space and form new sea floor. Convergent boundaries are plates which are moving towards each other causing one plate to submerge beneath the other. And transform boundaries are two plates which are sliding past each other without much disturbance.
South of Crete are two zones with convergent boundaries. The most near, the Ptolemeus trench, is an oceanic-continental convergence: this occurs when the denser oceanic Libyan Sea plate subducts beneath the less dense continental (Cretan) plate creating this trench. Once the subducting oceanic crust reaches a depth of about 100 kilometers, melting of the crust and the overlying mantle begins forming a magma. Some of this magma is pushed to the surface resulting in volcanic eruptions (Thera/ Santorini). In fact the south coast of Crete is pushed up by the Libyan Sea plate. Although VERY slowly, one can see clearly nowadays the old sea level, being about 150 cm above current water level!
South coast of Crete near Sougia