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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.
On March 18, 1953 the major Yenice-Gonen earthquake strikes West Turkey,
and 250 people die. On the Ionian islands it destroyed most buildings
on Zakynthos and the nearby island of Cephalonia, with 7,2 on Richter's
scale. More than 476 people were killed. It is said that this earthquake
affected Crete also, with for instance some damage to the water cisterns
in Loutro and some roofs collapsing.
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The worst earthquakes
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Earthquakes that caused terrible loss of life in the 20 + 21th
century
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| Deaths |
Magnitude |
Country |
Year
|
| 240,000 |
7.8 to 8.2 |
Tangshan, China |
1976 |
| 200,000 |
8.3 |
Xining, China |
1927 |
| 150,000 (est) |
8.6 |
Gansu, China |
1920 |
| 143,000 |
8.3 |
Yokohama, Japan
|
1923 |
| 70,000 |
7.7 |
Northern Peru |
1970 |
| 50,000 +? |
7.9 |
Gujarat, India |
2001 |
| 50,000 |
7.3 to 7.7 |
Northwest Iran |
1990 |
| 41,000 |
6.3 |
Southeast Iran, Bam |
2003 |
| 33,000 |
7.9 |
Erzincan province,
Turkey |
1939 |
| 28,000 |
8.3 |
Chillan, Chile |
1939 |
| 25,000 |
6.9 |
Turkish-USSR border |
1988 |
| 25,000 |
7.7 |
Northeast Iran |
1978 |
| 22,778 |
7.5 |
Guatemala |
1976 |
| 20,000 |
8.6 |
Santiago/Valparaiso, Chile |
1906 |
| 19,000 |
8.6 |
Kangra, India |
1905 |
| 17,000 |
7.2 |
Turkey |
1999 |
| 12,000 |
8.1 |
Central Asia |
1907 |
| 10,700 |
8.4 |
India/Nepal |
1934 |
| 9,500 |
8.1 |
Michoacan, Mexico |
1985 |
| 6,000+ |
7.2 |
Kobe, Japan |
1995 |
| 4-5,000 |
9.5 |
Chile |
1960 |
| 5,000 |
6.1 |
Northeast Afghanistan |
1998 |
| 2,990 |
8.9 |
Sanriku, Japan |
1933 |
| 2,500 |
7.1 |
Varto, Turkey |
1966 |
| 2,300 |
6.7 |
Turkey |
1975 |
| 1,530 |
8.7 |
India/Assam/Tibet
|
1950 |
| 1,500 |
7.1 |
Northern Iran |
1997 |
| 1,330 |
8.4 |
Tonankai, Japan |
1946 |
| 1,300 |
6.9 |
Turkey |
1983 |
| 1,300 |
6.0 |
Turkey |
1946 |
| 1,171 |
6 |
Western Colombia |
1999 |
| 1,100 |
7.3 |
Gediz, Turkey |
1970 |
| 1,000 |
8.9 |
Colombia |
1906 |
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Source: Associated Press, Reuters, MSNBC
research, Sfakia-Crete.com |
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 [1994], 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.
Earthquake
Severity |
This
is logarithmic scale, meaning that an increase of one unit
- for example, from 3.5 to 4.5 - represents an 10-fold increase
in the amplitude of the seismic waves and approximately a 30-fold
increase in the energy released. |
Richter
Magnitudes |
Earthquake
Effects |
Less than
3.5 |
Generally
not felt, but recorded |
3.5-5.4 |
Often felt,
but rarely causes damage |
Under 6.0 |
At most slight
damage to well-designed buildings. Can cause major damage to poorly
constructed buildings over small regions |
6.1-6.9 |
Can be destructive
in areas up to about 100 kilometers across where people live |
7.0-7.9 |
Major earthquake.
Can cause serious damage over larger areas |
8
or greater |
Great
earthquake. Can cause serious damage in areas several hundred
kilometers across |
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 ".
At the time of the first Thera Conference [Athens 1971] excavation had
clearly documented a widespread 'destruction horizon' on Cretan sites
datable to about the middle of the 15th century B.C. The date was deduced
from the pottery found in the burned and flattened buildings, pottery
which includes fine specimens of the style known as Late Minoan IB.
Prosperous towns like Mochlos, Pseira, and Gournia were wrecked and
abandoned. The palaces at Mallia, Phaestos, and Kato Zakro fell, and
were never rebuilt. The era of the 'villas' - opulent mansions serving
as district administrative centres - came to an abrupt end. The great
palace at Knossos survived the disaster, but suffered damage, and was
rebuilt and redecorated in a rather different style.
To read the 'full
story', click here.
Let us look at the
geological map:
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