Anatolian tectonic plate, north anatolian fault, historical earthquakes Greece, Attica, Earthquakes, Tsunami, , Hurricanes, Volcanic Eruptions and other Natural and Man-Made Hazards and Disasters - by Dr. George Pararas Carayannis

Tsunami, Earthquakes, Hurricanes, Volcanic Eruptions and other Natural and Man-Made Hazards and Disasters

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The Earthquake of September 7, 1999 in Athens, Greece

George Pararas-Carayannis

Introduction

On Tuesday, 7 September 1999, a powerful earthquake struck nea Athens, Greece. This was the strongest earthquake to hit Athens in nearly a century and the worst to hit Greece in nearly 20 years. The quake killed dozens of people. Dozens more were trapped beneath the rubble of collapsed buildings. Hundreds more were injured and thousands more were left homeless. Preliminary assessment shows that 672 homes were destroyed beyond repair while 2,217 more were in need of repair. The greatest damage to buildings, occurred in the Athens suburbs of Menidi, Ano Liosia, Nea Filadelphia, Nea Ionia, Kifissia and Zefyri. There was no apparent damage to the Acropolis, to the Temple of Zeus, and other antiquities in the area.

 

Earthquake Epicenter, Origin Time, Magnitude and Aftershocks

The earthquake's epicenter (38.13N 23.55E) was approximately 20 kilometers northwest of Athens, between Menidi and Mount Parnes, which is a national park and sparsely inhabited. According to the Athens Seismological Institute, the earthquake's magnitude was 5.9 on the Richter scale and it occurred at 23:07:41. It was felt across the Aegean Sea and as far away as the city of Izmir, Turkey, about 180 miles east of Athens.

In Athens and surrounding suburbs, the quake was felt with great intensity. This is attributed to the orientation of the fault that produced it, with the city; also because the depth of the quake's focus was relatively shallow (10 km). In the few days following the main earthquake more than 700 aftershocks were recorded, including eight with magnitude over 4, and one that registered 4.7.

 

 

 

 

Tectonic Setting - Geological Instability of the Eastern Mediterranean Basin

Modified After Nafi Toksoz of MIT/ERL

The eastern Mediterranean basin where the Anatolian plate extends, is one of the world's most intense seismic zones where large destructive earthquakes occur with frequency. The geological instability and the resulting earthquakes are caused by the active grinding of the Anatolian plate wedged against the continental plates of Africa, Eurasia and Arabia. As these larger tectonic plates grind against the Anatolian plate, Asia Minor and Greece move, smashing closer together in some places and drawing apart in other regions. The 500-mile North Anatolian fault marks the northern boundary of the Anatolian plate grinding against the Eurasian plate. This great fault zone appears to be moving eastward at the present time. At its western end it impacts on the North Aegean fault, a volcano-dotted undersea rift that runs down the middle of the Aegean Sea. The fault zone then meanders under Greece and heads up the Ionian and Adriatic Seas. As a result of active interaction and movement along these major faults, hundreds of earthquakes of all sizes are recorded every year throughout this region. Since 1964, more than 20,000 quakes have been recorded around Greece alone (see U.S.G.S map below (modified)).

 

 

 

 

The Seismicity of Central Greece

The long-term seismicity of central Greece is clearly different from that of neighboring Anatolia to the east, in that no earthquakes of magnitude greater than 7.0 occur - or at least nothing that large is known to have occurred in the last 500 years. In contrast, earthquakes with magnitude greater than 7 have occurred with greater frequency in Anatolia and in other parts of the Middle East in the last 100 years.

In recent times, in 1968, a magnitude 7.1 quake on the North Aegean fault near the island of Aghios Efstratios, killed 20 people. Up to now, the deadliest earthquake in Greece occurred in 1978 in the northern port of Thessaloniki. It had a magnitude of 6.5 and resulted in 45 deaths.

( Seismicity of Greece - Epicenter of the September 7, 1999 earthquake- Modified USGS map)

 

Was the earthquake of September 7, 1999 in Greece a consequence of the August 17 earthquake in Turkey? Is there a relationship with the Turkish earthquake ?

It appears that following the Turkish earthquake that struck north-western Turkey on August 17, 1999, there has been an increase in seismic activity in the entire eastern Mediterranean Sea and particularly in the Northern Aegean Sea where the North Anatolian Fault meets the Northern Aegean fault. Therefore, the Turkish earthquake appears to have an effect over a wider area than it would be ordinarily expected. After the quake in Turkey, it is very possible that the North Anatolian fault activated other faults in the Greek region. The two recent earthquakes in the sea region between the islands of Imvros and Samothraki,in the Northern Aegean Sea, are suggestive of such direct influence. However the earthquake in Central Greece occurred on a different and distant fault. Therefore there is no clear or apparent direct connection with the Turkish quake. The closeness in their timing may be just a statistical coincidence.

Nonetheless, it should be emphasized that the dynamics of the tectonics of the Anatolian plate are not well known or understood. Strong earthquakes, as the one that occurred in Turkey, appear to have a tendency to migrate and to stimulate a wider zone, resulting in subsequent crustal compensation along other areas of accumulated seismic strain. Therefore, an increased amount of seismic activity in one area of the Anatolian plate could be expected to affect other areas of the same plate. In the short term, the potential risk from earthquakes in this region must not be discounted. In the short term, additional seismic activity can be expected in the eastern Mediterranean basin .

Has the seismic strain been released by the September 7, 1999 Earthquake in the Athens Region? Is there danger of another large earthquake in the area in the near future?

Following the Athens earthquake of September 7, 1999, the strength of the aftershocks has been steadily decreasing. This reduction in the magnitude of the aftershocks indicates that most of the seismic strain along the fault, which gave rise to the main quake, has been released. There is no apparent danger of another destructive earthquake in the near future. However, smaller aftershocks should be expected to continue for some time.


Past earthquakes which have affected the City of Athens

Although numerous earthquakes have occurred in central Greece throughout history, most have been of medium size, relatively shallow in depth, destructive only in the immediate vicinity, and have resulted in relatively small losses of life. Historically, the city of Athens has not been struck very frequently or has been extensively damaged by past earthquakes. The September 7, 1999, earthquake was an exception in that the epicenter was much closer to the city than previous events.

According to Nicholas Ambraseys of the Imperial College of London, earthquake damage in the city of Athens during its long history has been very infrequent, small, and primarily caused from relatively large earthquakes with epicenters considerably distant from the city. According to Ambrassey's historical earthquake catalog, the strongest earthquake in the Athens area this century, occurred on October 17, 1914. It occurred between Thiva and Chalkis and had a magnitude 6.2. on the Richter scale and an epicenter approximately 47 Kilometers from Athens. This earthquake destroyed a small number of older houses, but caused relatively minor damage to a few buildings.

(Modified figure from Ambraseys showing the epicenters of important historical earthquakes (Ms 6.0 +) in Greece for the period between the 5th century BC and the 19th century. - Solid circles are for events after 1890, and their epicentral regions are shaded. - Solid squares are approximate epicentres of events of the 19th century. - Open squares are locations of events before 1800. - Pre-1890 events are labeled with the year of their occurrence: those that may have been smaller than Ms 6.0 are labeled with a bar. Dashed lines show major tectonic structures.)

Also, the historical record shows that, in addition to the 1914 event, damage was caused by earthquakes in the Athens area in 1705, 1894, 1928, 1930, 1981 and, more recently, in 1995. These quakes had epicenters which varied in distance from 40 to approximately 180 km from the city of Athens.

 

The April 22, 1928, earthquake had a magnitude 6.3. Its epicenter was near Corinth approximately 77 km, from Athens. It resulted in some minor damage to a small number of older houses.

The April 17,1930 earthquake was a 5.9 magnitude event which occurred on the SW coast of Gulf of Saronicos, approximately 59 km from Athens. Only 4 old houses collapsed in the city and few more had cracks.

The July 20, 1938 earthquake in Oropos, about 37 km north of Athens, had a magnitude 6.1. It was strongly felt in the city where it caused minor damage.

The July 6, 1965 earthquake had a magnitude 6.4 with its epicenter offshore in the Gulf of Corinth, about 122 km from Athens. It was generally felt in the city but it caused no damage.

Of all recent earthquakes, the quake of February 24, 1981, with magnitude 6.7 and its epicenter in the Alkionides near the port of Corinth, approximately 77 km southwest of Athens, was the most destructive in terms of loss of life and damage to property. It killed 20 people. Strong aftershocks followed, one of them five hours later with a magnitude 6.4 and epicentral distance of 60 km from Athens. The main shock and the aftershocks damaged about 500 houses and caused widespread but minor damage to a number of public buildings, including museums exhibiting archaeological antiquities.

Lastly, a 6.1 magnitude earthquake occurred in 1995 in Aegio, approximately 120 miles southwest of Athens. It was responsible for 26 deaths in the region.

 

Did the Earthquake of September 7, 1999 generate a tsunami?

No, it did not. Earthquakes in Central Greece, with the exception of those occurring near the Corinthian Gulf, are not known to produce tsunamis. However, this is not always the case elsewhere in Greece. The historical record shows that numerous large destructive earthquakes and tsunamis have occurred from antiquity to the present in the Eastern Mediterranean Sea and particularly in the Aegean Archipelago.

 

(Sparta earthquake of 413 B.C.)

 

 

 

References

Ambraseys N. , Earthquakes in Athens, Imperial College, London
Ambraseys, N. and Jackson, J., 1990.
Seismicity and associated strain of central Greece between 1890 and 1988. Geophys. Journ. Intern., 101: 663-708.

Angelier J., Lyberis N., Le Pichon X., Barrier E. and Huchon P. (1982). The tectonic development of the Hellenic arc and the Sea of Crete : a synthesis. Tectonophysics, 86, 159-196.

Angelier J. (1979). Recent quaternary tectonics in the Hellenic Arc: examples of geological observations on land. Tectonophysics, 52, 267-275.


Galanopoulos, A., 1953.
Katalog der Erdbeben in Griechenland fur die Zeit von 1879 bis 1892. Ann. Geol. Pays Helleniques, 5, Athens.
Galanopoulos, A., 1956.
I seismiki epikindynotis ton Athinon. Praktika Akadem. Athenon, Athens.

Links to other Web Sites for More Information on the September 7, 1999 Earthquake and the Potential for Destructive Tsunamis in Greece

National Observatory of Athens (NOA)

National Observatory of Athens, Institute of Geodynamics

European-Mediterranean Seismological Centre

Seismo-Surfing the Internet (Steve Mallone) ETH, Zurich

Tsunami Potential in Greece.html

 

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