EC project "Review of Historical Seismicity in Europe" (RHISE) 1989-1993

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Nicholas N. Ambraseys *
* Department of Civil Engineering, Imperial College of Science and Technology, London SW7 2BU, United Kingdom.

Material for the Investigation
of the Seismicity of Central Greece

The historical seismicity of central Greece is imperfectly known; its coverage is discontinuous and its record is grossly deficient. The period from 1800 onwards is relatively well-documented. However, further back in time it becomes increasingly difficult to find data, and the minimum size of the earthquake for which there is guaranteed detection increases.
Our studies show that the total number of important earthquakes in central Greece identified for the period between the 5th century BC and the 18th century AD amounts to just over eighty (Fig. 1). Seen by historical periods, we may identify a comparatively acceptable level of reported activity during the 5th century BC, literary sources providing information for twelve earthquakes. While it is certain that many small earthquakes must be missing from the record, we can reasonably assume that most of those of which details survived were relatively important events. It is also reasonable to assume that any major or damaging earthquakes in the vicinity of the larger urban centres of the time should be mentioned.
After the 5th century BC, however, the number of earthquakes identified gives way to a generally very low level in the Greco-Roman, Byzantine and Venetian periods. This decline has to be seen in terms of inferior reporting of events in sources lacking material of local origin rather than due to lack of seismic activity. After the 14th century the situation gradually improves, but our dependence on information from Venetian and Turkish sources gives the distribution of earthquakes a bias in favour of coastal areas, while data from places further inland continue to be almost totally lacking.
It is only after the 17th century that documents, such as consular correspondence, Turkish unpublished material, the diaries of European travellers and press reports, begin to preserve data that would otherwise have escaped notice in more general works and provide information about seismic activity. For this and earlier periods local, Greek sources are almost totally lacking. Fig. 2 shows the time distribution of the number of earthquakes per century in the region shown in Fig. 1.

Fig. 1  - Distribution of earthquakes of Ms>6.0 in central Greece (updated from Ambraseys and Jackson, 1990).

- 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

Fig. 2 - Time distribution of earthquakes in Greece for the period prior to 1750.
N is the number of earthquakes per century identified.

Thin bars show, for comparison, distribution of damaging earthquakes

in the Marmara Sea region (Ambraseys and Finkel, 1991).

Earthquakes in Athens
A typical example of the incomplete earthquake record of Greece is that for the city of Athens.
The historical record of Athens appears to have been almost free of destructive earthquakes. There is little in literary sources and no epigraphic material referring to earthquakes in Athens (Robert, 1978). Earthquake damage known to have occurred in the city during its 25-century long history has been very infrequent, small, and chiefly due to relatively large earthquakes originating either on land or offshore at some considerable distance from the city.

427 BC
The earliest known earthquake to have caused some concern in Athens occurred in the winter of 427 BC. During that period there were repeated shocks in Athens, but also in Evia and Boeotia, particularly at Orchomenos. The passage in Thucidides (Histories, iii, 87) that refers to these events clearly implies that these shocks originated some distance away from Athens, perhaps from the region just south of Atalanti, about 90 km from the city, where they caused no damage.

426 BC
Archaeological evidence suggests that an earthquake in 426 BC was responsible for the dislocation of the NE corner of the Parthenon, and for the displacement of about one third of the east facade of the temple by about 2.5 cm (Korres, 1985). The shock of 427 BC was apparently a precursor of the large magnitude earthquake that followed in the summer of 426 BC in the region between Atalanti and Scarpheia. However, Athens, at an epicentral distance of about 140 km, is not mentioned by Thucidides (Histories, III, 89), and by later writers such as Demetrius of Callatis in Strabo (Geography, I, 3, 20), and Diodorus (Library of History, XII, 59), who describe the destructive effects of this shock (see also Bousquet and Pechoux, 1981). An earthquake of this magnitude would certainly have been felt in Athens, but at such a large epicentral distance its effects should not have been serious. The silence of historical sources about damage to the Acropolis and to the city of Athens, as well as to other towns as far as Opus, contrasts strangely with the interpretation and dating of the damage of the Parthenon observed by Korres and Bouras (1983).
An earthquake sufficiently strong to cause damage to the solid structure of the Parthenon should have been more damaging or even destructive in the city of Athens, probably with casualties, and for this we have no evidence. It is possible that the Parthenon was damaged much later, possibly in the explosion of 1687, or alternatively, that its ruins were dislodged by earthquakes after the 18th century. Destructive earthquake effects in Athens clearly stretches the size of the 426 BC earthquake beyond the limits of the possible.

420 BC
The second and last earthquake of the classical period reported from Athens occurred in the summer of 420 BC (Thucidides, Histories, V, 45). The shock is described as slight by Plutarch (Lives, Nic., 271), presumably originating at some considerable distance from the city.

32 AD ca.
An earthquake in Athens is alluded to by Dionysius (Letters, 7, p. 303; p. 327) circa 32 AD. This was probably the result of a large earthquake in the Sea of Marmara, or more likely, a "supernatural" event associated with the Crucifixion.

2nd to 18th century
For the following 16 centuries we could find no information relating to earthquakes in Athens. During this period the city gradually ceased to be a centre of importance; it became remote and ultimately well removed from the great centres of literature and culture.

1705 September 3
Evidence for earthquake damage in Athens about the turn of the eighteenth century is found in a 4-page manuscript history of Athens (Ambraseys and Finkel, 1992; 1993). The shock caused considerable damage to various structures in the Acropolis as well in Athens, to buildings already weakened by the siege of 1687 and subsequently abandoned. Of the buildings affected by the earthquake in Athens, the church of St. Dionysius and the nearby residence of the Metropolitan, neither of which is extant, were located at the north foot of the rock of Areopagus. The cells of the monastery of Nikodimou must have been located in the vicinity of the modern Russian church. The location of the Vasiliki Ekklisia is not certain: perhaps it was located near the Stadium. There is no evidence that the earthquake caused any loss of life among the inhabitants and garrison of the Acropolis or serious damage in the town itself: for example, that the Cathedral of Athens must have survived the shock intact, since three days later it was safe enough to be used for congregation.
There is no reason to suppose, however, that other towns were not affected in this earthquake. It is possible that some of the damage reported in Turkish unpublished documents from Negreponte at about the same time could have been the result of the same earthquake.
Although some doubt must remain as to the actual date of the event, 3 September, the year of the earthquake can now be fixed in 1705, and the epicentre of the event located somewhere between Oropos and Plataeae, a location about 30 km distant from both Athens and Negreponte. Unfortunately, Venetian sources for this region become scarce after the Carlowitz treaty in 1699, and no information about this event so far has been found therein.
A shock reported from Zante in 1705, may well be a different event, belonging to the aftershock sequence of the earthquake of November 1704 in the Ionian islands.
However, the fact that as yet we have found no mention of an earthquake in 1705 in central Greece in the consular and Jesuit correspondence from towns in the Morea and neighbouring regions (Egina, Corinto, Napoli di Romania, Patrasso, Smyrna, Constantinople), as well as no mention in the European press of the time, implies that perhaps the shock that affected Athens in 1705 was not felt very far and did not cause any great concern in nearby towns - in other words, that the 1705 earthquake was not a large magnitude event.

1785 June 13
Another earthquake was felt in Athens on 13 June 1785 (Old Style) (Burnias, 1892), but it is very unlikely that it caused any damage; a number of travellers that passed through Athens shortly after that date do not mention any noticeable effects. However, we know that this earthquake originated most probably from the region of Oropos, 35 km N of Athens, where it caused considerable damage that extended to the fortress of Egribos (Chalkis), to Oropos and to farms in region of the Darbent of Sulesi.

19th century
With the advent of the 19th century, there is an obvious improvement in the volume and quality of data, which becomes more complete as we approch the 20th century. This is largely due to the availability of additional published sources of information. Archival material nevertheless continues to provide much useful data. Thus, the number of earthquakes identified in Greece for the 19th century amounts to over 1500. However, only very few caused concern in Athens, chiefly relatively large, distant earthquakes, the more important of which are the following.

1805 September (November) 17
On the night of 17 September a shock was felt in Athens as a result of which "some blocks of the western tympanum (of the Parthenon) were thrown down" (Dodwell, 1819). The context in which the event is recorded suggests the 17th, either of September or of November 1805. The shock caused no damage in Athens and it is not mentioned in other sources. Sieberg (1932) says that this earthquake was responsible for great damage in the town and this inaccurate information is repeated by later writers. This was a small, probably local shock that triggered the fall of pieces of marble from the Parthenon, loosened by the dismantling operations of 1801-1803.

1837 March 18
An earthquake in the southern part of the Saronic Gulf near Hydra, of a probable magnitude 6.4, caused great panic and some damage in Athens, 58 km away. In the Agora, blocks of marble from the gable-end of the tetrakionion were thrown down to the west (Schmidt, 1879).

1853 August 18
This earthquake had an estimated magnitude of 6.0, and affected the region of Thiva, about 53 km from Athens. The shock caused considerable panic in the city but no damage. We could fine no evidence for damage to historical monuments in Athens.

1874 January 17
In the morning there was an earthquake in Athens (pop. 65,000). According to Schmidt (1879), the shock caused the collapse of a part of the wall of the Acropolis built by Odysseus Androutsos in 1822. However, the press in Athens (Aeon, Ethnophylax, Alitheia, Eph. Syzitiseon) does not confirm Schmidt's information. Otherwise, the shock was not felt by the majority of the inhabitants of the city and it was not reported from other places. Galanopoulos (1956) says that this was a damaging shock, which is clearly an exaggeration. Apparently this was a small local earthquake and the collapse of the wall was due to its vulnerability rather than the severity of the earthquake.

1889 January 22
At 6h 15m an earthquake caused some damage to the monastery of Daphni, causing vertical cracks to open in the dome and walls of the church and in some of the cells (Galanopoulos, 1953).

1894 April 20
The earthquake of 20 April, had a magnitude 6.4, and originated in the region of Martino. In Athens (pop. 140,000), at an epicentral distance of 90 km, a few old houses were thrown down and several buildings were fissured. A block of marble fell from Adrian's Gate and the capital of an old column in the Agora was thrown down.

1894 April 27
A few days later, a larger earthquake of magnitude 6.9 occurred at Atalanti, 100 km NW of Athens. In the city the shock caused great panic and some minor damage, but no casualties. In the Acropolis a few existing cracks in the eastern pediment of the Parthenon were enlarged by the shock and small blocks of marble fell of the epistyle.

20th century
After 1900, the availability of documentary information continues to improve, supplemented by instrumental data. During this century few earthquakes pass unrecorded, generally only so far as minor events are concerned, only very few of them being of any consequence in Athens. Of the many hundreds of shocks felt in the city during this century, the following caused some concern or damage to historical monuments.

1914 October 17
This earthquake occured between Thiva and Chalkis and had a magnitude 6.2. In Athens, at an epicentral distance of 47 km, the shock caused panic; a small number of dilapidated houses collapsed and a few buildings suffered minor damage. There was no damage to the monuments on the Acropolis.

1928 April 22
This was a magnitude 6.3 earthquake near Corinth. In Athens, at an epicentral distance of 77 km, the shock caused some panic and minor damage to a small number of old houses. There is no evidence that it caused any concern to those involved at the time with the restoration of Acropolis monuments.

1930 April 17
This 5.9 magnitude shock occurred on the SW coast of the Saronic Gulf, 59 km from Athens. In the city, 4 old houses collapsed, and few were cracked. There is no indication that the shock caused any damage to historical monuments in the city.

1938 July 20
The earthquake in Oropos, 37 km north of Athens, had a magnitude 6.1. It was strongly felt in the city where it caused little panic, and insignificant damage. We could find no information about damage to historical monuments in Athens.

1965 July 6
This earthquake had a magnitude 6.4 and an offshore epicentre in the Gulf of Corinth, 122 km from Athens. It was generally felt in the city where it caused absolutely no damage.

1981 February 24
The main shock of the earthquake sequence in the Alkionides had a magnitude 6.7. In Athens (pop. 3,000,000) at an epicentral distance of 77 km, the main shock and its strong aftershocks ruined about 500 houses and caused widespread but minor damage to a number of public buildings. In archaeological museums a number of exhibits were broken and the Parthenon sustained minor damage; the SE, but chiefly the NE corner of the monument were displaced by a few centimetres and some of the joints were caused to open up (Zambas, 1985).
Five hours later, there was a strong aftershock of magnitude 6.4, originating from an epicentral distance of 60 km. This shock caused no further displacements and had no effect on the opening of the joints, (Korres and Bouras, 1983).

The general conclusion is that the long-term historical seismicity of central Greece for the period before the 18th century is very imperfectly known. The main factors seen to influence the completeness of data are the quality of the contemporary literary record, the prevailing historical circumstances, the geographical location of events and their magnitude. For this period, local sources seem to be the only fund of macroseismic information for the region, and in the absence of such sources our data is clearly incomplete.
Consequently, the apparent low earthquake hazard of a number of regions shown on modern seismic maps of central Greece, such as the Gulf of Argos, the Evrotas-Taygetos graben, northern Evia and Parnassos, may not be genuine, and it may reflect partly the lack of long-term observations. The only conclusion that we can draw from this re-evaluation is that there is no historical or instrumental evidence, so far, that any of the earthquakes identified on land have exceeded magnitude Ms 7.0 in size.

Our studies bring out a number of points that should be taken into consideration when historical events are used in earthquake hazard assessment.
The vulnerability of modern man-made structures differs in many respects from that of old ones. Historical monuments are sensitive to differential settlements and their lateral stability is almost exclusively dependant on frictional resistance. To judge the destructiveness of an historical earthquake from 20th century case histories can be seriously misleading.
We should not, as we consistently have done, presume that the orderly or disorderly distribution of masonry pieces always implies earthquake effects and presume that an earthquake, many tens or hundreds of miles from the site, however well attested in the sources, necessarily caused the damage.
Great care must be taken to establish the simultaneity of the destructive effects of early earthquakes at places located large distances apart. Quite often the amalgamation of the effects of two or more different historical events stretches the size of the earthquake beyond the limits of the possible, and leads to speculation and to the development of "catastrophe" theories.
The long-term seismicity of central Greece is clearly different from that of neighbouring Anatolia in that either (a) no earthquakes of magnitude greater than 7.0 occur, or (b) 500 years is not long enough to reveal such events in central Greece, whereas 100 years is more than adequate in Anatolia and in other parts of the Middle E