  | |
| Local date | 9 July 551 (0551-07-09) |
|---|---|
| Magnitude | 7.5Mw |
| Epicenter | 33°54′N35°30′E / 33.9°N 35.5°E /33.9; 35.5 |
| Areas affected | Phoenice Libanensis,Byzantine Empire (nowLebanon) |
| Max. intensity | MMI X (Extreme) |
| Tsunami | Yes |
| Casualties | Above 30,000 |
The551 Beirut earthquake occurred on 9 July with an estimated magnitude of about 7.5 on themoment magnitude scale and a maximum felt intensity of X (Extreme) on theMercalli intensity scale. It triggered a devastatingtsunami which affected the coastal towns of ByzantinePhoenicia, causing great destruction and sinking many ships. Overall large numbers of people were reported killed, with one estimate of 30,000 by theanonymous pilgrim of Piacenza forBerytus (modernBeirut) alone.[1]
What is nowLebanon lies astride theDead Sea Transform, which forms part of the boundary between theArabian plate and theAfrican plate. In Lebanon the fault zone forms a restraining bend associated with a right stepping offset of the fault trace.Transpressional deformation associated with this bend has formed a number ofthrust faults, such as the recently identifiedMount Lebanon thrust, which underlies the city and is interpreted to crop out at the seabed offshore.[2]
There is little in the way of detailed descriptions of the damage caused by this earthquake in contemporary accounts. Sources refer to the coastal cities from Tyre to Tripoli being reduced to ruins with many thousands of casualties.[1] TheAnonymous pilgrim of Piacenza reported that 30,000 people died in Beirut alone.[1] Reports of damage atPetra and other locations in the Jordan Valley associated with the 551 event have been re-analyzed, suggesting that a later earthquake was more likely to be responsible.[3]
The earliest[4] account of the earthquake comes fromJohn Malalas, who recorded in hisChronographia that in the year 551 AD, during the14th indiction, a catastrophic earthquake struck the regions of Palestine, Arabia, Mesopotamia, Antioch,Phoenice Maritima, andPhoenice Libanensis, with Tyre, Sidon, Beirut, Tripolis, Byblos, andBotrys being most affected, alongside parts of other settlements, where many people were trapped. In Botrys, part of the mountain known asLithoprosopon, located near the sea, broke off and fell into the water, creating a harbor large enough to accommodate very large ships. This was significant, as the city had never previously had a harbor.
In response to the disaster, the emperor provided financial assistance to all the affected provinces and undertook restoration efforts for parts of the damaged cities. During the earthquake, the sea receded by a mile, destroying numerous ships, but eventually returned to its original position.[5]
Agathias, a Greek poet and historian, also recounted the effects of the earthquake in hisHistories:
"[...] several cities both on the islands and the mainland were razed to the ground and their inhabitants wiped out. The lovely city of Berytus, the jewel of Phoenicia, was completely ruined, and its world-famous architectural treasures were reduced to a heap of rubble, practically nothing but the bare pavements of the buildings being left. Many of the local inhabitants were crushed to death under the weight of the wreckage, as were many cultivated young men of distinguished parentage who had come there to study the Law [...] At this point, then, the professors of law moved to the neighboring city of Sidon, and the schools were transferred there until Berytus was rebuilt. The restored city was very different from what it had been in the past, though it was not changed beyond recognition, since it still preserved a few traces of its former self [...]"[6]
The earthquake was felt over a wide area fromAlexandria in the southwest toAntioch in the north previously damaged by theearthquake of 526. The area of felt intensity of VIII or more extends fromTripoli in the north toTyre in the south. Estimates for the magnitude vary from 7.2 on thesurface-wave magnitude scale to a possible 7.5 on themoment magnitude scale.[2] The rupture length is estimated to be greater than 100 kilometres (62 mi) and possibly as long as 150 kilometres (93 mi). The origin of the tsunami was thought to have been due to an underwater landslide triggered by an earthquake on the Dead Sea Transform itself.[7] More recent analysis suggested that an offshore continuation of the Roum Fault mapped onshore may have been responsible.[8] However, seabed surveys have discounted this possibility and the discovery of geologically recent fault scarps at seabed indicate that movement on the newly identified Mount Lebanon Thrust was the cause of the earthquake and the resulting tsunami.Quaternary uplift recorded by a series ofmarine-cut terraces between Tripoli and Beirut are consistent with continuing upward movement of the hanging wall of the proposed thrust. At a smaller scale, an upliftedvermetid bench, which indicates vertical movement of about 80 cm, is dated to the sixth century A.D. Continued uplift above this thrust since the lateMiocene may explain the formation of theMount Lebanon range.[2]
The tsunami affected the whole of the coast from Tyre to Tripoli. Withdrawal of the sea by up to two miles was recorded in some contemporary accounts.[1]
The estimated return time for large earthquakes on the Mount Lebanon thrust is 1500–1750 years, although a shorter return time is also possible depending on the dating of the most recent sea-level highstand. This latter possibility would indicate that a repeat of this event may be long overdue.[2]
