The 551 Beirut earthquake occurred on 9 July with an estimated magnitude of about 7.5 on the

moment magnitude scale The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pa ...

and a maximum felt intensity of X (''Extreme'') on the

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the ef ...

. It triggered a devastating

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater exp ...

which affected the coastal towns of Byzantine

Phoenicia Phoenicia () was an ancient thalassocratic civilization originating in the Levant region of the eastern Mediterranean, primarily located in modern Lebanon. The territory of the Phoenician city-states extended and shrank throughout their his ...

, causing great destruction and sinking many ships. Overall large numbers of people were reported killed, with one estimate of 30,000 by the

Anonymous pilgrim of Piacenza The anonymous pilgrim of Piacenza, sometimes simply called the Piacenza Pilgrim, was a sixth-century Christian pilgrim from Piacenza in northern Italy who travelled to the Holy Land at the height of Byzantine rule in the 570s and wrote a narrative ...

for

Beirut Beirut, french: Beyrouth is the capital and largest city of Lebanon. , Greater Beirut has a population of 2.5 million, which makes it the third-largest city in the Levant region. The city is situated on a peninsula at the midpoint o ...

alone.

Tectonic setting

Lebanon lies astride the

Dead Sea Transform The Dead Sea Transform (DST) fault system, also sometimes referred to as the Dead Sea Rift, is a series of faults that run from the Maras Triple Junction (a junction with the East Anatolian Fault in southeastern Turkey) to the northern end of the ...

, which forms part of the boundary between the

Arabian Plate The Arabian Plate is a minor tectonic plate in the Northern and Eastern Hemispheres. It is one of the three continental plates (along with the African and the Indian Plates) that have been moving northward in geological history and colliding ...

and the

African Plate The African Plate is a major tectonic plate that includes much of the continent of Africa (except for its easternmost part) and the adjacent oceanic crust to the west and south. It is bounded by the North American Plate and South American Plate ...

. In Lebanon the fault zone forms a restraining bend associated with a right stepping offset of the fault trace.

Transpression In geology, transpression is a type of strike-slip deformation that deviates from simple shear because of a simultaneous component of shortening perpendicular to the fault plane. This movement ends up resulting in oblique shear. It is generally ve ...

al deformation associated with this bend has formed a number of

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

s, such as the recently identified ''Mount Lebanon thrust'', which underlies the city and is interpreted to crop out at the seabed offshore.

Damage

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. The

reported that 30,000 people died in Beirut alone. Reports of damage at

Petra Petra ( ar, ٱلْبَتْرَاء, Al-Batrāʾ; grc, Πέτρα, "Rock", Nabataean: ), originally known to its inhabitants as Raqmu or Raqēmō, is an historic and archaeological city in southern Jordan. It is adjacent to the mountain of Ja ...

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.

Characteristics

Earthquake

The earthquake was felt over a wide area from

Alexandria Alexandria ( or ; ar, ٱلْإِسْكَنْدَرِيَّةُ ; grc-gre, Αλεξάνδρεια, Alexándria) is the second largest city in Egypt, and the largest city on the Mediterranean coast. Founded in by Alexander the Great, Alexandri ...

in the southwest to

Antioch Antioch on the Orontes (; grc-gre, Ἀντιόχεια ἡ ἐπὶ Ὀρόντου, ''Antiókheia hē epì Oróntou'', Learned ; also Syrian Antioch) grc-koi, Ἀντιόχεια ἡ ἐπὶ Ὀρόντου; or Ἀντιόχεια ἡ ἐπ� ...

in the north previously damaged by the earthquake of 526. The area of felt intensity of VIII or more extends from Tripoli in the north to Tyre in the south. Estimates for the magnitude vary from 7.2 on the surface wave magnitude scale to a possible 7.5 on the

. The rupture length is estimated to be greater than and possibly as long as . 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. More recent analysis suggested that an offshore continuation of the Roum Fault mapped onshore may have been responsible. 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 The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million year ...

uplift recorded by a series of marine-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 uplifted

vermetid The Vermetidae, the worm snails or worm shells, are a Taxonomy (biology), taxonomic family (biology), family of small to medium-sized sea snails, marine (ocean), marine gastropod molluscs in the clade Littorinimorpha. The shells of species in the ...

bench, which indicates vertical movement of about 80 cm, is dated to the sixth century A.D. Continued uplift above this thrust since the late

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

may explain the formation of the

Mount Lebanon Mount Lebanon ( ar, جَبَل لُبْنَان, ''jabal lubnān'', ; syr, ܛܘܪ ܠܒ݂ܢܢ, ', , ''ṭūr lewnōn'' french: Mont Liban) is a mountain range in Lebanon. It averages above in elevation, with its peak at . Geography The Mount Le ...

range.

Tsunami

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.

Future seismic hazard

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.

References

{{Earthquakes in the Levant 0551 Beirut 6th-century earthquakes Lebanon in the Roman era 550s in the Byzantine Empire 551 Medieval tsunamis Earthquakes in the Byzantine Empire