The 551 CE Beirut earthquake was likely generated offshore Lebanon on the Mount Lebanon Thrust, a submarine thrust system identified by Elias et al. (2007). Their interpretation links historical reports of destruction and tsunami activity with radiocarbon-dated marine fossils, fresh submarine fault scarps mapped along the continental slope.

The coastal evidence comes from uplifted benches along the Lebanese coast, especially between Beirut and Tripoli. Previous researchers had suggested that these fossil benches were produced by past earthquake activity. Morhange et al. (2006) dated fossil vermetids preserved on the tops of these benches in order to determine when they last lived near mean sea level. Elias et al. (2007) then combined these dates with offshore geophysical evidence and concluded that the lowest bench, B1, was raised by about 80 ± 30 cm during the 551 CE event.

Figure DR7

Death age probability distribution of 17 ¹⁴C calibrated vermetid death ages on the B1 bench between Beirut and Palmier Island. The distribution shows a sixth-century cluster used by Elias et al. (2007) to associate uplift of the B1 bench with the 551 CE earthquake.

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Elias et al. (2007) supplemental

The offshore component of the interpretation was based on geophysical data collected during the SHALIMAR survey. Elias et al. (2007) identified an approximately 160 km long offshore thrust system, which they named the Mount Lebanon Thrust. The deep-towed sonar data revealed fresh west-facing scarps cutting the otherwise smooth seafloor. These scarps lie near the base of larger cumulative bathymetric escarpments and were interpreted as submarine seismic ruptures on the offshore thrust system.

Figure 2a

Bathymetric map of the proximal Lebanese offshore. Bold red lines mark fresh seafloor seismic breaks. The green line marks the survey path. Box b marks the location of the sonar image shown in Figure 3. Onshore, orange lines mark elevated benches, and red dots mark radiocarbon sample locations.

JW: Image is rotated compared to the original publication.

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Elias et al. (2007)

Elias et al. (2007) emphasized the seismic character of these scarps, stating that "given their geomorphic resemblance to sub-aerial, seismic dip-slip ruptures, and their position near the foot of cumulative bathymetric escarpments, the seismic origin of such submarine breaks is not in doubt, although assessing whether they result from one or several earthquakes will require further investigation." This statement is important because it identifies the scarps as earthquake-related features, while also leaving open whether the observed scarps record a single rupture or multiple events.

Figure 3

Seafloor seismic rupture. Sidescan sonar image of a fresh seismic rupture along the base of the continental slope west of Damour. Note the sinuous, segmented trace, west-facing main scarp, and smaller parallel scarps on the hanging wall and footwall.

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Elias et al. (2007)

For the 551 CE earthquake, Elias et al. (2007) estimated a rupture length of at least 100 km, and possibly as much as 150 km if the Rankine-Aabdeh lateral ramp was involved. They argued that raising the Tabarja benches by 80 ± 30 cm required about 1.5-3 m of seismic slip on east-dipping ramps in the upper 20 km of the crust, using elastic half-space dislocation modeling after Okada (1985). For a thrust rupture of this length, they estimated a moment magnitude of about M_w 7.4-7.6, consistent with the historical scale of destruction and with a tsunamigenic offshore source.

Figure 4

Most likely sources of the 551 CE earthquake and other large historical earthquakes in Lebanon. The open star marks the inferred epicenter of the 551 CE earthquake. Colored patches enclose areas where macroseismic intensities greater than VIII were reported. Blue corresponds to the 551 CE earthquake.

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Elias et al. (2007)

This offshore thrust interpretation also argues against the Yammouneh Fault as the source of the coastal uplift and makes it less likely that the uplift resulted from other major earthquakes in the region that may not have originated on the Mount Lebanon thrust, including the 303–306 CE Eusebius Martyr earthquake, the 347/348/349 CE Beirut Conversion earthquake, the 450–457 CE Tripoli earthquake, or the 502 CE Fire in the Sky earthquake. Elias et al. (2007) noted that strike-slip motion on the Yammouneh Fault produces only small local uplift, less than about 1 m over approximately 10,000 years, citing Daëron et al. (2005). They therefore ruled out the Yammouneh Fault as the mechanism responsible for uplifting the Lebanese shoreline north of Beirut. The coastal vermetid ages also indicate that the 1202 CE earthquake did not produce uplift along this shoreline, and that the eleventh- to fourteenth-century earthquake sequence did not rupture the offshore Mount Lebanon Thrust system.

Elias et al. (2007) further suggested that their survey showed no evidence for a large local submarine landslide and therefore argued that a large landslide could be ruled out as the source of historical tsunamis along the Lebanese coast, including the 551 CE tsunami. This conclusion should be treated cautiously. The available side-scan sonar coverage was limited, and the textual sources for the 551 CE Beirut earthquake describe an initial ebbing of the sea, a detail that may suggest tsunami generation by a large submarine landslide or at least by a mechanism that included significant seafloor displacement. Even so, the geographic coincidence between the uplifted coastal benches and the offshore scarps strongly supports the Mount Lebanon Thrust as a major source of ancient earthquakes and tsunamis along the Lebanese coast.

Taken together, the evidence suggests that the 551 CE earthquake was a large offshore thrust event, probably M_w ~7.5, involving roughly 100-150 km of rupture on the Mount Lebanon Thrust. The earthquake uplifted parts of the Lebanese coast, killed intertidal vermetids that had previously lived near sea level, and generated a tsunami recorded in historical sources.