Tabarja Benches
Figure 2BUplifted, karst-pitted vermetid benches near Tabarja.
Elias et al (2007)
- Tabarja, Lebanon and vicinity
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Tabarja, Lebanon and vicinity
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Elias et al (2007)
examined uplifted benches on the Lebanese coast between Sarafand and Tripolis; some in the vicinity of Tabarja
(~20 km. NE of Beirut).
They estimated ~80 cm. of uplift took place on the lowest bench (B1) in the 6th century CE. They additionally collected deep towed sonar data offshore of
the uplifted benches which showed fresh west facing fault scarps that cut the smooth seafloor. They associated these fault
scarps with seismic activity from the newly discovered offshore Mount Lebanon Thrust Fault system. They surmised
that ~100–150-km. of the Mount Lebanon thrust fault broke and generated an earthquake in 551 CE with a
moment magnitude (Mw) of ~7.5
Previous researchers had speculated that elevated fossil benches present on the Lebanese coast (largely between Beirut and Tripolis) had reached
their position due to past earthquake activity.
Morhange et al (2006) radiocarbon dated fossil Vermetids on the tops of these benches in order to
estimate when the bench top was last in the sub tidal zone (which approximates mean sea level). By examining the radiocarbon dates and engaging in some seismic sleuthing,
Elias et al (2007) determined that the well documented 551 Beirut Quake caused 80 +/- 30 cm. uplift of the lowest bench (B1) during this seismic event.
Figure DR7Death age probability distribution of 17, 14C calibrated Vermetid death ages on
"B1" bench between Beirut and Palmier Island (sum probability, normalized to unit height).
Elias et al (2007) supplemental
Elias et al (2007)
discovered an ~160 km. long offshore thrust system in the process of collecting and analyzing their geophysical data. They termed this new thrust fault system the
the Mount Lebanon Thrust.
Figure 2aBathymetric map of proximal Lebanese offshore.
-
Offshore
- Bold red lines are "fresh" seafloor seismic breaks.
- Green line is survey path.
- Box b indicates location of sonar image in Figure 3 above.
Onshore- Orange lines are locations of elevated benches
- Red dots are sample locations for radiocarbon dating.
JW: Image is rotated compared to original publication -
Elias et al (2007)
The deep towed sonar data showed fresh west facing seismogenic fault scarps on the smooth ocean floor which allowed them to map and characterize this thrust system. In commenting on the fault scarps, they stated 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.
Figure 3Seafloor seismic rupture. Sidescan sonar image of "fresh" seismic rupture along base of continental slope west of Damour (box "b" in Fig. 2A). Note sinuous, segmented trace, west-facing main scarp, and smaller parallel scarps on top of east (hanging) wall and away from main scarp base on footwall. Dark shades mark high backscatter. Insonification from top (west).
Elias et al (2007)
They later added that "direct dating of the sea-floor scarps will provide the ultimate proof". They noted that their survey "showed no evidence of submarine landslides except for small-scale slump scars and rockslides on or at the base of steep slopes south of Damour and near Batroun" concluding that it was "possible to rule out the occurrence of a large local submarine landslide as potential sources of historical tsunamis [e.g. in 551 CE] along the Lebanese coast." This last conclusion does not seem justified given the limited amount of side scan sonar data they collected. Further, the textual sources for the 551 CE Beirut Quake spoke of an initial ebbing of the sea which suggests that the tsunami was caused by a large submarine landslide. Nonetheless, the geographic coincidence of the stretch of coast exhibiting uplifted benches with the observed areal extent of submarine fault scarps appears to confirm that the Mount Lebanon Thrust fault was the source of large ancient earthquakes and tsunamis along the coast.
Figure 2CProjected total station measurements of bench elevations relative to LMSL (living vermetid surface in swash zone). Note signifi cant scatter in elevation measurements on fossil levels, particularly B1 and B4, likely because of sub-levels with rounded edges and of irregularities due to deep pitting by microkarst.
Elias et al (2007)
Elias et al (2007) estimated a moment magnitude (Mw) of ~7.4-7.6 for the 551 CE Beirut Quake and offered the following discussion:
To raise the Tabarja trottoirs [benches] 80 ± 30 cm above the LMSL [Local Mean Sea Level], simple dislocation modeling in an elastic half-space (Okada, 1985) requires 1.5-3 m of seismic slip on these ramps, assuming they dip -45° eastward in the upper 20 km of the crust (Data Repository item DR8). Such slip amounts are consistent with the estimated magnitude of the A.D. 551 earthquake, and sufficient to account for the tsunami observed. Historical evidence combined with the extent of vermetid death in the sixth century A.D. implies a rupture length of at least -100 km, and possibly up to 150 km if the Rankine-Aabdeh lateral ramp was involved (Figs. 1 and 4), as suggested by two ages on Palmier Island (Table DR6). For such rupture lengths on thrust faults, empirical scaling laws predict an Mw of ~7.4-7.6 (Wells and Coppersmith, 1994), consistent with macroseismic estimates. Because strike-slip motion on the Yammouneh fault has been shown to produce only small local uplift (less than ~1 m in ~10,000 yr; Daeron et al., 2005), the inference that events on this fault might raise shorelines north of Beirut (Morhange et al., 2006) can be safely ruled out. The coastal 14C vermetid ages confirm that the great A.D. 1202 earthquake, for instance, produced no uplift along the Lebanese shoreline. That benches offshore Tripoli are older than the seventh century A.D. in fact excludes the possibility that any of the earthquakes of the eleventh to fourteenth century A.D. sequence, including the A.D. 1063 event, ruptured the offshore Mount Lebanon thrust system. Hence, the destruction of Tripoli and Arqa by the latter earthquake may have been caused by slip on the Aakkar and/or Tripoli thrusts (Fig. 4).
Most likely sources of A.D. 551 (open star—inferred epicenter, this study) and other large historical earthquakes in Lebanon (modifi ed from Daëron et al., 2005). Colored patches enclose areas where macroseismic intensities >VIII were reported. Blue color corresponds to A.D. 551. (VIII isoseismal from Sieberg, 1932).
Elias et al (2007)