DISTINGUISHING DAMAGES OF TWO EARTHQUAKES – ARCHAEOSEISMOLOGY OF A CRUSADER CASTLE (AL-MARQAB CITADEL, SYRIA)
Walls of elongated structures are most sensitive to damaging vibrations at high angles to their orientation. Circular and square buildings (temple domes, stupas, chedis, prangs, donjons, church towers, minarets) might record strong vibrations of any direction. A succession of damaging vibrations have been identified and dated by archaeological survey of failed walls and restoration work.
Medieval Al-Marqab citadel (Latin: Margat) in coastal Syria was built by the Order of St. John (Hospitallers) in the 12-13th century. The hilltop fortification has masonry walls made with and without mortar, using the opus caementum technology (Roman concrete). Fallen keystones, V-shaped and U-shaped failures, single-corner and symmetrical corner collapses, in-plane and out-of-plane shift of ashlar masonry walls were surveyed. Dating was attempted by historical and archaeological methods.
Traces of two major, successive earthquakes were identified. A V-shaped extrusion at the top of the Crusader-built donjon indicates NE-SW vibration. This was formed certainly during the Crusader period (1170-1285), as no trace of similar shaking direction is displayed in Muslim-built structures. We suggest that the 1202 earthquake, the largest one ever recorded in the Near East, produced this damage. This quake yielded at least I = VIII intensity at Al-Marqab, higher than previously considered.
A subsequent, NW-SE directed vibration of lesser relative intensity damaged the Qalawun tower, a Muslim fortification, but left no traces on the Crusader donjon. Candidate earthquakes are the 1404 and 1759 events.
One cannot avoid to note that the epicentre of the 1202 earthquake was south of Al-Marqab, in the Mt. Lebanon range. Subsequent major earthquakes had epicentres to the north, in the Aleppo region. This coincidence may indicate that there are cases when a relationship between damage directions and epicentre directions cannot be excluded. (OTKA K67583)
Reference
Kázmér, M. & Major, B. (2010): Distinguishing damages of two earthquakes – Archaeoseismology of a Crusader castle (Al-Marqab citadel, Syria). In: Sintubin, M., Stewart, I., Niemi, T. & Altunel, E. (eds) (2010): Ancient Earthquakes. Geological Society of America Special Paper (in press)