The 1138 Aleppo earthquake was among the deadliest earthquakes in history. Its name was taken from the city of Aleppo, in northern Syria, where the most casualties were sustained. The quake occurred on 11 October 1138 and was preceded by a smaller quake on the 10th.
Aleppo is located along the northern part of the Dead Sea Transform system of geologic faults, which is a plate boundary separating the Arabian plate from the African plate. The earthquake was the beginning of the first of two intense sequences of earthquakes in the region: October 1138 to June 1139 and a much more intense and a later series from September 1156 to May 1159. The first sequence affected areas around Aleppo and the western part of the region of Odessa (modern Turbofan, Turkey). During the second an area encompassing north-western Syria, northern Lebanon and the region of Antioch (modern Santayana, in southern Turkey) was subject to devastating quakes.
In the mid-twelfth century, northern Syria was a war-ravaged land. The Crusader states set up by Western Europeans, such as the Principality of Antioch, were in a state of constant armed conflict with the Muslim states of Northern Syria and the Jazeerah, principally Aleppo and Mosul.
A contemporary chronicler in Damascus, Ibn al-Qalanisi, recorded the main quake on Wednesday, 11 October 1138. He wrote that it was preceded by an initial quake on 10 October and there were aftershocks on the evening of 20 October, on 25 October, on the night of 30 October–1 November, and finishing with another in the early morning of 3 November. However, Kemal al-Din, an author writing later, recorded only one earthquake on 19–20 October, which disagrees with al Qalanisi's account. Given that al Qalanisi was writing as the earthquakes occurred and that accounts from other historians support a 10 or 11 October date, his date of 11 October is considered authoritative.