the historians johann jakob reiske (18th century) and e. a. wallis budge (20th century) interpreted the sources as indicating an earthquake in the persian gulf rather than the caspian sea. robert mallet (19th century) instead interpreted the sources as indicating the location of the earthquake in the caspian sea. at least some of the primary sources locate the areas affected by the earthquake within the province of jibal in the abbasid caliphate.