Historical Earthquakes and Their Societal Impact
Mansurah, the eighth-century Arabic capital of Sindh province, Pakistan, flourished for a mere 200 yr. Its destruction by an earthquake ca. 980 A.D. was first proposed by archaeologists who reported the discovery of crushed skeletons amid dateable coins found among its rubble. An abrupt natural death to the city was challenged by others who noted that the absence of wood or valuables was consistent with the city being sacked and systematically looted. The recent discovery of four decorated door knockers beneath the collapsed walls of one of the largest structures in Mansurah, however, reopens the case for an earthquake, since an invading army would almost certainly have removed them as booty. We suggest that an earthquake not only destroyed the city and its suburbs (intensity ≈ VIII), but resulted in postseismic avulsion of the river on which its citizens depended for agriculture, sanitation, and trade. Since natural levees have been observed in India to collapse in intensity VII shaking, it is unnecessary to invoke coseismic uplift as a requirement for upstream river avulsion. The absence in the past two centuries of large earthquakes in the region has resulted in central Sindh being depicted as a region of low seismic hazard, yet in 1668, in the same province, an earthquake destroyed nearby Samawani and also initiated avulsion of the Indus. A case can be made for reevaluating the five millennia of archaeological ruins in Pakistan to establish a long-term view of seismicity unavailable from the short instrumental record.