Archaeological and architectural evidence of historical seismic activity along the Zāgros Main Recent fault at Borujerd (western Iranian Plateau); the 1316 C.E. earthquake
Manuel Berberian, Mohammad Moqaddas, Ahmad Kabiri, 2017. "Archaeological and architectural evidence of historical seismic activity along the Zāgros Main Recent fault at Borujerd (western Iranian Plateau); the 1316 C.E. earthquake", Tectonic Evolution, Collision, and Seismicity of Southwest Asia: In Honor of Manuel Berberian’s Forty-Five Years of Research Contributions, Rasoul Sorkhabi
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We present the first critical examination of the > 1300 yr history of the Borujerd old congregational mosque by means of scrutinizing ancient historical chronicles, archaeology, architecture, historical and modern seismicity, geology, and regional active faults. Our study resulted in recognition of at least four major phases of destruction, rebuilding, and renovations of the megastructure located 4 km to the northeast of the Zagros Main Recent fault in western Iran. The grand structure shows significant paleo-architectural and archaeological evidence of destruction and damage. Although some damage events, recorded in seven phases since the seventh century C.E., could have been due to the poor construction of early periods and their decay, there is strong evidence of at least one extensive, simultaneous, and abrupt destruction and damage pattern of mosque III (ca. post–1090/pre–1139 C.E.) in the early fourteenth century. We suggest that the poorly known 1316 C.E. strong earthquake (which destroyed more than 20 villages in the general area, with erroneous epicentral location in the historical seismic catalogues) was possibly responsible for the simultaneous sudden collapse of the Borujerd congregational mosque lofty dome chamber and its tall free-standing minaret; we infer that this earthquake occurred with intensity > VIII+ (modified Mercalli intensity scale) conceivably along a seismic gap zone of the Zagros Main Recent fault. No pre–1316 C.E. monument exists in the epicentral region, and no strong earthquake has occurred along that segment of the Zāgros Main Recent fault for the last seven centuries. Retrospectively, apparent indigenous paleo-architectural renovations were utilized during construction of the new congregational mosque (mosque IV: ca. post–1405/pre–1447 C.E.) to enhance the coherency and elasticity of the rigid brick structure to withstand future earthquake shear stress. The hazard-reducing efforts included: (1) retrofitting the surviving load-bearing structural elements; (2) avoiding grandeur and majesty and implementing simplicity by reducing the size, height, and shape of the dome chamber; (3) avoiding free-standing minarets; (4) minimizing the size and reducing the light/ventilation openings; and (5) utilizing several levels of timber bracings to neutralize earthquake strong ground motion. Our research reveals that the return period of large-magnitude earthquakes along the two major segments of the fault is in the range of 1000 and 2000 yr, thus making historical earthquakes unrecognizable through routine historical research. It also shows how the use of archaeoseismology and paleo-architectural investigations on deformed monuments may improve our knowledge of long-term seismicity and seismic hazards of a region. This kind of study permits us to hypothesize the occurrence of strong earthquakes in an area for which historical seismicity does not show significant earthquakes. Finally, based on the described historic seismic damage and destruction, the regional national monuments should be properly retrofitted to withstand future earthquake hazards.