A Major Medieval Earthquake on the Main Köpetdag (Kopeh Dagh) Fault, Turkmenistan

Abstract

ABSTRACTThe Main Köpetdag fault (MKDF) of Turkmenistan is one of the longest (≈500 km) and the most rapidly straining (9.1 ± 1.3 mm/yr) faults of the Arabia–Eurasia collision zone, and yet, in contrast to adjacent parts of Turkmenistan and Iran, it has little in the way of observed seismicity. Field observations indicate a fresh paleorupture along an ≈100 km long section of the MKDF with abundant streams offset across it. We use WorldView-2 optical satellite imagery to measure geomorphic offsets and compile them into a cumulative offset probability density (COPD) function. The COPD yields two peaks in offset density at 7.5 ± 1.5 and 12.5 ± 1.5 m, when considering the highest and the lowest quality measurements, whereas the medium-quality measurements present a single peak at 9 ± 3 m. We are unable to separate peaks into distinct events when exploring the contributions of offset measurements along strike that have significant variability. The paleorupture displaces archaeological remains, including extensive field boundaries of likely medieval age, and a paleoseismic trench brackets the age of the most recent displacement at 600–800 yr B.P. From our measurements of displacement, we estimate that the most recent paleoearthquake had a moment magnitude (Mw) 7.6 ± 0.4 and yet is missing from earthquake records in nearby Iran, indicating that the paucity of historical earthquakes in Turkmenistan may be misleading. The Baharly, Döwgala, and Gökdepe segments of the MKDF, which extend eastward toward Aşgabat, do not show fresh surface expression. Infrared-stimulated luminescence ages of 3.4 ± 0.5 and 2.5 ± 0.2 ka from a river-cutting exposure along the Baharly segment predate the most recent paleorupture. We conclude that individual segments of the MKDF are capable of rupturing in major (Mw>7.0) earthquakes, with ends of individual ruptures possibly controlled by segment boundaries. The occurrence of major earthquakes and rapid slip rate suggests that the MKDF constitutes a hazard to a populated region of Turkmenistan and northeast Iran.