Rupture of the 1949 Khait Earthquake on a Cryptic Fault: Implications for Earthquake Hazard

Abstract The 1949 M w 7.6 Khait earthquake, Tajikistan, was one of the most destructive of the Pamir—Tien Shan region, killing over 7,000 people by building collapse and landsliding. It occurred at the transition between the Pamir and Tien Shan mountain ranges, which converge along the narrow Vashkh river valley. Therefore, it is important in understanding the tectonic and structural development at the junction of these two major mountain ranges, as well as yielding important information regarding hazard and risk to local populations and infrastructure. Although widespread landsliding and intensities of shaking were recorded, no surface ruptures were mapped, and the earthquake epicenter and source are poorly known. To determine the location, magnitude, and focal mechanism of the 1949 earthquake we combine calibrated earthquake relocations with body‐wave amplitude ratios from digitized paper seismograms. Our analysis shows that the earthquake nucleated within the Tien Shan basement with strike slip kinematics. Using high‐resolution digital elevation models and orthophotos derived from high‐resolution satellite imagery, we identify earthquake ruptures within the epicentral zone consistent with NW‐SE right‐lateral faulting. Relocated aftershocks and later seismicity also follow a NW‐SE trend through the Tien Shan north of the Vakhsh river. Mapping tectonically offset moraines dated in previous literature, we find a Vakhsh Fault slip rate of only half the present day strain accumulation rate derived from GNSS. Our results suggest a significant proportion of the regional deformation may occur away from the geomorphologically well expressed Vakhsh and Darvaz faults, and on faults with little prior expression of activity.