Active tectonics and seismic hazard in Central Asia
Abstract:
In this thesis, I investigate the behaviour of active faults near major cities in Central Asia, to better understand the seismic hazard they pose. First, I explore past and potential earthquakes on the Zailisky Range Front fault, which lies at the northern boundary of the Tian Shan mountains and runs beneath Almaty, Kazakhstan’s largest city. I present the first paleoseismic trench on the fault, which reveals two earthquakes of at least Mw ~6.6 – 6.7. The penultimate earthquake occurred at 9.5 +/- 0.8 ka and the most recent event between 8.4 – 2.6 ka. I use high resolution digital elevation data to map and measure fault scarps along the ~300 km of the range front. Scarp heights are consistent with ruptures of at least Mw ~6.5 – 6.7 for the most recent event or events. A tentative mapping of fault segmentation and the historical record of the 1887 Mw ~7.2 – 7.7 Verny earthquake indicate that the fault is capable of rupturing in larger events. I estimate Quaternary slip rates of ~0.1 – 0.4 mm/yr. A higher geodetic slip rate derived from published GNSS velocities (1.9 +/- 0.7 mm/yr) indicates deformation is accommodated by several parallel structures, some of which may be unmapped.
The second study region is the northern part of the Tajik basin close to Dushanbe, Tajikistan’s capital. I present ~130 m resolution surface velocity maps of the Tajik basin from a Multi-Temporal InSAR analysis of ~5 years of Sentinel-1 data. The rate maps show aseismic slip on several faults associated with an evaporite horizon. The right-lateral Ilyak fault is creeping at ~ 6.7 – 8.7 mm/yr in the east and ~ 4.2 – 4.5 mm/yr in the west. The rate decreases from east to west as slip transfers to the basin thrust sheets. There are no signals indicative of strain accumulation at depth across the Ilyak fault in the surface velocity maps, but moderate earthquakes in the basement suggest that deformation around a locked fault may be obscured by the sedimentary cover. Finally, using satellite-derived elevation models, I document geomorphic evidence for an active thrust fault within the Dushanbe Trough which may be capable of producing a Mw 7.0 – 7.2 earthquake based on the mapped length, highlighting the need for field studies to determine its seismic potential. The research presented in this thesis highlights the seismic hazard posed to major cities in Central Asia and the need for further research to better characterise the active faults.