Dr Roberta Wilkinson

Abstract:

The study of surface ruptures is key to understanding the earthquake occurrence of faults especially in the absence of historical events. We present a detailed analysis of geomorphic displacements along the Dzhungarian Fault, which straddles the border of China and Kazakhstan. We use digital elevation models derived from structure-from-motion analysis of Pléiades satellite imagery and drone imagery from specific field sites to measure surface offsets. We provide direct age constraints from alluvial terraces displaced by faulting and indirect dating from morphological analysis of the scarps. We find that the southern 250 km of the fault likely ruptured in a single event in the last 4000 years, with displacements of 10-15 m, and potentially up to 20 m at one site. We infer that this Dzhungarian rupture is likely linked with a previously identified paleo-earthquake rupture on the Lepsy Fault through a system of splays in the intervening highlands. Though there are remaining uncertainties regarding consistency in age constraints between the two fault ruptures, most of the sites along the two faults are consistent with a most recent event 2000-4000 years ago. Rupture on the Dzhungarian Fault alone is likely to have exceeded Mw 8, and the combined Lepsy-Dzhungarian rupture scenario may have been up to Mw 8.4. Despite being at the upper end of known or inferred continental earthquake magnitudes, our proposed scenario combining the 375 km of the Dzhungarian and Lepsy ruptures yields a slip-to-length ratio consistent with global averages and so do other historical intra-continental earthquakes 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.