Probing the upper end of intracontinental earthquake magnitude: a prehistoric example from the Dzhungarian and Lepsy faults of Kazakhstan

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 4,000 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 2,000–4,000 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 intracontinental earthquakes in Central Asia

Assessing the activity of faults in continental interiors: Palaeoseismic insights from SE Kazakhstan

The presence of fault scarps is a first-order criterion for identifying active faults. Yet the preservation of these features depends on the recurrence interval between surface rupturing events, combined with the rates of erosional and depositional processes that act on the landscape. Within arid continental interiors single earthquake scarps can be preserved for thousands of years, and yet the interval between surface ruptures on faults in these regions may be much longer, such that the lack of evidence for surface faulting in the morphology may not preclude activity on those faults. In this study we investigate the 50 km-long ‘Toraigyr’ thrust fault in the northern Tien Shan. From palaeoseismological trenching we show that two surface rupturing earthquakes occurred in the last $\textbf{39.9±2.7 ka}$ BP, but only the most recent event (3.15–3.6 ka BP) has a clear morphological expression. We conclude that a landscape reset took place in between the two events, likely as a consequence of the climatic change at the end of the last glacial maximum. These findings illustrate that in the Tien Shan evidence for the most recent active faulting can be easily obliterated by climatic processes due to the long earthquake recurrence intervals. Our results illustrate the problems related to the assessment of active tectonic deformation and seismic hazard assessments in continental interior settings.This study was financed by NERC and ESRC (Earthquakes without Frontiers project, Grant code: EwF_NE/J02001X/1_1), and the Centre for Observation and Modelling of Earthquakes and Tectonics (COMET). KOMPSAT-2 imagery was obtained through a category-1 award to RTW. EJC thanks St. Edmund Hall for travel support. RTW was supported during this research by a University Research Fellowship from the Royal Society of London

Shortening accommodated by thrust and strike-slip faults in 1 the Ili Basin, northern Tien Shan

The Tien Shan accommodates a significant portion of the India‐Eurasia N‐S convergence. In its northern part a zigzag pattern of mountain ranges bounds the western Ili Basin. The role of this basin in the overall shortening and the regional kinematics is not well understood. Geodetic data and instrumental seismicity are not sufficient to infer the role of individual faults and fault systems. We analyze GPS data and earthquake slip vectors and present the results of fault mapping based on remote sensing and field campaigns in the western Ili Basin. These observations indicate that E‐W thrust faults are active at the basin margins, and oblique and strike‐slip faults, both in the basin and in the Paleozoic rocks within the mountain ranges, have been active in the Late Quaternary. We propose a regional tectonic model in which the left‐lateral strike‐slip faults at the NW margin of the basin accommodate ~3‐mm/year NE‐SW shear. Smaller right‐lateral oblique faults transfer the motion in between the left‐lateral faults, and also take up shortening by rotations about vertical axes. We see the onset of internal deformation within the Ili Basin, although it has a strong basement. Our kinematic model is consistent with geodetic data, earthquake seismology, historical, and prehistorical surface faulting, and describes the first‐order features of active deformation that can be observed in the northern Tien Shan. Our study illustrates the importance of combining these different data sets to understand the regional tectonics

Probing the upper end of intra-continental earthquake magnitude: a prehistoric example from the dzhungarian and lepsy faults of Kazakhstan

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

Shortening accommodated by thrust and strike-slip faults in 1 the Ili Basin, northern Tien Shan

The Tien Shan accommodates a significant portion of the India‐Eurasia N‐S convergence. In its northern part a zigzag pattern of mountain ranges bounds the western Ili Basin. The role of this basin in the overall shortening and the regional kinematics is not well understood. Geodetic data and instrumental seismicity are not sufficient to infer the role of individual faults and fault systems. We analyze GPS data and earthquake slip vectors and present the results of fault mapping based on remote sensing and field campaigns in the western Ili Basin. These observations indicate that E‐W thrust faults are active at the basin margins, and oblique and strike‐slip faults, both in the basin and in the Paleozoic rocks within the mountain ranges, have been active in the Late Quaternary. We propose a regional tectonic model in which the left‐lateral strike‐slip faults at the NW margin of the basin accommodate ~3‐mm/year NE‐SW shear. Smaller right‐lateral oblique faults transfer the motion in between the left‐lateral faults, and also take up shortening by rotations about vertical axes. We see the onset of internal deformation within the Ili Basin, although it has a strong basement. Our kinematic model is consistent with geodetic data, earthquake seismology, historical, and prehistorical surface faulting, and describes the first‐order features of active deformation that can be observed in the northern Tien Shan. Our study illustrates the importance of combining these different data sets to understand the regional tectonics

Assessing the activity of faults in continental interiors: palaeoseismic insights from SE Kazakhstan

The presence of fault scarps is a first-order criterion for identifying active faults. Yet the preservation of these features depends on the recurrence interval between surface rupturing events, combined with the rates of erosional and depositional processes that act on the landscape. Within arid continental interiors single earthquake scarps can be preserved for thousands of years, and yet the interval between surface ruptures on faults in these regions may be much longer, such that the lack of evidence for surface faulting in the morphology may not preclude activity on those faults. In this study we investigate the 50 km-long ‘Toraigyr’ thrust fault in the northern Tien Shan. From palaeoseismological trenching we show that two surface rupturing earthquakes occurred in the last 39.9±2.7 ka BP, but only the most recent event (3.15 – 3.6 ka BP) has a clear morphological expression. We conclude that a landscape reset took place in between the two events, likely as a consequence of the climatic change at the end of the last glacial maximum. These findings illustrate that in the Tien Shan evidence for the most recent active faulting can be easily obliterated by climatic processes due to the long earthquake recurrence intervals. Our results illustrate the problems related to the assessment of active tectonic deformation and seismic hazard assessments in continental interior settings

A creeping intracontinental thrust fault: past and present slip-rates on the Northern edge of the Tien Shan, Kazakhstan

We demonstrate that a continental interior reverse fault is deforming by aseismic creep, presently, and likely also in the long term. The Karkara Rangefront Fault, part of the larger Main Terskey Front, forms the northern boundary of the high Terskey Tien Shan in southeastern Kazakhstan and is a mature structure with evidence for high slip rates throughout the late Cenozoic. Combining field studies with a satellite stereo-image derived digital elevation model (2 m resolution), we map a series of fluvial terraces along the rangefront which are uplifted by up to ∼300 m above the present river level. Radiocarbon ages from one catchment constrain the ages of the lowermost two terraces to be ∼4–5 ka and ∼10–15 ka, consistent with prominent, regionally extensive terraces observed elsewhere in the Tien Shan. Based on conservative estimates for the fault dip under the displaced terraces, we estimate a slip rate along the fault plane of 3.5+1.7 −0.4 mm yr–1 on the Karkara Rangefront Fault and a further >0.8 mm yr–1 on a fold structure in the Kegen basin that we infer is driven by a detachment from the main rangefront. We therefore estimate a minimum shortening rate across the rangefront of 1.1–3.3 mm yr–1. Elastic modelling of the regional GPS velocity field suggests that the fault is presently creeping at ∼3 mm yr–1 (horizontal shortening), consistent with the upper limit of our Late Quaternary slip rate estimate. This is the fastest known slip rate in the northern Tien Shan and the only individual structure resolved in the regional velocity field. At present the fault is accumulating minimal strain, and there is evidence in the geomorphology that this creep is sustained in the long term, but whether or not it is also capable of generating earthquakes requires further study

Multisegment rupture in the 11 July 1889 Chilik earthquake ( M w 8.0-8.3), Kazakh Tien Shan, interpreted from remote sensing, field survey, and paleoseismic trenching: THE M w 8.0-8.3 CHILIK EARTHQUAKE

The 11 July 1889 Chilik earthquake (M-w 8.0-8.3) forms part of a remarkable sequence of large earthquakes in the late nineteenth and early twentieth centuries in the northern Tien Shan. Despite its importance, the source of the 1889 earthquake remains unknown, though the macroseismic epicenter is sited in the Chilik valley, similar to 100 km southeast of Almaty, Kazakhstan (similar to 2 million population). Several short fault segments that have been inferred to have ruptured in 1889 are too short on their own to account for the estimated magnitude. In this paper we perform detailed surveying and trenching of the similar to 30 km long Saty fault, one of the previously inferred sources, and find that it was formed in a single earthquake within the last 700 years, involving surface slip of up to 10 m. The scarp-forming event, likely to be the 1889 earthquake, was the only surface-rupturing event for at least 5000 years and potentially for much longer. From satellite imagery we extend the mapped length of fresh scarps within the 1889 epicentral zone to a total of similar to 175 km, which we also suggest as candidate ruptures from the 1889 earthquake. The 175 km of rupture involves conjugate oblique left-lateral and right-lateral slip on three separate faults, with step overs of several kilometers between them. All three faults were essentially invisible in the Holocene geomorphology prior to the last slip. The recurrence interval between large earthquakes on any of these faults, and presumably on other faults of the Tien Shan, may be longer than the timescale over which the landscape is reset, providing a challenge for delineating sources of future hazard