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Field workshop in paleoseismology – trenching the 1911 Chon Aksu (Mw 7.9) earthquake ruptures, Kyrgyzstan

July and August of 2022 were filled with a range of active tectonics fieldtrips within Central Asia, with varied research and training aims. Aidyn Mukambayev, Ian Pierce and Neill Marshall undertook trenching excavations along the Chinggis fault of the Kazakh platform – an important type example of active faulting deep in continental interior settings. Aidyn and a team from Oxford also undertook reconnaissance for trenching sites along the Zailisky rangefront, with importance for the hazard to Almaty city, and also within the Ili basin. In Kyrgyzstan Magali Rizza led fieldwork looking at geomorphic development within the tectonically active Naryn basin, and Ian Pierce led a trip south of Issyk Kul to examine preserved palaeo-earthquake ruptures and deformed fluvial terrace sequences. A few of us were able to visit central Mongolia, to work with colleagues on a study of active faulting there.

In late July all the groups converged on the Chon Aksu valley, north of Issyk Kul in Kyrgyzstan, for a field workshop in palaeoseismology. The Chon Aksu valley contains exceptional earthquake ruptures preserved from the 1911 Chon Kemin earthquake, and we chose the location to combine field training with novel research.

Map from Arrowsmith et al., 2016. The 1911 rupture zone is shown in red, and landslides/mass movements in yellow. The rupture is ~200 km in total length, and involved slip on several separate segments with substantial stepover distances between them. Through our investigations we hope to better assess the slip style and distribution in 1911, place age constraints on earlier events, and to see whether these earlier events formed from similar multi-segment complex sources as in 1911

In particular we aimed to assess:

(1) the extent to which the rupture scarps represent slip in a single earthquake, whether they are composite scarps produced by multiple earthquakes, and to place age constraint on the pre-1911 events.

(2) to assess whether earlier earthquake cycles show similar multi-fault complex rupture as exhibited in 1911.

(3) to better understand the geomorphic development of the Chon Aksu valley, which is sculpted through an interplay of tectonic, fluvial, and glacial processes.

Panoramic view of the ruptures

The workshop is one of the main training activities within our NATO SPS program on security from earthquakes in the Tien Shan, and follows on from the classroom-based training workshop in quaternary dating and hazard modelling in Aix-en-Provence earlier in the year. We assembled a team of fifteen scientists from Azerbaijan, France, Kyrgyzstan, Mongolia, the UK, and the USA. The workshop was led by Ramon Arrowsmith, with the team split into groups, some logging and interpreting trenches, and others taking roles in sample collection and drone surveying.

Ramon leading a tour of the ruptures.

Five trenches were excavated across the ruptures, in sites of varying landscape, elevation, and depositional environment ranging from coarse fluvial/alluvial gravels to finely bedded peats. Early career or less experienced team members led the interpretation of three of the trenches. On the final day of the workshop the whole team moved from trench to trench, listening to each team give their interpretations, and then brainstorming together to develop and refine the ideas.

One of the trenches
Daytime discussions at the excavations
Evening presentations and discussions

The trenches were really varied, not only in depositional environment, but also in the expression of faulting within them, and one take home message for all of us was in how much variability there can be in the surface manifestation of faulting even over short distances. Another lesson was in the importance of site selection, and the careful appreciation of the landscape and local environment in selecting insightful trench locations.

Beautiful scenery. A lake formed by drainage blocked by the scarp.

Regardless of experience level we all learnt a lot, and we were pleasantly surprised to find evidence of two events, including 1911, in each of the trenches. Once we have completed all the dating work we will hopefully be able to answer our scientific aims. More of that later …

End of trip dinner

The 2022 Tien Shan field season was supported through the NATO Science for Peace and Security multi-year project (G5690) and through the Leverhulme Trust ‘EROICA’ program (RPG-2018-371). Many thanks to the drivers and cooks and to the research team.

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