Characterizing the Present‐Day Activity of the Tunka and Sayan Faults Within Their Relay Zone (Western Baikal Rift System, Russia)

The Sayan and Tunka faults are located at the boundary between the northernmost mountain belt of Central Asia (the Sayan‐Baikal ranges) and the Siberian platform. These prominent crustal structures were involved in the opening of the southern Baikal rift system since the beginning of the Cenozoic and define large‐scale sharp morphotectonic features. Despite low instrumental seismic activity, Late Pleistocene‐Holocene morphotectonic features along the two faults indicate that the faults are active and have the capacity to produce strong earthquakes. A careful mapping of the most recent trace of activity, within the south‐eastern parts of the two faults where they merge within a relay zone, demonstrates that they correspond now to left‐lateral‐reverse faults, suggesting a recent inversion of their vertical component. We also show that the two faults are now structurally connected via a young surface rupture and that no obvious post‐Last Glacial Maximum ruptures are observed along the central part of the Sayan Fault beyond its junction zone with the Tunka fault. This suggests that the left‐lateral strike‐slip deformation is transferred from the eastern Sayan fault to the Tunka fault. A detailed morphotectonic study along the south‐eastern Sayan fault allows estimating a left‐lateral slip rate between 1.3 (min) and 3.9 mm/year (max). Finally, a critical review of Russian paleoseismic data, combined with our paleoseismological investigations, allows us to propose that the mean recurrence time along the two faults is on the order of 4 kyr and that they may have either ruptured together or during seismic clusters.