Earthquake geology of the Mondy fault (SW Baikal Rift, Siberia)

Abstract

The Mondy Fault is a 90 km long E-W trending active structure belonging to the southwestern part of the Baikal rift system, connecting the Tunka depression to the East to the Hovsgol rift to the West. The fault is well expressed in the morphology and formed during the Neogene within a transtensional strain regime (left-lateral + normal) contemporaneously with the opening of the Baikal Rift. On April 4th, 1950, the fault produced a large earthquake with a moment magnitude of Mw 6.9, and a left-lateral focal mechanism. Along with other structures of the Tunka depression such as the Tunka and Sayan faults, it represents a seismic hazard for the cities in the region as the Irkutsk agglomeration (1 million people). To characterize its potential activity, we combined morphotectonic and paleoseismological investigations at two sites along the eastern and western parts of the fault, respectively. Our study shows that cumulative left-lateral displacements are associated with a strong reverse component. This indicating that the previous Neogene normal vertical component has been reversed, consistently with the inversion of the tectonic regime observed within the SW Baikal Rift region in the Late Pleistocene-Holocene. We estimated the slip rate along the fault to be 0.9–1.5 mm/yr over the last ∼ 13 ka, and identified four large surface-rupturing events with minimum magnitude of 7.4 separated by an average recurrence interval of 3.9–4.6 ka.