Abstract

Coseismic soft-sediment deformation has been studied by structural and tectononophysical methods in the Selenga Delta area shaken by the devastating M ∼ 7.5 Tsagan earthquake in 1862. Among the documented deformation structures (seismites), clastic dikes are the most reliable paleoseismic indicators. The dikes have their sizes and extent showing proximity to the primary coseismic rupture zone and are closely associated with faults of different hierarchic levels. The Tsagan event occurred under SW–NE extension as motion on a stepped system of normal faults dipping at 300°–350°, ∠45°–75°.

The amount of vertical motion measured against a reference layer in a trench reached 2.83 m, and the maximum dip displacement measured in a single fracture was 0.5 m. The earthquake was generated by the Delta Fault that dips at 60° on average to the northwest.

The distribution of quantitative parameters of brittle and brittle-plastic deformation has been analyzed along two profiles, and two new parameters were introduced: indices of mean intensity (I) of clastic dikes and microdikes; the new parameters were calculated by specially developed equations. Summation of significant peaks in all parameters (SUMspp) allowed contouring the zone of most intense soft-sediment deformation near Dubinino Village.

Deformation mostly propagated in the NE–SW and N–S directions. The location of the 1862 Tsagan earthquake at 52.35° N and 106.67° E was inferred from the SUMspp value taking into account the dip of the causative fault plane and the average origin depth of earthquakes in the Baikal rift. The approach we used is applicable to locating preinstrumental events.

The recurrence of large earthquakes in the area of Proval Bay (Lake Baikal) has been estimated to be 1120–1230 years proceeding from alternating deformed and undeformed sediments in the sections, their thicknesses and deposition rates according to radiocarbon dating. The seismic activity has been associated with the same fault which can generate M ≥ 7 events.

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