Single-event throws along the Delta Fault (Baikal rift) reconstructed from ground penetrating radar, geological and geomorphological data

Abstract

Surface displacement is critical to assess the seismic potential of an active fault. Its distribution along strike is strongly variable, and its detection is sometimes ambiguous in soft and poorly consolidated sediments. Here we show that ground penetrating radar (GPR), supported by geological and geomorphological data, is effective in revealing the total throw on an active fault, including brittle and ductile deformational components. We studied throws that occurred along the Delta Fault during the 1862 Tsagan earthquake on Lake Baikal. The motion on that fault was measured along a stepped system of normal ruptures, in which the main plane dips to the NW at 56–76°. The total throws of Holocene sediments, from GPR data, vary from 1.81 to 9.59 m. The contribution of the ductile component to the offset was between 13 and 42%. Its maximum values correspond to minimum dip values of the main fault plane and the largest widths of the fault damage zone. The width also correlates well with the total vertical displacement. Consequently, the structural features of the rupture zone and the variable distribution of offsets in soft sediments of mixed behaviour should be properly considered when estimating the magnitudes of normal fault earthquakes.