High Resolution Magnetic Survey to study Hustai Fault (Northern Mongolia)

During the last century, earthquake activities in Mongolia made this country one of the most seismic active intra-continental area in the world. Some recent observations raise strong concern on still unknown structures around Ulaanbaatar (1 M inhabitants). Instrumental seismicity shows continuous activity with five M 4+ events since 1974 and another M 5.4 in the same year. A part of this recent activity is located along the Hustai fault, in the west of Ulaanbaatar, and occurs in the shallow crust above 10-20 km. A multi-disciplinary study - including GPR profiling, magnetic and GPS mapping, morphotectonic observations and paleoseismic trenching - was carried out in the fault area to assess its seismogenic potential. We present preliminary results of high resolution magnetic surveys using three axis fluxgate magnetic sensors. In Hustai area, about 1.5 km² were prospected with survey line spacing of 5 m to investigate the subsurface character of the active fault. The main fault is clearly detected as well as secondary branches that affect some paleostreams. The combined approach of morphotectonics observations and magnetic measurements was used to select the location of two paleoseismic trenches. The fluxgate equipment, being an easy, non-invasive and high-resolution way of mapping, was used inside excavation trenches. Micro magnetic surveys were conducted on the walls of the trenches along 30 m, with a vertical extent of 2 m and a spacing of 0.1 m between each line. These measurements are used to define different units of sediments with a very high level of detail particularly where the stratigraphic interfaces are poorly visible. Magnetic mapping reveals a fault zone in recent units that consists of intense deformational patterns. Simultaneous use of horizontal and vertical maps is relevant to propose a 3D interpretation of the repartition of sedimentary layers. Faulted units related to recent depositional process attest for the ongoing activity of the Hustai Fault. Our multidisciplinary approach may help to assess seismogenic potential for the nearby city of Ulaanbaatar.