Studying active faults by combing GPR images with morphotectonic and trenching results. Ulaanbaatar, Mongolia.

Summary Most of the Mongolian population 1.5 million is concentrated around Ulaanbaatar (UB) capital city, which is the main political and economical centre of the country. Hence, the seismic hazard assessment is of first importance for the country. The seismic activity observed in the vicinity of UB city increased since 2005. Several active faults have been identified around UB city and, according to their length and morphology, they can produce earthquakes of magnitude Mw = 6.5–7.5. The paleoseismology is the science that involves the search of ancient traces of earthquakes in the surface layers. It relies on methods including: quantitative geomorphology, trenches and various dating methods. However, the fault geomorphology is often smoothed due to erosion processes and a low slip rate, and the exact location of the fault is thus hidden. In such a context, the Ground Penetrating Radar (GPR) method gives good and useful results before paleoseismological trenching and can help to characterize faults by identifying offsets of radar reflections and buried fluvial channel deposits.