Characterization of active fault zones near Ulaanbaatar, Mongolia, from electrical resistivity models
Implications for seismic hazard assessment
Shoovdor Tserendug (Mongolian Academy of Sciences)
Matthew J. Comeau (TU Delft - Applied Geophysics and Petrophysics)
Enkhzul Bayartogtokh (Mongolian Academy of Sciences)
Erdenechimeg Batmagnai (Mongolian Academy of Sciences)
Alexey Kuvshinov (ETH Zürich, Institute of Solar-Terrestrial Physics, Moscow State University)
Odonbaatar Chimed (Mongolian Academy of Sciences)
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Abstract
Strong earthquakes have been mapped within the Ulaanbaatar region, Mongolia, near the capital city of Ulaanbaatar. From 1994 to 2016, 120 earthquake events were recorded between 3.4 and 5.6, and 978 earthquake events were recorded with a magnitude between 2.5 and 3.4 (Adiya, 2016; Al-Ashkar et al., 2022). Residents of Ulaanbaatar have felt several of these earthquakes.
Historical records dating back to 1905 show that Mongolia as a whole has experienced four major earthquakes with magnitudes larger than 8 and many moderate earthquakes with magnitudes larger than 5.5 (e.g., Adiya, 2016). However, the seismicity in Mongolia is mostly concentrated along the Mongolian-Altai and Gobi-Altai (south and west of the Khangai mountains), the Bulnay fault (north of the Khangai mountains), and around the Mogod area (east of the Khangai mountains) (e.g., Adiya, 2016), which are remote and sparsely populated areas. In contrast, the region around Ulaanbaatar is home to a large population; today, about 1.7 million inhabitants, or half of the country’s total population.
In the west of the Ulaanbaatar region, there are several prominent fault zones, some of which have only recently been identified. The majority of the seismic events in this region are related to three of these: the Khustai, Sharkhai, and Avdar fault zones (Adiya, 2016; Al-Ashkar et al., 2022). Seismicity is typically detected in the upper crust (above ~16 km depth; Ferry et al., 2010; Adiya, 2016). These fault zones are quasi-parallel and are ~100 km long (Figure 1). Historical seismic events are predicted to have produced vertical offsets of up to 10 m, with some sections showing cumulative horizontal offsets of up to 100 m (Al-Ashkar et al., 2022). Based on paleo-seismic surveys, it is estimated that these fault zones could produce earthquakes of magnitude 7+ (Ferry et al., 2010, 2012; Schlupp et al., 2013; Al-Ashkar et al., 2022).
These fault zones pose a serious threat and risk of damage to Ulaanbaatar. Because of this, we aim to characterize the subsurface structure of the active fault zones near Ulaanbaatar in order to better understand them. To do this, we measure magnetotelluric data and generate electrical resistivity models. We aim to give an integrated interpretation of the electrical conductivity structure of the subsurface with geomorphological and geological knowledge, in addition to geodetic measurements, paleo-seismic trenching, fault mechanical models, and near-surface ground-penetrating radar surveys. Understanding the subsurface structure of the region and characterizing the active faults is an important step for assessing seismic hazards.