Mogod Soum, Bulgan aimag, is located in the eastern part of Khangai Dome. During the winter, the soum is heavily affected by air pollution due to coal burning. Using geothermal resources in the region, manifested by hot springs, could dramatically reduce air pollution. To understand the nature of the geothermal reservoir feeding the hot springs, we conducted Magnetotelluric surveys in the Mogod hot spring region during the fieldwork in 2020, 2021, 2022 and 2024. To obtain a subsurface electrical conductivity model of the hot spring area with magnetotellurics, we inverted data from 60 unique sites. As a tool for inversion, we used a high-order finite element code available to locally refined unstructured meshes to ensure numerical accuracy with a sufficiently fine discretization of the inversion domain while keeping the computational cost feasible. We inverted the full impedance tensor to recover a 3-D electrical conductivity model. The best-fitting model provides important new insights into the subsurface structure of the Mogod region. ... Read more

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.
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Within the Ulaanbaatar region, Mongolia, 120 earthquake events were recorded with a magnitude between 3.4 and 5.6 and 978 earthquake events had a magnitude between 2.5 and 3.4, for the period from 1994 to 2016. Several of these have been strongly felt by residents of Ulaanbaatar. Historical records, since 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. The seismicity in Mongolia is concentrated along the Mongolian-Altai and Gobi-Altai, the Bulnay (north of the Khangai mountains), and around Mogod (east of the Khangai mountains).

In the west of the Ulaanbaatar region there are several prominent fault zones, some only identified very recently. The majority of the seismic events in this region are related to the Khustai, Sharkhai, and Avdar fault zones. Seismicity is typically detected in the upper ~16 km of the crust. These fault zones are 100+ km long and historical events are predicted to have produced vertical offsets of up to 10 m; some sections show a cumulative horizontal offset of up to 100 m. Based on paleo-seismic surveys, it is estimated that these fault zones could produce earthquakes up to magnitude 7. Therefore, these faults pose a serious threat and risk of damage to Ulaanbaatar.

In this presentation we aim to characterize the active fault zones near Ulaanbaatar with electrical resistivity models generated from magnetotelluric data. In mid-2024 we carried out measurements across the Khustai, Sharkhai, and Avdar fault zones and modeled the local features near the fault traces and the regional crustal features of the region. Preliminary models show several low-resistivity features (approximately <100 Ωm) in the near-surface. The upper crust (0-25 km depth) appears to have a generally high-resistivity (~10,000 Ωm), whereas the lower crust (25–50 km depth) appears to have a lower resistivity (approximately <100 Ωm).

We aim to give an integrated interpretation of the electrical conductivity structure of the subsurface with geomorphological and geological knowledge, geodetic measurements, paleo-seismic trenching, and near-surface ground-penetrating radar surveys. We also aim to discuss the relation with fault mechanical models and local fault damage zones, and the relevance of the low slip rate. Understanding the subsurface structure of the region and characterizing the active faults is an important step for assessing the seismic hazard.
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