The Central Mongolia Seismic Experiment: Multiple Applications of Temporary Broadband Seismic Arrays

As part of multidisciplinary research on intracontinental deformation and surface uplift, we deployed a temporary broadband seismic array in central Mongolia covering an ∼900 × 600 km area extending from Lake Khövsgöl in the north to the Altai Mountains in the south. A total of 112 broadband stations were deployed as three separate subarrays in two separate mobilizations. Each subarray recorded local, regional, and teleseismic earthquakes for a 21-month period. Although the primary purpose of the array is to characterize the lithosphere and sublithospheric mantle, the array recorded a number of events of potential interest to the broader geoscience community including the Chelyabinsk meteor explosion, North Korean nuclear tests, the deep Mw 8.3 Sea of Okhotsk earthquake, and large megathrust events offshore Chile and in Nepal. The array includes the first dense deployment of seismometers across the Hangay dome, a region previously believed to be relatively aseismic serving as a rigid block focusing strain to the west and south along the Mongolian and Gobi-Altai. Initial results from local earthquakes recorded by the array suggest that the Hangay is deforming rather than behaving as a rigid block and that the earthquake potential of faults within the Hangay should be incorporated in hazard analysis for Mongolia. Supplemental Content: Table of station location, sensor type, data range and recovery, sensor orientation, data quality, and site characteristics, and figures showing subarrays and station names of the central Mongolia seismic experiment plotted on Google Earth, and a plot of sensor orientation and histogram of uncertainty. INTRODUCTION Mongolia sits deep in the Asian continental interior between the Siberian craton to the north, the diffuse northern edge of deformation associated with the India–Asia collision to the south, and far-field subduction of the Pacific plate to the east (Fig. 1). Central and western Mongolia constitute a significant portion of the greater Mongolian plateau, an ∼2:6 million km2 area of central Asia with an average elevation of ∼1500 m. High-elevation low-relief surfaces are common on continents but are not predicted by plate tectonics. The origin and persistence of continental plateaus through time provides insight into the evolution of continents and interactions between mantle dynamics and surface processes. These large-scale topographic features impact the geologic record, climate, and biogeography. The high topography of the Mongolian plateau has been attributed to far-field effects of India–Asia convergence, Pacific plate subduction, mantle plume activity, convective mantle flow, and magmatic underplating (Molnar and Tapponnier, 1975; Windley and Allen, 1993; Cunningham, 2001; Petit et al., 2002, 2008; Yanovskaya and Kozhevnikov, 2003; Bayasgalan et al., 2005; Zorin et al., 2006). Within central Mongolia, the broad domal Hangay upland (∼200; 000 km2) is embedded in the greater Mongolian plateau. Elevations within the Hangay average 2500 m, approximately 1000 m above the regional trend. Locally, the highest peak Otgontenger reaches just over 4000 m. The drainage divide in the Hangay separates rivers flowing north to the Arctic Ocean from those flowing into internally drained basins to the west and south. A kinematic transition between predominantly compressional deformation to the south and extension adjacent to the Siberian craton takes place in Mongolia. In western Mongolia, northward-directed shortening related to the India–Asia collision decreases from south to north from ∼10 mm=yr south of the Altai, to∼4 mm=year in the Altai, and to 0 on the Siberian craton (Calais et al., 2003). Central and eastern Mongolia moves eastward at ∼4mm=yr (Calais et al., 2003). Major strike-slip faults within Mongolia accommodate the transition from shortening to extension and give rise to the Mongolian 1364 Seismological Research Letters Volume 90, Number 3 May/June 2019 doi: 10.1785/0220180360 Downloaded from http://pubs.geoscienceworld.org/ssa/srl/article-pdf/90/3/1364/4686492/srl-2018360.1.pdf by Missouri University of Science and Technology user on 07 December 2021 and Gobi-Altai ranges. The Bulnay and Gobi-Altai fault systems have sustained some of the largest recorded intracontinental earthquakes, four events M ≥ 8:0 within a 53 yr period in the early-to-mid-1900s (Tapponnier and Molnar, 1979; Khil’ko et al., 1985; Baljinnyam et al., 1993; Schlupp and Cisternas, 2007). Extension is accommodated by the Baikal, Tunka, Khövsgöl, Darkhad, and Busiingol rifts that wrap around the southern and eastern margin of the Siberian craton (Fig. 1). Mongolia has a complex geologic history. The crust is composed of Archean to early Proterozoic crystalline rocks modified by Paleozoic accretionary events associated with formation of the central Asian orogenic belt, a protracted and significant period of continental growth involving the opening and closing of ocean basins in the Neoproterozoic and early Phanerozoic (1000–250Ma; Şengör et al., 1993; Badarch et al., 2002; Jahn, 2004; Windley et al., 2007; Wilhem et al., 2012). The accreted terranes of Mongolia sit between the Siberian craton to the north and the Tarim and North China cratons to the south. Younger Cenozoic deformation and basalt volcanism continues today. The elevated low-relief landscape hosts a 30 Ma record of intermittent basalt magmatism sourced from the sublithospheric mantle (Barry et al., 2003; Yarmolyuk et al., 2008; Hunt et al., 2012; Ancuta et al., 2014; Carlson and Ionov, 2014; Ancuta et al., 2018). A number of these basalts contain mantle and lower crustal xenoliths (Stosch et al., 1995; Ionov, 2007; Carlson and Ionov, 2014). As part of a larger multidisciplinary project investigating the origin of high topography in continental interiors, we deployed temporary broadband seismic stations from 2012 to 2016 to determine the structure and composition of the lithosphere and sublithospheric mantle beneath the region. The deployment is a collaborative effort between Lehigh University, the University of Florida, and the Institute of Astronomy and Geophysics, Mongolian Academy of Sciences. The central Mongolia seismic experiment consists of 112 broadband seismic stations in two separate deployments covering an ∼900 × 600 km region in central Mongolia (Fig. 1). Although the primary purpose of the seismic experiment is to look at crustal and upper-mantle structure, the data recorded by the array are applicable to a number of studies. Local and regional earthquakes recorded by the array can be used to improve seismic hazard assessment in Mongolia. Data recorded by the array fill a significant gap in existing regional velocity and attenuation tomographic models for central and eastern Asia and can be used to improve geodynamic models and nuclear monitoring capabilities in this part of the world. Central Mongolia is antipodal to South America providing a window into deep Earth structure (Rial and Cormier, 1980; Butler et al., 1986; Poupinet et al., 1993; Butler and Tsuboi, 2010; Retailleau et al., 2014). Data recorded by the array have the potential to contribute to studies of surface and atmospheric processes. A number of the stations are located along rivers in the Hangay and Altai ranges and can be used to study fluvial processes and erosion (Burtin et al., 2008; Hsu et al., 2011; Schmandt et al., 2013; Roth et al., 2014). Central Mongolia is a major site of cyclogenesis in the lee of the Sayan, Hangay, and Altai ranges linked to loess deposition in China and midwinter storms in the North Pacific (Chung et al., 1976; Adachi and Kimura, 2007; Roe, 2009; Penny et al., 2010; Caves et al., 2015). Within the time period of the two deployments that make up the central Mongolia seismic experiment, we recorded a number of significant events: the Chelyabinsk meteor explosion in 2013, nuclear ▴ Figure 1. (a) Tectonic setting of Mongolia. AT, Altyn Tagh; BR, Baikal rift; DL, Mongolian depression of lakes; GA, Gobi-Altai; HD, Hangay dome; HR, Khövsgöl rift; KS, Kunlun Shan; MA, Mongolian Altai; QS, Qilian Shan; TB, Tunka basin; TS, Tien Shan. (b) Station locations central Mongolia seismic experiment, subarrays, and Global Seismic Network station ULN. More detailed station location maps including station names are included in E Figure S1 (available in the supplemental content to this article). The color version of this figure is available only in the electronic edition. Seismological Research Letters Volume 90, Number 3 May/June 2019 1365 Downloaded from http://pubs.geoscienceworld.org/ssa/srl/article-pdf/90/3/1364/4686492/srl-2018360.1.pdf by Missouri University of Science and Technology user on 07 December 2021 tests by North Korea in 2013 and 2016, the deep 2013Mw 8.3 Sea of Okhotsk earthquake and aftershock sequence, megathrust ruptures of the Chilean subduction zone in the 2014 Iquique Mw 8.2 and 2015 Illapel Mw 8.3 earthquakes, and the 2015 Mw 7.8 Gorkha earthquake in Nepal. In this article, we give a complete description of the central Mongolia seismic experiment, provide an overview of the data acquired, data quality, and noise characteristics, review events of interest recorded by the array, and present preliminary results of local seismicity in the Hangay. INSTRUMENT DEPLOYMENT A total of 112 broadband seismic stations were deployed in central Mongolia, extending from the Lake Khövsgöl region near the Mongolia–Russia border, through the Hangay dome region, and into the Gobi-Altai region near the Mongolia– China border (Fig. 1). The stations were deployed as three separate subarrays in two separate mobilizations. From June 2012 to April 2014, a 72-station seismic array was deployed across the Hangay dome in central Mongolia, 61 STS-2 sensors from Incorporated Research Institutions for Seismology–Program for the Array Seismic Studies of the Continental Lithosphere (IRIS-PASSCAL) and 7 STS-2.5 and 4 STS-2 sensors from the University of Flo