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Currently, the detailed structure beneath central-south Mongolia (103.5 degrees E-111.5 degrees E, 43 degrees N-49 degrees N) is poorly known from previous studies. In order to investigate the detailed structure of the study region, 69 portable broadband stations were deployed in central-south Mongolia from August 2011 to July 2013. This cooperation project in Mongolia provided dense seismic array data for the area for the first time.

Ambient noise tomography is increasingly used in studying the structure of the crust and upper mantle. We calculated the inter-station empirical green functions (EGFs) from cross-correlation using the vertical component of the continuous data recorded by these 69 broadband seismic stations from August 2011 to July 2013 in south-central Mongolia. In addition, a time-frequency analysis based on a continuous wavelet transform was used to extract the Rayleigh wave phase velocity dispersion curves. Through quality control and manual screening, we finally obtained a total number of 1478 phase velocity dispersion curves at periods ranging from 6 s to 30 s. The Ditmar & Yanovskaya method was utilized to obtain phase velocity maps of the Rayleigh waves at periods of 6 similar to 30 s in the study area.

Checkerboard tests showed that the tomographic results had a high resolution of 0.5 degrees X 0.5 degrees. The results revealed that the phase velocity maps of the Rayleigh waves had a perturbation of about +/- 2%. A phase velocity map with a short period (e.g., 6 s) was imaged, with high-speed anomalies corresponding to the mountain ranges in the north and low-speed anomalies coinciding with the sedimentary basin and Gobi Desert in the central-south region. As the period (15 s, 20 s) increased, the imaging still showed a high-velocity zone (HVZ) in the north and low-velocity zone (LVZ) in the middle. The phase velocity maps with a long period (e.g., 30 s) showed an HVZ in the north that expanded further to the south than those with shorter periods (e.g., 15 s and 20 s), which is associated with the thinner crust in the south compared to that in the north. On those maps with long periods (e.g., 20 s, 30 s), there were significant differences between the northern and southern sides of the main Mongolian lineament (MML). On maps with periods ranging from 6 s to 30 s, the middle Gobi area was imaged with an obvious low speed, while the Hangay-Hentey basin was always imaged with an obvious high velocity in the north. We compared the tomographic result at 15 s with that from the classic two-station method using earthquake data, and a phase velocity difference of only about 1% was found.

The S-wave velocity structure of the crust and upper mantle showed weekly lateral heterogeneity (a perturbation of about +/- 2%) in central-south Mongolia. The phase velocity distribution at a short period (e.g., 6 s) was effectively related to the geology tectonic units on the surface. However, the effect of the phase velocity distribution controlled by the surface geological structure was significantly weaker as the periods increased (e.g., 15 s, 20 s). In the phase velocity maps with a long period (e.g., 30 s), the phase velocity distribution was mainly associated with the crustal thickness. For the MML, this was not only a boundary for the topography and tectonics, but also for the crustal structure. The middle Gobi area always showed an LVZ, which could have been related to Cenozoic volcanism, while the Hangay-Hentey basin was always imaged with an HVZ, which could have been associated with the old, stable layers in the north.