Ambient Noise Tomography in Central‐South Mongolia

Relying on cooperation projects in Mongolia, “Observation and modeling on the geomagnetic, gravitational field and deep structure in the Far East Areas”, we got dense seismic array data in the area for the first time. By using the vertical component of continuous data, recorded by 69 broadband seismic stations from Aug. 2011 to Jul. 2013 in south-central Mongolia (103.5°E–111.5° E, 43°N–49°N), we calculated the inter-station Empirical Green functions (EGFs) from cross-correlation. 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 manually 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 wave at periods of 6∼30 s in the study area. Checkerboard tests showed that the tomographic results had a high resolution of 0.5° ×0.5°. The results revealed that the phase velocity maps of Rayleigh waves have a small 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 central-south. However, the effect of the phase velocity distribution controlled by the surface geological structure was significantly weaker. 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 Main Mongolian Lineament (MML), indicating that MML was not only a boundary for the topography and tectonics, but also for the crustal structure. The Middle Gobi area always showed an LVZ at periods from 6 s to 30 s, 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.