There are strong interactions between the crust and the mantle in the Abaga area. To study the structure of the crust and upper mantle in that area, the fundamental-mode Rayleigh-wave phase velocity dispersion along 3,331 inter-station paths were estimated by the continuous wavelet transformation method. The dispersion data were used to construct the first 2-D phase velocity maps for waves with periods of 12–80 s and a horizontal resolution of 0.5°×0.5° by applying a linear inversion. The results show that the short-period phase velocity distributions (12–20 s) are affected by exposed Cenozoic volcanoes and the thickness of the sedimentary layer. Phase velocity maps for waves with periods of 30–40 s indicate that the direction of the Solonker suture zone is in good agreement with the extension of the high-speed anomalous connection in the study area. It is concluded that the Solonker suture zone extends throughout the entire lithosphere. For periods of 30–80 s, stable low-velocity anomalies were observed near the Holocene volcanoes in the northeast and south of the study area, suggesting that the low-velocity anomaly is related to volcanic activity. Moreover, the two low-velocity anomalies may be connected, suggesting that the Darigan Volcano may have the same thermal source as the Honggeertu Volcano and even the Datong Volcano. And the thermal source of the two low-velocity anomalies is speculated to be upwelling of hot mantle material. In addition, the velocities in the crust and upper mantle are lower than those from the AK135 model and are close to the velocity for the destroyed North China Craton, and very similar to that for the East African Rift Valley. This may indicate that the lithospheric structure in the Abaga area is active.
The Central Asian Orogenic Belt (CAOB) is one of the largest Paleozoic orogenic belts in the world, and more than 50% of its crust is newly generated. Therefore, it is the most significant continental orogenic belt for continental crustal growth and modification in the global Phanerozoic continental crust (Sengör et al., 1993; Badarch et al., 2002; Xiao et al., 2003, 2004; Windley et al., 2007). It is also recognized as the region having the strongest tectonic movements and crust-mantle interactions during geological history (Petit et al., 2002; Jahn, 2004; Tiberi et al., 2008). It has generally experienced three stages of continental margin growth, post-collision, and intracontinental orogeny (Khain et al., 2002). The Solonker Suture (Soron-Sonid Zuoqi-Linxi area) is generally considered to be the final suture zone in the eastern part of the Central Asian Orogen (Figure 1, Tang, 1990; Sengör et al., 1993; Xiao et al., 2003). However, due to the complex tectonic evolution, the underground structure and dynamic deformation mechanism of the region have always been one of the frontiers of geoscience research.
Figure
1.
Tectonic sketch and distribution of seismic stations in the study region. Blue triangles are stations (NM** are the names of the stations). Red triangles represent the Abaga new generation volcanic group and red diamonds represent Holocene volcanoes. Red circles correspond to cities. Active faults are represented by black solid lines. The red solid line marked with AB is the profile position of Figure 11. (b) The blue solid rectangle is the study area shown in (a). AM: Altai Mountains; BCF: Bayan Obo-Chifeng fault; BL: Baikal Lake; DG: Darigan Volcano; DT: Darigan Volcano; HE: Honggeertu Volcano; HM: Kent Mountain; HP: Hangai Plateau; LXF: Linxi fault; MML: Mongolian Main Structural Line; MOS: Mongolian Okhotsk Suture; OB: Ordos Block; SC: Siberia Continent; SLS: Solonker Suture; SY: Sayan Mountains; TP: Tibetan Plateau; XHF: Xilinhot fault; XMF: Xilamulun fault; ZuF: Zuunbayan fault
This study area is located to the west of the Abaga volcano group in Inner Mongolia. It is on the border between China and Mongolia and belongs to the central and southern CAOB. There has been little research of this area. Cenozoic basalts developed widely in this area (Barry et al., 2003), and have undergone complex crustal deformation (Petit et al. 2002; Jahn, 2004; Xiao et al., 2004; Kröner et al., 2007; Tiberi et al., 2008) and strong crust-mantle interactions (Guo et al., 2016; Wang et al., 2013; Webb and Johnson, 2006). The Abaga volcanic group in this area is an intraplate volcanic group with the largest area and the largest amount of Quaternary volcanic basalts in East Asia (Bai et al., 2012). The origin of the intraplate volcanoes is still controversial. Some studies indicate that the formation of the intraplate volcanoes is related to the lithosphere structure (Ho et al., 2008) and the Pacific subduction plate (Wang et al., 2015; Guo et al., 2016). Other studies have suggested that the intraplate volcanoes formed due to the interaction between the mantle plume and the lithosphere (Windley and Allen, 1993) or due to asthenospheric convection and the delamination of the lower lithosphere (Smith, 2013).
Due to the lack of high-resolution observational data, previous studies on the region have mainly focused on northeastern China in the east of the study area (Zhang et al., 2013a, b; Pan et al., 2014) and central and southern Mongolia in the north of the study area (He et al., 2014; Zhang et al., 2014a, 2017; Pan et al., 2015; Yu et al., 2015, 2016; He et al., 2016, 2017; Hou et al., 2017; Qiang et al., 2017), but have rarely extended to the Abaga area on the China-Mongolia border. Deep seismic reflection and receiver function studies suggest that the Paleo-Asian Ocean subducted and closed in NS direction along the southern edge of the Solonker suture zone (Zhang et al., 2014b; Gong et al., 2016). The receiver function also found crustal thinning and a high wave velocity ratio in the Abaga volcanic group area, which are believed to be caused by the underplating of the Cenozoic volcanic activity (He et al., 2018). Only invalid shear wave splitting has been observed in the Abaga volcanic group, and there are no valid splitting results, which may be due to the local upwelling of mantle material (Qiang and Wu, 2019). However, the vertical resolution of body-wave imaging is not high, and it places weak constraints on shallow structures. Moreover, the receiver function is not sensitive to absolute velocity. In contrast, as one of the main seismological methods for detecting structures in the crust and upper mantle, surface-wave imaging has good vertical resolution and it can constrain low-velocity layers. The phase velocity is a comprehensive reflection of the vertical variation of the S wave velocity in a certain depth range of the ray path propagation area. The surface-wave phase velocity is more sensitive to the S wave velocity than the P wave velocity and rock density. The lateral change of the phase velocity for different periods can reflect that of the S-wave velocity at different depth ranges.
In this paper, we present tomography results for regional surface waves using almost three years of continuous seismic observation data obtained from 36 stations (NM arrays) set up in the Abaga area by the Institute of Geophysics, China Earthquake Administration. We first extracted the phase velocity dispersion curve of fundamental Rayleigh waves between two stations for waves with periods of 12–80 s. Then, the surface-wave tomography method proposed by Yanovskaya and Ditmar (1990) and Ditmar and Yanovskaya (1987) was used to reconstruct a two-dimensional phase velocity distribution map with a resolution of 0.5°×0.5° in the Abaga area. Finally, we discuss the geological significance of these results.
2.
Data and methods
From October 2012 to June 2015, broadband seismic observations were made by the Institute of Geophysics, China Earthquake Administration, to the west of the Abaga volcano group of Inner Mongolia. The distance between adjacent stations was about 40 km. Seismometers used include GURALP CMG-3ESPC, GURALP CMG-3T, and Nanometrics Trillium G120P. All seismometers used GPS times and positions to ensure consistency. Stations in the study area are located north of the Yinshan Mountains and south of the China-Mongolia border. The western stations are across the Solonker suture zone. There are Abaga Cenozoic volcanic groups in the northeast of the study area. In addition, there are exposed Holocene volcanoes on the north and south sides of the study area (Figure 1).
The surface-wave tomography in this paper is based on classical ray theory and obtained by the dual-station method. Compared with the single-station method, the surface-wave dual-station method is more accurate. Moreover, the stations are evenly distributed, which ensures that the dual-station method has uniform and good ray coverage. Here, S1, S2, E, and E' represent the far station, near station, earthquake focus, and epicenter, respectively. Referring to the seismic catalog and source parameters given by USGS, we selected vertical components that meet the following criteria:
1) The epicentral distances (α and β) were limited to the range 10°–120°, which reduces the adverse influences of near-source effects and high-order surface waves as much as possible.
2) Shallow source earthquakes with magnitudes between 5.5 and 7.5 and focal depth (Δ) less than 70 km were selected to ensure that distant earthquake events with relatively well developed surface-wave dispersion and high signal-to-noise ratios can be selected.
3) According to ray theory, if surface waves travel along the path of a large arc, the two stations must be on the same large arc as the epicenter. In practice, it is difficult for a station pair to meet this requirement. Therefore, when selecting station pairs, the deviation between the azimuth (η) from the far station to the near station and the azimuth (θ) from the far station to the seismic event was required to be less than 5° (Figure 2, Hou et al., 2017).
Figure
2.
Paths of two stations. E, the earthquake focus; E', the earthquake epicenter; S1, the far station; S2, the near station; α, the epicentral distance between E' and S1; β, the epicentral distance between E' and S2; Δ, the focal depth; η, the deviation between the azimuths of the far station and the near station; θ, the deviation between the azimuths of the far station and the seismic event
Thus, 356 global earthquake events (Figure 3) meeting the above criteria were selected for this study. The vast majority of earthquakes were in the Pacific Rim. However, for the stations in the study area, these earthquake events have a good overall coverage, making the dual-station method more evenly distributed and better resolved.
Figure
3.
Distribution of events. The red star is the center of the study area. The circles marked 30°, 60°, 90°, and 120° indicate where the epicentral distances between the earthquake epicenter and the station are 30°, 60°, 90°, and 120°, respectively. The blue lines represent the plate boundaries. The different circles represent the location of seismic events with magnitude indicated by the size and source depth indicated by the color
Before extracting the Rayleigh-wave dispersion, we first resampled the selected original seismic waveform records to 1 Hz. We removed the mean and linear trends, applied a symmetric taper function to both ends of the data, filtered and removed instrument responses. We excluded four stations (NM08, NM20, NM30, and NM31) as we did not obtain data from them due to instrument failure. After the aforementioned processing, a comparison of the waveforms before and after removing instrument responses for the three instruments is shown in Figure 4. It can be seen that the waveforms in each pair are different, but they are relatively close, because they all represent velocity data.
Figure
4.
Comparison of waveforms before (upper) and after (lower) removing the instrument response. (a) Data measured by seismometer GURALP CMG-3ESPC. P marks the arrival time of the P-wave. NM01 is the name of the recording station. 001 represents the vertical component. The date and time are the earthquake occurred. The number 7 and 15 in the lower right corner represents the QDP parameter in sac, which is used to control the resolution. (b) Same as (a) but for GURALP CMG-3T. (c) Same as (a) but for Nanometrics Trillium G120P
In this paper, a frequency-time analysis based on a continuous wavelet transform (Wu et al., 2009) was used to extract 3,331 fundamental Rayleigh-wave phase velocity dispersion curves between pairs of stations. After removing non-smooth dispersions, we finally got 3,005 high-quality dispersion curves.
For the same great circle ray path (station pair), there may be multiple earthquake events that meet the requirements, and multiple dispersion curves will be extracted, that is, repeated ray paths will appear. If the repeated ray paths (station pairs) in a certain period are processed uniquely, there are actually up to 186 different ray paths (station pairs). Figure 5 shows the distribution of phase velocity ray paths for periods of 12, 20, 30, 40, 50, 60, 70, and 80 s. The paths are densely distributed and have good azimuth coverage. There are two methods for dealing with a repeated path: ① normalize the cross-correlation waveform of the repeated path and then extract the dispersion curve after stacking and ② extract the dispersion curve corresponding to each earthquake event independently and then average these dispersion curves. This paper takes the latter method. Figure 6 shows a processing example. Focusing on one station pair as the research object, the great circle ray path was repeated for each of the 28 earthquake events from two opposite directions (Figure 6a). The measured phase velocity dispersion curves of each earthquake event in this example ray path and their average are plotted in Figure 6b.
Figure
5.
Distribution of paths for Rayleigh-wave phase velocity measurements. The blue triangles are the stations. A black line between two stations indicates that there is a path between them. The red polygon encloses the area covered by the stations
Figure
6.
Examples of data processing and measurement deviation for repeated paths. (a) Events distributed along the same great circle ray path. Red triangles correspond to the station pair. (b) Phase velocity dispersion curves for the same great circle ray path. The black lines represent the measured phase velocity dispersion curves of the example ray path in (a), and the red line represents the average value. (c) Variation of standard deviation with period. The black line represents the standard deviation of the example ray path in (a). The red line represents the standard deviation of all ray paths in the study area
This paper uses the methods of Ritzwoller and Levshin (1998) and Li et al. (2013) to calculate standard deviations. For a small number of single dispersion curves without repeated paths, the standard deviation between a single dispersion curve and the average value of all dispersion curves in the study area was treated as the measurement deviation of this ray path. For repeated ray paths, the standard deviation of multiple dispersion curves for the same ray path was calculated as the measurement deviation of the average dispersion curve of this ray path (Figure 6). Figure 6c shows the variation of the measurement deviation versus period for this example ray path and the average dispersion curves of all paths in the study area. Over 10–80 s, the deviation for long periods is higher than that for short periods, but the deviation is basically within the range 0.02–0.06 km/s.
This paper uses the 2-D inversion method proposed by Ditmar and Yanovskaya (1987) and Yanovskaya and Ditmar (1990), which is a generalization of the Backus-Gillbert one-dimensional inversion theory to two dimensions. This method obtains the phase velocity distribution of different periodic surface waves by minimizing the penalty function. In addition, the method also gives the horizontal spatial resolution of each grid point when inverting the phase velocity (Figure 7). It can be seen that the phase velocity resolution of the entire ray coverage area from 12 to 80 s is basically within 50 km, except for the marginal areas and areas not covered by rays.
Figure
7.
Horizontal resolution maps of each periods in the study region. The filled area represents the area covered by the stations
If the mesh elements are too large, some data may be wasted; if the mesh elements are too small, then artificial false anomalies may be introduced into the inversion results. Therefore, we performed a checkerboard test. We divided the study area into a 0.5°×0.5° grid. The initial model velocity was set as 3.9 km/s, and the associated a priori error was ±0.3 km/s. We added Gaussian noise with a standard deviation of 0.15 s to the travel time of a theoretical ray. Then, using the same inversion method and parameters settings as described above, we calculated the ray distributions for periods of 12, 20, 30, 40, 50, 60, 70 and 80 s. The results show that with a grid size of 0.5°×0.5°, the input model can be restored well in our ray coverage area (Figure 8). This also shows that it is appropriate to divide the study area into a grid of 0.5°×0.5° during the inversion.
Figure
8.
Checkerboard test of each periods in the study region. The black polygon encloses the area covered by the stations
Using the aforementioned inversion method and para-meters, the Rayleigh-wave phase velocity distributions for a 0.5°×0.5° grid was obtained for periods of 12, 20, 30, 40, 50, 60, 70 and 80 s (Figure 9). Since the Rayleigh-wave velocity is more sensitive to the S-wave velocity, by calculating the sensitive kernel, the phase velocity distri-bution of different periods can be linked to the S-wave velocity change at the corresponding depth (Figure 10, Yu et al., 2015). In the following, the relation between the Rayleigh-wave phase velocity distribution and the crust-mantle structure for the above periods will be discussed separately.
Figure
9.
Phase velocity maps of Rayleigh waves in the study region. The number in the upper left corner represents the period, and the number in the upper right corner is the average phase velocity (km/s) for each period
For short periods (12 and 20 s), the Rayleigh-wave phase velocity distribution is mainly related to the velocity structure of the middle and upper crust and its lateral changes (Figure 9). From the inversion results, the 12-s period of the NM38 station and the 20-s period of the NM35 station have low-velocity anomalies. This is in good agreement with the significant negative amplitude anomalies near the NM38 station at a depth of 15 km and the NM35 station at a depth of 30 km in the Abaga volcano area detected with the common conversion point stacking profile (He et al., 2018). Moreover, it also shows that the reliability of the inversion method used in this paper is relatively high.
The phase velocity results for periods of 12 and 20 s show that the northeastern part of the study area (near NM39) and the south-central part (near NM03, NM04, and NM06) also have low-velocity anomalies. The analysis suggests that the low-velocity anomalies in the northeast of the study area may be related to the Cenozoic volcanic rocks exposed in the area, while the low-velocity anomalies in the central and southern areas are not only related to the volcanic rocks exposed in the area, but also to the thickness of the sedimentary layer below it. In addition, for periods of 12 s, the southeast of the study area (near NM40 and NM38) has low-velocity anomalies. The southeastern part of the study area is the vast Hunshandak Desert, which has many sedimentary areas. The thick sedimentary layer in this area results in relatively low phase velocities.
3.2
Intermediate periods
For intermediate periods (30 and 40 s), the Rayleigh-wave phase velocity distribution mainly reflects the velocity structure and its lateral changes in the middle and lower crust and even the top of the upper mantle (Figure 9). Research based on the receiver function shows that the thickness of the crust in the study area is 35–44 km, with an average of about 40 km. On the whole, the thickness of the crust in the southwestern part of the study area is thicker, while that in the northeast is thinner. The crust at the edge of the Ordos Basin in the southwest is thicker, and the crust in the Abaga volcanic group in the northeast is significantly thinner (He et al., 2018). Thus, due to the variation in crustal thickness in this area, the phase velocity distribution for periods of 30–40 s should gradually increase from the southwest to the northeast. However, in addition to the above-mentioned overall trend, the phase velocity distribution obtained in this study also has other anomalies.
Anomaly 1: There is a low-velocity anomaly in the southern part of Darigan Volcano, which extends southward. To clarify the cause of this anomaly, the results for the area with stations NM39, NM35, NM23, and NM13 near Darigan Volcano and with stations NM07 and NM09 were compared with the area with purely ineffective splitting in the shear wave splitting study (Qiang and Wu, 2019). The two areas have a high spatial consistency. Moreover, the area with a high crustal wave velocity ratio (~1.84) obtained by the receiver function study (He et al., 2018), the area with a low-velocity anomaly found by the body-wave imaging research (Zhang et al., 2017), and the extensive basalt exposed area (Guo et al., 2016) all have certain similarities in spatial position with the low-velocity anomaly near Darigan Volcano in the northern part of the study region. In addition, there is an obvious low-speed anomaly between 44°N and 45°N for periods of 12–80 s (Figure 11). Therefore, we speculate that the low-velocity anomaly of the middle and lower crust is due to the upwelling of hot mantle material. This inference is consistent with the geochemical research that indicates that the volcanic source rocks in the Solonker suture zone are mainly mantle material and that there is partial melting of the newly formed lower crust material (Tan et al., 2017; Gao et al., 2018).
Figure
11.
Phase velocity map plotted along the AB profile. The average is 3.65 km/s
Anomaly 2: There are two obvious high-velocity anomalies in the Solonker suture zone. The extension of the line connecting the two anomalous positions is in the same direction as the Solonker suture zone, which runs near EW. It is speculated that these two high-velocity anomalies were originally distributed continuously in strips but were truncated as anomaly 1 extended southeast. This is consistent with the conclusion of the deep seismic reflection study that “the Moho surface of the Solonker suture zone is discontinuous and the tendency of the overlying crust is clutter, which is related to the intrusion of mantle material in the lithosphere” (Zhang et al., 2014b). On the one hand, the planar distribution of the strip-shaped high-velocity anomaly agrees well with the orientation and direction of the Solonker suture zone. On the other hand, it can also be seen from Figure 11 that the Solonker suture zone (between 42°N and 44°N) has a high-speed anomaly for periods of 30–40 s or even deeper. The significant characteristics of this anomaly further reveal that the Solonker suture zone, which may be the final closed boundary of the ancient Asian Ocean (Li, 2006; Li et al., 2014), should reach the bottom of the lithosphere.
3.3
Long periods
The phase velocity dispersion for periods greater than 40 s mainly reflects the velocity structure of the upper mantle and its lateral changes (Figure 9). The phase velocity distributions for periods of 50–80 s show that the northeastern part of the study area (around NM39, NM35, and NM36) has a low-velocity anomaly that extends from southwest to the south of the study area (near NM06). The low-velocity anomaly in the northeast of the study area is highly coincident with the distribution of the Darigan volcanic area, and the more northward the low-velocity anomalies are, the more significant the agreement is, indicating that they are likely to be affected by Darigan Volcano. The low-velocity anomalies in the central and southern regions of the study area are more obvious to the south. However, due to the invalid data from stations NM08 and NM30 and the boundary effects, the spread of the low-velocity anomalies to Honggeertu Volcano and Datong Volcano in the southern area is not obvious. On a larger scale, the southern part of the study area is close to the Ordos Block (Figure 1). Previous research shows that the Ordos Block behaves as a high-velocity anomaly (Huang and Zhao, 2006; Li et al., 2006; Tian et al., 2009; Obrebski et al., 2012), which is in stark contrast to the low-velocity anomaly near the Quaternary volcanoes presented here. The imaging results presented in this article show that the low-velocity anomalous zone extending in the southwest direction of the study area just connects the low-velocity anomalies of the two major volcanic distribution areas. The low-velocity anomaly runs through the upper mantle, reaching the mantle transition zone or the lower mantle top (Figure 9). In addition, body-wave imaging shows that the low-velocity anomalies below the Datong Volcano extend westward (Ding et al., 2009; Tian et al., 2009; Lei, 2012). Therefore, we speculate that the deep materials of Darigan Volcano, Honggeertu Volcano, and even Datong Volcano may be connected or interactive, or they may originate from the same upwelling of hot mantle material. This agrees with previous research: “the lower velocity anomaly in central and southern Mongolia and the lower velocity anomaly of Datong Volcano seem to be unified in the upper mantle” (Li et al., 2006; Li and Van der Hilst, 2010) and “the Gobi desert in central and southern Mongolia has the same thermal source as Darigan Volcano” (Zhang et al., 2014a).
3.4
Summary
In this article, we first averaged the dispersion curves of multiple paths for the same station pair over the entire study area to create a dispersion curve. Then, we averaged the fundamental-order Rayleigh-wave phase velocity dispersion curves for the paths between all station pairs. This effectively prevents multiple repeated paths for the same station pair from participating in the calculation and causing deviations. Finally, we obtained a dispersion curve that characterizes the entire Abaga area (Figure 12). Generally, for periods below 80 s, the dispersion curve in the Abaga area is lower than that calculated by the global continental average model (the AK135 model). This means that compared with the AK135 model, the crust in the study area is thicker and the upper mantle phase velocity is lower, which confirms the relatively young and active geological structural background in the area (Cunningham, 2001; Petit et al., 1998).
Figure
12.
Surface-wave phase velocity dispersion curves. The black lines are 3005 high-quality dispersion curves. The red diamonds, pink triangles, blue squares, orange circles and green solid lines represent the average phase velocity of this study, central and southern Mongolia (Yu et al., 2015), the eastern and western parts of the Great Rift Valley (Adams et al., 2012), the eastern NCC (Li et al., 2009) and the AK135 model respectively
For short periods (<20 s), the Rayleigh-wave velocity dispersion curve for the study area, as in south-central Mongolia (Yu et al., 2015), is significantly higher than that for the destroyed eastern North China Craton (NCC) (Li et al., 2009), indicating that the sediment in the Abaga area is thicker than in the eastern NCC. The middle-period (20–40 s) Rayleigh velocity in the study area is significantly higher than that in south-central Mongolia (Yu et al., 2015) and closer to the eastern part of NCC (Li et al., 2009), indicating that the lithosphere in the study area is thicker than in central-south Mongolia and close to the thickness in NCC. For long periods (40–80 s), the Rayleigh-wave velocity dispersion in the study area is close to that of south-central Mongolia (Yu et al., 2015) and the eastern and western parts of the Great Rift Valley (Adams et al., 2012), but slightly higher than that of the eastern NCC (Li et al., 2009). Therefore, it is inferred that the lithosphere in the Abaga area is geologically structurally active.
4.
Conclusions
Based on data from 36 seismic observation stations deployed in the Abaga region of Inner Mongolia from October 2012 to June 2015, a wavelet transform frequency-time analysis (Wu et al., 2009) was used to extract 3,331 fundamental Rayleigh-wave phase velocity dispersion curves from different station pairs. Phase velocity distributions for periods of 12–80 s were calculated for the study area using a two-dimensional linear inversion method (Ditmar and Yanovskaya, 1987; Yanovskaya and Ditmar, 1990). The results of this study show that:
1) The short-period phase velocity distributions (12–20 s) in the study area are related to the exposed Cenozoic volcanic rock and the thickness of the sedimentary layer.
2) For periods of 30–40 s, the Solonker suture zone passing through the Abaga study area is in good agreement with the high-velocity anomaly distribution extending east-west. It was inferred that the Solonker suture zone extends throughout the entire lithosphere.
3) For periods of 40–80 s, there are stable low-speed anomalies near the Darigan Volcano in the northeast of the study area and the Honggeertu Volcano in the south. These anomalies may be related to Holocene volcanic activity.
4) For periods of 30–80 s, the low-speed anomalies near Darigan Volcano and Honggeertu Volcano may be connected, indicating that they may have the same source of thermal material, which may be due to the upwelling of hot mantle material.
5) The crust-mantle phase velocity in the Abaga region of Inner Mongolia is lower than the global continental average and slightly higher than for the destroyed NCC region. It is similar to that for the Great Rift Valley in East Africa, revealing that the Abaga region has an active lithosphere.
Acknowledgments
This study is supported by National Natural Science Foundation of China (Nos. 41674094 and 41504073). We thank everyone who participated in the selection, erection, and maintenance of the stations. Some of the figures in this paper were drawn using the GMT program (Wessel and Smith, 1998). We thank the two reviewers for their invaluable guidance.
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DOI:10.3389/feart.2023.1131393
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Qing-Dong Ye
2.
The First Monitoring and Application Center, China Earthquake Administration, Tianjin 300180, China
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