## 4 Results
### Relationship between Low-Frequency Signal and Beneath Structure  
As described earlier, the morphological characteristics of the low-frequency signals recorded at DSM, WUH and SZS before the Ms5.8 earthquake in Azuoqi, Inner Mongolia, in 2015, are completely different, and the low-frequency abnormal signals lasting about 100 h before the main earthquake are extracted from the data recorded at WUH (Figures 6-8). Does the low-frequency abnormal signal form from the three seismic stations reflect the state and process of rock micro fracture below the epicenter? The different low-frequency anomaly patterns from the three seismic stations also reflect that the unstable change direction of the stress field is from south to north, that is, the unstable micro fracture from the stress is transmitted from below SZS to below WUH and then blocked under WUH. Until the main earthquake of the Ms5.8 earthquake occurs, the energy continues to be transmitted north to DSM. Therefore, we compared the positions of the temporary stations around the epicenter with DSM, WUH and SZS and found that the erection positions of the stations have the following characteristics: 64052 in L6 is equivalent to SZS, 15597 in L9 is equivalent to WUH, and 15670 in L7 is equivalent to DSM. The underground structure change characteristics of 64052, 15597 and represent the underground structure change characteristics of SZS, WUH and DSM, respectively. The results show that the underground structure of SZS (64052), located about 60 km from the epicenter of the Ms5.8 earthquake in Azuoqi, Inner Mongolia, in 2015, has changed from October 2013 to July 2014. Until the Ms5.8 earthquake in Azuoqi, Inner Mongolia, its underground morphological characteristics were constantly changing, which was consistent with the characteristics of the low-frequency signal and extreme instability of Shizuishan station (Figures 15 and 16).  
Figure 16: Variation characteristics of underground structure of 64052 (SZS) in different time periods.  
Figure 15: Low-frequency signal characteristics and 64052 in L6 line = SZS.  
The underground structure morphology of WUH (15597), which is located 42 km from the epicenter of the 2015 Azuoqi Ms5.8 earthquake in Inner Mongolia, did not change as significantly as that of DSM (15670) from October 2013 to July 2014. It only showed significant changes from June 2014 to April 2015. After the earthquake, the medium morphology recovered to a state similar to that of DSM (15670). The SZS appeared earlier than the WUH, in terms of the time when the instability of the underground structure began (Figures 17 and 18).  
The underground structure of DSM (15670), which is located 158 km north of the epicenter of the 2015 Azuoqi Ms5.8 earthquake in Inner Mongolia, was basically the same from October 2013 to July 2014 and from June 2014 to April 2015, but there were obvious structural changes from April 2015 to October 2016. In other words, the underground structure of DSM changed significantly under the influence of the earthquake after the Ms5.8 earthquake in Azuoqi, Inner Mongolia, in 2015. In terms of time, the structural changes of DSM, WUH and SZS are different. From the perspective of the time of the structural change under the monitoring stations, SZS changes first, followed by WUH, and finally DSM. This feature shows that the stress transmission direction is from south to north, which is consistent with the northeast compression of the Qinghai Tibet block on the Ordos block in the southwest, and that the stress transmission direction reflected by the maximum tracking results from the seismic wave spectrum envelope (Figures 19 and 20).  
Figure 17: Low-frequency signal characteristics and 15597 in L9 = WUH.  
Figure 18: Variation characteristics of underground structure of 15597 (WUH) in different time periods.