# Regional Seismicity of the Northeastern Tibetan Plateau Revealed by Crustal Magnetic Anomalies
## 7 Curie Point Depths
### Correlation Between the Curie Point Depth and Seismicity  
The NETP has a relatively shallow CPD, and its crust is tectonically active. Strong earthquakes occur along the geothermal gradient zone where the thermal stress is concen  
Figure 14: Nonlinear least-squares fit between the heat flow (Q\({}_{s}\)) and depth of the Curie surface (H) for different geological areas: (**a**) the NETP (mainly in the Qinling–Qilian orogenic belt), (**b**) the Qaidam Basin, (**c**) the Ordos Basin, and (**d**) the Sichuan Basin. trated. As such, it is possible that the regional seismicity is related to the CPD. Earthquake epicenters are superimposed on the CPD map in Figure 15.  
The CPD exhibits an obvious negative correlation with seismic activity. The Curie surface determined from the CPDs is a specific thermal dynamic surface, and its depth shows a good correlation with the background geothermal field and seismicity. Figure 15 shows that earthquakes occur mainly where the Curie surface is shallow or steeply dipping. The earthquakes are mainly observed along the western and northern margins of the Ordos Block, which corresponds to a transition zone between areas with shallow and deep Curie surface. In contrast, there is less seismicity in the interior of the block, corresponding to a relatively uniform and deep CPD. The Alxa Block is relatively stable and has little seismic activity within its interior. However, more seismicity activity is observed at the margin of the block, particularly in the transition zone between the block and Qilian orogenic belt where the Curie surface transitions from shallow to deep. The Curie surface exhibits a depth gradient at the boundaries of the NETP, especially near the Kunlun and Longmenshan faults. Based on the distribution of earthquakes along the fault zones, there is strong coupling between the earthquake distribution and Curie surface. Most earthquakes in the study area have occurred in the upper crust. Since 1900, there have been 379 earthquakes of Ms \(\geq\) 5.0. The number of earthquakes with a focal depth of \(<\)10 km is 273, and the number of earthquakes with a focal depth of 10-20 km is 64, which account for 72% and 17% of all earthquakes, respectively (Figure 16b). Earthquakes with a focal depth of \(\leq\)20 km are dominant in the NETP, where the CPD is shallow. Figure 16a shows the focal depths (black dot), the CPDs (blue dotted line), and upper crustal thicknesses (red line) from the global seismic crustal model (Crust 1.0) [49] at the epicenters in Table 1. One can find that most of the focal depths are located in the upper crust. The crustal thermal structure controls the mechanical properties of crustal rocks and variations in rock strength with depth, which define the crustal depth of the brittle-ductile transition and thickness of the seismogenic layer. The crustal temperature of  
Figure 15: Distribution of the CPD and earthquakes in the NETP and adjacent areas. For black lines, please see Figure 1. Earthquakes tend to occur where the Curie surface is shallow or has a marked depth gradient. the brittle-ductile transition is generally 300-500 \({}^{\circ}\)C [50]. Assuming the temperature of the CPD is 580 \({}^{\circ}\)C, the Curie surface is shallower than the upper crust in the NETP (Figure 16a), which indicates that the temperature conditions of the brittle-ductile transition extend into the upper crust beneath the NETP. The rocks in the middle and lower crust do not undergo brittle failure. The NETP is subjected to high stress as a result of India-Eurasia collision and blocking by the rigid Ordos, Sichuan, and Tarim basins, resulting in the middle and lower crusts being unable to withstand a high differential stress or store a large amount of elastic strain energy. As such, the middle and lower crust transfer the tectonic stress to the brittle rocks of the upper crust, resulting in a concentration of differential stress and the accumulation of strain energy. Therefore, earthquakes occur mainly in the upper crust beneath the NETP.