# Regional Seismicity of the Northeastern Tibetan Plateau Revealed by Crustal Magnetic Anomalies
## 8 Discussion  
There are several deep faults around the NETP, and strong earthquakes occur along fault zones. Deep faults are often the boundaries between positive and negative magnetic anomalies, as well as where the Curie surface exhibits a depth gradient. Mantle heat ascends along deep faults, which results in high crustal temperatures and a shallow Curie surface. During the upwelling of crustal material, rocks accumulate strain energy. When the rocks reach the threshold for elastic deformation, the accumulated energy needs to be released, which causes the rocks to fracture, and an earthquake occurs. There is a large difference in the stress in the transition zone across the Curie surface (i.e., the transition between high- and low-temperature zones), which leads to a variable velocity of the material above and below it. As such, energy accumulation due to frictional motion can generate earthquakes. The Qiangtang Block has a wide, gentle, and low Bouguer gravity anomaly, which reflects a gravity deficit. The heat flow is as high as 318 mW/m\({}^{2}\)[48], and the block has a very shallow CPD (18 km). In addition, seismic studies have revealed that the Qiangtang Block has a low Q value, low P\({}_{\rm n}\)-wave velocity, and only weak S\({}_{\rm n}\) waves [51; 52]. [PERSON] (1978) proposed that the high heat flow is due to tectonically induced magmatism [53]. Figure 16: **(a)** Plot of focal depths for Ms \(\geq\) 5.0 earthquakes versus the CPDs and upper crustal depths at the epicenters in Table 1. **(b)** Proportions of earthquakes at different depth intervals. Earthquakes with a focal depth of \(\leq\)20 km are dominant in the NETP. Most earthquakes in the study area have occurred in the upper crust. Magmatic additions to the crust in response to extensions might explain the thermal evolution of the Qiangtang Block [54]. The geotherm is heated at depths by upward convection in response to extension and is depressed at shallow levels by rapid sedimentation, thereby maintaining isostatic balance. Historically, there have been many strong earthquakes in the Qiangtang Block, with 148 earthquakes of Ms \(\geq\) 5.0. Some nearly E-W-trending faults occur around the block, and the fault zones act as channels for magma ascent. The heat flow in this block is as high as 318 mW/m\({}^{2}\), consistent with the presence of hot springs [48]. During magma upwelling, the pressure and temperature decrease, and the volume of upwelling material increases. Therefore, this process can release energy that causes earthquakes. The CPDs in the Qilian orogenic belt are different from those in the Qaidam Basin. A deep Curie surface is present in the Qaidam Basin, whereas a shallow Curie surface occurs in the Qilian orogenic belt (Figure 12). The crust of the Qaidam Basin is rigid. Recent seismic and magnetotelluric studies have revealed that the velocity and resistivity are both low beneath the Qilian orogenic belt, while the reverse applies to the Qaidam Basin [55]. The Qilian orogenic belt is in a compressional setting due to the subduction of the Eurasian Plate and the blocking effect of the rigid crust in the Qaidam Basin. The NETP has negative Bouguer gravity anomalies over a large area (Figure 7a). The low values in this area, coupled with negative/low magnetic anomalies (Figure 5), a shallow Curie surface (Figure 12), and a deep Moho [4], indicate the presence of a hot crust. From the southwest (i.e., the Qiangtang Block) to the northeast (i.e., the Qilian orogenic belt), the gravity anomalies increase and the Moho depth decreases (Figure 7a), which indicates the northeastward expansion of the Tibetan Plateau. The GPS-derived velocities show that crustal materials from the NETP are moving northeastward (Figure 1). Our study shows that the Tarim, Ordos, and Sichuan basins around the NETP have high gravity values and strong magnetic anomalies (Figure 5), a deep CPD (Figure 12), and shallow Moho [4], indicative of a cold and rigid lithosphere beneath these basins.