# Regional Seismicity of the Northeastern Tibetan Plateau Revealed by Crustal Magnetic Anomalies
## 6 Relationships Between the Magnetic Anomalies and Earthquakes  
To examine the correlation between the magnetic anomaly distribution and seismic activity, we superimposed the earthquake locations onto the surface magnetic anomaly map (Figure 6). The total intensity \(\Delta F\) and vertical component \(\Delta Z\) of the RTP magnetic anomalies are shown in Figure 6a,b, respectively. The earthquake distribution in the study area is mainly concentrated in the NETP and along the main fault zones. The seismic activity varies significantly amongst the different tectonic units. The most frequent seismic activity is in the Qiangtang Block and at the boundary of the Songpan-Ganzi Plateau. There are few earthquakes in the Ordos Block, which is consistent with its stability. Figure 6: Distribution of crustal magnetic anomalies and earthquakes since 1900 in the NETP and adjacent areas for (**a**) \(\Delta F\) component, (**b**) \(\Delta Z\) component, (**c**) \(\Delta H\) component, (**d**) \(\Delta X\) component, (**e**) \(\Delta D\) component, (**f**) \(\Delta Y\) component, and (**g**) \(\Delta I\) component. Strong earthquakes have occurred mainly in regions where the magnetic anomalies were negative or that had a strong–weak transition. The interior of the Songpan-Ganzi Plateau is dominated by weak and wide negative magnetic anomalies, and the boundary of the plateau is marked by transitions or strong negative magnetic anomalies. Bouguer and isostatic gravity anomalies from WGM2012 are shown in Figure 7[34]. The isostatic gravity anomalies on most of the NETP are within \(\pm 20\) mGal, which are considered to indicate a state of gravitational equilibrium. In contrast, gravitational disequilibrium (i.e., positive or negative anomalies) is mostly observed near the boundary zones of blocks and particularly in the Longmen orogenic belt. This belt has an isostatic gravity anomaly as high as 50 mGal, corresponding to a non-isostatic gravity anomaly and the strong tectonic displacement of a deep structure. Most earthquakes occur in the region of the non-isostatic gravity anomalies near the block boundaries, and there is a transition between positive and negative magnetic anomalies or large negative magnetic anomalies. Figure 7: Gravity anomalies in the NETP and adjacent areas. (**a**) Bouguer gravity and (**b**) isostatic gravity anomalies. For black lines, please see Figure 1. The Bouguer gravity anomalies are negative in the NETP but positive in its vicinity, ranging from \(-600\) to 50 mGal. Gravity disequilibrium (positive or negative) is mostly observed near block boundaries, particularly in the Longmen orogenic belt. The Songpan-Ganzi Plateau is adjacent to the West Qinling tectonic belt, Sichuan-Yunnan Block, and Sichuan Basin, and it frequently experiences strong seismic activity. The region is not only the transition zone from the Tibetan Plateau to the Yangtze Platform, but also a convergence zone for material flowing from west to east and from a fast to a slow velocity. The plateau contains a range of geological structures and horizontal displacement velocities (based on GPS observations) as compared with the surrounding blocks (Figure 1). The deep structures also indicate that the plateau is an important gravity anomaly gradient zone and a transition zone where the Mohorovic surface (Moho) shallows abruptly from west to east. The uplift of the Tibetan Plateau as a result of the India-Eurasia collision has led to the eastward flow of deep materials, which are blocked by the rigid Sichuan Basin. Compression, accretion, and lateral flow all occur in the Songpan-Ganzi Plateau. This explains why this area, in which earthquakes occur, is dominated by weak negative magnetic anomalies in its interior, whereas its boundary is a transition zone between positive and negative anomalies or has large negative magnetic anomalies. Most earthquakes, especially those of Ms \(\geq 7\), occur along the deep and large faults in the NETP, which mark the boundaries between different blocks.