# Correlation of Gravity and Magnetic Field Changes Preceding Strong Earthquakes in Yunnan Province
## 4 Discussion  
During the earthquake preparation process of the four strong earthquakes in Yunnan from 2011 to 2021, the trends of gravity and magnetic field changes tend to be consistent in the three years before the earthquake. After the data are normalized, the amount of data overlap increases year to year, and the nuclear density difference decreases year to year. In particular, there is a number of overlaps of the gravity and magnetic field fusion data in the year before the earthquake reaches the maximum, the overlapped data distribution interval is concentrated, and the nuclear density difference is less than 0.25. In the year of the earthquake, the number of coincident data decreases, the distribution interval is scattered, and the nuclear density difference increases. The correlation coefficient of gravity and the magnetic field increases year to year during the seismogenic process and reaches the maximum value one year before the earthquake. The correlation coefficient decreases sharply in the year of the earthquake. The gravitational field is formed by the combined force of the Earth's gravitational force on an object and the inertial centrifugal force produced by the Earth's rotation. The lithospheric magnetic field is produced by lithospheric magnetic material, and the two fields are unrelated [18; 19; 20]. Under normal conditions (constant density and magnetic susceptibility), the gravitational field and the lithospheric magnetic field are both stable field sources. During the seismogenic process of a strong earthquake, along with tectonic movement, the rock is stressed, and the density and magnetic susceptibility change. The gravity value and geomagnetism change the total strength [21; 22]. The change in the  
Figure 4: The strong earthquakes in Yunnan Province from 2011 to 2021 and the changes in the correlation coefficient of gravity and magnetic field within the three-year earthquake area before the earthquakes. (**a**) Yingjiang \(M_{6}\).1 earthquake in 2014; (**b**) Ludian \(M_{6}\).5 earthquake in 2014; (**c**) Jinggu \(M_{6}\).6 earthquake in 2014; (**d**) Yangbi \(M_{6}\).4 earthquake in 2021.  
tectonic stress on the field source causes the change in the mass distribution of the local area, resulting in the zoning pattern of gravity on the field and the abnormal zone along the structure [23; 24]. At the same time, the change in the rock stress causes the magnetic susceptibility to change. This results in a regional anomaly feature of the change in the lithospheric magnetic field [25]. The gravity anomaly in the strong earthquake danger zone is characterized by the occurrence of long-term and large-scale multipoint anomalies in the epicentral area, and the zoning pattern is distinguished along the active structure and the cascade and anomaly zones distributed along the structure in the periphery. The characteristics of mobile geomagnetic anomalies in strong earthquake danger areas are as follows: The annual variability of mobile geomagnetic anomalies near the epicenter is concentrated in a certain spatial distribution. During the six months to one year before the earthquake, there are several abnormal regions of total intensity of geomagnetic points near the epicenter and in specific geological structures [26]. Under the action of plate tectonics, the gravity and magnetic fields gradually tend to be consistent in the process of strong earthquake preparation and weaken or reverse after the earthquake [27; 28] The results show that the compression or tension between plates produces large stress flow during tectonic activities [29]. The stress flow propagates in the rock circle [30]. When encountering areas with drastic changes in medium structure and density (such as faults), the propagation channel encounters obstacles, and the stress flow will change the gravity field and geomagnetic field near the epicenter.