# Exploring the Dynamics of Global Plate Motion Based on the Granger Causality Test
## 4 Discussion  
The geodynamic mechanism is the research focus and core issue of plate motions and plate tectonics. It is also the greatest mystery in Earth science, and the evidence of plate motion remains the focus of researchers' studies. During the last century, an increasing number of advanced technologies have been applied to research patterns of plate motion. The global positioning system (GPS) has opened up new ways of monitoring and studying crustal movement, and many research papers concerning the global plate motion field have emerged [11, 58, 59, 60]. General plate motion fields on a global scale can show the direction of plate movement, but they cannot describe the mechanism of plate motions. Due to the natural property of earthquakes, the Earth's crust accumulates stress during the movement of plates, and crustal shaking occurs when the stress exceeds the critical point. Based on the earthquake catalog, the main purpose of this paper was to study the relationship and causality of energy release between two plate boundaries to further understand the movement mechanism from another point of view. By comparing the global plate motion field and Figure 4, an interesting point can be revealed: the directions of plate movement are opposite to the directions of Granger causality between plate boundaries. For example, the causality direction between a subduction zone and a mid-ocean ridge in one plate is mostly from the subduction zone to the mid-ocean ridge, which is opposite to the directions of plate motions, especially for the Australia, Nazca, and Pacific plates. Mantle convection may be the most famous hypothesis, and it includes the following forces that existed in plate motion: the push force of mid-ocean ridge, the force that drive solid lithosphere to move, and the pull force that downward-moves the subduction zone; the crust in the subduction zone sinks, carried by the cold, heavy, downward-moving mantle, inducing movement of the oceanic crust behind. Nevertheless, this theory does not indicate which force, e.g., subduction effect or the expansion effect, is the dominant role in global plate motion. The causality direction of energy release at plate boundaries, estimated from the Granger test based on global earthquake records from 1904 to 2014, has shown that the energy released at a subduction zone Granger causes the energy release at a mid-ocean ridge in the Pacific, Nazca, African, Australian, Eurasian, and Philippine sea plates. Due  
Figure 4: A map of Granger causality between the energy releases at plate boundaries is reported. The red marks indicate Bidirectional Granger Causality between the energy releases at plate boundaries. The green marks indicate unidirectional Granger causality between the energy releases at plate boundaries of Ocean plates. The yellow marks indicate unidirectional Granger causality between the energy releases at the plate boundaries of the continental plates. In the results of unidirectional Granger causality, the energy release at the boundary is shown by the triangular mark, and the Granger cause energy release at the boundary is shown by the circular mark. to the earthquakes caused by crustal movement, the conclusion can be made that seismic energy is first released in the subduction zone and then released in the mid-ocean ridge, as well as that the speed of plate motion at the subduction zone is much higher than that at the mid-ocean ridge. It can be further inferred that the pull force is the main driving force of plate motion in the Pacific, Nazca, African, Australian, Eurasian, and Philippine plates regarding the global plate motion field. This interpretation is in agreement with the findings by [PERSON] [32], who suggests that the forces acting on the downgoing slab control the velocity of the oceanic plates and are an order of magnitude stronger than any other force. In addition, the conclusion that subduction's pull force provides the main driving force for plate motion has also been recognized in other studies [31; 61].