## 4 Discussion  
Based on five recent electrical field negative anomaly cases before EQs in China and the 23 cases in Table 1, it can be concluded that the atmospheric electric field always shows negative anomalies 2-48 h before significant inland EQs (Ms \(\geq\) 3.0) and a shock time of 2-48 h may be largely possible. Based on our current statistical results, the time interval between the occurrence of most EQs and atmospheric electric field anomaly signal does not exceed 24 h; it means in most cases that people have no more than 24 h to leave their work and homes around the epicenter. A possible physical mechanism for why the electric field negative anomaly signal occurs so close to the EQ occurrence is depicted in Figure 3.  
Figure 3: Logical flow diagram illustrating the hourly scale physical processes leading up to an earthquake. Subfigures a-f are the sequential physical logic diagrams in order. Figure 2: Negative atmospheric electrostatic field Ez signal before the Mianning M3.6 earthquake. The circle indicates the epicenter and the star indicates the Mianning Station. Figure 3 demonstrates a systematic process in which an underground rock mass is first subjected to high pressure, resulting in the formation of a large number of microfractures. The interface between the two sides of a fault develops into a state that is ready to slip and rupture further; thus, once the stress that is continuously imposed on the fault reaches a threshold, an EQ is triggered. Therefore, once an Ez precursor appears, it signifies that the whole fault is becoming unstable. Observational experience suggests that the duration of time between this occurrence and the onset of an EQ is only approximately several hours to one day. The above process illustrated in Figure 3 describes a possible hourly scale critical state. The rock mass at the hypocentral depth is in a critical state during the final stage of microrupture expansion before an EQ. However, the stress field acting on the interface and fracturing the rock varies depending on different sources of stress; consequently, the time period in which an EQ occurs differs among different locations at the hourly scale. Nevertheless, all of the evidence indicates that, at this stage, the time interval over which a rock mass can remain unfractured at the hypocentral depth is approximately several hours to one day. This is likely to be a common feature of inland EQs worldwide that are triggered by the eventual cracking of rock masses under driving stresses. In addition, abundant rock fractures are interconnected at the subsurface and are prepared to slip and rupture. Furthermore, the high-temperature and high-pressure environment deep underground drives the emission of gases (such as CO\({}_{2}\), helium, radon and water vapor) [25; 26] from the hypocentral depth to the atmosphere, which enhances the output of thermal radiation and causes sudden changes in the atmospheric composition and local meteorological conditions. These radioactive gases can, in turn, undergo decay and emit \(\alpha\) particles [11; 27; 28; 29; 30; 31; 32], which may further ionize the local atmosphere within minutes. With the increasing emission of radioactive gases, many positively and negatively charged particles are injected close to the surface near the epicenter. Then, a large number of electron-ion pairs disperse in the air and cations and anions, which carry positive and negative charges, respectively, are separated by the fair-weather electrostatic static field Ez, gravity and thermal convection and are transported down and up, respectively. Then, a newly established upward electric field is formed in the atmosphere. This whole process can be completed within minutes. This illustrates that, in the final stage leading up to an EQ, there is a large release of radioactive gases such as radon near the impending fracture region.