## 2 Temporal Characteristics of Near-Surface Atmospheric Electrostatic Abnormalities before an EQ  
First, we describe four abnormal atmospheric electrostatic signal cases that always appear several hours to one day before an inland EQ occurs and present a list of inland EQs with their shock time, epicentral location, magnitude, weather conditions, precursor beginning time and precursor duration. Figure 1 shows the four negative atmospheric electrostatic field Ez cases before four EQs (the Beijing Ms 3.0, Rongcounty Ms 4.7, Changing Ms 6.0 and Wenchuan Ms 8.0 EQs). The red circles indicate the epicenters and the red stars represent the stations. One case  
is shown in Figure 1a. Before the Beijing EQ, which occurred on 14 April 2019, in Beijing, China, the weather was identified as fair. An abnormal phenomenon was observed when the normal positive Ez became negative (approximately \(-\)100 V/m) at 09:00 in the morning LT (3.8 h before the EQ), as shown in Figure 1a. A medium-magnitude EQ was predicted to occur several hours later based on previous analytic results. Later, an Ms 3.0 EQ was confirmed to have occurred at 12:47 LT with the epicenter 40 km north of where the scientific instrument was located. Similarly, as shown in Figure 1b, the Rongxian Ms 4.7 EQ occurred at 05:38 LT on 24 February 2019, in Sichuan Province (longitude = 104.49\({}^{\circ}\) E, latitude = 29.47\({}^{\circ}\) N), and Ez became negative 11 h earlier under fair air conditions at Zigiong Station, which was situated 30 km from the epicenter. Ez suddenly decreased to a minimum of \(-\)410 V/m at 23:24 LT (the maximum negative Ez value). Although there were two short intervals (tens of minutes) of positive Ez values, Ez showed a negative anomaly for more than ten hours before the EQ began. Figure 1c also shows that the Changing Ms 6.0 EQ occurred at 23:30 LT on 17 June 2019, in Sichuan Province (longitude = 104.49\({}^{\circ}\) E, latitude = 29.47\({}^{\circ}\) N) and the Ez became negative approximately 23.5 hours before the EQ under fair weather conditions at the Yibin Station, which was situated 50 km from the epicenter. Ez suddenly decreased to a minimum of \(-\)10,000 V/m at 23:30 LT (the maximum negative Ez value). The negative Ez signal lasted approximately 70 min, and then Ez recovered to normal positive values. Twenty-three hours later, EQ occurred. It is important to note that there were also significant positive Ez signals before that EQ, but these signals were not considered because they are not ubiquitous; i.e., not every EQ involves a positive Ez signal several hours beforehand. Hence, positive Ez signals may be irrelevant. Another case shown in Figure 1d is that for the Wenchuan Ms 8.0 EQ that occurred at 14:28 LT on 12 May 2008 (longitude = 103.32\({}^{\circ}\) E, latitude = 31.00\({}^{\circ}\) N). Despite a data gap, the Ez signals in Pixian and Wenjiang Counties showed negative values for 7 h before  
Figure 1: Negative atmospheric electrostatic field Ez before earthquakes. (**a**) Beijing Ms 3.0 earthquake on 14 April 2019, with a station located 40 km from the epicenter. (**b**) Rongxian Ms 4.7 earthquake on 24 February 2019, with a station located 30 km from the epicenter. (**c**) Wenchuan Ms 8.0 earthquake on 12 May 2008, with two stations located 50 and 55 km from the epicenter. (**d**) Changning Ms 6.0 earthquake on 17 June 2019, with a station located 50 km from the epicenter. The data gap in panel (**c**) is due to a power outage. In the maps on the right, the stars denote the stations and the circles denote the epicenters. the EQ. Note that the weather in Wenchuan, Pixian and Wenjiang counties was very fair all day as well. The peak negative values of Ez at the Pixian and Wenjiang stations were \(-2750\) V/m and \(-750\) V/m, respectively. The Pixian Station is 50 km from the epicenter, while the Wenjiang Station is 55 km from the epicenter. There are many other examples, such as these four cases, where abnormal atmospheric electrostatic signals appear several hours to one day before an inland EQ occurs.