In addition, atmospheric electrostatic field monitoring has recently become more popular. Atmospheric electric field anomalies prior to EQs have been widely studied [4; 5; 6; 7; 8; 9; 10; 11]. Since the Tangshan EQ in 1976, China has set up several monitoring stations of atmospheric electric fields, and some obvious anomalous cases have been observed [5; 10]. Under normal conditions, the atmospheric electric field is mainly modulated by meteorological conditions and solar activities. On a fair day without the influence of human activities, if a negative anomaly of the atmospheric electric field appears, it is likely to be followed by an EQ event.  
There are various methods for measuring the atmospheric electric field; commonly used methods involve ground-based rotating electric field meters, roller electric field meters, sphere-borne double-ball electric field meters and microrocket electric field meters. In the past, the most common methods were potential probes and burning fuses, and these methods are still used by some observatories today. The most widely used method today is the electric field mill (EFM), which provides good exposure to atmospheric electric fields. It usually consists of one or more electrodes that are alternately shielded and exposed to the atmospheric electric field. Long-term observational studies of atmospheric electric fields have been executed since the 1980s by different research teams [12; 13; 14] using EFM100 in Sichuan, Yunnan, Beijing, Hebei, etc. The atmospheric electric field meters that were used for measurements in this paper are also EFM100 with a measurement accuracy \(<\)5% and resolution of 10 V/m.  
Recent observations [12; 15] revealing the hourly scale characteristics of inland EQ precursors indicated that detecting these precursors is feasible. Compared with preseismic anomalies in low-frequency geoacoustic emissions and in ultralow frequency (ULF) and VLF electromagnetic emissions, preseismic anomalies within the atmospheric electrostatic field occur closer in time to strong inland EQs. [PERSON] et al. [11] noted that anomalous radon emissions trigger great decreases in the atmospheric field of the lower atmosphere (from the ground to an altitude of 2 km), as observed around the time of the Kobe EQ in 1995. They further suggested that the behavior of radon in terms of the atmospheric electrostatic process can explain the seismic precursors observed near the ground. [PERSON] et al. [11] further suggested that the behavior of radon in terms of the atmospheric electrical quasistatic process can explain seismic precursors observed near the ground. [PERSON] et al. [4] described the characteristics of the vertical atmospheric electrostatic field as negative 7-12 h before an EQ according to the statistics of 30 EQ events of various classes over northern India. [PERSON] [9] reported more than one hundred cases featuring negative vertical Ez anomalies approximately one day in advance for Ms 4-6 EQs, but similar to the relationship between Ez and magnitude, the Ez value and epicentral location do not exhibit an obvious relationship. Furthermore, previous studies indicated that the near-surface atmospheric electrostatic field is unique in that it becomes anomalously negative (the downward direction of Ez is defined as positive under fair-weather conditions) just before some EQs under fair-weather conditions [10; 16], but these studies did not summarize the features of the electrostatic field at an hourly scale. Thus, this paper examines the hourly scale features of anomalously negative signals of the atmospheric electrostatic field as precursors. These signals could be the most stable and reliable indicator warning of an imminent EQ several hours to one day in advance and could therefore be leveraged to alert people of an impending emergency.