  Although earthquake prediction is a big challenge in the world, some simple
observational tools can capture many physical signals and demonstrate that an
earthquake (EQ) may be forthcoming in short period. Many researchers have
studied the significant variation of atmospheric electrostatic field related to
the forthcoming earthquake. However, until now, there is not a compelling
physical mechanism which can explain why atmospheric electrostatic abnormal
signal could appear just before an earthquake. Here we present a precursor
signal and propose a brief physical interpretation. Under fair air conditions,
if the near-surface atmospheric electrostatic field Ez (oriented down when it
is positive) presents a very stable negative anomaly (from -100 V/m to -5000
V/m), it will forebode that an earthquake (seismicity from 3-8) would take
place in the next several to tens of hours within a distance less than 100 km.
We name this prediction technique as "DC Ez Determination"(DED). In addition,
the mechanism of such abnormal quasi-static electric field before a forthcoming
earthquake has been proposed here: (1) Radon gas releases from the rock clefts
near the epicenter during seismogenic process. (2) {\alpha} particles are
produced due to the radioactive decay of Radon gas in the air. (3) {\alpha}
particle ionizing radiation creates more positive particles in the air, which
is much more effective than that {\beta} and {\gamma} particles produced during
Radon radioactive decay. (4) The new positive particles change formal positive
atmospheric electric field (Ez) into stably negative near the earth-surface.
(5) The closer the instrument is to the epicenter, the more likely it is to
observe the negative Ez signal related to the earthquake. It is recommended to
establish an instrument network to capture the reliable precursor and make some
warnings before the earthquake disaster.
