Since the Mesozoic, the North China has experienced intensive seismic activity. Given its special seismo-tectonic background and unusually high level of seismic potential, a network of deep wells, primarily monitoring fault and crustal deformation precursors to destructive earthquakes, has been deployed in this area since late 1960s. Although these deep well-aquifer systems can effectively capture the tectonic transients, numerous spike-like signals associated with non-tectonic sources are always recorded in the groundwater levels; however, their physical mechanism have not been confirmed on a case-by-case basis up to now.
The squall line is a common type of meso-scale convective system usually defined as a group of violent thunderstorms or storm cells arranged in the form of a narrow （a few tens of kilometers） or long （several hundreds of kilometers） line in a Doppler radar. Furthermore, the typical life-span of a squall line can last several hours, and its propagation speed is approximately 60 km/h. The surface area affected by squall line is typically accompanied by abrupt changes in surface pressure and air temperature.
Squall lines occur with a high frequency in North China, but it is still unclear that how this kind of meso-scale severe weather disturbs the groundwater levels in deep wells. In view of the current situation, a target research is needed to reveal the features of hydraulic responses. On 21 September 2017, a squall line passed across the central North China, and significantly disturbed the groundwater levels in four wells deeper than 1 km, which are Wuji, Xinji, Yongqing, and Ningjin wells. Here we mainly adopt the spectrogram and linear regression methods to systematically examine the unique signals stimulated by this squall line. The results show that: ① From a macro perspective, the barometric pressures recorded at the deep wells can abruptly jump induced by the passage of squall line; instantly, the strong pulse-like disturbances were observed on groundwater level graphs associated with pressure jumps; ② The duration of disturbance could last as long as about 127 minutes for this case; ③ In the highfrequency band, the barometric waves induced by squall line have a period between 15 cpd （cycles per day） and 25 cpd. Correspondingly, the cross-correlation coefficients between barometric waves and groundwater changes can be lower than −0.95 for the four deep wells in this special frequency band. Furthermore, the barometric pressure response coefficients vary from about −4.9 mm/hPa to −6.9 mm/hPa for the four wells, determined as the linear regression coefficient between groundwater level and barometric pressure in the 15−25 cpd band-passed data. Our analysis shows that this work can extend our understating of the signatures caused by squall line in the field of groundwater research. Additionally, owing to the large amplitude of barometric waves, the squall line can help quantify the precise responses of groundwater levels in deep wells to barometric pressures in the high frequency band.
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