Detection of the Late Quaternary activity of the Liaocheng-Lankao Fault in the south-central part of the North China Plain: Discussion on the seismogenic mechanism of the 1937 Heze M 7.0 earthquake

Objective The North China Plain (NCP) is one of the most populated and economically developed areas in China and is a region with a high level of seismic hazards. Studying the Quaternary activity of the faults and the seismogenic mechanism of the large earthquakes in NCP is conducive to exploring the seismogenic pattern of intraplate earthquakes and reducing the damage caused by seismic hazards. The Liaocheng-Lankao fault (LLF) is an important buried deep major fault in the south-central part of the NCP. The activity of the LLF and its relationship with the 1937 Heze M 7.0 earthquake is still highly controversial. Methods In this study, the activity of the Liaocheng-Liaokao fault is finely studied by combining shallow seismic exploration, drilling, and Quaternary dating methods. Results Shallow seismic reflection profile ZF-2 reveals that the strata below 145 m are obviously displaced, and the strata above 145 m are disturbed. The Bachengsi drilling profile reveals 16 sets of marker layers and three west-dipping normal faults Fa, Fb, and Fc; they form a "compound Y" structure in the profile, of which Fa displaces the bottom boundary of the Holocene (burial depth of approximately 38.9 m) and is an early Holocene active fault. It also reveals four paleoseismic events since the Late Pleistocene, with vertical displacement of 1.2±0.2 to 3.7±0.2 m for a single event. Based on the stratigraphic offsets in the boreholes, the average vertical slip rate of this fault is calculated to be about 0.1±0.05 mm/a for the early Late Pleistocene and 0.35±0.04 mm/a for the late Late Pleistocene-middle Holocene. The fitted age-depth curves by the test results of seven 14C samples and four OSL samples can be divided into two segments. Within the depth range of 0 to 86.0 m (approximately 21 to 0 ka), the age and depth of the strata conform to the formula y=(253.69±16.56)x+(924.72±681.36), from which the average deposition rate of this section is calculated to be 3.94±0.26 mm/a. Within the depth range of 102.9 to 145.4 m (approximately 128 to 59 ka), the age and depth of the strata conform to the formula y=(1470.67±259.91)x+(-95061.92±30190.73), from which the average deposition rate of this section is calculated to be 0.68±0.12 mm/a. The vertical slip rate of the LLF and the sedimentation rate of the Dongpu Sag have increased significantly since the late Late Pleistocene. The intensity lines of the Heze M 7.0 earthquake show an asymmetric butterfly shape. Conclusion The 1937 Heze M 7.0 and M 6\begin{document}${\raise0.7ex\hbox{$3$} \!\mathord{\left/{\vphantom {3 4}}\right.}\!\lower0.7ex\hbox{$4$}} $\end{document} earthquakes formed "Z" -shaped ground fissure zones, which can be divided into three sections: the southeastern section (section A), the middle section (section B), and the northwestern section (section C). The long axis of the intensity lines and the distribution of the surface rupture of the 1937 Heze M 7.0 coincide with the NNE-striking Xiaoliu-Xieyuanjie and NWW-striking Dongming-Chengwu faults in location and striking. The analysis of the intensity lines, surface rupture distribution, focal mechanism solution of the 1937 Heze M 7.0 earthquake and M 6\begin{document}${\raise0.7ex\hbox{$3$} \!\mathord{\left/{\vphantom {3 4}}\right.}\!\lower0.7ex\hbox{$4$}} $\end{document} earthquakes, and regional stress implies that the Xiaoliu-Xieyuanji fault and the Dongming-Chengwu fault are the seismogenic faults of the 1937 Heze M 7.0 earthquake. The LLF, as the deep major fault in the region, controlled the accumulation of stress, stimulated the earthquake with its deep movement, and reduced the effect of the seismic energy westward, acting as the regional seismic controlling fault of the 1937 Heze M 7.0 earthquake. Significance This article proposes a method for fine detection of the localization, structure, latest activity age, sliding rate, and paleoseismic sequences of the buried fault and also proposes a pattern of seismicity in which seismogenic faults do not coincide with the regional seismic controlling fault. It provides new insights into the characterization of seismicity within the NCP and can provide the geological basis for urban and rural planning, high-speed railway design, and earthquake prevention and disaster reduction project construction in this region.