A Review of the Mechanics of the Abnormal Geo-electrical Resistivity Preceding the 1976 Tangshan Earthquake Informed by Present Groundwater Conservation

In recent years, new groundwater conservation policies in the Beijing-Tianjin-Hebei region have led to a rapid increase in groundwater level and a synchronous decrease in geo-electrical resistivity (GER) around Tianjin. In constrast, during the few years preceding the 1976 Tangshan M7.8 earthquake, both groundwater levels and GER around Tianjin decreased synchronously. This significant contrast seems to indicate that abnormal GER changes in Tianjin and Tangshan before the M7.8 earthquake may serve as seismogenic precursors. Therefore, in this paper, we employ combined hard inclusion models to quantitatively calculate the body strains preceding the M7.8 earthquake, and analyze their relationship with GER decreases. So far in this field, very few researches have performed theoretical calculations on the definite parameters such as the size, elastic modulus, and spatial attitude of the rheological hard inclusions, which can reasonably explain the distribution of ground deformation and seismic gaps in Tangshan and surrounding areas, as well as the asynchronous and unequal rate of GER decline. The calculation results indicate that for the 1976 Tangshan earthquake, the distribution and evolution of the precursor can be well studied by setting three ellipsoidal rheological hard inclusions with different elastic moduli through aftershock distribution. This work provides a valuable case for applying hard inclusion theory to the medium-term and short-term evolution of other strong earthquake.