Study on the variation characteristics of the mobile gravity field before and after small earthquakes around the Yishu belt under weak seismic activity

To explore the correlation between small earthquakes and gravity field changes in Shandong Province under the weak seismic activity, this study takes the area around the Yishu fault zone as the core region. It selects earthquakes with magnitudes of M_L3.0 and above, including the 2013 Laizhou M_L4.9 earthquake, the 2015 Pingyi M_L3.9 (collapse-induced) earthquake, and the 2020 Changqing M_L4.6 earthquake. The data from the provincial mobile gravity observation network of the Shandong Earthquake Agency during 2010 to 2022, were processed using the classical adjustment method in the LGADJ software. Subsequently, the characteristics of gravity field changes before and after the earthquakes were analyzed across different time scales.The results show that: In the gravity field changes between adjacent phases before and after the occurrence of small earthquakes, areas with changes exceeding the threshold appear around the epicenters, and the epicenters are mostly located at the bends of the 0 μGal or 10 μGal contour line. In terms of time scales, the changes near the epicenters are significant in the maps of adjacent phases and 1-year interval changes, whereas the changes are not obvious in the maps of multi-year accumulations (2 years and above). The measuring points with significant changes show drastic variations only before and after the earthquakes and remain stable at other times. This study provides a scientific basis for the monitoring and medium-term prediction of small earthquakes in weak earthquake regions.