Anomalous Characteristics of Z Component of the Lithospheric Magnetic Field near the Epicenter A Few Days before Small Earthquakes

In order to take advantage of the short seismogenic time to capture effective seismomagnetic anomaly information, the lithospheric magnetic field variation models from April 2019 to April 2020 and from April to September 2020 were calculated after the Renqiu M_S3.2 earthquake in Hebei and Helinger M_S3.0 earthquake in Inner Mongolia. It is found that the relative anomaly of Z component near the epicenter is more obvious, and there is a certain area of high positive anomaly. After removing the components of trend change, the local outliers were about 2~3 nT, and the influence range was about 50 km. In addition, there were significant negative anomalies near the epicenter of Renqiu M_S3.2 in the second stage, which were similar in scope and magnitude to those before the earthquake, and may be related to the recovery of the lithospheric magnetic field shape after the earthquake.