Conductivity Anomalies before M > 6 Earthquakes in China during 2014 – 2019

<p>The North-South Seismic Belt of China is one of the most active seismic areas on the Chinese continent.&#160; More than ten strong earthquakes (Ms > 6) have occurred in this region since 2010.&#160; However, Earthquake-related conductivity anomalies are rarely reported for those earthquakes.&#160; In this study, 3-component geomagnetic data recorded at sixty geomagnetic stations are selected to compute the Parkinson vectors to monitor the changes of conductivity before and after the earthquakes.&#160; Considering most fluxgate magnetometers have only been installed since 2014, we concentrate on six Ms > 6 earthquakes occurred during 2014&#8211;2019.&#160; To mitigate artificial disturbances, low noise data during the 00:00 &#8211; 5:00 LT are utilized.&#160; We compute the background distribution and monitoring distribution using the azimuth of the Parkinson vectors at each station within six years (2014 &#8211; 2019) and a 15-day moving window, respectively.&#160; The background distribution is subtracted from the monitoring distributions to mitigate the influences of underlying inhomogeneous tectonic structures.&#160; The obtained difference distributions binned by 10&#176; within 400 km from each station are superimposed during 60 days before and after the earthquake to construct integrated maps.&#160; To analyze the potential frequency characteristics, we compute the results from low to high frequency band.&#160; The results show that for four earthquakes, the conductivity anomalies areas appear near the epicenter 10 to 20 days before earthquakes, while the rest two earthquakes have no anomaly.&#160; The conductivity anomalies appear at all study frequency band from 0.0005 Hz to 0.1 Hz, and significantly at 0.001 &#8211; 0.005 Hz before earthquakes.&#160; Meanwhile, we find that the lower frequency band corresponds to larger anomalies area.&#160; These results suggest the change of underlying conductivity near the hypocenter is a possible phenomenon for strong earthquakes, and the frequency characteristics of the seismo-conductivity anomaly during the earthquake are helpful to understand the pre-earthquake anomalous phenomena.</p>