By calculating the diurnal-variation correlation of the daily geomagnetic vertical component,we have studied the boundary(lines of centrally distributed induced currents from geomagnetic diurnal-variations)between geomagnetic stations with anti-phase variation in the North-South seismic belt of the Chinese mainland from March 1 to October 31 between 2008 and 2018. We identified 23 times of superimposed anomalies that lines of centrally distributed induced currents with superimposed lengths of more than 500km occurred within 16 days. Within 18 months after the superposition,earthquakes above magnitude M_S6 occurred in 20 superimposed sections,leading to a probability of 87% earthquake occurrence. The earthquakes mainly occurred within 250km of the end of the superimposed sections,and one or more superposition may occur before an earthquake. The strike of the superimposed sections is consistent with high-conductivity belts in the upper mantle and crust. It is speculated that these superimposed anomalies are near the high-conductivity zones in the upper mantle and crust,which provides information as earthquake precursor near the high-conductivity zones in the upper mantle and crust. Strong earthquakes in the Chinese mainland mainly occur in areas where the high-conductivity zones in the upper mantle and crust have been uplifted,even though it is still not clear when they will occur at present. It is proposed that the superimposed anomalies may be reliable precursors to strong earthquakes occurring within 1~2 years near the high-conductivity zones in the crust and upper mantle.