摘要
Since 1970,13 earthquakes of M≥ 5.0 occurred in the Northeast China,of which 8 medium-strong earthquakes are independent in space and time.Having studied small earthquakes before the 8 medium-strong earthquakes systemically,we found that these small earthquakes activities increased.Combining with characteristics of the small earthquake activities in the Northeast China,we proposed and applied the new method whether the spatial changing pattern of small earthquake activities change the small earthquake frequency.The result showed that the small earthquake frequency calculated by using new method could pick up anomaly frequency of small earthquakes before medium-strong earthquake.The authors could get medium-short-term abnormal indexes effectively before the medium-strong earthquakes in Northeast China.