Abstract: Based on the study results of literature (regarding the Region-Time-Length algorithm and its application in North China), the improved RTL algorithm has been used to do the RTL spatial scanning for 35 moderately strong earthquakes (M_S≥5.0) in North China area. The results show that for 24 of 35 earthquake samples, the obvious RTL anomaly appears at the epicentral area, or at the surrounding regions but nearby the mainshock (the distance from anomalous area to mainshock is smaller than 1°, most of them are smaller than 0.5°) during 3 months before the mainshock. In this case, the potential site of coming mainshock could be roughly judged according the RTL anomaly distribution. For 3 of 35 earthquake samples, the RTL anomaly appears at the epicentral area during the 4~12 months before the mainshock. But during 3 months before the mainshock, the anomaly at the epicentral area disappears and migrates to outside region surrounding the mainshock. For 8 of 35 earthquake samples, there is not any TRL anomaly found in the region surrounding the mainshock during 1 year before the mainshock. In other words, the 3 months short-term RTL anomaly is remarkable for about 69% among total earthquake samples, and the ratio for 1 year moderate and short-term anomaly is about 77%, and no RTL anomaly occurrence within one year accounts for 23% of the total earthquake samples. Our study results also show that, for a few earthquake samples, the obvious RTL anomaly appears at the epicentral area during the 3 months short-term before the mainshock, but in the same time the RTL anomaly also appears in the other regions, this will bring more difficulty for the site judgment of the coming earthquakes. The approximate computing approaches and rough indexes for judgment of RTL anomaly will be given at the end of the paper.

Key words: RTL algorithm, Spatial scanning, Seismic activity level, Medium-or short-term forecast, North China area