Abstract: Huailai Basin in Hebei Province, has developed faults, with frequent strong earthquakes in history and frequent medium-small earthquakes today. According to the high signal-to-noise small earthquake event waveforms recorded by Shacheng Seismic Station from October 2008 to April 2018, the shear-waves splitting characteristics of Huailai area are studied. The results show that the average polarization direction of fast shear-waves in Huailai area is 87.3°±33.3°, which suggests the direction of maximum horizontal principal compressive stress is NEE or nearly EW. This is consistent with the principal compressive stress direction of the regional tectonic stress field in the Capital zone and North China obtained by other predecessors, which reflects local information of regional tectonics. The dominant direction of fast shear-waves of Shacheng station is inconsistent with the strike (NW) of Huangtuyao-Tumu fault near the station. During the periods of seismogenesis or small earthquake swarm, the polarization direction of fast shear-waves in Huailai area is relatively disordered, with no obvious dominant direction. Before the small earthquake swarm activity in Huailai area, there was a significant decrease in the time delay of slow shear-waves, which can provide some supports for the short-impending earthquake prediction in this area. During the period of January 2017 to April 2018, the time delay of slow shear-waves is only 1.24 ms·km^-1, which is lower than other periods, probably due to the absence of major earthquakes.

Key words: Shear wave splitting, Fast wave polarization direction, Slow wave delay time, Horizontal principal compressive stress, Earthquake swarm