The active period of the faults in the Qiongzhou Straits occurred principally in the Tertiary. The rupture zone is situated in the nearly E-W striking tectonic lowland between the Leizhou Peninsula and the Hainan Island, which was preformed by the southward and northward compressions due to the expansion of the South China Sea Basin. During the interglacial periods in the Quaternary, the lowland was inundated and deepened gradually due to erosion by sea curfent. It was until the last uprising of sea level during the postglacial period that the Qiongzhou Straits assumed its present appearance.