Abstract: Through studying the data of earthquake swarms from 1970 to 1991 in North China, we find: ① Six or seven years before Tangshan earthquake, the enhancement of the earth-quake swarm activity showed a fluctuation feature. After the Tangshan earthquake, the regional swarm activity decreased generally; ② Before the Tangshan earthquake with M =7. 8, the distribution of the swarms showed in the form of zones, but after the earthquake, the zones disappeared; ③ Before the main shock, earthquake swarms showed a phe-nomenon of migration; ④ A large number of precursory swarms satisfying the comprehen-sive indexes only occurred around the mainshock epicenter within 5 years before the earth-quake.

Key words: Earthquake swarm, Precursory swarm, Tangshan earthquake, Spatiotem poralevolution characteristics