Abstract: Ninghe earthquake sequence occurred in April of 2003 was clearly felt by the majority area of Tianjin municipality and the epicentral intensity is V. The analytical results indicate that Ninghe earthquake sequence has the basic characteristics of earthquake swarm in the distribution of space, time and intensity. However, it is different from the traditional earthquake swarm because it had only a few aftershocks and its frequency and intensity attenuated rapidly. Although Ninghe earthquake sequence occurred at the southwestern end of Tangshan aftershock zone, it has the spatial characteristics of its own. The focal-mechanism solutions of three M≈4 earthquakes are not consistent, which might be related to the complicated tectonics in the focal region. It is a kind of manifestation of the stress adjustment and variation in the focal region, indicating that the stress in the focal region is still being adjusted.

Key words: Ninghe earthquake sequence, Focal-mechanism solution, Complicated tectonics, Tianjin area