The Taipei basin, historically low in seismicity, is located in northern Taiwan. A dense broadband seismic array was deployed in the basin in June 2004 to monitor seismic activity. During the period of operation, three felt earthquakes occurred near the eastern part of the Taipei basin, about 3 km to the south of Taipei 101 then the tallest building in the world. Relocated earthquakes show a southeast-dipping distribution of hypocenters beneath the Taipei basin. The seismicity pattern and focal mechanisms of the three felt events suggest the existence of a blind normal fault whose surface projection is along the river channel in the middle of the basin.