In early December 2006, a massive fluid injection was carried out at 5 km depth below the city of Basel, Switzerland, for geothermal reservoir enhancement. During the six-day stimulation, approximately 13,000 induced microearthquakes were detected by a borehole network. The largest of the induced earthquakes, which had a magnitude of ML 3.4, was strongly felt in the Basel area and led to the termination of the project after only six days of stimulation. We analyzed the approximately 3500 locatable events of this induced earthquake sequence, which is one of the most densely monitored deep fluid-injections in the world. The seismic monitoring system consisted of six borehole seismometers at depths between 300 m and 2700 m near the injection well and of numerous surface stations in the Basel area. In this article, we report on the analysis of the sequence using exclusively data from the down-hole instruments. We show how a refinement of arrival time picks by cross-correlation techniques and subsequent high-precision relocations lead to significant improvements of the hypocenter locations compared to routinely applied manual procedures. We also analyze focal mechanisms determined from both first-motion polarities and amplitudes of signals recorded by the borehole sensors alone and compare the results to the focal mechanisms of the larger events recorded also by the surface networks. Our findings indicate that the induced sequence consists to a large part of repeating earthquakes that repeatedly rupture the same fault patches, and that the activated seismogenic structure is not a single fault plane but a heterogeneous fault zone. (C) 2014 Elsevier Ltd. All rights reserved.