A detailed 3-D P-wave velocity structure of the crust in the epicentral area of the 17 January, 1994 Northridge earthquake is determined by using 104,709 arrival times from 1673 Northridge aftershocks and 2948 other local earthquakes. A test performed using the data from the nearby portable stations suggests that the aftershock hypocenters relocated with the obtained 3-D model are accurate to about 2 km. We found that regions with high aftershock activity are generally associated with faster P-wave velocities. The velocity is high around the main south-dipping fault of the 1994 Northridge earthquake and the north-dipping fault of the 1971 San Fernando earthquake. A linear distribution of strike-slip aftershocks was found along a NE-SW boundary between high-velocity and low-velocity structures. To the west of this boundary a cluster of large shallow aftershocks with mixed mechanisms occurred in or near the border of a low-velocity area, while to the east aftershocks with thrust mechanisms occurred in a high-velocity area. These observations suggest that lateral variations of crustal properties are closely related to the fault segmentation in the Transverse Ranges. A better understanding of these features is important for long-term seismic hazard assessment in the Los Angeles area. ¿ American Geophysical Union 1995