The distribution of earthquakes at the surface and at depth in the Los Humeros geothermal field, Puebla (México), is analyzed from 1997-2004. Data for 95 earthquakes were registered at more than five permanent and temporary stations installed by the Comisión Federal de Electricidad and the Instituto de Ingeniería of the Universidad Nacional Autónoma de México. The duration magnitudes of the quakes are equal to or lower than 3.6 Md and the focal depths do not exceed 4.0 km. Simple focal mechanisms and moment tensor inversions'were made, and the number of earthquakes registered by two stations of the permanent network (numbers S05, S06) was compared with water-injection and steam-production volumes over a certain period. The results at the surface and at depth show seismic activity occurring in the northern zone of the field around injection wells 129 (well H-29) and 138 (well H-38); whereas, the simple focal mechanisms and moment tensors demonstrate stresses of heterogeneous origin, suggesting that part of the seismic activity in Los Humeros is probably "induced", mainly by injecting water.