3 Seismic Activity

The Tucunduba Dam is about 290 km West of Fortaleza, Ceará state. The seismic monitoring of the area, with a local network, has began on June 11, 1997, soon after the occurrence of the event with magnitude 3.2 mb on June 09, 1997. The monitoring was done with one the analogical station (used for magnitude duration measurement and statistical control activity) and seven digital stations. The digital stations, with three components each, were operated from June to November 1997. In this work, the data collected during digital monitoring was analyzed to determine hypocenters and focal mechanism. To determine hypocenters, the HYPO71 program was used, with halfspace model, whose parameters were 5.95 km/s, for P-wave velocities, and 1.69 for the ratio between P and S-wave velocity. The active zone was nearly 1 km length, with depth between 4.5 and 5.2 km. With 16 events recorded in the same six stations, we determined the direction of the fault plane (NE-SW). The fault mechanism is strike-slip with a small normal component. The dip estimate was 65◦SE with FPFIT and 80◦SE with FOCMEC. Preliminary estimates of maximum horizontal compressive stress, using P-axis direction, were in accordance with Ferreira et al.(1998). The small difference is probably due to influence of the sedimentary basin in the regional stress. The active area is in accordance with seismicity described by Assumpção (1998), i. e., the combination regional stress, local flexural effect from thick sedimentary loads, and a presumably weaker crust from Mesozoic thinning.