Seismic activity in senador SÁ-CE, Brazil, 1997-1998

Abstract

The Tucunduba Dam is about 290 km West of Fortaleza, in Ceará state. The seismic monitoring of the area, with a local network, began on June 11, 1997, soon after the occurrence of an event with magnitude 3.2mb on June 09, 1997. The monitoring was done with one analog station (used for magnitude measurement and statistical control of the activity) and seven digital stations. The digital three-component stations 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 the hypocenters, we used a half-space model, whose parameters were 5.95 km/s, for P-wave velocities, and 1.69 for the ratio between P and S-wave velocities. The active zone was nearly 1 km long, 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 was estimated to be 65°SE using composite focal mechanisms and 80°SE from the P/S amplitudes ratios. Preliminary estimates of maximum horizontal compressive stress, from the P-axis direction, were in agreement with Ferreira et al.(1998). The small difference is probably due to influence of the sedimentary basin on the regional stress. The active area is in accordance with seismicity described by Assumpção (1998). © 2004 Sociedade Brasileira de Geofísica.