Earthquake sequences in the southern block of the Pernambuco Lineament, NE Brazil: Stress field and seismotectonic implications

摘要

The Pernambuco Lineament (PeL) is an E–W-striking, 700 km long shear belt located in the central part of the Borborema Province, Brazil. Seismic sequences have been monitored in the region since 1991, and they are clearly correlated with the main shear belt and its NE-trending branches. A new sequence of small earthquakes occurred in April 2010 away from the main belt in the southern block of the lineament. We monitored this seismicity using a ten station network for 154 days and used the observations to determine the nature of the seismicity and the stress patterns. In addition, we used data from five previous local seismograph networks to determine the stress tensor. The results indicate that the seismicity is concentrated in three clusters, two of which are associated with normal faults: the first strikes 96°, dips 51°, and has a rake of ?65°, and the second strikes 253°, dips 64°, and has a rake of ?120°. These faults are 2.2 km and 1.5 km long and 2.0–3.0 km and 3.0–3.5 km deep, respectively, and are not correlated with the Pernambuco Lineament or its NE-trending branches. The inversion of focalmechanisms fromour investigation and previous studies indicates E–W-trending compression and N–S-trending extension that affects both the northern and southern blocks of the PeL. The new data indicate that the fault behavior in the shear zone area is more complex than has previously been observed along reactivated mylonitic belts.