A continuing intraplate earthquake sequence near João Câmara, northeastern Brazil‐Preliminary results

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Abstract
Shallow seismicity along a well‐defined section of reactivated Precambrian basement near João Câmara, Brazil (5.5°S, 33.7°W) has continued since August 1986. A magnitude 4.3 event on 21st August with a decaying aftershock sequence and another main shock‐aftershock sequence which started in September were followed by a magnitude 5.1 event on 30th November and 11 events of magnitude 4 or above up to February 1987. A network of up to four vertical component short period seismometers with smoked paper recorders has been deployed since late August 1986. Several thousand events have been recorded above magnitude 0, and activity was still being recorded (about 10 events per day) in May 1987. Simple velocity structure, good transmission and impulsive S arrivals on almost all seismograms combine to facilitate reliable hypocentral locations even for this sparse network, and epicenters define a 25 km linear pattern of seismicity with northeasterly strike, approximately coincident with a Precambrian structural trend. A composite fault plane solution shows dextral shear along this strike with a small normal component. Fault plane solutions and epicenter distribution in Northeastern Brazil suggest an overall picture of E‐W regional compression.

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