The April 1991 Iraçuba (CE) Seismic Event and its Aftershocks

A sequence of seismic events occurred near Taperuaba, district of Sobral-CE, following the main event of April 19 that reached magnitude 4.8 mb, the second largest ever recorded in this state. A cooperation effort among the Seismological Observatory, the Special Secretary of Civil Defense and the Municipality of Sabral was done to study these events within the period April 20 to 24. Macrosseismic survey was also realized and the maximum intensity found was VI (Modified Mercalli Scale). The data recorded in four local stations were analyzed and appointed a low activity confined in a narrow zone with NW-SE linear trend, and depths between 7 and 11 km. A composite focal mechanism, using the main event and its aftershocks, indicates a dextral strike-slip motion with small component of normal faulting. The NW-SE P axis is inconsistent with the roughly E-W stress field orientation for the northeastern region and may indicate local process related to the Potiguar Basin.