Seismicity in a low-strain area: Example of the Irauçuba fault in NE Brazil

Abstract

The present study illustrates an example of methods and results of fault examination in a slow strain and elastic attenuation area in intraplate Brazil. We investigate the Irauçuba seismogenic fault, which occurs in the Precambrian crystalline basement of the Borborema Province, the Equatorial margin of Brazil. The focal mechanisms in the region show normal, reverse, and mainly strike-slip faulting indicating a regional strike-slip stress regime in the upper crust up to 12 km depth with maximum horizontal compression (SHmax) roughly following the coastline trend. Under such context, we present earthquake data from a swarm sequence, the Irauçuba fault, at which the seismicity does not correlate with surface features. However, events with a magnitude as small as 1.5 mb are felt by the local population, and, in some cases, events with magnitude 2.5 mb or greater have damaged poorly built constructions. Therefore, suitable aftershock monitoring demands detailed macroseismic surveillance and engagement of the affected population and the Civil Defense. This engagement is crucial for awareness of the affected cities and the sensor network design and sustainability of the many stages of the aftershock monitoring such as deployment and maintenance. The earthquake data reveals that the Irauçuba NW-SE-striking fault occurs at depths between 8 and 9 km and dips ~70° to NE. This fault has not been previously reported and is solely defined by the spatial distribution of hypocentres that coincides with the focal mechanism solution, and no obvious surface expression is observed. The fault plane reveals an initial rupture at shallower depths that progressively migrated to deeper portions of the crust. Therefore, we suggest that the initial seismic period transferred stresses to other near-critical areas of the fault, which allowed pore pressure to migrate within permeable portions of the fault zone within the upper crust.