Earthquake Preparatory Process in pull-apart systems: reservoir-triggered seismicity in Castanhão, NE Brazil

The manmade changes to the environment can pose a risk of triggering seismic activity within the regions affected by such alterations. The seismic activity can be triggered by many factors, such as underground mining, hydrocarbon extraction, CO2 sequestration, wastewater injection, etc. One of the impacting factors is reservoir impoundment related to artificially created bodies of water. 

The artificial reservoirs play a significant role in the modern environment. They are built for the purpose of flood prevention, water storage, irrigation, hydropower, etc. Building the dam over rivers, however, can pose a risk to local societies in case of its damage. Such a case took place in 1967 and was related to the Koyna earthquake, which was triggered by reservoir impoundment, causing over 200 fatalities and leaving a few thousand people injured. 

In the following study, we investigate the Reservoir-Triggered Seismicity (RTS) within the biggest artificial lake – Castanhão Reservoir – in the State of Ceará, NE Brazil. For the purpose of study, we employ tools available within the EPOS EPISODES Platform, PyMPA Template Matching package, Hypo71, HypoDD, and KIWITool. We observed 227 earthquakes on the study site, between December 2009 and August 2008, whose moment magnitudes vary between 0.0 and 2.7. We also investigate the role of pore pressure variation in triggering earthquakes within the reservoir. 

Our results show that changes in pore pressure in the underlying medium can cause the swarm-like seismicity within the vicinity of the reservoir located within pull-apart systems. We compare them to other known sites within similar tectonic settings. The study shows that the pull-apart basins are prone to reservoir-triggered seismicity and should be treated as such during seismic hazard assessment. Such an investigation requires a multidisciplinary approach within Solid Earth science disciplines such as seismology, geology, tectonics, as well as scientific branches such as hydrology and modelling. The result of this study is in preparation to be published as one of the new EPISODES on the EPOS Episodes Platform.