Characteristics of an Ongoing Seasonally Rain-Induced Seismicity in South-East Brazil

We examine the ongoing rainwater induced seismicity in the interior of the state of São Paulo, Brazil. The seasonal precipitation increase precedes the onset and increase of seismicity. The seismicity fluctuates from about 130 events per month with magnitudes between -0.2 to 2 Mw during the heavy rain season, to the absence of seismic events during the "dry" months. 

With more than 1500 seismic events underneath an area of 20 km^2 between 2016 and the beginning of 2019, we follow the seismicity variations and perform full moment tensor analysis when possible. We identify two main regions where events are more frequently occurring and have mostly prevalent sub-horizontal dipping planes, the very shallow events between 100 and 200 m and from 600 to 700 m depth. 

They suggest that the confined aquifer characteristics of intermittent sandstone layers and fractured basalt rocks influence the characteristics of the seismicity occurring underneath our study area. The facilitating factor of uncased drilled water wells that support the transport of water from upper to lower aquifers, and their location in the area of persistent seismicity, promote that the stress conditions of the fractured basaltic rock inside the shallow confined aquifers can be affected by the intrusion and percolation of significant amounts of rainwater. We see this induced seismicity as a phenomenon that could be more commonly occurring in other regions under similar weather conditions and aquifer and host rock characteristics, such as the Paraná basin.