Seismological Observations of the Seasonal Rain and Aquifer Induced Seismicity in Southeastern Brazil

We update our analysis on the ongoing seasonal induced microseismic activity in southeastern Brazil, in the interior of the state of Sao Paulo.  This is an area that not evidenced any active seismicity before 2016. We monitor this phenomenon as it is similar to other episodes of seasonal seismicity in other regions of Brazil, under similar aquifer and host rock conditions, commonly associated with those of the Parana Basin. 

This induced seismicity is seemingly triggered yearly during the high-rain season in Southeast Brazil, between December and May, and ceases as soon as the heavy rain season ends each year.  In these periods of increased precipitation during the annual onset of seismicity, we have found more than 1500 seismic events of magnitudes up to M2.0 in since 2017, after we deployed seismic stations in this area. Using phase weighing earthquake locations algorithms, we examine the clustering of the seismicity around recently drilled water wells, and seismicity rate changes, as it is modified by variations in the precipitation.

We perform full moment tensor analysis when possible to find the seismic activity is not only clustering horizontally, but at depth as well.  We identify two main regions where events are more frequently occurring and have mostly prevalent sub-horizontal dipping planes: The shallow events between 100 and 200 m and from 600 to 700 m depth. 

This phenomenon is facilitated mainly by the inadequate water well perforation practices in the region. Uncased water wells promote the transport of both rainwater and groundwater from upper to lower aquifers during higher precipitation months. The stress conditions of the fractured basaltic rock inside the confined aquifers are affected by the intrusion and percolation of significant amounts of water, which produce pore-pressure changes inside the host rock, and facilitates stress release though the microseisms.  This implies that the confined aquifer characteristics of intermittent sandstone layers and fractured basalt rocks from the Parana Basin condition the characteristics of the seismicity occurring in this region of Brazil.