Rainwater and Aquifer Factors of Seasonally Induced Seismicity in Southeast Brazil

Date: 4/26/2019

Time: 04:30 PM

Room: Cascade I

While seasonal rainwater is normally not considered as a determining factor in cases of induced seismicity, it has been identified in southeastern Brazil as the driving factor behind seasonally induced seismicity in Jurupema, located in the interior of the state of Sao Paulo. While induced seismicity occurs mostly as a by-product of particular man-made practices and structures, induced seismicity from rainwater is one of the less documented phenomena of this type. Even though this type of seismicity is still scarce and underreported worldwide, this seismicity already has a precedent in Brazil, in Bebedouro, about 50 km away from our study area.

We have been monitoring the seasonality of this phenomenon since 2016, where the trends in the regional precipitation data coincide with trends of seismicity increase of the more than 1300 seismic events between 2016 and 2018. We follow the seismicity variations and perform full moment tensor analysis when possible, to identify two main regions where events are more frequently occurring and have mostly horizontal nodal planes, the very shallow events between 100 and 200 m and from 600 to 700 m depth.

We find that the significant similarities of the seismicity to that of the previously recorded earthquake sequences in Bebedouro, not only correlate to the seasonal precipitation changes, but the seismicity also correlates with the location of water wells used for irrigation drilled contemporary to the onset of seismicity, and the type of underlying rock that hosts the confined and unconfined aquifers.

The behavior and characteristics of this seismicity seems to be promoted by the type of rock hosting the aquifers in this region, where the stress conditions of the fractured basaltic rock inside shallow confined aquifers can be affected by the intrusion and percolation, coming from the addition of rainwater affecting the pore-pressure conditions, given the facilitating factor of water wells that permit permeation of water from upper to lower aquifers.

Presenting Author: Jaime Convers

Authors