CARLOS COSTA E SILVASimulation: a hypothetical earthquake damages the main church in BebedouroCARLOS COSTA E SILVA
A team from the IAG, the Astronomy, Geophysics and Atmosphere Sciences Institute of the University of São Paulo (USP) verified that light seismic movements have constantly occurred in the district of Andes in the city of Bebedouro since January 2004. A few months before, 10 deep water wells – with depths ranging from 120 to 200 meters – had been dug on a farm to irrigate orange groves during the dry season. Ten seismograph stations installed in the region since 2005 helped researchers conclude that the wells had caused the approximately 3 thousand seismic movements registered in the last five years – an average of two such movements a day, (most of them during the wet season, when water is not pumped out of the wells). At first, the inhabitants of Andes attributed these seismic movements to explosions in stone quarries, even though no such quarries exist in the vicinity.
As it rains a lot from December through March, the wells are not pumped out and remain full of water. “The water penetrates easily through the basalt rocks that form one of the underground layers and this facilitates the movements of loose rocks,” says geophysicist Marcelo Assumpção, coordinator of the team from IAG contacted by the inhabitants of the region in early 2005. The magnitude of the stronger seismic movements reached 2.6 and 2.9 on March 11 and March 30, 2005.
“Normally, earthquakes whose magnitude ranges from 2 to 3 points on the Richter scale are not perceived by people. These earthquakes are common in Bebedouro because they are close to the surface, at a depth of approximately 200 meters,” he says. Stronger earthquakes are not that common. In March 2005, the seismic equipment registered at least 100 earthquakes a week, 20 to 30 of which were felt by the inhabitants of the district of Andes. The stronger earthquakes shake the houses and make floors and windows vibrate, cause cracks on flooring and walls, make pictures and doors swing back and forth and scare people.
The seismographs indicated that the earthquakes can migrate, as detailed in the paper to be published in the Water Resources Research journal. “Each year, the earthquakes begin near wells where the outflow of water is stronger,” says Assumpção. “Eventually, the epicenters move away for a few kilometers, and follow the movement of the water pressure in the deeper layers.”
The infiltration in the cracks of the rocks can cause stronger quakes in the vicinity of dams in hydroelectric power plants. Specialists attribute changes in the soil, caused by the changes provoked by the building of dams and reservoirs, as the origin of approximately 100 earthquakes that have occurred around the world. The most serious of these was the earthquake associated with the construction of the Zipingpu dam in China, with a magnitude of 7.9 degrees on the Richter scale. It resulted in the death of 80 thousand people in May 2008. A team from IAG and from the University of Brasília (UnB) identified 16 hydroelectric power plants that could cause earthquakes in Brazil. Magnitude 4 quakes occurred in the 1970s in the vicinity of the Volta Grande and Marimbondo reservoirs, both in the vicinity of Rio Grande, which lies less than 100 kilometers away from Bebedouro.
“The lands in the north and northwest of the State of São Paulo have geological flaws that cause earthquakes,” says Tereza Higashi Yamabe, a retired professor from São Paulo State University, currently working as a collaborating researcher at IAG. Professor Yamabe is also doing research on the region. In general, she says, most of the hundreds of wells in the area lie at a depth of less than 100 meters and the water flow is less than 10 cubic meters per hour. This flow does not go through the sandstone rocks that form the first rocky layer under the ground. People who need more water usually dig deeper wells, down to the basalt rock layer, which is thicker and more compact than the sandstone rocks. The basalt rocks, however, have cracks that could be filled with water. “The deep wells that cause the seismic movements allow the water running through the layers of sandstone to go down to deeper layers and put pressure on the rocks already under geological pressure and about to slide,” says Tereza.
Hearings
These phenomena changed from being purely natural ones. In 2006 and 2007, Tereza and Assumpção went to two public hearings in the district of Andes, called by the City Council of Bebedouro. “We explained our interpretation that the seismic movements had been caused by the digging of new wells on the Fazenda Aparecida farm and recommended that the shallow part of the wells be sealed off to prevent the water from flowing down into the basalt layers, especially during the season when water is not being pumped out of the wells,” says Assumpção. He adds that these recommendations were forwarded to DAEE, the Water and Electric Power Supply Department of the Bebedouro city government and to the town’s district attorney. These recommendations, however, were not transformed into actions that could prevent the earthquakes. “The population affected by the earthquakes wanted the wells to be closed off, but the city councilmen refused to do so,” he says. “Each group accepted our conclusions or not, depending on their own interests.” Assumpção says that the owner of the farm with 14 deep wells refused to accept the fact that the source of the earthquakes could be found on his property. “The manager of Saaeb, the water supply company of Bededouro, also refused to believe in the results of the study.”
Finding anybody willing to talk about the earthquakes in Bebedouro is a tedious task. Nobody from Fazenda Aparecida farm talks about this matter. The technicians from Saaeb say that they are waiting for a decision from Daee. Queries to the city government of Bebedouro were not answered. Most of the city council members were elected in 2009 and current city government employees were hired in the same year; as a result, the city’s previous local government is getting the blame. “There is no law that concerns this situation and therefore no government entity with the legal authority to order that the upper part of the wells be sealed,” says Assumpção.
After a relatively quiet second semester in 2009, the earthquakes started again in mid-December, repeating the same pattern of the previous years. It is possible to leave things as they are, until the rocks settle and all the energy is spent. The risk is that future earthquakes might be even stronger. Tereza recalls that Nuporanga, 80 kilometers away from Bebedouro, is another case of earthquakes caused by the digging of wells. The inhabitants of Nuporanga hve already become accustomed to hundreds of light earthquakes that they have felt since 1977, and were frightened by a more severe earthquake that occurred 12 years later, whose magnitude was 3.2 points on the Richter scale.
The Project
Seismic activity in Bebedouro, SP, triggered by water wells (nº 07/04325-0); Type Regular Line of Aid for Research Project; Coordinator Marcelo Sousa de Assumpção – IAG/USP; Investment
R$138,495.60 (FAPESP)
Scientific article
Assumpção, M. et al. Seismic activity triggered by water wells in the Paraná basin, Brazil. Water Resources Research (in press).
