Induced seismicity in the Castanhão reservoir, NE Brazil — Preliminary results |
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| Authors: | JM Ferreira GS Frana CS Vilar AF do Nascimento FHR Bezerra M Assumpo |
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| Institution: | aUniversidade Federal do Rio Grande do Norte, Centro de Ciências Exatas e da Terra, Departamento de Física, Natal, RN, Brazil;bUniversidade Federal do Rio Grande do Norte, Centro de Ciências Exatas e da Terra, Departamento de Geologia, Natal, RN, Brazil;cUniversidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Departamento de Geofísica, São Paulo, SP, Brazil |
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| Abstract: | The Castanhão reservoir was built in the state of Ceará, a dry region in Northeastern Brazil, to regulate the flow of the Jaguaribe River, for irrigation, and for power generation. It is an earth-filled dam, 60 m high, with a water capacity of 4.5 × 109 m3. The seismicity in the area has been monitored since 1998, with a few interruptions, using one analog or one digital station and, during a few periods, a three-station network. The first earthquakes likely to be induced events were detected in 2003, when the water level was about 20 m high. In early 2004 a very heavy rainfall season quickly filled the reservoir. Shortly after, an increase in the seismic activity occurred and many micro-earthquakes were recorded. We suggest that this activity resulted from an increase in pore pressure due to undrained response. Therefore, we may classify this cluster of micro-earthquakes as “initial seismicity.” We deployed a network with four analog stations in the area, following this activity, to determine the epicentral zone. At least three epicentral areas under the reservoir were detected. The spatio-temporal analysis of the available data revealed that the seismicity occurs in clusters and that these were activated at different periods. We identified four sets of faults (N–S-, E–W-, NW–SE-, and NE–SW-oriented), some of which moved in shallow crustal levels and as recently as the Quaternary (1.8 Ma). Under the present-day stress regime, the last two sets moved as strike-slip structures. We suggest a possible correlation between dormant faults and the observed induced seismicity. |
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| Keywords: | Reservoir-induced seismicity NE Brazil Undrained response Pore pressure diffusion |
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