Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010geoji.181.1566e&link_type=abstract
Geophysical Journal International, Volume 181, Issue 3, pp. 1566-1574.
Astronomy and Astrophysics
Astronomy
5
Permeability And Porosity, Fracture And Flow, Fault Zone Rheology, Earthquake Source Observations
Scientific paper
We relocate precisely micro-earthquakes induced by the Açu reservoir in Brazil and observe seismicity migration consistent with pore-pressure diffusion on a single fault zone. Fluids are believed to play a major role in triggering tectonic earthquakes; reservoir induced seismicity provides a natural laboratory in which to investigate the spatio-temporal evolution and triggering of earthquakes caused by fluid and pore-pressure diffusion.
Between 1994 and 1997, 267 earthquakes (ML <= 2.1) were recorded and located beneath the Açu reservoir. The seismicity increased several months following annual water level peaks, implying that pore-pressure diffusion is the principal triggering mechanism. The small station spacing and very low-attenuation, Precambrian basement, rock enabled starting earthquake locations with uncertainties of only a few hundred metres. We relocate 155 earthquakes from the largest cluster at Açu using waveform cross-correlation to obtain groups of similar events. We use these groups to improve the pick accuracy (to subsample accuracy in 200 sample per second data), and then invert for more accurate hypocentral locations. Our uncertainties are on the order of 10 m, and our locations are more tightly clustered. We observe temporal migration of the earthquakes, both along strike, and to increasing depth. We observe a seismicity migration rate between 15 and 58 m d-1. The rate is highest during the time of peak seismicity rate, and there is some suggestion that the rate decreases with increasing depth. Peak depth in seismicity is reached 175 d after the water peak, that is 192 d after the water low, and the maximum depth then decreases at a similar rate to the rate of increase. Our observations are consistent with triggering by pore-pressure diffusion within a heterogeneous fault zone with an average hydraulic diffusivity of ~0.06 m2 s-1 and fracture permeability of ~6 × 10-16 m2.
Abercrombie Rachel E.
Do Nascimento Aderson F.
El Hariri Maya
Rowe Charlotte A.
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