The role of fluids in triggering earthquakes: observations from reservoir induced seismicity in Brazil

Abstract

S U M M A R YWe 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 (M L ≤ 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 m 2 s -1 and fracture permeability of ∼6 × 10 −16 m 2 .

Seismic hazard assessment in Polyphyto Dam area (NW Greece) and its relation with the "unexpected" earthquake of 13 May 1995 (<i>M</i><sub>s</sub> = 6.5, NW Greece)

“…The calculated increase of both the number of earthquakes and of PGA values may be attributed to the impoundment and the water level fluctuations of the Polyphyto Dam due to the pore pressure increase (Talwani and Acree, 1985;Roeloffs, 1988;El Hariri et al, 2010). Nevertheless, these factors did not affect the magnitude of the 13 May 1995 event.…”

Violations of Gutenberg–Richter Relation in Anthropogenic Seismicity

“…In 1989, additional three stations were installed, and the next four stations in the years 1990-1991 [Ferreira et al, 1995]. Between 1994 and 1997, the reservoir was monitored by at least 8 stations in 5 different configurations [do Nascimento et al, 2004, El Hariri et al, 2010. The data from the vicinity of Açu reservoir was acquired from the longest (10-year) observation period from the datasets analyzed in the present work.…”

Magnetic transfer function entropy and the 2009 <i>M</i><sub>w</sub> = 6.3 L'Aquila earthquake (Central Italy)

“…The figure shows also the magnitude of earthquakes with M ≥ 2.4: they all lie close to the solid oblique line. This line would correspond to a downward earthquake migration with apparent velocity of 12 m day −1 , a value which is comparable with those occurring in other situations; for instance, it is similar to the velocities found in reservoir induced seismicity where pore pressure diffusion causes hypocentre vertical migration rates between 15 and 58 m day −1 (El Hariri et al, 2010). We also notice that the first magnetic entropic burst occurs just after the largest deep event (M L = 2.6), while the other two occur between the last two deepest earthquakes preceding the main shock, just during the downward migration of hypocentres.…”