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Figure 2 Decline of the pore pressure due to production at the Lacq gas reservoir and number of recorded
earthquakes with magnitude M 3, with time, gas pressure (in MPa; 1 MPa is equal to 10 bars) (circles, left
scale), and number of M > 3 earthquakes per year (solid line, right scale). The number of earthquakes increased
with decreasing pressure. SOURCE: Segall (1989).
(Figure 2). The epicenters of 95 percent of the well-located events and all of the M > 3 events were within the
boundaries of the gas field (Grasso and Wittlinger, 1990).
An analysis of the stress changes above and below the reservoir indicates that the induced seismicity is
consistent with a thrust fault regime where the least compressive stress is vertical. Furthermore, the maximum
shear stress change is calculated to be about 0.1 MPa (1 bar) for a pressure drop of 30 MPa (300 bars), sug-
gesting that the in situ stress prior to production was close to causing frictional failure of the rock.
SOURCES: Segall (1989); Segall et al. (1994); Segall and Fitzgerald (1998); Grasso and Wittlinger (1990);
Grasso (1992).
 
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