Waste water injection is thought to have induced earthquakes by changing the pore pressure regime in proximity to pre-existing zones of weakness. Is a fault subject to compression, extension, or strike-slip strain more likely to move as a result of a change in pore pressure from water injection?
In the case of fluid injection you have to consider only for the additional hydraulic pore pressure which will reduce each of the 3 principal stresses. Thus the Mohr circle in Mohr's representation of failure (diagram of shear stress versus normal stress) is moved towards the left, thus enhancing the risk of failure for any stress regimes, i.e. for all earthquake mechanisms.
The problem of induced seismicity is different in the case of a reservoir. Here there are two effects, the load of the lake and the hydraulic pressure:
For a thrust fault the load results in an increase of the vertical principal stress equal to the load and an increase of the horizontal principal stresses by ca. 1/3 of the load. The radius of Mohr's circle gets smaller, and this is only partly compensated by the reduction of all pricipal stresses ( moving the now smaller circle towards the left). So a thrust fault (compression) might be stabilized.
For a strike-slip fault the least and the largest principal stresses ( both horizontal) are equally reduced by 1/3 of the load. The increased pore pressure will exceed this reduction by moving the reduced circle by the full amount of the load towards failure.
The largest effect occurs in the case of a normal fault ( extension).Here the radius of Mohr's circle is enlarged due to the load and moved towards failure by the full amount of the increased pressure, thus enhancing the earthquake risk the most.
Regards
Rainer
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