This is a fascinating new study of the earthquakes in Oklahoma related to fluid injection. I have not seen it out in the media yet.
T.H.W. Goebel, M. Weingarten, X. Chen, J. Haffener, E.E. Brodsky. Earth and Planetary Science Letters. Volume 472, 15 August 2017, Pages 50-61.
It was a mystery as to why two additional areas, up to 40 km away from the injection site were suddenly experiencing seismicity. This new thinking about induced seismicity changes the seismic hazard assessments.
In short, there are several ways that fluid injection, even at shallow depths, can cause earthquakes. The most commonly known means was to increase the pore pressure in the rock (pressure exerted by fluid in the rock spaces) which subsequently reduces stress that holds the fault in place and can cause them to release and move. (It’s not “lubrication” but a bit more complicated than that.) The second way was for fluids to travel deeper through more transmissive “damaged” rock around the fault. Areas that had a hydrologic connection to this fault would be influenced. Another way is for the surrounding rock to react to movements in adjacent rock (a propagating pressure release).
A new mechanism, tested here, is that of poroelasticity stress – the fluid-filled pores experience a change in pressure under a mechanical stress, which, in turn, leads to fluid motion. As a reaction to this change in pore volume, the solid material shifts and deforms elastically.
It’s difficult to imagine the big effects that are caused by the small changes and tiny movements that occur at this microscopic level. But it’s how things work. In this situation, it was important to note that the faults may not have been “ready to slip” but reactivated by other local deformation events. The pore pressure effects are local but the other means can go much farther than the classic model considered. The injection wells were close together and the faults were situated in such a way that they were affected by the increase in fluid into the subsurface.
Bottom line: the zone of influence of injection wells may be larger than commonly assumed. But, it’s not a simple calculation. There is a LOT going on and it takes a while to figure it out.
If I have any of the above translation from scientese wrong, or if there is additional info, please let me know.