Groundwater unloading and corresponding changes in fault zone stick-slip behavior

Question

I was intrigued by this Slate piece, more or less a summary of Amos et al. (2014)'s Nature article, which essentially states that groundwater depletion and subsequent crustal unloading causes a decrease in the stress normal to the San Andreas fault, thus bringing the fault "closer to failure" (to quote from the abstract).

Does that mean, as Eric Holthaus, the author of the Slate post suggests, Amos and his co-authors believe that due to the drought, the next "major" earthquake may not be quite as significant in magnitude? (The idea being that an increase in small-magnitude slip events over several years will help ease the intensity of short-period rupture propagation during a larger event?) Or, does it mean that the released stress is just being redistributed to locked sections of the fault system, which I assume experience decreased Mohr–Coulomb forces due to the region wide groundwater draw down? Elsewhere in the Slate article, for example, Holthaus notes that the August 2014 Napa Earthquake actually redistributed some of its released stress onto the Hayward Fault, which doesn't quite seem to support the "easing" hypothesis.

Am I wrong in thinking that the two ideas referenced (easing vs. redistribution) might be mutually exclusive? I sort of wish Holthaus had delved a little deeper in his analysis, since it seems that the clarity of the Slate article takes a fair hit from this inconsistency.

Answer 1

Changes in water level effect the pore pressure at depth around the fault. The general assumption is that the pore pressure is proportional to fault-normal stress which can be thought of clamping stress (i.e., the stress that prevents it from slipping). Basically the higher the clamping stress the more shear stress is required to make it slip. Now the shear stress on San Andreas accumulates due to plate motion between the North American and Pacific plates and that rate remains unchanged. But the normal stress can change slightly due to water level changes, so it can either advance or retard the occurrence of an earthquake in time. But under no circumstances can it stop them from occurring.

The main idea in the paper is that the normal stress on faults have slightly decreased (due to water level drop) which helps to unclamp the fault(s) which in turn increases the background seismicity rates.

So given all this, the first argument by (Holthaus) is correct. However the unclamping can also advance (in time) the occurrence of the big one. Having said that please remember that stress changes we are talking about are rather small.

With regards to the Napa earthquake: The point of an earthquake is to relive the accumulated shear strain. If there are two faults in parallel (as is the case in the region) and both are accumulating strain then an earthquake on one will decrease the 'effective shear stress' on the other fault, directly parallel to it. However at the same time it will increase the shear at the two ends. So things are slightly more complicated.