Friday, October 26, 2012

Groundwater: Too Much or Too Little - Earthquake!

Groundwater hydrologists have known for a long time that injecting water into the subsurface can change the hydrology enough to trigger small earthquakes.  Just listen to all the current rhetoric regarding hydraulic fracturing and the increased incidents of small regional tremblors in oil and gas producing areas.  The injections not only pressure up the local systems, but also lubricate the faults that slip and slide - releasing the stored energy of a mobile, but temporarily stuck crust.

But a recent study of the May 5, 2011 earthquake in Lorca, Spain has concluded that groundwater pumping was a significant factor in that earthquake.  First, this was a very shallow quake - only .6 miles below the land surface.  Such a shallow earthquake is unusual.  In this immediate area the groundwater declines have been extreme - on the order of 800+ feet over the past 50 years.  In essence, the dewatering was bounded by a geologic fault (the Alhama de Murcia fault), and, according to the study, the weight of the removed water actually increased the stresses on the fault in those specific areas. The researchers used satellite imagery and GPS stations to come to these conclusions. 

It was a surprise to the study scientists that the relatively small stress changes due to dewatering could have had sufficient impact on such a large scale fault system.  In fact, the calculated stress increases due to the dewatering were not much more than normal atmospheric pressure changes.  The study concluded that the energy released by the 5.1 quake actually included not only the increased stress due to dewatering, but also several centuries of normal stress buildup due to the much more common regional deformation. Kind of like the dewatering was the straw that broke the camel's back.

On the surface of it all, it would seem now that we can neither take out, nor put in, water underground without fearing earthquakes - at least in certain circumstances.  And perhaps even more importantly, had this quake happened in Italy, would the hydrologists have been tried for culpability along with the risk assessment scientists?

The report findings are detailed in the October 21, 2012 issue of the journal of Nature Geosciences.

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