By Dan Oancea - Twitter

 

 "Civilization exists by geological consent, subject to change without notice." William Durant

Still waiting for the big one? What do you know about smaller quakes that could have been triggered by your reputable power company?

The environmental impact of geothermal power plants is assessed by the Geothermal Literature Assessment: Environmental Issues paper which notes that Air Emissions (As, CO2, H2S, Hg, nitrogen oxides and particulate matter) are low to non-existent; the Water Quality and Brine are easy to deal with (re-injection of spent brine); Solid Wastes (geothermal sludge) have to be considered hazardous and need to be properly disposed; Noise Pollution exists (do not build a geothermal station close to residential areas); Land Use conflicts and problems generated by Subsidence and Induced Seismicity also exists.

In New Zealand they found that mercury infiltrates the water in geothermal wells, and at the same time is being discharged in air by cooling towers. Arsenic and boron were also reported as important natural geothermal pollutants.

A good online paper is the Induced Seismicity Associated with Enhanced Geothermal Systems study. I will only highlight a few of its finds beginning with the presentation of several well-documented cases:

- At Soultz, France fluid injection in two deep reservoirs (3,500 m and 5,000 m) generated micro earthquakes (up to 2.9) that determined the local population to intervene for curtailing the activity;

- At the Geysers Geothermal Field in Northern California, U.S.A., a strong correlation has been proved between injection and seismicity – a maximum 4.6 event occurred in 1982; numerous faults cut through the area;

- At the Cooper Basin, Australia, magnitude 3.0 earthquakes were detected since the beginning of injection / hydrofracturing; it is a sparsely populated area so no local resistance has been encountered by geothermal energy developers;

- Berlin, El Salvador, a seismically active region, it is marked by plate subduction and volcanic activity; some 2,500 m deep injection and production wells were located on the flanks of an inactive volcano; weeks after the shut-in of pumping operations a 4.4 magnitude event hit the area (it was the largest of a string of smaller magnitude earthquakes).

Their conclusions:

“…several conditions must be met for significant (damaging) earthquakes to occur. There must be a fault system large enough to allow significant slip, there must be forces present to cause this slip along the fault (as opposed to some other direction), and these forces must be greater than the forces holding the fault together (the sum of the forces perpendicular to the fault plus the strength of the material in the fault). Also, as pointed out above, the larger earthquakes that can cause damage to a structure usually can only occur at depths greater than 5 km. “

What kind of forces are acting in the case of injection induced earthquakes?

Pore pressure is defined as the pressure value of the fluid stored by a rock in its pores and fissures. By pumping pressurized fluid into an underground rock a drastic increase in pore pressure could be noticed. The water (pore pressure) is actually pushing apart the walls of the fault.  Japanese researchers tested the theory by pumping water in active faults. No need to say what happened next.

So what went wrong in the Basel, Switzerland case? In the first place a well-known historical account was much disregarded.

On the evening of October 18, 1356 a 6.0 to 6.9 magnitude earthquake leveled the city of Basel. It is considered to be the most powerful intra-plate earthquake known to have occurred in central Europe. Historical records reported that “no church, tower, or house of stone in this town or in the suburb endured, most of them were destroyed…” Hundreds to thousands perished. Other medium to large earthquakes have also been reported during the last hundred of years.

The city lies on an important geological feature: the seismically active Rhine Graben. Intensive geological studies have managed to identify the fault responsible for the 1356 catastrophic event: an active scarp fault which has moved the surface upward by almost 2 meters. If an earthquake of that magnitude were to occur today, damages would top 50 billion dollars. Injecting fluid in a seismically active zone could turn out to be a pretty expensive business.

An Australian company drilled its geothermal wells only a few kilometers away from the Olympic Dam mine, which is located on top of a heat anomaly. Their ‘mini-fracture stimulation’ program will probably take place in September 2007. Because of its close location to the mine site this might be an interesting story to follow. Good news is that their well is not that deep – some 2 km deep, while in order to generate a ‘significant’ earthquake a 4-5 km deep well is generally required. On the other hand we don’t know anything about how close the injection well is to any major active fault; anyways, I’m also thinking that a ‘low magnitude’ 3.5 to 4.0 earthquake would still be able to generate major damages to any man-made underground structures and cavities.

Nowadays the role of the water within the crust and mantle is being reconsidered.  New studies have found that a subducting slab of crust could carry to depth not only minerals and rocks but a lot of water too; an underground ocean of water actually resides deep in the mantle underneath Asia.

A recent New Zealand study found that ‘hot fluids’ in lower mantle are more than necessary in triggering ‘swarms of earthquakes’ (i.e. small earthquakes occurring simultaneously).

And here comes a really intriguing theory. Laurent Bollinger of the French Commission for Atomic Energy looked at earthquake frequency in Nepal and found that the amount of rain brought by the summer monsoon influences the frequency of winter earthquakes. He considers two explanations as being possible:

- The summer time tremors have been suppressed by the sheer weight of the water accumulated in underground reservoirs (we’re talking up to six meters of rainfall per year);

- Or, the water simply seeps down for several months until it reaches the 10 km deep thrust plane and triggers winter earthquakes (remember that the magnitude of pore pressure is also given by the weight/height of the water column).

The latter is by far my favorite. The water doesn’t really need to seep down that deep; any major active fault would definitely react to a significant increase in pore pressure.

The future of geothermal energy looks brighter than ever. At the same time safety concerns represent major issues that have to be dealt with prior of even thinking of putting a hole in the ground.

If the chosen location is close to a city, dam or mine, while the targeted ‘hot rock’ lies at a depth of 5 km or more, in a well known seismically active (faulted) zone, chances are that your innocent looking injection hole would trigger a swarm of earthquakes. And you could be held responsible for that. Not to mention that the local population would withdraw their support to your project and will do whatever it takes to get your operations closed for good.

While dealing with an increased pressure to find green, renewable, alternative power sources all geoscientists should always bear in mind their profession’s core values: the safety, health and welfare of the public and environment.

Good luck.