Enhanced geothermal systems (EGS) deliberately induce seismicity — earthquakes — in order to access hot, subsurface rocks for use in geothermal power generation. Recent quakes around the world have frightened those living near EGS sites and sparked controversy over the technique. We asked experts to provide EGS technical details and to evaluate the seismic risk the process poses.
On Aug. 15, just after 2 p.m. on a Saturday, an earthquake registering 2.7 on the Richter scale rocked the German city of Landau in the province of Rheinland-Pfalz, close to the French border. Residents reported hearing a loud boom, prompting more than 200 calls to the local police. No damage or injuries were reported, but shockwaves of panic began to radiate throughout the country — and beyond.
Alarm spread as the state geological survey for Rheinland-Pfalz claimed the epicenter of the quake was centered 450 meters (500 yards) from a novel geothermal plant’s drilling site, at about 3.3 km (2 miles) underground — the same depth at which the steam reservoir from which the plant was drawing heat lay (Figure 1). It was the first time tremors had been felt from the commercial plant commissioned in late 2007. One of the first of its kind, the €15.5 million ($23.2 million) enhanced geothermal system (EGS) project had been sponsored by Germany’s environment ministry, and it provided 3.2 MW of power and heat to a district-heating scheme. It also benefited from the country’s feed-in-tariff program.
1. Shaken up. This August, a 2.7-magnitude temblor rocked the German city of Landau, close to the French border. Because it was centered on one of the first commercial enhanced geothermal systems in the world, the quake triggered concerns about the novel technology and its seismic risk. Courtesy: Geox
In Australia, attention focused on a similar proof-of-concept project that bored two wells 4.4 km into the yellow crust of the Cooper Basin, in a desert deep in the interior, to demonstrate economic heat extraction from rocks heated to about 250C (482F). The government and Geodynamics admitted that project had in December 2003 generated tremors with magnitudes of up to 3.7. In northeastern France, two test wells had been drilled at 5 km to harness heat from rocks at 200C at Soultz-sous-Forêts, in Alsace. In 2003, a 2.9-magnitude quake prompted restrictions on that project’s activities.
The Landau shake-up was even compared to a 3.4-magnitude trembler that shook residents, put cracks in buildings, and shattered roof tiles in Basel, Switzerland, on Dec. 8, 2006. The city of medieval cathedrals (Figure 2) is built in the region of that country that is most prone to seismic activity. Switzerland already sees three or four quakes of that size a year, but Basel was particularly vulnerable, officials said in 2006, having been leveled by a 6.5-magnitude earthquake once before, in 1356. Geopower Basel, the company that had been executing a trial deep heat mining project within the city — at Neuhaus Street and Shafer Lane — admitted that high-pressure injection of water 5 km into underlying superheated rocks could have caused the event. Authorities immediately halted the project, pending findings of a detailed seismic risk study, the results of which are due this December.
2. Basel rocks. On Dec. 8, 2006, a trial enhanced geothermal systems (EGS) project within the city of Basel, Switzerland, triggered a 3.4-magnitude quake that cracked buildings, shattered roof tiles, and made waves around the world. On average, Switzerland experiences three or four earthquakes of that size per year, but Basel is particularly vulnerable, having been leveled by a 6.5-magnitude quake in 1356. Courtesy: Wikimedia Commons
In September at Landau, as in Basel, concern mounted as plant operator Geox attempted to explain that it had relied on underground "fracturing" of the "hot" underground rock mass to enhance its permeability so that more water could be circulated to deliver heat to the surface for power conversion.
In other words, Geox had actually induced an earthquake by pumping fluid under high pressure for the subsequent extraction of energy.
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The larges seismic event at Rosemanowes site in the UK was 1.9 and 3.1 as shown in your chart. I was the seismologist at the site. Please amend this to refeect the true figure.