Among the many impacts of anthropogenic activity on the Earth, one that has caused particular public disquiet in recent years is “induced seismicity”, that is minor earthquakes and tremors caused by industrial processes. A review article recently published in Reviews of Geophysics examined some of the current challenges in monitoring and managing induced seismicity, with particular reference to European cases. The editors asked two of the authors some questions about this highly controversial issue.

What kinds of human activity cause induced seismicity?

Many industrial activities that alter the state of stress or the pore pressure within the Earth’s crust can, in principle, induce earthquakes, although most do not. As a general rule, the deeper the activities are, the larger the stress alteration is, and the closer seismogenic faults are the more likely earthquakes are. The most common operations causing induced seismicity are underground mining, water reservoir impoundment, oil and gas production (from both conventional and non-conventional resources), geothermal energy exploitation, waste water disposal, and and natural gas storage operations. The largest number of induced seismic events at global scale have been related to mining operations while the largest induced events in terms of magnitude have so far been caused by water impoundment.

Is this a recent phenomenon and is it getting worse?

The relationship between industrial operations and induced seismicity has been known since the beginning of the twentieth century, particularly being exposed in relation to coal mining. However, the topic has received considerable attention in recent years. The sharp increase in seismicity rate in the central United States observed after 2009 and its association with wastewater disposal operations raised the interest of the scientific community and the general public.

On the one hand, the increasing number of reported induced seismicity cases can be attributed to the growing demand for energy that requires an extensive exploitation of natural resources. On the other hand, the number of reported induced earthquakes also has apparently increased because seismic monitoring infrastructure is now available and covers wide regions, making it easier to identify seismic events potentially associated to industrial activities. Last but not least, public tolerance for induced earthquakes has dramatically decreased, in line with lower risk tolerance for other technologies.

Can you give a recent example of an induced seismicity case?

In Europe, there have been several earthquakes induced by underground industrial activities. The Castor Project in Spain, for example, was an underground gas storage facility located off the coast of the Valencia Gulf. It was considered a strategic priority for the Spanish government. Industrial operations started in September 2013 and were followed by a sudden increase in seismicity. In fact, more than 1000 events with magnitudes between 0.7 and 4.3 were recorded in the locality within about 40 days. Such seismic activity was unusual for the area and triggered significant concern among the local population. Although the operations were halted after just a few weeks, the seismic sequence continued after the shutdown of operations and culminated with a magnitude 4.3 quake the following month, the largest earthquake ever associated with gas storage operations.

What happens to companies whose activities cause the earthquakes?

In many cases, quakes big enough to be felt have resulted in public outcry causing the blamed industries to halt their operations. In the Castor case in Spain, public concerns pushed the Spanish government to definitively close the project and open an investigation. About 20 people who took part in the transaction and approval of the Castor Project have since been indicted. In other cases, such as gas extraction in Groningen, The Netherlands, industrial activities were substantially reduced following felt seismicity, amounting to billions of dollars in lost or delayed revenues for the companies and the Dutch government.

How can human-induced earthquakes be controlled or reduced?

There are no simple solutions. We still have an incomplete understanding of all the relevant processes and of the site-specific conditions that matter (i.e. location of faults and their stressing state). A comprehensive risk governance strategy must include a suitable monitoring infrastructure, sophisticated data analysis techniques for real-time seismicity characterization, quantitative risk analysis, and transparent decision protocols. All these elements combined would help to evaluate the system response before the occurrence of critical events, allowing a risk mitigation strategy to be implemented before it is too late.

What additional data, modeling or other research efforts are needed?

Induced seismicity is inherently an interdisciplinary problem which requires a combination of seismological, hydrogeological, geodetic, and industrial data plus a wide range of modeling approaches. Bringing these elements together is only in its infancy. In addition, validation of models and approaches is a necessity. It would also help if open data policies were adopted both to enhance transparency and also to enable scientific research.

—Francesco Grigoli and Stefan Wiemer, Swiss Seismological Service, ETH-Zurich; email: [email protected]