22-02-2023 | Posted by Joaquín Martí
Earthquakes most commonly originate by the sliding of a crustal block with respect to another, all this usually being known as a fault. Whether or not the relative movement occurs, and the mode and speed of its occurrence, are a function of the normal and shear forces at the surface of the fault, the state of stress in its surroundings, and the mechanical properties of the ground, chiefly its equivalent friction coefficient.
Various human actions may alter the state of stress in the crust and thereby modify the natural seismic activity, a process termed induced seismicity.
The actions are usually associated with:
- Surface operations, particularly adding or removing large masses, such as filling or emptying reservoirs or tailings ponds.
- Extractions from the surface, as in mining operations, water extraction, geothermal operations, and extraction of hydrocarbons.
- Injections of fluids from the surface, for secondary oil recovery, geothermal operations, hydrocracking, and gas storage.
- Large explosions, as in nuclear and large high-explosive detonations.
There is no reliable way to predict the future seismic activity at a site and it is even more complex to discriminate between natural and induced seismicity. But at least a differentiation can be established between natural, induced, and triggered seismicity.
Induced earthquakes are those in which most of the stress relieved during the event had accumulated by human action. By contrast, in triggered earthquakes, most of the stress relieved had a tectonic origin; human action only contributed a small proportion of the stress which, when added to that naturally accumulated, triggered the earthquake. In other words, essentially the human action only affected the timing of the release of the previously accumulated stress.
One can also distinguish between two types of anthropogenic seismicity. In the first one, the induced seismicity normally occurs while the process is taking place, as in hydrocracking, and is often monitored. This simultaneity is not usually expected in the second type, as in hydrocarbon extraction.
Apart from a large number of studies of the natural seismicity all over the world, Principia has also carried out a smaller number of projects dealing with the seismicity potentially induced by human activities. Those studies have included reviews of the existing technical literature and are mainly related to the extraction of hydrocarbons.
The problems encountered by the already abandoned Castor Project, for underground storage of gas under the sea bottom, highlight the major role that induced seismicity issues may play in certain projects.