20.8. Induced Seismicity Due to Human Activities

Deep Well Injection

• Disposal of industrial fluids, wastewater, or CO₂ in deep wells can alter pore pressure and stress regimes.
• Documented cases in Oklahoma, USA, where seismicity increased significantly due to fracking and injection.

Deep well injection involves the disposal of fluids into the ground, often in deep wells designed for this purpose. When industrial fluids, wastewater, or carbon dioxide are injected, they can change the pressure in the surrounding rock layers. This change can, in turn, affect the stress on nearby faults which might eventually lead to earthquakes. Notably, there have been instances, particularly in Oklahoma, where this practice has been linked to increased seismic activity, indicating a correlation between human activity and natural seismic events.

Think of the Earth's crust as a tightly sealed can of soda. When you shake the can and then pop it open, the pressure inside causes the soda to spray out. Similarly, injecting fluids into the ground can shake up the pressure in underground rocks, causing them to slip and create earthquakes.

Hydraulic Fracturing (Fracking)

• High-pressure injection of fluid to fracture shale rocks.
• While microseismicity is expected, improper control can trigger larger seismic events.

Hydraulic fracturing, commonly known as fracking, is a technique used to extract natural gas or oil from underground rock formations. In this process, high-pressure fluid is injected into shale rocks to create fractures, allowing oil or gas to flow more freely. While small earthquakes or microseismicity can occur as a normal part of this process, if the operation is not carefully controlled, it can lead to larger earthquakes. This highlights the need for stringent monitoring and regulation in fracking operations.

Imagine trying to poke holes in a sponge with a straw. If you poke gently, small holes form where the liquid flows easily. However, if you push too hard, you could create a big tear, causing a larger mess. In fracking, if the injection pressure is too high without proper management, it can lead to significant seismic events like that larger tear in the sponge.

Mining and Quarrying

• Blasting operations, excavation, and mine collapse can lead to small earthquakes.
• Long-term mining alters the stress field in the Earth's crust.

Mining and quarrying involve removing materials from the ground through processes that often include blasting and excavation. These activities can disturb the Earth's crust, sometimes leading to small earthquakes, especially if the stress that builds up in the rocks exceeds their strength. Over time, as mining progresses, the stress distribution in the surrounding area can change, potentially triggering seismic events due to the altered stress conditions.

Consider how digging a hole in the sand can cause the sand nearby to shift and collapse. If you dig too much in one area, the surrounding sand may no longer be stable and can give way, similar to how mining can disturb the ground and lead to small quakes.

Geothermal and Oil Extraction

• Withdrawal of fluids can create voids or reduce subsurface pressure.
• Ground subsidence and fault reactivation may follow.

When fluids such as water or oil are extracted from the ground through geothermal or oil extraction processes, it can reduce the pressure in the subsurface. Over time, this can create voids or spaces underground where the fluid used to be. The resulting changes in pressure can lead to ground subsidence, where the ground sinks, and also increase the likelihood of existing faults becoming active again, potentially resulting in earthquakes.

Imagine a balloon filled with water. If you start to let the water out, the balloon’s shape changes and can even collapse in on itself. Similarly, when fluid is removed from underground, it can cause the land above to 'sink' and create stress on faults, increasing the risk of earthquakes.

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