There have been more and more earthquakes around the world in recent years, raising questions about whether they are natural or human-induced.

With the increase in the frequency of drilling and hydraulic fracking around the world, concerns about the spread of earthquakes caused by human activities have increased. The number of oil wells being drilled annually has reached 100,000, and the International Energy Agency (IEA) has stated that geothermal energy exploitation projects may increase by six times by 2050.

The extraction of large amounts of fossil fuels or the flooding of fractured rocks with liquids in hydraulic fracking may upset the balance of forces pressing on the Earth’s crust and thus increase the frequency of earthquakes.

Scientists believe that the ground under our feet can be unstable, as it consists of layers of varying thickness moving in different directions and is full of cracks and fractures permeated by narrow streams of fluids. There are sediments, mud and bedrock, not to mention the giant tectonic plates rubbing against each other or pulling apart.

Taking preventive measures against earthquakes provoked by human activities using expert knowledge could minimise or avoid much of the damage. Earthquakes may occur for natural or human-induced reasons. Depending on their degree of magnitude, the damage can be very different. Examples of human causes for earthquakes include fluids being either injected into or extracted from deep earth layers (such as groundwater extraction), different mining activities, and the construction of dams and tunnels.

Researchers have found that the effects of human-caused earthquakes may be similar to those caused by nature, but are often seen in areas with little or no previous seismic activity. While most natural earthquakes occur along fault lines, often found where tectonic plates meet, human-induced earthquakes can occur far from tectonic plate boundaries.

A 2017 study published in the US journal Seismological Research Letters identified 730 sites where human activity has caused earthquakes over the past 150 years. The discovery that surprised the researchers the most was that human activity had caused earthquakes of up to 7.9 in magnitude, and that the number of earthquakes was clearly on the rise in some regions of the world.

Just like earthquakes caused by nature, earthquakes caused by humans can be deadly, and geoscientists are beginning to understand their repercussions on people and the environment.

Ahmed Khaled, an assistant professor of environmental sustainability at the Faculty of Engineering at Ain Shams University in Cairo, said that an earthquake can be defined as any sudden shaking of the ground caused by the passage of seismic waves through the Earth’s crust. Seismic waves are produced when some form of energy stored in the Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and “slip”.

Earthquakes occur most often along geological faults, narrow zones where rock masses move in relation to one another. The major fault lines of the world are located at the fringes of the huge tectonic plates that make up the Earth’s crust. While little was understood about earthquakes until the emergence of seismology at the beginning of the 20th century, this has now yielded answers to such long-standing questions as why and how earthquakes occur.

Earthquakes have varied effects, including changes in geological features, damage to man-made structures, and impacts on human and animal life. Most of these effects occur on solid ground, but, since most earthquake foci are actually located under the ocean bottom, severe effects are often observed along the margins of oceans.

One of the most evident human constructions that can induce earthquakes and landslides is the Three Gorges Dam in China, Khaled said. This sits on two major fault lines, and geologists fear that rapid changes in water pressure when the dam’s reservoir levels are changed during flood season could activate already shaky ground and trigger an earthquake, a phenomenon known as reservoir-induced seismicity.

In the seven months following the 2006 increase in the water level, geologists recorded 822 tremors around the reservoir. The raising and lowering of the water level in the reservoir also destabilises the land around it. Water seeps into the soil in the cliffs surrounding the reservoir, causing enough erosion to make the ground slip.

According to one study, the shore of the Three Gorges reservoir has already collapsed in more than 90 places. Villagers in the area have reported mudslides and cracks appearing in their backyards. In 2003, 700 million cubic feet (20 million cubic metres) of rock slid into the Qinggan River just a couple of miles from where it flows into the Yangtze. According to Khaled, the rockslide spawned 65-foot (20-metre) waves that killed 14 people.

Reservoir-induced earthquake seismicity (RIS) refers to the seismicity in or near the reservoir area induced by water storage and release activities. Multiple earthquakes at 167 sites were triggered by what one study calls water reservoir impoundment, or dam building. In 2008, an estimated 80,000 people died or went missing following a 7.9 earthquake in China’s Sichuan province. Scientists believe it was triggered by the weight of 320 million tons of water that had been collected in the Zipingpu Reservoir over a well-known fault line.

OTHER CAUSES

Khaled said that mining accounts for the highest number of human-induced earthquakes worldwide (many earthquakes clustered around 271 sites).

The removal of material from the Earth can cause instability, leading to sudden collapses that trigger earthquakes. It can be argued that mining played a large part in the destruction wrought in 1989 upon Newcastle in the UK, owing to its proximity to underground black-coal mining.

Newcastle’s first coalfield opened in 1801. While the region has prospered economically since the 1950s as a result of exporting black coal to China and Japan, it has also experienced an undesirable consequence of booming mining operations in the form of frequent damaging seismic activity that has included earthquakes in 1841, 1868, 1925, 1989, and 1994.

Due to its unusually high seismic activity, this mining region is referred to as the “Newcastle Triangle Seismic Hazard Zone”. Earthquakes with a magnitude of greater than five on the Richter Scale occur, on average, every 40 years in this locale. Mining is expected to increase in scale, however, and today’s mines are bigger than ever and reach miles underground. All this activity could lead to more instability and more or larger earthquakes.

In the US, the debate around human-induced earthquakes has largely centred around fracking for oil and natural gas, given the rapid spread of the technology in many states.

According to the US Geological Survey, fracking can induce seismic activity, both directly and from disposing of the wastewater used in the process as the byproduct of the water, sand, and chemicals used to hydraulically fracture hydrocarbons from the rock. This high-pressure wastewater can crack rocks and lubricate faults.

In one study, the authors found 29 project sites where earthquakes have been induced by fracking itself, 36 sites where quakes were induced by post-fracking wastewater disposal, and 12 sites with tremors induced by unspecific oil and gas wastewater disposal.

In Canada, most of Alberta’s large earthquakes have been linked to fracking, and in the US many people who live near fracking sites have filed lawsuits for injuries sustained in earthquakes and for damage to their homes.

A report published in March 2018 by a team of experts investigating the causes of the Pohang earthquake in South Korea in November 2017 concluded that the earthquake was caused by human activities.

As a result of the earthquake, 135 people were injured, 1,700 people were evacuated from their homes, and thousands of buildings were damaged, with losses amounting to $75 million. Some pointed the finger at drilling operations for a project to generate electricity from geothermal energy, prompting the South Korean government to assign a team of seismologists to research the extent to which industrial activity was linked to the earthquake.

The day after the quake, the company Next Geo, which was operating an experimental geothermal power plant in the area, denied responsibility for the tremors. But the evidence found by the international investigation team indicated otherwise. The team studied data on earthquakes in the region as well as information on drilling activities undertaken by Next Geo.

Geothermal power plants generate electricity using heat in the ground in many ways, including by the use of steam emitted from the heat stored in the rocks in the ground. But in some cases, the liquid in the Earth’s interior may not be enough to conduct heat to the surface in the form of vapour. That is why Next Geo planned to pump liquid into a drilled hole to break up the rock and extract heat from far below the surface.

But when the team began drilling, the drill caused the rock to break up in the ground, and the team resorted to pumping a relatively dense liquid called “drilling mud” into the centre of the hole to remove it so that the liquid could reach the bottom and push the rock up towards the surface.

However, the drilling then collided with an area full of cracks and fissures at a depth of 3.8 km, and instead of the mud going up, it penetrated into the cracks, and then increased the pressure on the rocks.

Scientist Bill Ellsworth of the Center for the Study of Earthquakes at Stanford University in the US who participated in the international team of investigators to uncover the causes of the Pohang earthquake said that the liquid leaked for some reason out of the hole, and when the drilling workers pumped more liquid into it, the intense pressure on the rocks caused seismic waves.

The seismic waves were so slight at first that no one noticed them. But by analysing the light seismic waves, the investigators later found that the drilling workers had penetrated the fault line, or the boundary between two tectonic plates. Workers usually check the area that will be subject to drilling or pumping liquid, to avoid approaching the faults, but in this case no signs of a fault appeared at the surface, and it did not occur to the team that the drilling machines had crossed the fault.

Ellsworth said that any change in the rocks surrounding faults may lead to their movement, and that this is what happened in this case.

The slight seismic waves indicated that the drilling had caused a problem in the ground, and after a few weeks the city was hit by a 5.5-magnitude earthquake. The South Korean government has accepted that the earthquake was caused by human activity and has said that it will dismantle the geothermal power plant.

* A version of this article appears in print in the 25 May, 2023 edition of Al-Ahram Weekly