Can human activity cause earthquakes?
Many people are confident that human activity cannot induce earthquakes – but what does science say about this question?
But before going down this road, let’s look at what an earthquake is and what does cause them.
An earthquake is caused by a sudden slip on a fault. The tectonic plates are always moving slowly, but sometimes they get “stuck” at their edges due to friction. When the stress on the edge overcomes this friction, there is a sudden release of energy in the form of waves that travel through the earth’s crust. The shaking induced by these waves we feel as an earthquake. Sometimes the fault adapts to movement by constant “creep”, resulting in many tiny shocks. But when creep is not constant, strain can build up over hundreds of years, producing large earthquakes when they do release.
This all leads to the question: Can we cause earthquakes? And the related question: Can we prevent them?
We must accept that the answer to the first of these questions is – yes. Earthquakes induced by human activity have been documented at many locations around the world – including Australia. It seems that earthquakes can be induced by a wide range of activities including impoundment of reservoirs, surface and underground mining, withdrawal of fluids and gas from the subsurface, and injection of fluids into underground formations. These activities may cause what we term induced earthquakes. While it is true that most induced earthquakes are small and present little hazard, larger and potentially damaging manmade earthquakes have occurred.
A good example of a larger induced earthquake close to home is the 1989 quake in Newcastle which had a magnitude of 5.6 on the Richter scale. This event killed 13 people, put 160 in hospital and caused billions of dollars of damage. This quake was attributed to deep coal mining. Furthermore, Newcastle has a very long history of coal mining and, probably as a consequence, has experienced earthquakes in 1841, 1868, 1925, and 1994 in addition to the 1989 event. What is now called the “Newcastle Triangle Seismic Hazard Zone” experiences earthquakes of magnitude greater than 5 on the Richter scale, on average, every 40 years. Other areas in NSW experience such events, on average, every 170 years.
The next obvious question is: Could it be a coincidence?
Research data, analysed over decades, suggest that mining has caused this increased activity. This has been supported by data from many overseas locations. We have to accept that earthquakes induced by human activities are a real possibility.
The more recent earthquake (August 2024) in Muswellbrook (magnitude 4.8) was also likely caused by coal mining – this time at the Mount Arthur mine (the quake sequence was directly under the operating mine). There were also many aftershocks with magnitudes of up to 4.6. The mining company seems to have acknowledged that the mining activity was responsible for the quake.
But care is needed before assigning blame – after all, correlation does not imply causation. For example, recent studies indicate that even large open pit mines do not appreciably change the stress along faults in the near-surface. Calculations show faults would need to be already tectonically stressed almost to the point of failure for mining to affect the timing of an earthquake. It has to be stated that currently available evidence for these Muswellbrook quakes does not allow us to say unequivocally whether they are related to mining.
Our current level of expertise suggests that the following are examples of induced Australian earthquakes:
- Jindabyne 1959, magnitude 5.0 – induced by the filling of Eucumbene reservoir.
- Warragamba 1973, magnitude 5.5 – induced by Warragamba Reservoir filling.
- Thomson Dam 1996, magnitude 5.0 – induced by Thomson Reservoir filling.
Of the seven deepest Australian water reservoirs, five have induced earthquakes – Warragamba, Thomson, Talbingo, Gordon, and Eucumbene. Significantly, the deepest, Dartmouth, did not induce a quake. The three largest reservoirs all triggered earthquakes (Gordon, Argyle, and Eucumbebe).
One of the most reliable ways of triggering small earthquakes is to inject fluids or gas into highly stressed rock. This sort of thing is done primarily to increase hydrocarbon extraction, develop porosity through hydraulic fracturing, and for long-term storage of waste fluids and gas.
However, it is now recognized that a site susceptible to induced earthquakes generally has a pre-existing susceptibility to natural earthquakes.
So now back to the second question we posed at the beginning: Can we prevent earthquakes?
We cannot prevent natural earthquakes, but we can significantly mitigate their effects by building safer structures and identifying hazards.
The hazards posed by manmade earthquakes can be mitigated by minimizing or in some cases stopping the activity that is causing them. For example, earthquakes linked to wastewater disposal in deep wells in Colorado, Ohio, and Arkansas stopped occurring after injection was halted.
Induced seismicity under large reservoirs can be delayed for up to several years after reservoir filling due to the slow increase in pore pressure as groundwater slowly permeates to greater depth. The activity can then continue for many years, typically 10-30 years before a new equilibrium is established and seismicity returns to normal levels.
On the other hand, seismicity resulting from fluid injection occurs soon after injection starts, and ends within days or weeks of the end of injection.
Conclusions?
Induced seismicity associated with fluid injection, infiltration and extraction is a commonly observed phenomenon. While these events are generally small, they can exceed magnitude 5 on the Richter scale.
There is much research yet to be done to enable a better control of induced earthquakes.
