Induced Seismicity Explained
“It’s not the unknown we fear, but what we think we know about the unknown.”
What is Seismicity?
Seismicity, tremors, earthquakes, seismic events. All of these phrases can be used to describe ground and sub surface movement. For millions of years our planet’s crustal plates have been moving around; changing the shape of the continents and creating mountains and volcanoes. The most active regions are at the boundaries of these tectonic plates, where they are moving apart, sliding past each other, or colliding. This is also where some of the world’s largest geological faults are found, like the San Andreas Fault in California. However, faults come in all shapes and sizes and are found all over the world, even in Cornwall! As the Earth’s tectonic plates move, pressure builds up along every size of fault until the rock can’t withstand it anymore and then they slip, releasing energy as a seismic event.
Natural earthquakes happen in Cornwall too!
Seismicity can only be felt by people if the shock waves are strong enough to reach the surface and can only cause damage if they are strong enough to shake the ground forcibly. If an earthquake is small or very deep, the energy is absorbed by the surrounding rocks before it reaches the surface and may not be felt at all.
What is Induced Seismicity?
The testing and development of geothermal reservoirs in Cornwall is likely to cause some minor seismicity. Most of these induced events are so small that they’re not felt at the surface, but sometimes a slight rumbling sound or momentary ‘shiver’ can be detected in the local area. This is not dangerous, and no damage can be caused to local infrastructure at these low levels. Once the power plant is online and the geothermal system is in operation, it is extremely unlikely that any seismicity will be felt.
Below is a GEL bespoke animation about natural and induced seismicity.
Managing Induced Seismicity
Seismic Monitoring
GEL has installed a network of seismometers and ground motion detectors around Cornwall that can detect earthquakes at extremely low levels. The data will be monitored continuously throughout development to alert GEL of any increase in seismic activity and to allow mitigating action to be taken, if necessary. The British Geological Survey also reports activity on their website and if any activity occurs in response to geothermal activity, GEL will post on social media to ensure complete transparency.
As well as providing environmental monitoring, the monitoring system is a valuable tool to help geologists and engineers understand the geothermal reservoir because it can locate any seismic events caused by water circulating through the granite.
Mitigation
To manage seismicity, GEL uses existing British Standards (BS 6472-2:2008) and Cornwall Council’s planning guidelines for blasting, quarrying and mining activity. Acceptable levels are based on how much ground vibration is measured at the surface, referred to as Peak Ground Velocity, or PGV. This is believed to be a more accurate depiction of what will be felt by people in the local area compared to the magnitude (ML) of an event.
GEL operates at very conservative vibration limits. A maximum PGV of 8.5mm/s for a single event is set as a limit by Cornwall Council’s daytime blasting regulations. Previous research suggests that humans cannot generally feel movements with a PGV of less than 2mm/s. However, at United Downs a maximum PGV of 0.8 mm/s was felt. Therefore, operations at all GEL sites will enter a ‘caution’ state if events with a PGV greater than 0.5mm/s are detected during the day. Above this, operations will be closely monitored and if further significant events are detected, operations will cease until the reservoir stabilises.
