There are two types of seismicity: natural and induced
Natural seismicity in the UK is low by world standards, however induced seismicity is a common occurrence
Millions of hydraulic fracturing operations have been conducted safely and without incident worldwide
What is natural seismic activity and induced seismicity?
Naturally occurring earthquakes are caused by the release of stresses (tectonic stresses). The majority of these earthquakes occur along plate boundaries, but natural seismic activity can and regularly does occur anywhere in the world.
Induced seismicity are minor earthquakes or tremors that are caused by human activities including mining, hydraulic fracturing, geothermal operations, water reservoir dams and certain construction activities, such as foundation laying. Most induced seismicity is of a low magnitude. Both induced and natural occurrences of earthquakes and tremors are referred to as seismic events.
How are earthquakes and tremors measured?
The strength of an earthquake or tremor is measured using the Richter scale. The Richter Scale is logarithmic, meaning that when measured from the same distance, a seismic event of magnitude 5 produces vibrations with amplitudes 10 times greater than those from a magnitude 4 event and 100 times greater than those from a magnitude 3.0 event.
In terms of energy, this means an earthquake or tremor measuring 5 on the Richter scale releases 32 times more energy than one measuring 4 and about 1000 times more energy than an event measuring 3.
Earthquakes or tremors measuring below 3.0 on the Richter Scale are rarely felt on the surface and require sensitive surface or specialist "listening" equipment, buried at depth to measure.
Do earthquakes occur in the UK?
Yes. Natural seismicity in the UK is low compared to more seismically active regions of the world, such as Japan and the Western USA because the UK is not near a plate tectonic boundary, nor does it have any active volcanoes. However, between 20 to 30 earthquakes are felt by people each year, and a few hundred smaller ones are recorded by sensitive instruments. The majority of these are very small and cause no damage.
On average, the UK experiences seismic events measuring 5 (felt by everyone nearby) every 20 years and events measuring 4 (felt by many people) every three to four years.
The British Geological Survey (BGS) publishes a record of all earthquakes around the British Isles in the past 50 days. Based on a search conducted on the 27th April 2020, there were 22 recorded earthquakes in the previous 50 days, including one in Brackley, Northamptonshire at a depth of 4km which measured 2.2
According to the BGS, an earthquake is only considered to be "significant" if it measures above 4 and an earthquake measuring below 3 is generally not felt at the surface.
For example, a minor earthquake at 9.1 km in depth and measuring 4.2 on the Richter scale was detected in Sandwich, Kent on the 22th of May 2015. Neither Kent Police nor Kent Fire & Rescue Services reported any damage or injury.
Can hydraulic fracturing (fracking) cause earthquakes in the UK?
Yes, like other industrial processes, the method of hydraulic fracturing can cause tremors associated with induced seismicity.
In 2011, two earth tremors, measuring 1.5 and 2.3 on the Richter scale were registered at Preese Hall near Blackpool, Lancashire. After an extensive study into these events, a report published by the Department of Energy and Climate Change in conjunction with the BGS concluded that this seismic activity was caused by hydraulic fracturing in the area.
However, the report concluded that the sizes of the seismic events at Preese Hall were considered to be somewhat unique due to the surrounding geology of the region as hydraulic fracturing released pre-existing stress in the subsurface.
The report also concluded that further small earthquakes cannot be ruled out, however the risk from these tremors is low, and structural damage is extremely unlikely.
Further operations in Lancashire
In 2018 and 2019, a further programme of hydraulic fracturing was undertaken in Lancashire by Cuadrilla Resources this time at their Preston New Road site near Blackpool. Fracturing operations at the first well produced induced seismicity. During the 100 days that included the period of hydraulic fracturing, British Geological Survey data showed that there were 88 seismic events in the UK, 54 of which were associated by time and location to the hydraulic fracturing operations in Lancashire. None of the Lancashire events were in the top 10 by order of magnitude, and only one was in the top 25. But the unprecedented presence of large numbers of highly sensitive seismic monitoring instruments at the site meant that seismic events well below local magnitude (ML) 1 were recorded when they otherwise wouldn't be. In fact, there are many thousands of these tiny events estimated to occur naturally in Britain every year
During the fracturing operations on the second well, the first 5 of the planned 41 stages were undertaken per frac plan design. However, c. 2 hours after completion of the sixth stage a 1.5ML tremor was recorded. As a consequence, the operator pumped reduced volumes on the seventh stage. Three days after the completion of stage 7 a 2.9ML tremor was recorded and was felt at the surface by a number of people. The tremor, which lasted about 3 seconds, was associated with ground motion that peaked at 8.2 mm/second. For context, British Standard BS 5228-2 presents guidance that intermittent ground vibrations caused by construction activities should not exceed 15 mm/second in order to safeguard against any cosmetic damage (e.g. cracks to plasterwork) to residential buildings. Operations were again suspended.
The Oil and Gas Authority has recently reviewed the data from the first well and concluded in its interim report that the causes and impacts of seismicity should be considered on a local geology basis and that these cannot and should not be generalised.
How is seismicity currently regulated in the UK?
Currently, the Oil and Gas Authority regulate seismicity for oil and gas operations onshore through the use of a traffic light system (TLS)
Where hydraulic fracturing is planned, the OGA requires the operator to produce a 'Hydraulic Fracturing Plan', detailing how the risks of any potential induced seismicity will be managed. This ensures that the risk of felt seismicity is very low and within existing background levels.
The traffic light system was put in place to manage the risk of induced seismic during hydraulic fracturing and after pumping had ceased ("trailing events"). The 0.5ML 'red light' Richter Scale (Local Magnitude) level for use during operations was set by Government. If a 0.5 is detected, the operator must pause for a period dictated by the hydraulic fracture plan, which is approved and regulated by the Oil and Gas Authority (OGA), and subsequent consent must be given by the regulator to proceed. The operator must also do a number of checks - including verifying well integrity - and consider any modification in terms of subsequent frac operations before continuing.
Operationally the process involves a large number of highly sophisticated monitoring stations which are able to pick up activity well below 0.0 ML. This very low magnitude micro-seismicity is to be expected as the equipment records tiny fractures being created in the shale. The sophisticated and dense seismic array temporarily monitoring around operational sites also results in many more events being detected locally which can and do occur naturally in the UK but are missed by the typical monitoring network. It is estimated that there are over 10,000 of these events in the UK every year which are not felt at the surface, the majority of which go undetected.
The OGA (Oil and Gas Authority) www.ogauthority.co.uk regulate manmade, or induced, seismicity from oil and gas operations in the UK.
Comparisons with the rest of the world
The current rules for shale gas extraction are the strictest in the world and are much stricter than for any other industry involved in creating seismicity in the UK. In the US the 'red light' limits range from 1ML to 4ML and in Canada they are largely set around 4ML. This means that UK operations must stop at a seismic event over 3,162 times weaker than operators in North America must.
Comparison with other industries
Induced seismicity refers to typically minor earthquakes and tremors that are caused by human activity. Most induced seismicity is of a very low magnitude. There are many different ways in which human activity can cause induced seismicity including geothermal operations, reservoir impoundment (water behind dams), mining, extraction of ground water, construction, wastewater injections, and oil and gas stimulation operations such as hydraulic fracturing.
The hydraulic fracturing plan required by the OGA has two measurement levels associated with seismicity. The first is local magnitude (ML) as discussed above. The other is a measurement of ground motion, known as peak particle velocity (PPV), which is what you would typically see being used to regulate other mineral extraction such as quarrying or the construction activity.
At Preston New Road in Lancashire the maximum ground motion recorded during the 2.9ML seismic event was 8.2mm/sec and the average during the same event was 5.4 mm/sec. This compares to the following British Standards for ground motion for other industries:
Other measurements have included jumping on a wooden floor (8mm/sec) and a lorry travelling across the road at a distance of 8m (2mm/sec).
You can read our seismic protocol guidance here