Seismicity

 

  • Natural seismicity in the UK is low by world standards

  • Risk of tremors from hydraulic fracturing is low

  • Risk of any structural damage caused by hydraulic fracturing extremely unlikely

 

What causes earthquakes?

Naturally occurring earthquakes are caused by tectonic activity. The majority of these earthquakes occur along plate boundaries but natural seismic activity can occur anywhere in the world.

 

How are earthquakes measured?

Earthquakes measuring below 3.0 on the Richter Scale are generally not felt on the surface

The size or magnitude of an earthquake is measured using the Richter scale. The way the scale works means that when measured from the same distance, an earthquake of magnitude 5 produces vibrations with amplitudes 10 times greater than those from a magnitude 4 earthquake and 100 times greater than those from a magnitude 3.0 earthquake.

Therefore, an earthquake of measuring 5 on the Richter scale releases 32 times more energy than an earthquake of measuring 4 and about 1000 times more energy than an earthquake measuring 3.

 

Are there earthquakes in the UK?

Yes. Natural seismicity in the UK is low compared to the rest of the world. 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 28th September 2015, there were 18 recorded earthquakes in the previous 50 days, including one in Oakham, Rutland which measured 2.8.

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, let alone causing any damage.

For example, a minor earthquake measuring 4.2 on the Richter scale was measured in Sandwich, Kent on the 22th of May 2015. There were no reports of any damage or injury from Kent Police nor Kent Fire & Rescue Services.

 

Can hydraulic fracturing (fracking) cause earthquakes?

Unlike natural seismicity, induced seismicity is caused by human activities such as mining, deep quarrying, hydrogeological extraction or fluid disposal and activities associated with non-conventional hydrocarbon extraction (fracking).

seismicityAccording to the Royal Society and Royal Academy of Engineering the level of seismicity brought on by hydraulic fracturing (fracking) is likely to be even smaller than mining. It is now being widely accepted that the magnitude of seismicity induced by hydraulic fracturing would be no greater than 3 (felt by few people and resulting in negligible, if any, surface impacts).

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 induced by hydraulic fracture treatments 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. The state of stress which was released by these events was pre-existing. Magnitudes of any earthquakes recorded during hydraulic fracture stimulation in reservoirs such as the Barnett Shale in the US are typically less than 1.0 and are therefore not observable by humans.

The report also concludes that further small earthquakes cannot be ruled out, however the risk from these tremors is low, and structural damage is extremely unlikely.

 

What is the industry doing to mitigate induced seismicity?

New controls are being introduced to address the possibility of any seismic risks. A Hydraulic Fracturing Programme (HFP) is prepared by the Operator and signed off by the Oil and Gas Authority (OGA) before drilling can commence.

Operators will be required to confirm that wells are not to be drilled into, or close to, existing faults which could provide the mechanism for triggering an earth tremor. Background seismicity and “real time” seismic monitoring will be used before and during activities and a “traffic-light” warning protocol will be employed. If any site records a seismic event of magnitude greater than 0.5, operations will be halted and pressures immediately reduced. This magnitude is well below the energy level that could be felt at the surface, and the protocol would enable a review of the possible causes of the event and allow further steps to be taken to prevent the occurrence of larger events.

free templates joomla

KEY FACTS

4.2
Strength on Richter scale of natural earthquake recorded in Sandwich, Kent on 22nd May 2015

 

 

2.3
Strength on Richter scale of tremor at Preese Hall, Lancashire in 2011 caused by hydraulic fracturing