OR/18/020 Seismicity

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R S Ward1, G Allen2, B J Baptie1, L Bateson1, R A Bell1, A S Butcher1, Z Daraktchieva3, R Dunmore4, R E Fisher5, A Horleston6, C H Howarth3, D G Jones1, C J Jordan1, M Kendall6, A Lewis4, D Lowry5, C A Miller3, C J Milne1, A Novellino1, J Pitt2, R M Purvis4, P L Smedley1 and J M Wasikiewicz3. 2018. Preliminary assessment of the environmental baseline in the Fylde, Lancashire. British Geological Survey Internal Report, OR/18/020.

Background

The primary aim of the seismicity work package is to deploy a network of seismic sensors to monitor background seismic activity in the vicinity of proposed shale-gas exploration and production in the Fylde, Lancashire. The data collected will allow reliable characterisation of baseline levels of natural seismic activity in the region. This will facilitate discrimination between any natural seismicity and induced seismicity related to future shale-gas exploration and production. A further aim is to make recommendations for a suitable traffic-light system to mitigate earthquake risk. The initial design requirement for the seismic monitoring network was reliable detection and location of earthquakes with magnitudes of 0.5 and above in the area of two potential sites for shale gas exploration at Preston New Road and Roseacre Wood.

Network performance

The seismic monitoring network consists of six near-surface sensors (red squares in Figure 51). We also receive real-time data from 4 stations installed and operated by Liverpool University. The latter were installed independent of this project and data from these is not guaranteed.

Continuous data are transmitted in near real-time to the BGS office in Edinburgh, where the data are processed and archived. The completeness of these data can be checked easily to gain an accurate picture of network performance. The completeness levels are shown in Figure 52.

All BGS stations show high levels of data completeness for the time period 1/4/2017 to 30/6/2017, with over 96% available from all stations except AQ10, installed in May 2017.

Figure 51    Ordnance Survey map of the Fylde peninsula overlain by superficial geology. Red squares show UK array sensors and the orange squares show the locations of Liverpool University sensors. The green star shows the location of the site of possible hydraulic fracturing at Preston New Road. © Crown Copyright and/or database right 2018. Licence number 100021290 EUL.
Figure 52    Data completeness for the stations on the Fylde Peninsula 1/4/2017 to 30/6/2017; AQ10 was installed on 4/5/2017.

The Liverpool stations show significantly lower levels of completeness.

The level of data completeness is an improvement on the values between 1/4/2016 and 31/3/2017. A value of over 95% is extremely good for data transmitted in near real-time using mobile phone networks and is better than many of the BGS permanent monitoring stations that use similar technology. Data losses result from failure of outstation hardware, communications problems, or failure of central data processing. The data acquisition is able to recover from short breaks in communications links to outstations by re-requesting missing packets of data from local data buffers, but failure of outstation hardware requires intervention by local operators or maintenance visits. No maintenance trips were required in the period 1/4/2017 to 30/6/2017.

Station noise and performance

We use power spectral density (PSD), calculated from one-hour segments of continuous data, to characterize noise levels at a range of frequencies or periods for each of the stations. A statistical analysis of the PSDs yields probability density functions (PDFs) of the noise power for each of the frequency bands at each station. Figure 53 shows the median noise levels calculated at each BGS station. These stations are noisier than most other stations in the BGS network and noisier than those in the Vale of Pickering network. This is because the Fylde Peninsula is densely populated, with many sources of cultural noise, while also having no bedrock near the surface. Rather, the near-surface geology comprises clays that are extremely poor transmitters of cultural noise. The permanent station ESK is given for comparison: this is one of the ‘quieter’ stations in the UK network. The only way to improve the signal-to-noise properties of this network would be to re-site some of the stations within boreholes.

Figure 53    Median noise levels at all BGS stations on the Fylde Peninsula. ESK is a quiet national network station included for comparison.

Noise analysis has shown that:

  • In general, all of the stations in Lancashire are noisier than those established in the Vale of Pickering, Yorkshire due to increased industrial noise and population density.
  • There is a noticeable seasonal variation at all sites at low frequencies due to storm noise in the winter.
  • The most prominent and pertinent feature for this study is the diurnal difference which is observed in both summer and winter. The day-time anthropogenic noise in the high-frequency range (5–45 Hz) can cause an increase in the mode of the station noise of over 20 dB at some sites, although it is more generally between 8 and 15 dB variation. This increase in the noise level could cause problems for the detection of small events since they will fall within this frequency range.

Data processing and analysis

Continuous data from all stations are transmitted in real-time to the BGS offices in Edinburgh and have been incorporated in the data acquisition and processing workflows used for the permanent UK network of real-time seismic stations. A simple detection algorithm is applied to the data from the Fylde Peninsula stations to detect possible events. All detections are then reviewed by an experienced analyst. Apart from teleseisms, only one event was detected in the period from 1/4/2017 to 30/6/2017. This was a 1.7 ML earthquake at Kents Bank in Cumbria, approximately 41 km north of the Preston New Road site.

Data availability

Helicorder plots showing 24 hours of data from each station are available online and can be found on the BGS Earthquake Seismology Team web site at https://www.earthquakes.bgs.ac.uk/helicorder/heli.html.

Regional seismicity

Figure 54 shows recorded seismicity within a 100 km square centred on the Preston New Road site. The Fylde Peninsula is an area of low seismicity even for the UK. Apart from the seismicity related to hydraulic fracturing operations at Preese Hall in 2011, most of the nearby seismicity is offshore in the Irish Sea. For example, a magnitude 2.5 ML earthquake occurred 5 km south- west of Blackpool in 1970.

Figure 54    Recorded seismicity within a 100 km square centred on the Preston New Road site (yellow star). Grey circles show earthquakes prior to 1970. Red circles show earthquakes recorded between 1970 and 31/03/2017. Yellow circles show earthquakes from 1/4/2017.

A magnitude 3.3 ML earthquake was recorded in the Irish Sea on 25/08/2013 at 09:58, with an epicentre approximately 25 km west of Fleetwood, Lancashire. This event was preceded by a magnitude 2.5 ML foreshock in the same location at 05:37 and followed by a magnitude 2.8 ML aftershock on 31/08/2013. These were the largest earthquakes to have occurred in the Irish Sea since a series of three earthquakes, with magnitudes ranging from 3.8 to 5.0, on 16 and 17 March 1843.

BGS received over 60 reports from members of the public who felt the earthquake. Almost all of these came from inhabitants of the Lancashire coast at distances of up to 40 km from the epicentre.

The epicentres were immediately east of the Bains gas field, leading to speculation that these earthquakes could have been related to hydrocarbon production. However, the Bains field ceased production in 2009, and although there is a long history of induced earthquakes related to gas extraction in places such as Groningen, The Netherlands, the historic earthquakes in the area show that natural seismicity predates any production.

A magnitude 0.9 ML earthquake in the Irish Sea was recorded on 5/4/2017. This was approximately 45 km west of the Preston New Road well site. It was not detected by the stations on the Fylde Peninsula.

The magnitude 3.7 Ulverston earthquake on 28 April 2009 was also felt in Lancashire. Historically, the largest earthquake in the region was a magnitude 4.4 earthquake near Lancaster in 1835 with a maximum intensity of 6 EMS.