OR/17/049 Seismicity
| Ward, R S1, Smedley, P L1, Allen, G2, Baptie, B J1, Daraktchieva, Z3, Horleston, A7, Jones, D G1, Jordan, C J1, Lewis, A4, Lowry, D5, Purvis, R M4, and Rivett, M O6. 2014. Environmental baseline monitoring project: phase II - final report. British Geological Survey Internal Report, OR/17/049.
(1) British Geological Survey, (2) University of Manchester, (3) Public Health England, (4) University of York (National Centres for Atmospheric Science), (5) Royal Holloway University of London, (6) University of Birmingham, (7) University of Bristol |
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 near Kirby Misperton. 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 within a 20 km by 20 km area around the Kirby Misperton site.
Design and deployment of the monitoring network
The seismic monitoring network consists of seven near-surface sensors (red squares in Figure 88) and four sensors installed in boreholes (orange squares in Figure 88). The latter comprised of three downhole geophones and a downhole broadband seismometer. The sensors are situated at a depth of approximately 30 m below the surface and are all close to the Kirby Misperton drill site. Installing instruments in boreholes is intended to improve the signal-to-noise ratio of the recorded data and allow smaller events to be detected and located. This will be particularly important for reliable detection and location of any small earthquakes that may be induced by hydraulic fracturing, as well as for the baseline monitoring. Surface sensor have also been installed at the site of the downhole broadband seismometer and at one of the downhole geophones in order to assess the improvement in signal-to-noise ratio and characterise the effect of the near-surface geology on the propagation of seismic waves and to help predict possible ground motions from induced earthquakes in the region.
Network detection capability
We used theoretical models of the expected amplitude of seismic waves as a function of magnitude and distance, together with estimates of seismic noise at each monitoring station to model the current detection capability of the network for earthquakes of different magnitudes. The results are shown in Figure 89 and suggest that the existing stations should allow any earthquakes with a magnitude of 0.5 or above across most of the study area to be detected. In the centre of the network, earthquakes magnitudes of 0.0 or greater are likely to be detected, and earthquakes with negative magnitudes should be detected around the drill site.
Station performance
Continuous data from all stations 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 easily checked to gain an accurate picture of network performance. All stations show high levels of data completeness, with over 95% available from all stations except AU08, which is 90% complete. The completeness levels are shown in Figure 90. 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.
Seismograms always contain noise from ambient Earth vibrations as well as transient recordings from earthquakes. Seismic noise from human activity is often referred to as ‘cultural noise’ and originates primarily from the coupling of traffic and machinery energy into the Earth. This cultural noise propagates mainly as high-frequency surface waves (>1–10 Hz, 1–0.1 sec) that attenuate within a few kilometres of the noise source and often shows very strong diurnal variations. High noise levels can limit the ability to detect and reliably locate small transient signals from earthquakes or other disturbances.
We use power spectral density (PSD), calculated from one hour segments of continuous data, to characterize noise levels in a range of frequencies or periods at each of the installed 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 and component. PDFs calculated from PSDs of the vertical component of ground motion for six of the sensors are shown in Figure 91. A colour scale gives the probability of a given noise power (magneta to red). The solid grey lines show the low and high noise models for seismic stations obtained by Peterson (1993)[1]. The mode of the PDF and the tenth and ninetieth percentiles are shown by the black and white dashed lines respectively.
The noise power probability for each station varies smoothly as a function of period (1/frequency), with a central peak between 1–10 s, which generally falls to lower powers at longer and shorter periods. This mirrors the shape of the high and low noise models. In the cultural noise band (periods less than 1 s), there is evidence of diurnal variations in noise at all six stations. In addition, the noise power varies from station to station, with AU08 generally showing significantly lower levels of noise than the other stations. Noise levels at the highest frequencies (shortest periods) at station AU20 are significantly higher than the other five stations shown.
PDFs calculated from PSDs calculated for borehole and surface sensors at stations AU13 and AU14 are shown in Figure 92. These show that the noise levels on the surface sensor is at least 20 dB higher than the borehole sensor in the frequency band 2–50 Hz. This emphasise the advantage of deploying sensors in even shallow boreholes over surface sensors.
Figure 93 shows the median noise levels calculated for all stations in the network along with the low and high noise models and allows a detailed comparison of the noise levels at all stations. Median noise levels at frequencies between 2 and 50 Hz vary from -140 to -110 dB, which corresponds to an increase in noise power by a factor of 1000. The surface sensor at station AU13 close to Kirby Misperton 8 show the highest noise levels, but the borehole sensor at this site shows much lower levels of noise. The surface sensors at AU09 and AU20 are also noisy a higher frequencies. The borehole sensors at AU10 and AU16 show higher levels of noise than a number of the surface sensors in quieter locations. The borehole geophones at AU10, AU14 and AU16 are not designed to record signals at very low frequencies. As a result these show high noise levels below frequencies of 1 Hz.
Data processing and analysis
Continuous data from all installed stations are transmitted in real-time to the BGS offices in Edinburgh and have been incorporated in the data acquisition and processing work flows used for the permanent UK network of real-time seismic stations operated by BGS. A simple detection algorithm is applied to the data from the Vale of Pickering stations as well as data from permanent BGS monitoring stations in the region to detect possible events. All detections have been reviewed by an experienced analyst. Detected events in the Vale of Pickering region in the time period from 1/4/2016 to 31/3/2017 are shown in Figure 94. The proximity of the calculated locations to quarries where blasting is known to take place, along with the recorded waveforms that are characteristic of a shallow source, suggests that all these events are of an explosive origin, i.e. quarry blasts. No earthquakes have been detected in the immediate locality of the Vale of Pickering, however, a number of other earthquakes from elsewhere, along with and quarry blasts have been detected. The largest earthquake was a magnitude 1.9 ML earthquake near Hexham, Northumberland on 15/10/2016. Magnitude 1.5 and 1.4 ML earthquakes were detected near Caistor in Lincolnshire on 17/2/2017.
Twenty-one events located just north of Pickering are in close proximity to the Newbridge quarry. These are shown in Figure 95. The magnitudes of these events range from 0.5 ML to 1.2 ML and the recorded ground velocities at the nearest stations (AU20) are less than 1 mm/s, significantly less than the daytime limit for blasting (10 mm/s) set out in BS 6472-2. Figure 97 shows the recorded ground motions from a Newbridge quarry blast at 10:59 on 26/7/2016. This had a local magnitude of 1.0 ML. The event is well recorded with impulsive P-wave arrivals on the closest stations, but signal to noise ratios decrease with distance from the epicentre as the amplitude of the signals decays with distance.
Figure 98 shows a histogram of the number of events at different times of the day for detected events. There is a strong peak in the number of events during the middle of the day, which suggests that the events are more likely to be anthropogenic rather than natural (natural earthquakes are usually randomly distributed throughout the day). All of the 21 detected events from Newbridge quarry occur between 10:00 and 16:00 UTC, with nearly all events between 11:00 and 14:00.
Regional seismicity
Figure 99 shows both historical and instrumentally recorded earthquake activity within a 100 km by 100 km square centred on the Kirby Misperton 8 well from the BGS earthquake catalogue. This catalogue is a combination of the historical catalogue of Musson (1994)[2] and later revisions for the period up to 1969, and earthquake parameters determined from instrumental data recorded by the UK National Seismic Monitoring Network thereafter (Musson, 1996[3]; Baptie, 2012[4]). It contains almost 10 000 instrumentally recorded local earthquakes from 1970 to present.
The Vale of Pickering region appears to be an area of low seismicity even for the UK with little significant recorded earthquake activity. Historically, the largest earthquake in the region was a magnitude 3.7 earthquake near Market Weighton in 1885. This had a maximum intensity of 5 EMS in the epicentral area, equivalent to shaking strong enough to cause buildings to tremble and top-heavy objects to topple. There have been a number of instrumentally recorded earthquakes in the region in the last 40 years with magnitudes in the range of 2–3 ML. These include: magnitude 2.9 and 3.0 ML earthquakes near Selby, North Yorkshire in 1978 and 1984 respectively; a magnitude 2.4 ML earthquake near Westerdale North Yorkshire in 1984; a magnitude 2.1 ML earthquake near Sledmere, Humberside in 1992; two earthquakes near York in 2003 and 2005 with magnitudes of 2.3 and 2.5 and, more recently, a magnitude 2.9 ML earthquake near Loftus, Cleveland in 2012. None of these earthquakes was within 20 km of Kirby Misperton.
Earthquake activity rates
The relationship between the magnitude and number of earthquakes in a given region and time period generally takes an exponential form that is referred to as the Gutenberg-Richter law (Gutenberg and Richter, 1954[5]), and is commonly expressed as:
log10 𝑁 = 𝑎 – 𝑏𝑀 (1)
Where, N is the number of earthquakes above a given magnitude M. The constant a, is a function of the total number of earthquakes in the sample and is known as the earthquake rate. This is commonly normalised over period of time, such as a year. The constant b gives the proportion of large events to small ones, and is commonly referred to as the b-value. In general, b-values are close to unity. This means that for each unit increase in magnitude, the number of earthquakes reduces tenfold. Plotting earthquake magnitudes against the logarithm of frequency gives a straight line
The rate parameter typically varies from place to place, such that an active region will have a higher value of than a less active region. Also, in a region of homogeneous seismicity, the value of the rate parameter in any sub-region scales with relative size of the two regions. For example, a region where seismicity is homogeneous and a = 3, will have 1000 earthquakes above a magnitude of zero each year. A sub-region, whose area is ten times smaller will have a = 2, i.e. 100 earthquakes above a magnitude of zero each year. This has important implications for baseline monitoring in small regions, particularly where activity rates are low, since the number of earthquakes in a given period of time may be very low, so longer durations of baseline monitoring are required to reliably determine seismicity rates.
The UK average values for a and b are 3.23 and ~1 respectively. This means that within a polygon enclosing mainland Britain we might expect around 17 earthquakes with a magnitude of 2.0 MW or above each year. Assuming that seismicity is homogeneous and scaling this number to a smaller area of 400 km2 the size of the Vale of Pickering study region, suggests that there will be an earthquake with a magnitude of 2.0 MW or above only every 65 years, and three earthquakes with a magnitude of 0.0 MW or above every two years. This highlights the challenge of reliable estimation of background activity rates in low seismicity regions, since it may require many decades of baseline monitoring to reliably determine rates in small areas if the levels of natural seismicity are low.
Applying the UK average seismicity rate parameter to the 100 km2 square centred on Kirby Misperton suggests we might expect approximately two earthquakes with a magnitude of 2.0 MW or above every five years. This is largely consistent with the observed number of earthquakes, as 17 events with this magnitude or greater have been recorded in the last 40 years.
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 www.earthquakes.bgs.ac.uk/research/BaselineMonitoring.html and at www.earthquakes.bgs.ac.uk/helicorder/heli.html. The web pages also contain background information on the baseline monitoring project as well as educational material to explain the scientific context. Recordings of ground motions from all stations are stored in a publicly open-data archive and can also be downloaded from the web-site. These data are available in the standard data formats developed in the international seismological community for data exchange. In the future, processed event data (automatically determined and manually revised event parameters) will also be made available through this website.
Conclusions
A network of 11 seismometers installed to monitor background seismicity in the Vale of Pickering area has been successfully operated throughout the reporting period. Models suggest that this network is capable of detecting events with magnitudes of 0.5 ML and above across the study region. All but one stations show levels of data completeness that are over 95%.
There are significant variations in noise levels across the network which effects detection capability. A number of surface stations show noise levels that are more than two orders of magnitude higher than the quietest stations, in terms of signal power. Borehole sensors generally show noise levels that are around 100 times lower than surface sensors at the same sites. However, the noise levels at these sites are still higher than surface sensors at the quietest stations.
The network has successfully detected a number of events in the area of interest throughout the reporting period. The proximity of the calculated locations to quarries where blasting is known to take place, along with the recorded waveforms that are characteristic of a shallow source, suggests that all these events are quarry blasts. All the suspect blasts occurred during the daytime, which adds further evidence to an anthropogenic origin. The magnitudes of these events range from 0.5 ML to 1.2 ML and the recorded ground velocities at the nearest stations (AU20) are less than 1 mm/s, significantly less than the daytime limit for blasting (10 mm/s) set out in BS 6472-2.
References
- ↑ Peterson, J. Observation and modelling of seismic background noise, U.S. Geol. Surv. Tech. Rept., 93–322, 1–95, 1993.
- ↑ Musson, R M W. 1994. A catalogue of British earthquakes. British Geological Survey Global Seismology Report, WL/94/04.
- ↑ Musson, R M W. 1996. The seismicity of the British Isles. Annali di Geofisica 39, 463–469.
- ↑ Baptie, B. 2012. UK earthquake monitoring 2011/2012: Twenty-third Annual Report. British Geological Survey Commissioned report, OR/12/092 (Edinburgh).
- ↑ Gutenberg, B and Richter, C F. 1954. Seismicity of the Earth and Associated Phenomena, 2nd ed. Princeton, N.J.: Princeton University Press.