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Radon anomalies preceding earthquakes which occurred in the UK, in summer and autumn 2002
Science of the Total Environment 364 (2006) 138 – 148 www.elsevier.com/locate/scitotenv
Radon anomalies preceding earthquakes which occurred in the UK, in summer and autumn 2002 R.G.M. Crockett a,*, G.K. Gillmore b, P.S. Phillips a, A.R. Denman c, C.J. Groves-Kirkby c a
b
School of Applied Sciences, University College Northampton, Boughton Green Road, Northampton, NN2 7AL, UK Department of Geography and Environmental Science, University of Bradford, Bradford, West Yorkshire, BD7 1DP, UK c Medical Physics Department, Northampton General Hospital, Cliftonville, Northampton, NN1 5BD, UK Received 20 January 2005; received in revised form 15 July 2005; accepted 1 August 2005 Available online 13 September 2005
Abstract During the course of an investigation into domestic radon levels in Northamptonshire, two hourly sampling real-time radon detectors were operated simultaneously in separate locations 2.25 km apart in Northampton, in the English East Midlands, for a 25-week period. This period of operation encompassed the period in September 2002 during which the Dudley earthquake (magnitude – 5.0) and smaller aftershocks occurred in the English West Midlands, UK. We report herein our observations regarding the occurrence of simultaneous short-period radon anomalies and their timing in relation to the Dudley, and other, earthquakes which occurred during the monitoring period. Analysis of the radon time-series reveals a short period when the two time-series displayed simultaneous in-phase short-term (6–9 h) radon anomalies prior to the main Dudley earthquake. Subsequent investigation revealed that a similar period occurred prior to another smaller but recorded earthquake in the English Channel. D 2005 Elsevier B.V. All rights reserved. Keywords: Radon; Earthquake; Seismology; Radon anomaly; Real-time monitoring
1. Introduction Globally, earthquakes resulted in the deaths of 2.7 million people during the period from 1900–1976.
* Corresponding author. Tel.: +44 1604 893107. E-mail address: [email protected] (R.G.M. Crockett). 0048-9697/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2005.08.003
Hence the interest in the literature on identifying geophysical precursors to predict earthquakes, such as the velocity of P-wave changes, ground tilt and uplift, decreases in electrical resistivity of rocks, underground water level fluctuations and increases in radon emissions. According to Bolt (2004) earthquakes have been classified into five stages (precursor stages I–III; earthquake at stage IV; rapid stress-relief, aftershocks
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at stage V) with changes in auspicious parameters associated with these stages. One of these parameters is radon emission: this increases during stage II, levels off in stage III and returns to normal in stage V (Bolt, 2004). During stage II, microcracks form in the rocks resulting in an increase in the surface area of the rocks (Asada, 1982), as P-wave velocity decreases and the ground uplifts and tilts. The increased surface area exposes more of the radon to water in rocks and the radon can then be dissolved into it. As the radon dissolved in water is forced out of the rock during stage II the radon may be released (Meyer, 1977) and the microcracks act as escape pathways for the gas (Asada, 1982). During stage III, P-wave velocity increases, ground uplift and tilt decrease, microcracks stop forming and radon emissions decrease. Changes during stage II may enable geophysicists/geologists to make short/medium-term predictions, although this is complicated by the fact that precursors such as radon emissions differ from area to area. Changes in groundwater radon concentrations were noted by Asada prior to the Tashkent earthquake in the USSR in 1966 (Asada, 1982). He also commented on similar changes in radon concentration prior to the Songpan–Pingwu earthquakes in China in 1976: he noted increases in radon beginning 2 or 3 years prior to the earthquake, continuing until a radon peak approximately 6 days before the first (magnitude – 7.2) earthquake. Asada pointed to a correlation between the distance to the epicentre, the magnitude of change, and the timing of peaks in radon concentration, with the amplitude of the change being greater closer to the epicentre. Similarly, water samples taken from wells within a 30 km radius of the 1995 Kobe earthquake epicentre showed increases in radon levels 2 months before the earthquake, these rising to 12 times normal 10 days before the earthquake. Radon gas concentration increase (and temperature decrease) in groundwater was noted near Izu–Oshima–Kinkai in Japan in 1978 as an earthquake precursor by the IASPEI report (IASPEI, 1997). Koch and Heinicke working at Bad Brambach, Vogtland, Germany recorded radon anomalies associated with numerous microquakes (magnitude – 4.0 or less) (Koch and Heinicke, 1994). Since the initial observations in the 1960s and 1970s that radon anomalies could occur in groundwater and soils prior to earthquakes, many attempts
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have been made to use both as earthquake predictors (Finkelstein et al., 1998; Zmazek et al., 2000; Planinic et al., 2000; Plastino et al., 2002). However, whilst earthquake-related radon anomalies have been detected at distances on the order of hundreds of kilometres from the epicentre, so far these have proved to be unreliable as earthquake predictors (Wakita, 1996). Also, it is important to observe that Climent et al., using time-series analysis, found no relationship between radon levels and earthquakes in Japan (Climent et al., 1999). As well as the well-catalogued problems of interpreting the influences of factors (e.g. geological, meteorological) upon the emission of radon from the ground (Wakita, 1996; Climent et al., 1999; Chyi et al., 2001), most, if not all, of this work has been further hampered on two counts. Firstly, there is often reliance upon using integrating detectors (e.g. track-etch) which aggregate over whole exposure periods (e.g. weeks or months) and thus cannot record short-term variations (e.g. hours to days) within such periods. Secondly, very few have monitored radon in real-time at more than one location simultaneously and so are unable to confirm the general spatial nature of any short-term variation: a single real-time detector can record short-term variations only at its location and so cannot determine whether the variations are highly localised or more widespread. Sources of localised variations include vibrations induced by trains, street-traffic and building activities, for example, at distances up to several tens of metres, dependent upon factors such as type of vibration and type of bedrock. Sources of more widespread variations include earthquakes and large explosions. In the study reported herein, two real-time detectors were deployed at separate locations: deploying two (or more) real-time detectors in different locations provides the possibility of distinguishing localised radon variations from more general widespread ones.
2. Geology and seismicity Seismicity in the UK is often underestimated according to Musson (1996): in fact, small and moderate earthquakes are quite common in the UK with an average of ca. 20 earthquakes a month. This underestimation was such that it was really only with the
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expansion of nuclear power in the 1970s that there was any real awareness of the potential impact from UK earthquakes. Large shallow earthquakes in the UK are certainly possible: UK earthquakes may not be as spectacular as those in, say, Japan but neither should such events be ignored when considering natural hazards and risk management. Prediction of such events in the UK therefore has real value. Musson (1996) suggests that spatial distribution of earthquakes in the UK is strikingly non-uniform although the pattern itself has been stable for some time (a map of UK earthquakes for example over the last 20 or so years would give a pattern comparable to that of a map of earthquakes over the last 300 years). There are some regions of the UK that also have a long history of seismic activity – such as Swansea – which had a damaging earthquake in 1906. Two of the UK’s largest earthquakes (ca. magnitude – 5) occurred in the Dover Straits in 1382 and 1580 and both caused much damage in London. Interestingly, Musson points out that although seismicity in the UK is fairly stable, populations have grown considerably, particularly in the London area, since the 1382 and 1580 events. Therefore, many more people are at risk due to increased vulnerability of society to such earthquakes. The solid geology around Northampton, in the English East Midlands, essentially consists of sedimentary rocks. These are mainly Lower and Middle Jurassic to Upper Lias sediments, predominantly Northamptonshire sandstone ironstone, with Inferior Oolite in the west and south of the region (Hains and Horton, 1969). These Middle Jurassic sediments contain an ironstone at the base which is overlain by a yellow or brown sandstone (the Variable Beds). In the ironstone facies the sediments consist of sideritic, chamositic oolite and limonitic oolite (Hains and Horton, 1969; Poole et al., 1968). There is also significant unconsolidated surficial material that sometimes overlies the solid geology in the area. This is basically a mixture of matrix-supported glacial diamict, periglacially derived material or dheadT deposits, sandy outwash material or pre- or post-glacial river gravels and there is considerable spatial variability of the occurrence of these deposits within the area. Fluvio-glacial deposits, for example, are often found intimately associated with glacial tills (Hains and Horton, 1969), as are pre-glacial river gravels (e.g. Boulton, 1992; Smith et al., 2000; Toghill, 2003). In
the Northamptonshire sand ironstone field, head deposits (caused by frost-shattering of the underlying solid geology) consist of sandstone, ironstone and limestone fragments. These often mask slopes forming stony and silty deposits which may cover small valley floors (Hains and Horton, 1969). Similarly, East Midland post-glacial river terraces, such as those found along the river Nene (with associated alluvium) may pass laterally into deposits of angular or subangular material that is very difficult to distinguish from head deposits. The variability of the overlying surficial deposits in the general area influences radon levels due to variations in gas permeability. The Northamptonshire sand (maximum thickness of 21 m) underlies the area containing both radon detectors and is associated with raised radon levels, as is Lincolnshire limestone also found in the surrounding area. Regarding the 2002 Dudley earthquake in the English West Midlands, the Sedgley and Dudley Anticline occurs in the region which consists of 3 faulted whaleback folds with a north–north-westerly trend, arranged en eche´lon, giving rise to inliers of Silurian Wenlock and Ludlow beds as three denuded and faulted folds which form the steep sided Dudley Castle Hill, Wren’s Nest Hill and Hunt Hill. South-east of Dudley the Sedgley and Dudley Anticline is replaced by a fault (Hains and Horton, 1969).
3. Radon monitoring and observations Over the period 27th June 2002 to 19th December 2002, the University College Northampton Radon Research Group operated two hourly sampling realtime RAD-7 (Durridge Inc.) radon detectors simultaneously as part of an investigation into domestic radon levels in Northamptonshire (UCN-RRG, 2003). The two detectors were operated in two separate, but similar, domestic basements 2.25 km apart on an east–west axis on Northampton sand geology. The Dudley earthquake occurred at 23:53 GMT, 22nd September 2002 (00:53 BST, 23rd September). The earthquake was centred close to Dudley, west of Northampton, and was widely noticed by members of the public in the Northampton area. During the monitoring period, there were also smaller, more distant earthquakes in the English Channel and at Manchester, whose effects went unnoticed in the Northampton
R.G.M. Crockett et al. / Science of the Total Environment 364 (2006) 138–148 Table 1 Earthquake data (USGS, 2003) Location
Date/time (GMT)
Dist (km)
ML
Depth (km)
English Channel Dudley
26-Aug-2002 22-Sep-2002 23-Sep-2002 24-Sep-2002 21-Oct-2002 21-Oct-2002 22-Oct-2002 22-Oct-2002 23-Oct-2002 24-Oct-2002 24-Oct-2002 25-Oct-2002 25-Oct-2002 25-Oct-2002 29-Oct-2002
250.2 90.8 90.1 90.1 156.4 164.8 163.0 161.4 162.7 164.4 164.1 164.1 165.4 163.8 164.1
3.0 5.0 3.2 1.2 3.7 4.3 2.9 3.5 3.3 3.8 2.8 2.5 2.5 2.6 3.1
4 9 9 7 5 5 5 4 5 3 5 5 5 5 5
Manchester
23:41 23:53 03:32 09:29 07:45 11:42 03:39 12:28 01:53 08:24 15:46 00:19 00:20 17:24 04:42
area. The earthquake data are summarised in Table 1 (distances are between Northampton town centre and the epicentres) and the locations are shown in Fig. 1.
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In light of previous observations regarding the (potential) relationship of radon anomalies with earthquakes, as noted above, and the occurrence of large (by UK standards) earthquakes during the 25-week monitoring period, it was decided to investigate the time-series for the presence of potentially earthquakerelated anomalies. It was further reasoned that as the two detectors are close together in relation to their distance from the epicentres of the relevant earthquakes, any earthquake-related anomalies would occur simultaneously (within the 1-h resolution of the detectors) in the two time-series (TS1a, TS1b). The readings in both TS1a and TS1b are highly lognormally distributed: the correlation coefficients for both to lognormal distributions are 0.91. Autocorrelation reveals evidence of 24-h cycles in both timeseries, clarified by Base-Number Correlation, a novel technique under development at UCN (Crockett et al., 2001) to reveal weak intermittent 24-h cycles peaking at 18:00 in TS1a and more consistent 24-h cycles peaking at 16:00 in TS1b. These cycles will result
Fig. 1. Map of the Southern UK showing Northampton and the earthquake locations (crosses).
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from occupancy patterns of the houses concerned. There is also evidence of some longer-period variations, durations ca. 15 and 30 days, which are possibly attributable to lunar–tidal forces and also under investigation (Groves-Kirkby et al., 2004). Both time-series were investigated for meteorological dependences, using meteorological data obtained as part of the DEFRA-funded research programme. Both TS1a and TS1b showed marginal correlations to rainfall, with 14-day and 10-day lags, respectively. These cannot explain the observed radon anomalies and, other than this, no meteorological dependences were observed. It is possible that in such a pair of time-series, as well as radon anomalies occurring simultaneously due to external influences, some anomalies will occur simultaneously randomly by chance. However, it is reasonable to assume that if an earthquake (or other seismic shock) triggers simultaneous radon anomalies, these should be similar to those – if any – triggered by other earthquakes and potentially different to random anomalies. It was decided to search for simultaneous features in the time-series by cross-correlating over set durations rolled forward through the time-series on an hourly basis, these durations ranging from 1 day (24 pairs of hourly readings) to 30 days (720 pairs of hourly readings). The reasoning behind this is that random simultaneous anomalies are more likely to be significant over shorter correlation periods (e.g. a fewhour anomaly comprises a significant proportion of a 24-h or 48-h period) but become less significant as the correlation period increases. Conversely, if an earthquake triggers simultaneous anomalies, these are more likely to occur over longer periods and so be significant over longer correlation periods (of course, even these will become blurred as the correlation extends beyond the period in which the anomalies occur). These detectors were not actually synchronised to sample at the same time, there having been no requirement for this within the context of the original monitoring programme. The average interval between the two detectors taking their hourly readings was 22 min, i.e. 11 min ether side of the central time. Consequently, two sets of correlations were performed. Firstly, one with the two time-series das isT, i.e. unsynchronised, with an average mismatch of 11 min between each 60-min reading and the central time
(i.e. 18%). Secondly, with each time-series being 3-h moving-averaged, i.e. more closely synchronised but with some loss of information, with an average mismatch of 11 min between each 180-min reading and the central time (i.e. 6%). The result of this rolling cross-correlation is timeseries of correlation coefficients for each correlation period. These are summarised in the contour plots in Figs. 2 and 3 in which each of these time-series is plotted horizontally across the plot, with the correlation period marked along the vertical axis and the central date/time of that period along the horizontal axis. The variation in correlation coefficient has been linked across the different correlation periods to form contours: the correlation coefficients vary from highest positive correlation (white) to highest negative correlation (black). The two plots are very similar and, for the most part, are shaded medium-grey indicating periods of weak correlation, both positive and negative (|R| b 0.5), including extensive periods of no significant correlation (|R| b 0.3). There are, however, periods of medium correlation, (0.5 b |R| b 0.7) and high correlation (0.7 b |R| b 1), both positive (light/white) and negative (dark/black). It is apparent that for correlation periods of three days or less, the resultant time-series of correlation coefficients is noisy, with little apparent pattern or structure, owing to random simultaneous anomalies. However, it is also apparent that for longer correlation periods up to ca. 10–12 days, the time-series of correlation coefficients shows less noise and evidence of longer-period correlations in the form of light or dark wedges tapering upwards from the horizontal axis. These indicate longer-duration simultaneous anomalies, with the strength/duration of the anomalies increasing with wedge-height. The most significant positive correlation occurs around 23rd September 2002, with the correlation wedge extending to ca. 12-day periods before becoming blurred by the inclusion of increasing periods of non-simultaneous behaviour either side of the central simultaneous core period. There is a second smaller positive correlation core period around 25th August 2002, extending to 6 days and some – weak – evidence for a third period around 6th–7th October. Also, there is a period of high negative correlation from late October to mid-November.
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Fig. 2. Rolling cross-correlation of radon time-series, 1 h data.
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Fig. 3. Rolling cross-correlation of radon time-series, 3 h moving-averaged data.
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The most significant core period relates – in time – to the Dudley earthquake, as shown in Fig. 4, which shows the two radon time-series (3-h moving-averaged, with the times of the earthquakes indicated by the crosses just above the date–time axis). It is clear that some of the simultaneous anomalies forming the core period occur before the earthquake. The mean period between the simultaneous peaks is marginally less than 24 h, at approximately 23.75 h. Fig. 5 is the corresponding graph for the second, smaller core period, relating – in time – to a smaller earthquake some 250 km from Northampton in the English Channel. Again, some of the simultaneous anomalies forming this core period occur before the earthquake and, in this case, the mean period between the simultaneous peaks is marginally greater than 24 h, at approximately 24.5 h. With regard to the Dudley earthquake, the period of highest positive correlation starts on 21st September. This period is characterised by several ca. 6–9-h dspikesT in the radon levels occurring in-phase in both TS1a and b. The first two of these spikes occur at 16:00–17:00, 21st September and 14:00–16:00, 22nd September, preceding the Dudley earthquake which occurred at 23:53, 22nd September. These two spikes are preceded by two smaller spikes at 10:00, 20th
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September and 08:00, 21st September, and also by troughs of similar duration. The period around the English Channel earthquake is similar, with in-phase 6–9-h spikes. The third period around the 6th–7th October does not display 6–9-h spikes and there were no UK-local earthquakes at this time. Also, the period of negative correlation from late October to mid-November is characterised by out-of-phase variations having ca. 4–5 day periods with, at 18:00–19:00, 8th November, a single large dip in TS1a coinciding with a very large spike in TS1b, followed by second similar but less pronounced combination at 16:00– 17:00, 10th November (with no UK-local earthquakes during this period). Performing a rolling correlation of modelled earthquake-period radon-variations, comprising 6–9 h spikes as characteristic of both time-series around the Dudley and English Channel earthquakes, with TS1a and TS1b reveals only two periods of significant correlation (R = 0.6–0.8) in either series. The correlation is higher for TS1b than TS1a due to the greater amplitude of the variations in TS1b, as evident in Figs. 4 and 5. These two periods occur simultaneously in the two time-series: the stronger centres on 21st–22nd September, i.e. around the Dudley earthquake, and the weaker on 25th–26th
Fig. 4. Anomalies associated with the Dudley earthquake.
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Fig. 5. Anomalies associated with the English Channel earthquake.
August, i.e. around the English Channel earthquake. This confirms that such radon-variations only occur simultaneously to both time-series and do not occur elsewhere in either time-series, supporting the supposition that these simultaneous radon anomalies are indeed earthquake-related.
4. Discussion With regard to the UK-local earthquakes which occurred during the period of continuous radon monitoring, the radon anomalies presented herein were observed solely because real-time radon detectors were being operated as part of another project. However, two periods of simultaneous radon anomalies, yielding high positive correlation coefficients, have been observed. These two periods are characterised by the presence of short-term, 6–9-h duration, inphase peaks and troughs in the atmospheric radon levels: no such periods occur at other times in the time-series. These anomalies were not detected by the 1-week, 1-month and 3-month integrating detectors simultaneously deployed with the real-time detectors in both locations: at best, the integrating detectors
would have shown (small) increases in radon concentration, as observed in other research, but would not have shown the short-term dspikingT. Whatever the nature of the relationship of the anomalies to the underlying geology, we have observed two periods in which in-phase short-duration radon anomalies precede identifiable earthquakes. One of these periods clearly precedes the Dudley earthquake and the other precedes an earthquake in the English Channel. However, during the period 21st–29th October 2002, Manchester (160 km from Northampton) underwent a series of earthquakes (magnitude – 4.3 or less) (USGS, 2003) which does not correspond to any observation of simultaneous anomalies in the radon time-series, but is immediately trailed by a period of high negative correlation. There are no other earthquakes recorded within 250 km of Northampton during the monitoring period (USGS, 2003). As has been observed by other workers, radon anomalies occurring over large areas precede earthquakes. Our observations indicate that two (or more) real-time radon detectors, operating simultaneously 2.25 km apart in this instance, can be used to distinguish between highly localised radon anomalies
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occurring in one location (or very small area) and those occurring over larger areas. Thus, in principle, the use of more than one real-time radon detector can be used to distinguish between the large-scale radon anomalies detected (approximately) simultaneously in more than one location, as arising from earthquakerelated or other seismic causes, and localised radon anomalies, e.g. anomalies detected at only one detector, such as might be caused by traffic or building works. Our observations are of atmospheric radon whereas the majority of other observations are of radon in soils or groundwater. Anomalies in atmospheric radon associated with earthquakes have been observed by Bella and Plastino in Italy (Bella and Plastino, 1999), who obtained simultaneous timeseries (12-h intervals) of radon in air and groundwater at the same location. However, they also indicate an earthquake-precursor element in their data, for earthquakes of similar magnitudes at similar distances, specifically attributing spiking anomalies in the ratios between groundwater and atmospheric radon levels as emphasising a dpre-co seismic characteristicT. Further research is required to confirm any general nature that these observations might have in relation to earthquakes: as noted, we obtained our time-series of radon readings over periods when (rare) dlargeT UK earthquakes occurred entirely fortuitously. As an ongoing study for this and other radon research, real-time radon detectors are being deployed at different locations a few kilometres apart to provide further data should further dlocalT earthquakes occur during the monitoring period.
Acknowledgements The authors would like to acknowledge the UK Department for Environment, Food and Rural Affairs (DEFRA) for funding the research programme into domestic radon levels under which the radon timeseries data were collected. The authors would also like to acknowledge Dr. Roger Musson of the BGS, Drs. Mark Stirling, David Rhoades and Russell Robinson of the NZ GNS and Dr. Eugene Schweig of the USGS for their advice during the investigation of the radon anomalies.
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Radon anomalies preceding earthquakes which occurred in the UK, in summer and autumn 2002 R.G.M. Crockett a,*, G.K. Gillmore b, P.S. Phillips a, A.R. Denman c, C.J. Groves-Kirkby c a
b
School of Applied Sciences, University College Northampton, Boughton Green Road, Northampton, NN2 7AL, UK Department of Geography and Environmental Science, University of Bradford, Bradford, West Yorkshire, BD7 1DP, UK c Medical Physics Department, Northampton General Hospital, Cliftonville, Northampton, NN1 5BD, UK Received 20 January 2005; received in revised form 15 July 2005; accepted 1 August 2005 Available online 13 September 2005
Abstract During the course of an investigation into domestic radon levels in Northamptonshire, two hourly sampling real-time radon detectors were operated simultaneously in separate locations 2.25 km apart in Northampton, in the English East Midlands, for a 25-week period. This period of operation encompassed the period in September 2002 during which the Dudley earthquake (magnitude – 5.0) and smaller aftershocks occurred in the English West Midlands, UK. We report herein our observations regarding the occurrence of simultaneous short-period radon anomalies and their timing in relation to the Dudley, and other, earthquakes which occurred during the monitoring period. Analysis of the radon time-series reveals a short period when the two time-series displayed simultaneous in-phase short-term (6–9 h) radon anomalies prior to the main Dudley earthquake. Subsequent investigation revealed that a similar period occurred prior to another smaller but recorded earthquake in the English Channel. D 2005 Elsevier B.V. All rights reserved. Keywords: Radon; Earthquake; Seismology; Radon anomaly; Real-time monitoring
1. Introduction Globally, earthquakes resulted in the deaths of 2.7 million people during the period from 1900–1976.
* Corresponding author. Tel.: +44 1604 893107. E-mail address: [email protected] (R.G.M. Crockett). 0048-9697/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2005.08.003
Hence the interest in the literature on identifying geophysical precursors to predict earthquakes, such as the velocity of P-wave changes, ground tilt and uplift, decreases in electrical resistivity of rocks, underground water level fluctuations and increases in radon emissions. According to Bolt (2004) earthquakes have been classified into five stages (precursor stages I–III; earthquake at stage IV; rapid stress-relief, aftershocks
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at stage V) with changes in auspicious parameters associated with these stages. One of these parameters is radon emission: this increases during stage II, levels off in stage III and returns to normal in stage V (Bolt, 2004). During stage II, microcracks form in the rocks resulting in an increase in the surface area of the rocks (Asada, 1982), as P-wave velocity decreases and the ground uplifts and tilts. The increased surface area exposes more of the radon to water in rocks and the radon can then be dissolved into it. As the radon dissolved in water is forced out of the rock during stage II the radon may be released (Meyer, 1977) and the microcracks act as escape pathways for the gas (Asada, 1982). During stage III, P-wave velocity increases, ground uplift and tilt decrease, microcracks stop forming and radon emissions decrease. Changes during stage II may enable geophysicists/geologists to make short/medium-term predictions, although this is complicated by the fact that precursors such as radon emissions differ from area to area. Changes in groundwater radon concentrations were noted by Asada prior to the Tashkent earthquake in the USSR in 1966 (Asada, 1982). He also commented on similar changes in radon concentration prior to the Songpan–Pingwu earthquakes in China in 1976: he noted increases in radon beginning 2 or 3 years prior to the earthquake, continuing until a radon peak approximately 6 days before the first (magnitude – 7.2) earthquake. Asada pointed to a correlation between the distance to the epicentre, the magnitude of change, and the timing of peaks in radon concentration, with the amplitude of the change being greater closer to the epicentre. Similarly, water samples taken from wells within a 30 km radius of the 1995 Kobe earthquake epicentre showed increases in radon levels 2 months before the earthquake, these rising to 12 times normal 10 days before the earthquake. Radon gas concentration increase (and temperature decrease) in groundwater was noted near Izu–Oshima–Kinkai in Japan in 1978 as an earthquake precursor by the IASPEI report (IASPEI, 1997). Koch and Heinicke working at Bad Brambach, Vogtland, Germany recorded radon anomalies associated with numerous microquakes (magnitude – 4.0 or less) (Koch and Heinicke, 1994). Since the initial observations in the 1960s and 1970s that radon anomalies could occur in groundwater and soils prior to earthquakes, many attempts
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have been made to use both as earthquake predictors (Finkelstein et al., 1998; Zmazek et al., 2000; Planinic et al., 2000; Plastino et al., 2002). However, whilst earthquake-related radon anomalies have been detected at distances on the order of hundreds of kilometres from the epicentre, so far these have proved to be unreliable as earthquake predictors (Wakita, 1996). Also, it is important to observe that Climent et al., using time-series analysis, found no relationship between radon levels and earthquakes in Japan (Climent et al., 1999). As well as the well-catalogued problems of interpreting the influences of factors (e.g. geological, meteorological) upon the emission of radon from the ground (Wakita, 1996; Climent et al., 1999; Chyi et al., 2001), most, if not all, of this work has been further hampered on two counts. Firstly, there is often reliance upon using integrating detectors (e.g. track-etch) which aggregate over whole exposure periods (e.g. weeks or months) and thus cannot record short-term variations (e.g. hours to days) within such periods. Secondly, very few have monitored radon in real-time at more than one location simultaneously and so are unable to confirm the general spatial nature of any short-term variation: a single real-time detector can record short-term variations only at its location and so cannot determine whether the variations are highly localised or more widespread. Sources of localised variations include vibrations induced by trains, street-traffic and building activities, for example, at distances up to several tens of metres, dependent upon factors such as type of vibration and type of bedrock. Sources of more widespread variations include earthquakes and large explosions. In the study reported herein, two real-time detectors were deployed at separate locations: deploying two (or more) real-time detectors in different locations provides the possibility of distinguishing localised radon variations from more general widespread ones.
2. Geology and seismicity Seismicity in the UK is often underestimated according to Musson (1996): in fact, small and moderate earthquakes are quite common in the UK with an average of ca. 20 earthquakes a month. This underestimation was such that it was really only with the
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expansion of nuclear power in the 1970s that there was any real awareness of the potential impact from UK earthquakes. Large shallow earthquakes in the UK are certainly possible: UK earthquakes may not be as spectacular as those in, say, Japan but neither should such events be ignored when considering natural hazards and risk management. Prediction of such events in the UK therefore has real value. Musson (1996) suggests that spatial distribution of earthquakes in the UK is strikingly non-uniform although the pattern itself has been stable for some time (a map of UK earthquakes for example over the last 20 or so years would give a pattern comparable to that of a map of earthquakes over the last 300 years). There are some regions of the UK that also have a long history of seismic activity – such as Swansea – which had a damaging earthquake in 1906. Two of the UK’s largest earthquakes (ca. magnitude – 5) occurred in the Dover Straits in 1382 and 1580 and both caused much damage in London. Interestingly, Musson points out that although seismicity in the UK is fairly stable, populations have grown considerably, particularly in the London area, since the 1382 and 1580 events. Therefore, many more people are at risk due to increased vulnerability of society to such earthquakes. The solid geology around Northampton, in the English East Midlands, essentially consists of sedimentary rocks. These are mainly Lower and Middle Jurassic to Upper Lias sediments, predominantly Northamptonshire sandstone ironstone, with Inferior Oolite in the west and south of the region (Hains and Horton, 1969). These Middle Jurassic sediments contain an ironstone at the base which is overlain by a yellow or brown sandstone (the Variable Beds). In the ironstone facies the sediments consist of sideritic, chamositic oolite and limonitic oolite (Hains and Horton, 1969; Poole et al., 1968). There is also significant unconsolidated surficial material that sometimes overlies the solid geology in the area. This is basically a mixture of matrix-supported glacial diamict, periglacially derived material or dheadT deposits, sandy outwash material or pre- or post-glacial river gravels and there is considerable spatial variability of the occurrence of these deposits within the area. Fluvio-glacial deposits, for example, are often found intimately associated with glacial tills (Hains and Horton, 1969), as are pre-glacial river gravels (e.g. Boulton, 1992; Smith et al., 2000; Toghill, 2003). In
the Northamptonshire sand ironstone field, head deposits (caused by frost-shattering of the underlying solid geology) consist of sandstone, ironstone and limestone fragments. These often mask slopes forming stony and silty deposits which may cover small valley floors (Hains and Horton, 1969). Similarly, East Midland post-glacial river terraces, such as those found along the river Nene (with associated alluvium) may pass laterally into deposits of angular or subangular material that is very difficult to distinguish from head deposits. The variability of the overlying surficial deposits in the general area influences radon levels due to variations in gas permeability. The Northamptonshire sand (maximum thickness of 21 m) underlies the area containing both radon detectors and is associated with raised radon levels, as is Lincolnshire limestone also found in the surrounding area. Regarding the 2002 Dudley earthquake in the English West Midlands, the Sedgley and Dudley Anticline occurs in the region which consists of 3 faulted whaleback folds with a north–north-westerly trend, arranged en eche´lon, giving rise to inliers of Silurian Wenlock and Ludlow beds as three denuded and faulted folds which form the steep sided Dudley Castle Hill, Wren’s Nest Hill and Hunt Hill. South-east of Dudley the Sedgley and Dudley Anticline is replaced by a fault (Hains and Horton, 1969).
3. Radon monitoring and observations Over the period 27th June 2002 to 19th December 2002, the University College Northampton Radon Research Group operated two hourly sampling realtime RAD-7 (Durridge Inc.) radon detectors simultaneously as part of an investigation into domestic radon levels in Northamptonshire (UCN-RRG, 2003). The two detectors were operated in two separate, but similar, domestic basements 2.25 km apart on an east–west axis on Northampton sand geology. The Dudley earthquake occurred at 23:53 GMT, 22nd September 2002 (00:53 BST, 23rd September). The earthquake was centred close to Dudley, west of Northampton, and was widely noticed by members of the public in the Northampton area. During the monitoring period, there were also smaller, more distant earthquakes in the English Channel and at Manchester, whose effects went unnoticed in the Northampton
R.G.M. Crockett et al. / Science of the Total Environment 364 (2006) 138–148 Table 1 Earthquake data (USGS, 2003) Location
Date/time (GMT)
Dist (km)
ML
Depth (km)
English Channel Dudley
26-Aug-2002 22-Sep-2002 23-Sep-2002 24-Sep-2002 21-Oct-2002 21-Oct-2002 22-Oct-2002 22-Oct-2002 23-Oct-2002 24-Oct-2002 24-Oct-2002 25-Oct-2002 25-Oct-2002 25-Oct-2002 29-Oct-2002
250.2 90.8 90.1 90.1 156.4 164.8 163.0 161.4 162.7 164.4 164.1 164.1 165.4 163.8 164.1
3.0 5.0 3.2 1.2 3.7 4.3 2.9 3.5 3.3 3.8 2.8 2.5 2.5 2.6 3.1
4 9 9 7 5 5 5 4 5 3 5 5 5 5 5
Manchester
23:41 23:53 03:32 09:29 07:45 11:42 03:39 12:28 01:53 08:24 15:46 00:19 00:20 17:24 04:42
area. The earthquake data are summarised in Table 1 (distances are between Northampton town centre and the epicentres) and the locations are shown in Fig. 1.
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In light of previous observations regarding the (potential) relationship of radon anomalies with earthquakes, as noted above, and the occurrence of large (by UK standards) earthquakes during the 25-week monitoring period, it was decided to investigate the time-series for the presence of potentially earthquakerelated anomalies. It was further reasoned that as the two detectors are close together in relation to their distance from the epicentres of the relevant earthquakes, any earthquake-related anomalies would occur simultaneously (within the 1-h resolution of the detectors) in the two time-series (TS1a, TS1b). The readings in both TS1a and TS1b are highly lognormally distributed: the correlation coefficients for both to lognormal distributions are 0.91. Autocorrelation reveals evidence of 24-h cycles in both timeseries, clarified by Base-Number Correlation, a novel technique under development at UCN (Crockett et al., 2001) to reveal weak intermittent 24-h cycles peaking at 18:00 in TS1a and more consistent 24-h cycles peaking at 16:00 in TS1b. These cycles will result
Fig. 1. Map of the Southern UK showing Northampton and the earthquake locations (crosses).
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from occupancy patterns of the houses concerned. There is also evidence of some longer-period variations, durations ca. 15 and 30 days, which are possibly attributable to lunar–tidal forces and also under investigation (Groves-Kirkby et al., 2004). Both time-series were investigated for meteorological dependences, using meteorological data obtained as part of the DEFRA-funded research programme. Both TS1a and TS1b showed marginal correlations to rainfall, with 14-day and 10-day lags, respectively. These cannot explain the observed radon anomalies and, other than this, no meteorological dependences were observed. It is possible that in such a pair of time-series, as well as radon anomalies occurring simultaneously due to external influences, some anomalies will occur simultaneously randomly by chance. However, it is reasonable to assume that if an earthquake (or other seismic shock) triggers simultaneous radon anomalies, these should be similar to those – if any – triggered by other earthquakes and potentially different to random anomalies. It was decided to search for simultaneous features in the time-series by cross-correlating over set durations rolled forward through the time-series on an hourly basis, these durations ranging from 1 day (24 pairs of hourly readings) to 30 days (720 pairs of hourly readings). The reasoning behind this is that random simultaneous anomalies are more likely to be significant over shorter correlation periods (e.g. a fewhour anomaly comprises a significant proportion of a 24-h or 48-h period) but become less significant as the correlation period increases. Conversely, if an earthquake triggers simultaneous anomalies, these are more likely to occur over longer periods and so be significant over longer correlation periods (of course, even these will become blurred as the correlation extends beyond the period in which the anomalies occur). These detectors were not actually synchronised to sample at the same time, there having been no requirement for this within the context of the original monitoring programme. The average interval between the two detectors taking their hourly readings was 22 min, i.e. 11 min ether side of the central time. Consequently, two sets of correlations were performed. Firstly, one with the two time-series das isT, i.e. unsynchronised, with an average mismatch of 11 min between each 60-min reading and the central time
(i.e. 18%). Secondly, with each time-series being 3-h moving-averaged, i.e. more closely synchronised but with some loss of information, with an average mismatch of 11 min between each 180-min reading and the central time (i.e. 6%). The result of this rolling cross-correlation is timeseries of correlation coefficients for each correlation period. These are summarised in the contour plots in Figs. 2 and 3 in which each of these time-series is plotted horizontally across the plot, with the correlation period marked along the vertical axis and the central date/time of that period along the horizontal axis. The variation in correlation coefficient has been linked across the different correlation periods to form contours: the correlation coefficients vary from highest positive correlation (white) to highest negative correlation (black). The two plots are very similar and, for the most part, are shaded medium-grey indicating periods of weak correlation, both positive and negative (|R| b 0.5), including extensive periods of no significant correlation (|R| b 0.3). There are, however, periods of medium correlation, (0.5 b |R| b 0.7) and high correlation (0.7 b |R| b 1), both positive (light/white) and negative (dark/black). It is apparent that for correlation periods of three days or less, the resultant time-series of correlation coefficients is noisy, with little apparent pattern or structure, owing to random simultaneous anomalies. However, it is also apparent that for longer correlation periods up to ca. 10–12 days, the time-series of correlation coefficients shows less noise and evidence of longer-period correlations in the form of light or dark wedges tapering upwards from the horizontal axis. These indicate longer-duration simultaneous anomalies, with the strength/duration of the anomalies increasing with wedge-height. The most significant positive correlation occurs around 23rd September 2002, with the correlation wedge extending to ca. 12-day periods before becoming blurred by the inclusion of increasing periods of non-simultaneous behaviour either side of the central simultaneous core period. There is a second smaller positive correlation core period around 25th August 2002, extending to 6 days and some – weak – evidence for a third period around 6th–7th October. Also, there is a period of high negative correlation from late October to mid-November.
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Fig. 2. Rolling cross-correlation of radon time-series, 1 h data.
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Fig. 3. Rolling cross-correlation of radon time-series, 3 h moving-averaged data.
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The most significant core period relates – in time – to the Dudley earthquake, as shown in Fig. 4, which shows the two radon time-series (3-h moving-averaged, with the times of the earthquakes indicated by the crosses just above the date–time axis). It is clear that some of the simultaneous anomalies forming the core period occur before the earthquake. The mean period between the simultaneous peaks is marginally less than 24 h, at approximately 23.75 h. Fig. 5 is the corresponding graph for the second, smaller core period, relating – in time – to a smaller earthquake some 250 km from Northampton in the English Channel. Again, some of the simultaneous anomalies forming this core period occur before the earthquake and, in this case, the mean period between the simultaneous peaks is marginally greater than 24 h, at approximately 24.5 h. With regard to the Dudley earthquake, the period of highest positive correlation starts on 21st September. This period is characterised by several ca. 6–9-h dspikesT in the radon levels occurring in-phase in both TS1a and b. The first two of these spikes occur at 16:00–17:00, 21st September and 14:00–16:00, 22nd September, preceding the Dudley earthquake which occurred at 23:53, 22nd September. These two spikes are preceded by two smaller spikes at 10:00, 20th
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September and 08:00, 21st September, and also by troughs of similar duration. The period around the English Channel earthquake is similar, with in-phase 6–9-h spikes. The third period around the 6th–7th October does not display 6–9-h spikes and there were no UK-local earthquakes at this time. Also, the period of negative correlation from late October to mid-November is characterised by out-of-phase variations having ca. 4–5 day periods with, at 18:00–19:00, 8th November, a single large dip in TS1a coinciding with a very large spike in TS1b, followed by second similar but less pronounced combination at 16:00– 17:00, 10th November (with no UK-local earthquakes during this period). Performing a rolling correlation of modelled earthquake-period radon-variations, comprising 6–9 h spikes as characteristic of both time-series around the Dudley and English Channel earthquakes, with TS1a and TS1b reveals only two periods of significant correlation (R = 0.6–0.8) in either series. The correlation is higher for TS1b than TS1a due to the greater amplitude of the variations in TS1b, as evident in Figs. 4 and 5. These two periods occur simultaneously in the two time-series: the stronger centres on 21st–22nd September, i.e. around the Dudley earthquake, and the weaker on 25th–26th
Fig. 4. Anomalies associated with the Dudley earthquake.
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Fig. 5. Anomalies associated with the English Channel earthquake.
August, i.e. around the English Channel earthquake. This confirms that such radon-variations only occur simultaneously to both time-series and do not occur elsewhere in either time-series, supporting the supposition that these simultaneous radon anomalies are indeed earthquake-related.
4. Discussion With regard to the UK-local earthquakes which occurred during the period of continuous radon monitoring, the radon anomalies presented herein were observed solely because real-time radon detectors were being operated as part of another project. However, two periods of simultaneous radon anomalies, yielding high positive correlation coefficients, have been observed. These two periods are characterised by the presence of short-term, 6–9-h duration, inphase peaks and troughs in the atmospheric radon levels: no such periods occur at other times in the time-series. These anomalies were not detected by the 1-week, 1-month and 3-month integrating detectors simultaneously deployed with the real-time detectors in both locations: at best, the integrating detectors
would have shown (small) increases in radon concentration, as observed in other research, but would not have shown the short-term dspikingT. Whatever the nature of the relationship of the anomalies to the underlying geology, we have observed two periods in which in-phase short-duration radon anomalies precede identifiable earthquakes. One of these periods clearly precedes the Dudley earthquake and the other precedes an earthquake in the English Channel. However, during the period 21st–29th October 2002, Manchester (160 km from Northampton) underwent a series of earthquakes (magnitude – 4.3 or less) (USGS, 2003) which does not correspond to any observation of simultaneous anomalies in the radon time-series, but is immediately trailed by a period of high negative correlation. There are no other earthquakes recorded within 250 km of Northampton during the monitoring period (USGS, 2003). As has been observed by other workers, radon anomalies occurring over large areas precede earthquakes. Our observations indicate that two (or more) real-time radon detectors, operating simultaneously 2.25 km apart in this instance, can be used to distinguish between highly localised radon anomalies
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occurring in one location (or very small area) and those occurring over larger areas. Thus, in principle, the use of more than one real-time radon detector can be used to distinguish between the large-scale radon anomalies detected (approximately) simultaneously in more than one location, as arising from earthquakerelated or other seismic causes, and localised radon anomalies, e.g. anomalies detected at only one detector, such as might be caused by traffic or building works. Our observations are of atmospheric radon whereas the majority of other observations are of radon in soils or groundwater. Anomalies in atmospheric radon associated with earthquakes have been observed by Bella and Plastino in Italy (Bella and Plastino, 1999), who obtained simultaneous timeseries (12-h intervals) of radon in air and groundwater at the same location. However, they also indicate an earthquake-precursor element in their data, for earthquakes of similar magnitudes at similar distances, specifically attributing spiking anomalies in the ratios between groundwater and atmospheric radon levels as emphasising a dpre-co seismic characteristicT. Further research is required to confirm any general nature that these observations might have in relation to earthquakes: as noted, we obtained our time-series of radon readings over periods when (rare) dlargeT UK earthquakes occurred entirely fortuitously. As an ongoing study for this and other radon research, real-time radon detectors are being deployed at different locations a few kilometres apart to provide further data should further dlocalT earthquakes occur during the monitoring period.
Acknowledgements The authors would like to acknowledge the UK Department for Environment, Food and Rural Affairs (DEFRA) for funding the research programme into domestic radon levels under which the radon timeseries data were collected. The authors would also like to acknowledge Dr. Roger Musson of the BGS, Drs. Mark Stirling, David Rhoades and Russell Robinson of the NZ GNS and Dr. Eugene Schweig of the USGS for their advice during the investigation of the radon anomalies.
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