Radon Changes Associated with the Earthquake Sequence in June 2000 in the South Iceland Seismic Zone

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1 Pure appl. geophys. (28) Ó Birkhäuser Verlag, Basel, 28 DOI 1.17/s Pure and Applied Geophysics Radon Changes Associated with the Earthquake Sequence in June 2 in the South Iceland Seismic Zone PÁLL EINARSSON, 1 PÁLL THEODÓRSSON, 2 ÁSTA RUT HJARTARDÓTTIR, 1 and GUðÓN IGUðJÓNSSON 2 Abstract An earthquake sequence at the transform plate boundary in South Iceland, that included two magnitude 6.5 earthquakes in June 2, was anticipated on the basis of a centuries-long seismicity pattern in the area. A program of radon monitoring in geothermal water from drill holes, initiated in 1999, rendered distinct and consistent variations in radon in association with these events. All seven sampling stations in a km zone covering the epicentral area showed a consistent pattern. Four types of change could be identified: 1) Preseismic decrease of radon. Anomalously low values were measured days before the earthquakes. 2) Preseismic increase. Spikes appear in the time series at six stations days prior to the earthquakes. These anomalies were large and unusual if compared to a 17-years history of radon monitoring in this area. 3) Coseismic step, most likely related to the coseismic change in groundwater pressure observed over the entire area. 4) Postseismic return of the radon values to the preseismic level about three months later, also concurrent with groundwater pressure changes. Key words: South Iceland Seismic Zone, radon, earthquake precursor, co-scismic changes. 1. Introduction Various studies have shown that an increase in the concentration of radon in groundwater is an earthquake precursor, see e.g., reviews by HAUKSSON (1981) and KING (1985), and more recent studies by WAKITA (1996), ROELOFFS (1999), TRIQUE et al. (1999), and ZMAZEK et al. (22). Even though numerous examples of premonitory radon anomalies have been identified and described in the literature, statistical analysis of the relationship between radon and earthquakes has been difficult because of the lack of long-time series from a network of recording stations in active seismic or volcanic areas. A program of radon monitoring was initiated for this purpose in the plate boundary areas of Iceland in 1977 (HAUKSSON and GODDARD, 1981; HAUKSSON, 1981). 1 Institute of Earth Sciences, University of Iceland, Sturlugata 7, 11 Reykjavík, Iceland. palli@hi.is 2 Science Institute, University of Iceland, Dunhaga 3, 17 Reykjavík, Iceland.

2 P. Einarsson et al. Pure appl. geophys., A network of 11 sampling stations was established, nine of which were located within or near the South Iceland Seismic Zone; a transform zone where the most damaging historical earthquakes in Iceland have occurred. Judged from the time pattern of previous events a sequence of large earthquakes was expected in this zone with high probability (STEFÁNSSON et al., 1993; EINARSSON et al., 1981). This labor-intensive radon program gave promising results (JóNSSON and EINARSSON, 1996) but was discontinued in 1993 because of declining funding and deteriorating instruments. Radon monitoring was resumed in 1999 with new and improved instruments and time-saving sample preparation (THEODóRSSON, 1996; GUDJóNSSON and THEODóRSSON, 2) during which the sample preparation time was reduced from 3 hours to less than 1 minutes. The expected earthquakes occurred in June 2 (EINARSSON et al., 2; STEFáNSSON et al., 2) within the network of monitoring stations and after one year of radon monitoring. The earthquake sequence occurred along a 9-km stretch of the plate boundary and contained two magnitude 6.5 (M x ) earthquakes (on June 17 and 21) and several magnitude 5+ events (see e.g., PEDERSEN et al., 23; ÁRNADóTTIR et al., 24). This paper documents the radon time series and reports on the radon changes detected in association with these events. 2. The South Iceland Seismic Zone The South Iceland Seismic Zone is a transform-type plate boundary; a branch of the mid-atlantic plate boundary that crosses Iceland (Fig. 1). Plate divergence in the southern part of Iceland is accommodated by two sub-parallel rift zones: the Western and the Eastern Volcanic Zones. The gap between them is bridged in the south, near 64 N, by a zone of high seismic activity, the South Iceland Seismic Zone, which takes up the transform motion between the Reykjanes Ridge and the Eastern Volcanic Zone (EINARSSON, 1991). The two rift zones and the transform demarcate a block or a microplate, the Hreppar microplate. It has been argued that rifting is dying out in the Western Rift Zone, and is being taken over by the Eastern Rift Zone, or that the partition of rifting between the rift zones may be uneven and changes with time (SIGMUNDSSON et al., 1995). The South Iceland Seismic Zone has been defined by destruction areas of historical earthquakes, Holocene surface ruptures and instrumentally determined epicenters. It is oriented E-W and is 1 15 km wide. Destruction areas of individual earthquakes and surface faulting (Fig. 2) show, however, that each event is associated with faulting on N-S striking planes, perpendicular to the main zone. The overall left-lateral transform motion along the zone, i.e., between the Hreppar microplate to the north and the Eurasia plate to the south, thus appears to be accommodated by right-lateral faulting on many parallel, transverse faults and counter-clockwise rotation of the blocks between them, bookshelf faulting (EINARSSON et al., 1981).

3 Radon Changes and the South Iceland Earthquakes of June 2 Figure 1 The active plate boundary of Iceland passes near the center of the Iceland hotspot. The radon program is centered on the transform zone of South Iceland, SISZ. The Western and Eastern Volcanic Rift Zones are marked with W and E, respectively. Arrows show the direction of relative plate movements across the plate boundaries. The rate is 18.5 to 19.5 mm/year. Earthquakes in South Iceland tend to occur in major sequences in which most of the zone is affected. These sequences last from a few days to about three years. Each sequence typically begins with a magnitude 7 event in the eastern part of the zone, followed by smaller events farther west. Sequences of this type occurred in 1896, 1784, , , , 1339 and Apart from the historic gap between 1391 and 163, the sequences thus occur at intervals that range between 45 and 112 years (EINARSSON et al., 1981), and it has been argued that a complete strain release of the whole zone is accomplished in about 14 years (STEFÁNSSON and HALLDóRSSON, 1988). The lengthy time since the last sequence led to a long-term forecast published in 1985 (EINARSSON, 1985), later refined by STEFÁNSSON et al. (1993), of a major earthquake sequence within the next decades. The original forecast gave a 8% probability for the occurrence of a major earthquake sequence within the next 25 years (i.e., within the time window). The magnitude of the first event was estimated in the range and the most likely location was given in the eastern part of the seismic zone. In the refined version the magnitude range was reduced to and two seismic gaps were identified, at 2.3 W and 2.7 W. The forecast was fulfilled in June 2 when

4 P. Einarsson et al. Pure appl. geophys., Figure 2 The location of the radon sampling stations within the South Iceland Seismic Zone shown with triangles. Thin lines show Holocene surface fractures, formed in earthquakes before 2. The two long, thick lines show the source faults of the two earthquakes of June 17 and 21, 2 as delineated by aftershocks; the former one on the easternmost fault, the latter on the western fault. The sense of faulting was right-lateral strike-slip on both faults. Elevation contours are at 5 m intervals. two magnitude 6.5 events occurred in the zone; one at 2.37 W and the other at 2.71 W. 3. The 2 Earthquake Sequence The earthquakes of 2 were the largest in the zone since 1896 and They occurred within the SIL-network of seismographs operated by the Icelandic Meteorological Office (see e.g., website STEFÁNSSON et al. 1993). The sequence began on June 17 at 15:4 with a magnitude 6.5 event in the eastern part of the zone (Fig. 2). This immediately triggered a flurry of activity along at least a 9-km-long stretch of the plate boundary to the west, apparently triggered by the passing S waves from the first event. Among them was an event with an anomalously low seismic radiation but a moment equivalent to a magnitude 5.9 (ÁRNADóTTIR et al., 24). An earthquake of magnitude 5.7 (m b ) followed two minutes later on a small, parallel fault, about 3 4 km to the west of the first shock. An event of magnitude 4.9 (m b ) then occurred on the Reykjanes Peninsula, 9 km to the west, about 5 minutes after the first shock. Several other significant shocks also occurred this day along this segment of the plate boundary (PAGLI et al., 23). A second mainshock similar in magnitude to the first event occurred about 2 km west of the first one on June 21 at :51. It was clearly preceded by a clustering of microearthquakes along the eventual source fault (STEFÁNSSON et al., 2). ÁRNADóTTIR et al. (23, 24) present evidence that triggering played a large role in the occurrence of

5 Radon Changes and the South Iceland Earthquakes of June 2 events within the sequence, both dynamic triggering by the passing waves and triggering by regional changes in the Coulomb stress due to faulting. The aftershock distribution, moment tensor inversions, distribution of surface faulting, and modeling of surface deformation measured by GPS and InSAR confirm that the mainshocks of the sequence occurred on N-S striking faults, transverse to the zone itself (CLIFTON and EINARSSON, 25; ÁRNADóTTIR et al., 21; PEDERSEN et al., 23). The sense of faulting was right-lateral strike-slip conforming to the model of bookshelf faulting for the South Iceland Seismic Zone. The two mainshocks occurred on pre-existing faults and were accompanied by surface ruptures consisting primarily of en-échelon tension gashes and push-up structures (CLIFTON and EINARSSON, 25). The main zones of rupture were about 15 km long and fractured the crust down to 1 km depth. The surface faults coincided with the epicentral distributions of aftershocks. Fault displacements were of the order of.1 1 m. Faulting along conjugate, left-lateral strike-slip faults also occurred, but was less pronounced than that of the main rupture zones. The maximum fault displacement at depth, determined by modeling of geodetic data, was m. The source faults of the two largest earthquakes are shown in Figure 2. Large hydrological changes were observed in a wide area surrounding the seismically active zone. Pressure changes in boreholes followed a regular pattern conforming with crustal stress changes (BJöRNSSON et al., 21; JóNSSON et al., 23). Pressure decreased in areas to the NE and SW of the epicenters but increased in the quadrants to the NW and SE. These changes were large, but were reversed and equilibrated in less than three months. A post-earthquake crustal deformation signal was detected by InSAR that correlates with the water pressure changes (JóNSSON et al., 23). 4. Previous Radon Studies in South Iceland The relationship between radon and earthquakes has been studied in this area since 1977, when the first equipment for this purpose was installed. The instruments were operated until The radon monitoring network consisted of up to 9 stations. Samples (.6 l) of geothermal water were collected from drill holes every few weeks and sent to the laboratory for radon analysis. The resulting time series varied in length from 3 to 16 years. Many earthquake-related radon anomalies were identified (JóNSSON and EINARSSON, 1996). They are represented by both positive and negative excursions from the mean values, and occur mostly prior to the seismic events, i.e., within a few weeks. For a statistical analysis of the anomalies and comparison with the seismicity time series, significant earthquakes were selected according to the criteria of HAUKSSON and GODDARD (1981): M 2:4 log D :43 and M 2; ð1þ where M is the magnitude and D is the distance to a radon monitoring station. Thus 98 independent seismic events were selected. They were in the magnitude range The main conclusions were as follows:

6 P. Einarsson et al. Pure appl. geophys., 1. Radon anomalies were observed before 3 of the significant events % of all observed anomalies were related to seismicity. 3. 8% of the anomalies observed before earthquakes were positive. 4. If a positive anomaly is detected at one station, the probability of a significant earthquake occurring afterwards is 38%. 5. Some sampling sites were found to be more sensitive than others. The sensitivity appears to depend on local geological conditions. 6. A few radon anomalies appeared to be related to eruptive activity of the neighboring Hekla volcano. 5. Revival of the Radon Monitoring A new radon program was initiated in 1999 using a new time-saving technique and an instrument developed at our institute. It is based on a novel liquid scintillation technique where counting only Bi-218/Po-218 pulse pairs gives high sensitivity with a simple construction (THEODóRSSON, 1996; GUDJóNSSON and THEODóRSSON, 2). Scintillations other than those associated with radon-222 are thus excluded. Samples of 2 ml are taken from the geothermal drill holes at the sampling sites (Table 1) and analyzed in the laboratory. About 6% of the radon from the water samples is transferred to a scintillator in a 15 ml liquid scintillation counting vial by circulating and bubbling air for four minutes between the two liquids. The scintillation liquid is mineral oil. The scintillations are subsequently counted in a laboratory-made automatic sample changer. 226 counts per hour correspond to 1 Bq/l of radon in the water. The system represents a significant progress in the radon measuring technique where high sensitivity is needed. The technique also saves considerable time compared to previous procedures. Sample preparation time was reduced from 3 hours to less than 1 minutes. Sampling from geothermal wells in the South Iceland Seismic Zone began a year before the destructive earthquakes of June 2 occurred. Water samples were taken Table 1 Location, depth and temperature of geothermal holes sampled for radon Station Latitude Longitude Depth, m Teperature, C Bakki Öxnalækur Selfoss, hole & 85 Hlemmiskeið Flúðir, hole Kaldárholt Laugaland, hole

7 Radon Changes and the South Iceland Earthquakes of June 2 about twice a week from geothermal drill holes at seven sites (Fig. 2, Table 1) and sent to the Science Institute, University of Iceland, for analysis. These holes, ranging in depth between 38 m and 11 m, were mostly the same as used in the earlier radon program. The June 2 earthquakes originated within our sampling network. The nearest station, Laugaland, is only 2 km away from the source fault of the first earthquake (Fig. 2). 6. The Radon Changes The radon time series for the seven monitoring stations are shown in Figure 3. The raw data are plotted as individual points connected by thin lines. The numbers denote counts of disintegrations per hour in the sample. The thick, broken line shows a sevenpoint running average of the data. The vertical bars give the time of significant earthquakes in the area. The height of the bar reflects the excess magnitude of the event, M E, defined on the basis of the criteria of equation (1): M E ¼ M ð2:4 log D :43Þ: Largest probabilities of radon anomalies are expected prior to events with positive excess magnitude. The scale in the plots is arbitrary and the relative height of the bars is only presumed to reflect the likelihood that the respective earthquake is associated with a radon excursion at that particular station. The radon variations form a distinct pattern that can be related to the earthquakes. A typical behavior is seen at the station Hlemmiskeid. The radon values prior to the earthquakes of June 2 are relatively stable, varying by less than ± 5% around the mean. The largest deviations are positive, spike-like excursions that occur 59 and 115 days before the earthquakes. These are the highest values measured at this station during the two years of operation. Several of the lowest values are measured in a limited time interval days before June 17. A pronounced coseismic step is observed at this station. The mean value drops by about 5% at the time of the earthquakes. About three months later the mean value returns to the pre-earthquake level. The main features of the Hlemmiskeið time series can be identified in the other time series as well, except at Kaldárholt. The Öxnalækur series has a stable level in the beginning, low values days prior to the earthquakes, and a peak value 114 days before them. At Selfoss the same behavior is observed, but subdued. Low values occur days before, and high values 114 and 144 days before the events. At Bakki anomalous values are observed, but the background level of radon is very low. Therefore the negative deviations are more difficult to identify. The values are apparently low in the time interval days before the earthquakes, and 3 out of the 4 highest values are recorded 57, 5, and 43 days before June 17. At Flúðir the negative excursions are not prominent, although a large peak is seen 54 days before the earthquakes. The Laugaland series has well developed negative deviations ð2þ

8 P. Einarsson et al. Pure appl. geophys., prior to the events but positive spikes are not seen, except possibly a high value 58 days before the events. The Kaldárholt time series is quite irregular and does not display the same pattern as the other stations. Kaldárholt is located quite close to the source fault of the June 17 earthquake. In summary, the preseismic spikes are observed at Öxnalækur, Flúðir, Bakki, and possibly at Laugaland. The preseismic low is seen at Öxnalækur, Selfoss, Laugaland, and subdued at Bakki. The coseismic drop is observed at Öxnalækur, Laugaland, and possibly at Selfoss. At Kaldárholt and Flúðir the sampling was discontinued because of the coseismic pressure drop in the geothermal system. The radon values had returned to normal at all stations about three months after the earthquakes. The pressure in the geothermal systems also returned to normal at about this time. 7. Discussion The changes in radon concentration at our stations occur on a regional scale as shown by the similarity between the time series in Figure 3. The distance between the stations Selfoss and Laugaland is 27 km. This argues for a common cause of the changes. A meteorological cause is considered highly unlikely. The samples are taken from deep geothermal boreholes. No evidence for correlation with precipitation has been found. Seasonal effects are also considered unlikely. Radon time series are available for the same boreholes for the time period Seasonal effects were only seen at one of the stations (Bakki) and only for a part of the observation period (JóNSSON, 1994). All the boreholes are producing geothermal holes, used for house heating. The production is at a maximum during the winter months. Therefore, if a seasonal effect is responsible the radon concentration is expected to be low in the winter and high in the summer. This is not consistent with the changes observed in 2 (Fig. 3). The most robust result of this study is the demonstration of the coseismic drop in radon concentration and its postseismic return to previous values. Because of its temporal correlation with observed changes in groundwater pressure, we suggest that there is a causal relationship between these parameters. We note, however, that the radon concentration at all stations dropped during the earthquakes, regardless of whether they were located within areas of increasing or decreasing water pressure. It would seem that both increasing and decreasing groundwater pressure leads to a decrease in radon flow from the crustal rocks. This may not be as unreasonable as it seems at first sight. BJöRNSSON et al. (21) and JóNSSON et al. (23) point out that the pressure variations show a spatial pattern consistent with stress changes due to the faulting during the earthquakes. Pressure increases in areas where coseismic volumetric compression occurs in the crustal rocks. Pressure decreases where volumetric expansion occurs. The volumetric changes and water pressure are related through the porosity of the rock which in this case is to a large degree due to fractures. High water pressure is thus caused by the closure of cracks. Since the

9 Radon Changes and the South Iceland Earthquakes of June J F M A M J J A S O N D Öxnalækur 2 15 J F M A M J J A S O N D Hlemmiskeið Kaldárholt Laugaland Flúðir Selfoss 3 2 Bakki N day N day Figure 3 Time series of radon activity at the seven sampling stations. The activity is given in counts per hour. 226 counts per hour correspond to 1 Bq/l of radon in the water. The data points are connected by a thin line. The dashed lines show a seven-point running average of the data. Vertical bars with stars on top give the time of significant earthquakes in the area. Their heights are proportional to the excess magnitude of the events, as defined by equation (2) and depend on the earthquake magnitude and epicentral distance to the radon station. release of radon into the water takes place across fracture walls, the closure of fractures leads to reduced radon concentrations in the water. In the areas where coseismic dilatation takes place, increasing pore volume leads to a drop in pressure which inhibits the flow of water out of the rock. This will also lead to a reduced flux of radon out of the crustal rocks. Chemical components or physical parameters other than radon have not been monitored at our stations. However, a multi-parameter approach is highly desirable for a more meaningful interpretation of the causes of the changes as shown by the work of CLAESSON et al. (24, 27) in North Iceland.

10 P. Einarsson et al. Pure appl. geophys., 8. Conclusions Four different patterns can be identified in the radon time series within the South Iceland Seismic Zone in association with the earthquake sequence of June 2: 1. Pre-seismic decrease of radon. Anomalously low values were measured in the period days before the earthquakes. 2. Preseismic increase. Positive spikes appear in the time series days prior to the earthquakes. 3. Coseismic step. The radon values decrease at the time of the first earthquake. This is most likely related to the coseismic change in groundwater pressure observed over the whole area. 4. Postseismic return to preseismic levels about three months after the earthquakes, probably also linked with the pressure equilibration in the geothermal systems. In view of the positive results of the project, we are developing and testing a new, automatic radon instrument, Auto-Radon, based on the same design that continuously monitors the radon concentration in the geothermal groundwater (THEODóRSSON and GUDJóNSSON, 23; JóNSSON et al., 27). The instruments are located at the drill hole stations, measuring radon four times a day. Acknowledgements The radon programs in South Iceland have been supported by grants from several agencies, including the Icelandic Research Council, the SEISMIS Project, and the European Union under the projects PRENLAB and PREPARED. Numerous persons have participated in this radon project and measurements. We particular like to mention Gísli Jónsson and the attendants of the sampling sites in South Iceland, Guðlaugur Sveinsson, Stefán Ólafur Ólafsson, Valdimar Þorsteinsson, Vilhjálmur Eiríksson, Guðrún Magnúsdóttir, Olgeir Engilbertsson, and Hannes Bjarnason. REFERENCES ÁRNADóTTIR, Th., HREINSDóTTIR, S., GUðMUNDSSON, G., EINARSSON, P., HEINERT, M., and VöLKSEN, C. (21), Crustal deformation Measured by GPS in the South Iceland Seismic Zone due to two Large Earthquakes in June 2, Geophys. Res. Lett. 28, ÁRNADóTTIR, Th., JóNSSON, S., PEDERSEN, R., and GUDMUNDSSON, G. (23), Coulomb stress changes in the South Iceland Seismic Zone due to two large earthquakes in June 2, Geophys. Res. Lett. 3, doi:1.129/ 22GL16495, no. 5. ÁRNADóTTIR, Th., GEIRSSON, H., and EINARSSON, P. (24), Coseismic stress changes and crustal deformation on the Reykjanes Peninsula due to triggered earthquakes on June 17, 2, J. Geophys. Res. 19, B937, doi:1.129/24jb313.

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