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The South Iceland Seismic Zone (SISZ), Reykjanes Peninsula (RP), plate motion directions, transform motion along SISZ and opening are indicated. The outlines of the Iceland plume (hotspot) at 300-400 km depth are shown by the purple contour.

The South Iceland Seismic Zone (SISZ), Reykjanes Peninsula (RP), plate motion directions, transform motion along SISZ and opening are indicated. The outlines of the Iceland plume (hotspot) at 300-400 km depth are shown by the purple contour.

Contexts in source publication

Context 1
... ongoing PREPARED-project is a multidisciplinary project to collect the experiences gained in the June 2000 earthquakes and for modelling the processes with the aim of enhancing hazard assessments and warnings for dangerous earthquakes ( Stefánsson et al. 2002). The westward prolongation of the SISZ is on the Reykjanes Peninsula (RP) (Figure 1) where destructive earthquakes also occur. Even if its tectonic character is different from SISZ it is necessary to deal with SISZ and RP as a whole, as well as with the adjacent volcanoes and rift zones. ...
Context 2
... relative plate motion across the ridge is estimated 18.9 mm/+-0.5 mm/year in direction 102.9°+-1.1° east of north ( DeMets et al. 1990;DeMets et al. 1994) (Figure 1). ...
Context 3
... structure and the activity of the plate boundary are strongly influenced by the Iceland Plume beneath Central Iceland ( Tryggvason et al. 1983;Stefánsson and Halldórsson 1988) (Figure 1). The relative motion of the plate boundary with respect to the plume leads to ridge jumps, propagating rifts, oblique rifts, and other complexities. ...
Context 4
... geodetic measurements show that most of the spreading is occurring across the Eastern Rift, with about 10% taken up across the Western Rift at the present time, i.e. during the last decade or so ( LaFemina et al. 2005). The SISZ, where the two large earthquakes of year 2000 occurred, is a transform zone between these two rifts as seen in Figure 1. ...
Context 5
... WVRZ branches off the main plate boundary at the Hengill triple junction and extends for about 100 km to the NE with a trend of about 30° (Figure 1). At least 3 volcanic systems have been identified within this zone but their boundaries are not clear, partly because of the glacial cover of Langjökull. ...
Context 6
... crustal deformation prior to the June 2000 earthquakes is shown by a surface velocity field map of Southwest Iceland obtained from campaign and continuous GPS data collected between 1992 and 2000 ( Figure 10). The pre-seismic velocity field is affected by plate spreading, inflation at Hengill and Hekla Volcanoes and subsidence at the Svartsengi geothermal area. ...
Context 7
... pre-seismic velocity field is affected by plate spreading, inflation at Hengill and Hekla Volcanoes and subsidence at the Svartsengi geothermal area. The velocity field shown in Figure 10 is used to calculate the strain field in South Iceland in the pre-seismic period 1992-2000. The shear strain rate is composed of several components of the gradient of the velocity field in different directions. ...
Context 8
... shear strain rate is composed of several components of the gradient of the velocity field in different directions. The northward gradient of the eastward velocity component, i.e. dVe/dN, is most indicative of left-lateral motion on an EW transform, or right-lateral motion on an NS fault, as observed in the SISZ (see Figure 11). Figure 11 shows that the strain rate is high in the center of the SISZ, and decreases as we move north or south from 64°N. ...
Context 9
... northward gradient of the eastward velocity component, i.e. dVe/dN, is most indicative of left-lateral motion on an EW transform, or right-lateral motion on an NS fault, as observed in the SISZ (see Figure 11). Figure 11 shows that the strain rate is high in the center of the SISZ, and decreases as we move north or south from 64°N. This pattern demonstrates that strain was concentrated in the SISZ prior to the June 2000 earthquakes. ...
Context 10
... y is the distance perpendicular to the plate boundary. Figure 12. GPS station velocities in the SISZ parallel to the plate boundary (blue squares) as a function of distance along a NS profile (A-A'). ...
Context 11
... from nonlinear inversion estimating 4 model parameters (deep slip rate, locking depth, velocity shift and profile shift) are shown with solid lines. (Figure 10b from Árnadóttir et al. 2006). ...
Context 12
... the pre-seismic GPS network is not dense enough to confine the width of the SISZ, Figure 12 shows that the strain rate is highest in a 20 km wide strip along the SISZ. This observation is to be compared to the concentration of microearthquakes and length of mapped NS faults within a 10-15 km wide zone in the SISZ. ...
Context 13
... Árnadóttir et al. (2006) generate more complex model to explain the pre-seismic GPS velocity field. Their preferred plate boundary kinematic model is a 3D dislocation and point source model with left-lateral slip along the plate boundary on the Reykjanes Peninsula and below the SISZ, and opening across the Western and Eastern Volcanic Zones ( Figure 13). The model parameters cannot be uniquely determined from the GPS data. ...
Context 14
... 1998). Figure 14 shows that the inflation in Hengill and left-lateral slip below 7 km depth along the plate boundary on Reykjanes Peninsula combine to increase the Coulomb failure stress on NS, right-lateral strike-slip faults on the Reykjanes Peninsula, such as ruptured in the triggered earthquakes on June 17, 2000. The Hengill inflation appears to have acted to decrease the Coulomb failure stress on NS faults east of Hengill. ...
Context 15
... m 3 /year, to the rate of seismic moment release in historical times, estimated as 2.0-2.3x10 7 m 3 /year ( Stefánsson and Halldórsson 1988;Hackman et al. 1990 (Figure 15) they conclude that the areas of large co-seismic stress increase east of the June 17 and west of the June 21 ruptures, continue to be loaded by the post-seismic deformation. ...
Context 16
... periods selected are 1999 to June 17, 2000, June 17 to June 21,2000 and June 21 to end of year 2000. Significant evolution of stress axes directions are seen along the NS strike of the fault but also with depth (see Figure 16). These results show a modification of the state of stress in the southwestern part of the Hestfjall Fault occurring after the earthquake of June 21, 2000 and a larger deviation of the stress at 5 to 10 km depth, i.e. ~40° deviation, instead of ~20° deviation at 0-5 km depth. ...
Context 17
... scale estimations studied by Lund et al. (2005) show that in the area of the first large earthquake, June 17, the stress state in the lower part of the crust is different from the upper part in the period from 1996 to just before the initial large earthquake. In this period the earthquakes deeper than 7.5 km show great variations in the maximum horizontal stress (Figure 17). It is inferred also that at 7.5 km depth there is a transition from predominantly strike-slip at shallow depths to another stress regime below, possibly indicating more normal faulting. ...
Context 18
... horizontal compressions with time from 1991 to 2000 in the depth range below 7.5 km have high standard deviations, suggesting high- level of heteorogeneity, making stress inversions unsecure. However, the horizontal compressions show large spatial variations as seen in Figure 17. These variations in standard deviations and apparent horizontal compressions seem to persist from 1996 until the release of the 2000 earthquakes. ...
Context 19
... variations in standard deviations and apparent horizontal compressions seem to persist from 1996 until the release of the 2000 earthquakes. In the earthquake on June 21 there are no such depth variations as observed in WP2.4 (Figure 18). In the aftershocks there are interesting variations in the horizontal maximum stress east of the nothern part of the fault, where EW compressions are observed. ...
Context 20
... of these descriptions appears in Figures 19, 20 and 21. Figures 19 and 20 describe positions of microearthquakes in the area 10 weeks prior to the first large earthquake and show that the locations of microeathquakes start to move fast up and down, south and north, along the fault direction, 1-2 weeks before the earthquake. It starts at 11 km depth and near the northern end. ...
Context 21
... motion is hampered a day before the earthquake by the hard core asperity, which is gradually fractured in earthquakes concentrated there, mostly during the last day before the earthquake. A simple algorithm to visualize this motion is shown in Figure 21, which simply accumulates the distances between consecutive microearthquakes on weekly basis after each new microearthquake, i.e. at each earthquake point we see from the abscissa how much earthquakes have been moved along the fault plane during one week before it. Selecting a day for the plot would make the "long-term precursor" weaker but the asperity nucleation precursor stronger. ...
Context 22
... build-up time for enough fluid amount for triggering a large earthquake seems to be a few hundred years, may be of the order of 300 years. This is indicated already in Figure 1, if the two 2000 earthquakes are skipped. These earthquakes were relatively small, being also early in the 140 years cycle (Halldórsson and Stefánsson 2006). ...

Citations

... The earthquake activity in the RPOR is however found to be very different from what is seen in the SISZ. The average thickness of the seismogenic crust ranges from 5 to 9 km, with the majority of earthquakes occurring at a hypocentral depth of 4-6 km (Klein et al. 1973, 1977, Stefansson et al. 1993, Tryggvason et al. 2002, Stefánsson et al. 2006. Although the RPOR has historically been considered less active than the SISZ, there are reliable reports that large earthquakes (i.e., 6.0≤ ≤6.5) can occur there (e.g., the 6.2 1929 Hvalhnúkur and the 6 1968 ...
... proposed a seismic moment rate release of 1.6×10 7 m /yr using a 100 years catalogue, and subsequently, Stefánsson et al., (2003) revised it as 1.2-1.7×10 7 m /yr using a 140 years catalogue (Árnadóttir et al. 2005(Árnadóttir et al. , Stefánsson et al. 2006. In contrast, the seismic moment rate estimates for the RPOR computed from a compilation of local magnitude earthquakes greater than four recorded between 1926 and 2006, reported to be in the range of ~0.4-0.7×10 ...
... We note however that the fault lengths inferred for historical earthquakes in the region are all grossly overestimated by scaling laws from other shallow interplate regions (and their widths far exceed the maximum reported seismogenic depths)(Wells & Coppersmith 1994, Somerville et al. 1999, Mai & Beroza 2000, Hanks & Bakun 2002, Ellsworth 2003, Leonard 2010, Yen & Ma 2011, Thingbaijam et al. 2017. None of those came close to predicting the small fault areas typical of recent SISZ earthquakes, Roth 2004, Stefánsson et al. 2006, Dubois et al. 2008, Hreinsdóttir et al. 2009, Decriem et al. 2010. These studies show that earthquakes in the SISZ are associated with a relatively large global stress drop for a given magnitude, resulting in a much larger slip on relatively small fault planes, compared to strike-slip earthquakes of shallow crustal interplate regions worldwide. ...
Article
The largest earthquakes in Iceland occur in the South Iceland Seismic Zone (SISZ) and the Tjörnes Fracture Zone in the Northeast. With the latter being primarily offshore, the seismic risk in Iceland is highest in the relatively densely populated SISZ. Past probabilistic seismic hazard assessment (PSHA) efforts in Iceland have however been based on statistical analyses of various historical earthquake catalogues, and limited ground motions models (GMMs), all subject to varying types and degrees of uncertainties. Moreover, they relied on simplistic source descriptions and largely ignore that the unique ‘bookshelf’ strike-slip fault system of the SISZ extends along the plate margins towards the West and over the entire Reykjanes Peninsula Oblique Rift (RPOR) zone. Namely, the bookshelf fault system in Southwest Iceland is twice as long as previously thought and it dominates the strain release of transcurrent plate motion in Southwest Iceland, having potentially important implications for PSHA. In this study therefore, we propose a new 3D finite-fault model of the Southwest Iceland bookshelf transform zone. The model has been calibrated on the basis of first principles to the rate of transcurrent plate motions across the transform zone and constrained by the salient features of the fault system geometry as reported in the literature. We model the systematic spatial variability of the seismogenic potential along the zone by its provisional subdivision into six distinct zones. The fault system model allows both for deterministic and random fault locations, with each realization completely specified in terms of the maximum expected magnitude of each fault, its maximum dimensions, and its long-term slip rate. The variability of the model has been estimated through sensitivity analyses of its key parameters. The total seismic moment rates produced by the fault system model are completely consistent with those reported in the literature. The new model allows the derivation of simple but self-consistent zone-specific Gutenberg-Richter (GR) relationships, and the total long-term seismic activity predicted by the new 3D fault system model effectively explains the historical earthquake catalogue of the SISZ-RPOR transform zone in Southwest Iceland. We are therefore confident that the model can serve as the foundation for future time-independent physics-based PSHA for Southwest Iceland. Moreover, the consistency and versatility of the model allows its application in conventional approaches to PSHA, which has the potential of bridging the gap between physics-based and conventional approaches to PSHA in Southwest Iceland. Such efforts will improve our understanding of the key elements that affect the hazard, thus improving the reliability of hazard estimates, with important practical implications for the optimized assessment of seismic risk.
... The earthquake activity in the RPOR is however found to be very different from what is seen in the SISZ. The average thickness of the seismogenic crust ranges from 5 to 9 km, with the majority of earthquakes occurring at a hypocentral depth of 4-6 km (Klein et al. 1973, 1977, Stefansson et al. 1993, Tryggvason et al. 2002, Stefánsson et al. 2006. Although the RPOR has historically been considered less active than the SISZ, there are reliable reports that large earthquakes (i.e., 6.0≤ ≤6.5) can occur there (e.g., the 6.2 1929 Hvalhnúkur and the 6 1968 ...
... proposed a seismic moment rate release of 1.6×10 7 m /yr using a 100 years catalogue, and subsequently, Stefánsson et al., (2003) revised it as 1.2-1.7×10 7 m /yr using a 140 years catalogue (Árnadóttir et al. 2005(Árnadóttir et al. , Stefánsson et al. 2006. In contrast, the seismic moment rate estimates for the RPOR computed from a compilation of local magnitude earthquakes greater than four recorded between 1926 and 2006, reported to be in the range of ~0.4-0.7×10 ...
... We note however that the fault lengths inferred for historical earthquakes in the region are all grossly overestimated by scaling laws from other shallow interplate regions (and their widths far exceed the maximum reported seismogenic depths)(Wells & Coppersmith 1994, Somerville et al. 1999, Mai & Beroza 2000, Hanks & Bakun 2002, Ellsworth 2003, Leonard 2010, Yen & Ma 2011, Thingbaijam et al. 2017. None of those came close to predicting the small fault areas typical of recent SISZ earthquakes, Roth 2004, Stefánsson et al. 2006, Dubois et al. 2008, Hreinsdóttir et al. 2009, Decriem et al. 2010. These studies show that earthquakes in the SISZ are associated with a relatively large global stress drop for a given magnitude, resulting in a much larger slip on relatively small fault planes, compared to strike-slip earthquakes of shallow crustal interplate regions worldwide. ...
Presentation
Geohazards, volcanic and seismic activity, are pronounced in Iceland due to its location on the Mid-Atlantic Ridge. The strongest earthquakes in Iceland occur primarily in two transform zones, the Tjornes fracture zone in the North, and the South Iceland seismic zone (SISZ) and Reykjanes Peninsula oblique rift (RPOR) in the South. The left-lateral transcurrent motion across the SISZ-RPOR is accommodated by “bookshelf faulting” on an array of NS near-vertical dextral strike-slip faults oriented near perpendicular to the vector of plate motions. In this study, we have developed new 3D physics-based bookshelf fault system models for the SISZ-RPOR. The models have been calibrated to the steady-state relative velocity of plate extension in southwest Iceland and are constrained by the geometry of the fault system and its spatially variable seismogenic potential. We model this spatial variability by through distinct subzones of faulting and allow for both deterministic and random fault locations across the SISZ-RPOR. The fault system models are fully specified in terms of fault locations, fault dimensions, expected maximum magnitudes, long-term slip rates, and moment rates on individual faults. The long-term cumulative rate of seismic moment predicted by the new fault system models are fully consistent with those calculated from the various long-term earthquake catalogues for the region. The slip-rate along the zone is shown to vary systematically and increasing towards the west. This allows us to designate average slip-rates for each subzone and calibrate zone-specific magnitude-frequency relationships (i.e., Gutenberg-Richter). In other words, we present seismic area source zones of different maximum magnitudes and distinct a and b-values that are equivalent to the activity of the 3D physics-based fault system model of this study. The cumulative seismicity rate of the area source zones is in complete agreement with the earthquake catalogues, including the new ICEL-NMAR. The findings presented here form the basis for physics-based probabilistic seismic hazard assessment of the bookshelf fault system in Soutwest Iceland.
... The last part of the moment was released farther east in 1912 (i.e., 16 years later; Fig. 4), with a magnitude 7 earthquake. During a period of eight years before the 1912 earthquake, smaller earthquakes were felt near the fault area (Stefánsson and Halldórsson, 1988;Stefánsson, Gudmundsson, and Roberts, 2006;Stefánsson, Bonafede, et al., 2006). The 1912 earthquake did not trigger significant earthquakes to the west in the SISZ. ...
... The spatial development of microseismic activity just described here has a tendency of episodic shallowings. In light of the emerging ideas that high (near-lithostatic) porefluid pressures can be brought upward with time in response to strain (and modify the fracturing conditions there), a possible explanation of the episodic shallowing is the occurrence of upward pulses of high pore pressure from below the brittle crust (Zencher et al., 2006;Stefánsson, Bonafede, et al., 2006; see also the modeling described in the section Emerging New Model for Upward Migration of Magmatic Fluids). ...
Article
Full-text available
The purpose of this paper is to describe and to model long- and short-term processes that preceded two large earthquakes that occurred in 2000 in the South Iceland seismic zone (SISZ). The results are based on some key findings from multinational earthquake-prediction research projects in the SISZ since 1988. It involves a fusion of significant old and new results. The paper presents a new hypothesis for earthquake build-up processes in the region, followed by a discussion on how interseismic and preseismic observations can be explained by this model and a discussion about a plausible earthquake cycle in light of the new hypothesis. The research described here started in 1988 with the South Iceland Lowland (SIL) project. A significant outcome was the development of a seismic acquisition and evaluation system, the SIL system, retrieving source information from earthquakes down to magnitude zero. The research continued with the use of more multidisciplinary and multinational projects, revealing new information about crustal processes related to large earthquakes in the area. The validity of the work was tested in 2000 when two magnitude 6.6 (M-S) earthquakes occurred in the central part of the SISZ. The earthquakes had a long-term assessment of place. A short-term warning ("within short") about the location and size of a probable impending earthquake was issued 25 hours before the second earthquake.
... (b) Tectonic lineament map as observed from aerial photographs (modified from Khodayar and Einarsson, 2006). Clifton and Einarsson, 2005), epicentre of June 21, 2000 earthquake (Stefánsson et al., 2006), epicentres and damage zones of historical earthquakes (Björnsson, 1976;Einarsson and Björnsson, 1979;Björnsson, in press;Einarsson and Eiríksson, 1982), and hot springs (Iceland Geosurvey-ÍSOR) in the SISZ. (b) Riedel shear model and schematic location of geothermal manifestations. ...
Article
We studied fracture-controlled geothermal fields in the Hreppar Rift-Jump Block (HRJB), a micro-plate bounded by two NNE rifts and the E–W transform zone of the South Iceland Seismic Zone (SISZ). Distinguishing whether the extensional rift swarm or the transform zone shear fractures host the geothermal activity is challenging. GPS mapping of 208 springs and tectonic analysis indicate that six Riedel shear fracture sets of an older transform zone in the HRJB are permeable. Northerly dextral strike-slip faults are the principal permeable faults, although the highest discharge and temperature are found at their intersections with other fracture sets. Two northerly faults from the HRJB connect to the source faults of the major 1784 and 1896 earthquakes within the active SISZ. The 1784 earthquake caused pressure changes as far north as the studied springs, indicating that earthquakes keep faults permeable over hundreds of years.
... This wave path effect is in full agreement with the fault directions of microearthquakes, which is a source effect. Both effects are here interpreted as indicating high fluid pressures migrating from the bottom of the seismogenic zone as inferred in the SISZ (Stefansson et al. 2006a). ...
Article
Full-text available
The Tjörnes facture zone (TFZ) connects the EW extension of the Mid-Atlantic ridge north of Iceland to the extension of the North volcanic zone (NVZ) of Iceland. Earthquakes up to magnitude 7 (Ms) can occur in TFZ, volcanic eruptions have been observed and large crustal deformations are expected in similar way as have been observed in the NVZ. Most of the zone is below ocean, which limits the historical information and geological observations. For studying the dynamics of the zone we must rely on interpretation and modelling based on seismic observations, especially on microearthquake observations for the last 10 years. In this paper we demonstrate how microearthquakes can be applied to map the details of the plate boundary, and how this information can be applied to find epicenters and fault planes of large historical earthquakes, also how seismic information can be applied in dynamic modelling and to infer spatial and temporal interplay in activity, and to enhance hazard assessment.
... • High pore pressures can migrate from below the brittle/ductile transition to shallower depths. Thus episodes of fluid migrations can increase pore pressures up to lithostatic values and thus increase the instability of the faults (Zencher et al. 2006;Stefánsson et al. 2006). ...
Chapter
Regardless of the nature and origin of hot spots and whether they are fixed or mobile, the fact remains that there is a major thermal anomaly under Iceland, commonly referred to as the “Icelandic hot spot”. The Mid‐Atlantic Ridge migrated westward relative to the Icelandic hotspot, but after a few million years, the westward‐trailing part of the rift gradually became inactive, and a new rift was the apex of the mantle plume. The Icelandic rift is located more than 100 km away from the accretion axis of the oceanic ridge. Present‐day deformations in Iceland are well documented by data acquired through the networks established and developed since the late 1980s. The Icelandic rift presents notable structural differences compared to the submerged parts of the ridge. The main feature of fracturing in the Icelandic rift is the deformation partitioning between normal faults, open fractures and fractures injected by volcanic material.
Article
Full-text available
Since the end of 1993, the digital, automatic seismic system, known as the SIL-system, has acted as the national seismic network in Iceland. The number of stations in the network has increased gradually over the 14 years from 1994 to 2007. The detection capability of the system has increased at the same pace, especially along the volcanic zones. The general seismicity of Iceland, as monitored by the growing network, is presented in this paper. The main activity is, as expected, along the plate boundary, but seismicity has also been observed at intraplate locations. Instead of setting a lower limit to the size of earthquakes to be considered in this paper, changes in the sensitivity are discussed along with the seismic activity. Several major events have been observed during the observation period. In June 2000, two magnitude 6.6 earthquakes occurred in the South Iceland Seismic Zone, followed by a few earthquakes larger than magnitude 5. A few episodes caused by magma injections were recorded and four volcanic eruptions monitored. In addition to the large earthquakes in June 2000, seven earthquakes larger than magnitude 5 occurred in this period, two in Bárðarbunga in the years before the 1996 Gjálp eruption, two near the triple junction between the Reykjanes Peninsula, the Western Volcanic Zone and the South Iceland Seismic Zone, two in the Tjörnes Fracture Zone and one on the Reykjanes Peninsula. Intraplate earthquakes were recorded in the northwestern part of Iceland, Vestfirðir, in 1994 and 2006, and in Guðlaugstungur, between Langjökull and Hofsjökull, in 2004. Additionally, earthquakes are located near Surtsey every year. Two episodes of activity have been recorded in the Esjufjöll central volcano in the southeastern part of Vatnajökull, and several swarms have been detected under Öraefajökull. All the main ice caps cover seismically active central volcanoes, explaining the high seismicity beneath them.
Article
Full-text available
A swarm of earthquakes of magnitudes up to M L = 3.8 stroke the region of West Bohemia/Vogtland (border area between Czechia and Germany) in October 2008. It occurred in the Nový Kostel focal zone, where also all recent earthquake swarms (1985/1986, 1997, and 2000) took place, and was striking by a fast sequence of macroseismically observed earthquakes. We present the basic characteristics of this swarm based on the observations of a local network WEBNET (West Bohemia seismic network), which has been operated in the epicentral area, on the Czech territory. The swarm was recorded by 13 to 23 permanent and mobile WEBNET stations surrounding the swarm epicenters. In addition, a part of the swarm was also recorded by strong-motion accelerometers, which represent the first true accelerograms of the swarm earthquakes in the region. The peak ground acceleration reached 0.65 m/s2. A comparison with previous earthquake swarms indicates that the total seismic moments released during the 1985/1986 and 2008 swarms are similar, of about 4E16 Nm, and that they represent the two largest swarms that occurred in the West Bohemia/ Vogtland region since the M L = 5.0 swarm of 1908. Characteristic features of the 2008 swarm are its short duration (4 weeks) and rapidity and, consequently, the fastest seismic moment release compared to previous swarms. Up to 25,000 events in the magnitude range of 0.5 < M L < 3.8 were detected using an automatic picker. A total of nine swarm phases can be distinguished in the swarm, five of them exceeding the magnitude level of 2.5. The magnitude–frequency distribution of the complete 2008 swarm activity shows a b value close to 1. The swarm hypocenters fall precisely on the same fault portion of the Nový Kostel focal zone that was activated by the 2000 swarm (M L ≤ 3.2) in a depth interval from 6 to 11 km and also by the 1985/1986 swarm (M L ≤ 4.6). The steeply dipping fault planes of the 2000 and 2008 swarms seem to be identical considering the location error of about 100 m. Furthermore, focal mechanisms of the 2008 swarm are identical with those of the 2000 swarm, both matching an average strike of 170° and dip of 80° of the activated fault segment. An overall upward migration of activity is observed with first events at the bottom and last events at the top of the of the activated fault patch. Similarities in the activated fault area and in the seismic moments released during the three largest recent swarms enable to estimate the seismic potential of the focal zone. If the whole segment of the fault plane was activated simultaneously, it would represent an earthquake of M L ~5. This is in good agreement with the estimates of the maximum magnitudes of earthquakes that occurred in the West Bohemia/Vogtland region in the past.
Article
The South Iceland Seismic Zone (SISZ) is an E-W trending active transform zone with high seismicity. N-S trending right-lateral strike-slip faults accommodate the left-lateral transform motion. Using seismological data recorded from 1991 to 2007, we carried out stress inversion of focal mechanisms of 1340 earthquakes that affected the Skard and Leirubakki faults, eastern SISZ. Not only did the inversion show typical deviations of stress across the faults, it also revealed anticlockwise and clockwise rotations of stress axes with time. Numerical models of the Leirubakki Fault show that these rotations are consistent with the mechanical effect of a lowered friction coefficient during the post-seismic period relative to the pre-seismic period. The Skard Fault reveals a more complex behaviour associated with a higher post-seismic friction, resulting from a higher density of pre-existing fracturing and probable stress interaction between faults. Our results suggest that faults where micro-earthquakes occur during neighbouring major seismic events may undergo significant stress changes at the scale of several kilometres and on time-scales of several years.