Ragnar Slunga

Seismotectonic analysis of the Tjörnes Fracture Zone, an active transform fault in north Iceland

The Tjornes Fracture Zone is a transform fault connecting the rift zone of the Kolbeinsey Ridge with that of north Iceland. The main transform motion takes place on the Husavik-Flatey Fault, a major 7–9 Myr old right-lateral fault. In addition to this fault, there are two major seismic lineaments associated with the Tjornes Fracture Zone; the Grimsey lineament and the Dalvik lineament. These lineaments are marked by concentrations of seismicity with the largest earthquakes reaching magnitude 7. The maximum depth of earthquakes is 10–12 km and increases with distance from the spreading axis. We determined accurate relative locations and focal mechanisms of more than 800 earthquakes in 62 clusters on the principal seismic lineaments. The estimated relative location uncertainty for most of the relocated earthquakes is 2–20 m. The best fitting plane through each cluster is assumed to coincide with the fault plane of the group of earthquakes. For clusters near the Husavik-Flatey Fault the fault planes are right-lateral and strike N122°E–N140°E, similar to the overall strike of the Husavik-Flatey Fault. This agrees with the right-lateral displacement on the fault as well as with field observations of numerous transform-parallel right-lateral faults associated with the main fault. By contrast, earthquake clusters on the lineaments of Grimsey and Dalvik define (mostly) left-lateral planes striking roughly north-south, i.e., at 40°–90° to the overall trend of these lineaments. Field observations show that left-lateral, north-south trending fault planes are also common in the on-land parts of the Dalvik lineament. The different style of faulting probably represents transform faults at different stages of development.

The SIL data acquisition system—at present and beyond year 2000

Abstract The South Iceland Lowland (SIL) data acquisition system presently consists of 33 digital, three component seismic stations connected to a common data center. The automatic, on-line, earthquake analysis performed by the SIL network can be divided into three categories. (1) Single-station analysis performed at the site stations producing information about all incoming phases. A short message with data on the phase is sent to the center. (2) Multi-station analysis done at the center, using the phase reports from the stations and producing information about all detected events including estimates of location, magnitude and fault plane solutions. (3) Alert reporting to notify the operators of the network in cases of a priori defined changes in parameters derived from the single- and multi-station analysis. The system is designed for maximum automatic operation and minimum operational cost and has shown to be capable of automatic evaluation of more then 1000 earthquakes per day or episodically several earthquakes per minute. While no attempt is made to detect and locate teleseismic events, teleseismic data is automatically saved, based on e-mail messages from global seismological networks. Groups of events are analyzed using correlation techniques to obtain accurate absolute and relative locations of earthquakes with similar waveforms. In some areas within the network, most of the earthquakes correlate very highly with each other. Based on this a new approach is being taken regarding the automatic operation of the network. A geographically indexed data base will be created where different classes of earthquakes are stored. As new earthquakes are recorded by the network the system automatically looks for similar waveforms in this data base and, if found, takes the onset and first motion direction picks from there. The algorithm is planned to be implemented in late 1999. New methods have been developed to estimate the stress tensor based only on the microearthquake focal mechanisms and accurate relative locations. This is planned to be implemented into the automatic on-line procedures. Methods and related software are being developed for real-time monitoring of fault movements based on the high accuracy locations and fault plane solutions.

Stress tensor inversion using detailed microearthquake information and stability constraints: Application to Ölfus in southwest Iceland

Using detailed microearthquake data, we present a stress tensor inversion scheme with new methods for selecting the fault planes and allowing for errors in the focal mechanisms. The nonuniqueness of earthquake focal mechanisms is accounted for in our inversion scheme through the introduction into the inversion of a range of well-fitting focal mechanisms for each event. The range of focal mechanisms significantly improves the quality of the estimated stress tensor. Relative localization of clusters of microearthquakes is used to obtain information about which nodal plane could be correct fault plane. The clusters frequently fall on a common fault plane, and if there are acceptable focal mechanisms where one nodal plane has orientation similar to the common plane, we assume this is the correct fault plane for the event. If there is no predefined fault plane, we utilize a simple Mohr-Coloumb failure criterion to obtain a physical choice of fault plane between the two nodal planes in the focal mechanism. The nodal plane with highest relative instability is chosen as the fault plane. Differences between the instability and the standard slip angle criterion are investigated. The new inversion scheme has been applied to microearthquake data from the Olfus area in the vicinity of the southwest Iceland triple junction. We estimate an oblique strike-slip state of stress, maximum horizontal stress at N30°E, and minimum horizontal stress at N60°W, with significant normal faulting influence. The instability fault selection criterion predicts very well the orientation of faults mapped by relative localization.

The Baltic Shield earthquakes

Abstract The Baltic Shield area is a low-seismicity area with few seismic events exceeding M L = 4. Since 1979 dense regional seismic networks have been in operation and have allowed accurate location, including focal depths and fault plane solutions, of more than 200 earthquakes in the range M L = 0.6–4.5. The location of the southern Sweden seismicity shows that a long part of the Protogine zone, the border between the younger western and older eastern crustal rocks, marks a change in seismic activity. The focal depths of the earthquakes indicate three seismically different crustal layers: upper crust (0–13 km in northern Sweden, 0–18 km in southern Sweden), middle crust down to 35 km, and the seismically quiet lower crust. The Gutenberg b -value is smaller for the middle crust than for the upper crust. The fault plane solutions show that strike-slip is dominant. Along the Tornquist Line (the southwestern border of the Baltic Shield area) normal faulting occurs. In northern Sweden reverse faulting is more common than normal faulting. The stresses released by the earthquakes show a remarkable consistency considering the small sizes of the earthquakes. The data are consistent with a N60W direction of the regional principal compression, supporting the view that the Baltic Shield seismicity is related to plate tectonic processes rather than to uplift. The spatial distribution of the Baltic Shield seismicity is consistent with a model in which the earthquakes are considered as sudden breakdowns of asperities of normally, stably sliding faults. Aseismic sliding is estimated to be more than 20,000 times more extensive than seismic sliding. The rate at which southern Sweden presently deforms horizontally is estimated to be ⩾ 1 mm/yr.

Single and joint fault plane solutions for microearthquakes in South Iceland

Abstract We obtain fault plane solutions for a swarm of twelve events and two single events ranging in magnitude between −0.3 and 3.1. The events were recorded by a local network in South Iceland, consisting of eight three-component stations with approximately 30 km between the stations. For events larger than M L −1 the solutions are well constrained. The non-uniqueness of the fault plane solutions increases with diminishing magnitude. To enhance the uniqueness of the fault plane solutions for the earthquake swarm, we search for mechanisms with a common fault plane but possibly varying slip directions in that plane. Data for all the events are consistent with faulting on a common plane. Relative location of the events shows that they all occurred less than 11 m from a single plane. Fault plane solutions for all the analysed events of the swarm, as well as the two single events, are consistent with strike-slip-dominated faulting on faults striking roughly north-south. The results are in agreement with the strikes of mapped surface fractures and focal mechanisms of larger earthquakes in the area. This indicates that the microseismicity is related to the same pattern of regional deformation as the major earthquakes in South Iceland.

Perturbation of stress and oceanic rift extension across transform faults shown by earthquake focal mechanisms in Iceland

Abstract Major sources of stress perturbation are expected in the brittle crust, and documented by local studies in Iceland. Whether or not a significant average stress state may emerge from regional-scale inversion of very large sets of focal mechanisms is thus subject to doubt. We carried out stress inversion of double couple focal mechanisms recorded by the Icelandic seismological network within a set of 126 588 shallow earthquakes from July 1991 to July 1999. We performed mass inversion of two main data sets of 12 191 and 71 889 focal mechanisms in the transform zone areas of North and South Iceland respectively. The inversion reveals surprisingly high levels of consistency within these data sets, with regard to the expected dispersion. Adopting a threshold value of +40% in an individual fit scale from −100% (total misfit) to +100% (perfect fit), the proportion of acceptable data is as high as 78% for 65 571 focal mechanisms of the major regime in these two areas (53% for 18 519 mechanisms of the minor regime). The major stress regimes thus calculated show nearly vertical intermediate principal axes and the azimuths of extension (minimum stress σ 3 ) are 065° in the Tjornes Fracture Zone and 140° in the South Iceland Seismic Zone. With respect to the 105° azimuth of plate separation, these directions show nearly symmetrical angular deviations: 40° anticlockwise in the right-lateral transform zone area of North Iceland and 35° clockwise in the left-lateral one of South Iceland. Such large deviations reveal first-order stress perturbation in the areas where major rift offset resulted from Late Cainozoic rift jumps related to plate boundary migration with respect to the Iceland Mantle Plume.

Tomographical mapping of the lithosphere and asthenosphere beneath southern Scandinavia and adjacent areas

Abstract In this paper we report on a seismic tomographical survey of the lithosphere-asthenosphere beneath southern Scandinavia and adjacent areas. The ACH-inversion technique has been used, and the observational data in the form of P-wave travel-time residuals have been obtained from seismogram records from networks in Denmark, southern Norway and southern Sweden. The volume under investigation, subdivided into four levels (layers) of 7 × 7 knots (blocks) each for estimation of velocity perturbations, covers the area 54°–62°N and 7°–20°E and depth range 0–600 km. Major findings are as follows. Level 1 (0–100 km): pronounced velocity lows beneath southern Jutland, Zealand (the Variscan Foredeep) and the Sveco-Norwegian province of southeastern Norway (1500 Ma old). The Svecofennides of southeastern Sweden exhibit relatively high velocities, while the Sveco-Norwegian province of southwestern Sweden (1700 Ma old) is characterized by moderately high velocities. The transition between velocity highs and lows coincides with the Tornquist-Teisseyre Line (TTL) (or Fennoscundian Border Zone). Level 2 (100–300 km): velocity anomalies are less pronounced at this level, although velocity lows are found beneath the northern Jutland-Zealand area and slightly northwestwards of the Oslo Rift. The only velocity high is found beneath the southeasternmost part of Sweden. The TTL does not appear to have a clear counterpart in the velocity anomaly pattern in this depth range, although a clearly unambiguous answer here would require far more extensive observational data—a research goal of the future. Level 3 (300–500 km) and Level 4 (500–600 km): velocity anomalies are weak, which is not unexpected for the upper mantle beneath shield areas.

Fault mechanisms of Fennoscandian earthquakes and regional crustal stresses

Abstract The eleven currently available (May 1980) source mechanisms of Fennoscandian earthquakes are analysed. The orientations of the horizontal stresses involved in the faulting are estimated from the fault solutions. The NW-SE orientation of the largest horizontal compression found in a large number of various investigations in continental Europe north of the Alps, is confirmed by the eleven source mechanisms. This result is shown to be statistically significant. The dominating type of faulting is strike-slip on close to vertical fault planes. The agreement between these earthquakes and the European regional stress field strongly suggests that the Fennoscandian earthquakes are related to tectonic forces.

Estimates of current Icelandic stress tensors from the inversion of microearthquake fault plane solutions

Estimates of current Icelandic stress tensors from the inversion of microearthquake fault plane solutions