South Iceland Lowland Research Articles

The Modern Swedish National Seismic Network: Two Decades of Intraplate Microseismic Observation

AbstractThe Swedish National Seismic Network (SNSN) was modernized and rapidly expanded during the period 1998–2012. The network currently operates 68 permanent seismic stations, all with broadband instruments supplying real-time continuous data at 100 samples per second. Continuous data from 10 stations are shared with the international community via Orfeus, and approximately 10 stations of their individual choice are shared with institutes in neighboring countries (Denmark, Finland, Norway, and Germany). The SNSN uses the South Iceland Lowland (SIL) system as the primary system for automatic detection and event definition. In addition, an in-house system based on migration and stacking is used for automatic detection of small events, and implementations of SeisComP (SC) and Earthworm are used primarily for rapid detection of larger regional events. Global monitoring is performed with SC, using approximately 250 global stations, and we operate a continuous rapid risk assessment system serving Swedish crisis management authorities. Since the start of automatic processing in August 2000, the SNSN has recorded and interactively analyzed more than 171,000 seismic events, of which 10,700 were earthquakes with local magnitudes ranging from around −1 to 4.3. The microearthquake activity detected in the last 20 yr has significantly improved the identification and understanding of seismically active structures in Sweden.

Earthquake-Prediction Research and the Earthquakes of 2000 in the South Iceland Seismic Zone

Abstract 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.

Earthquake response of a base-isolated bridge subjected to strong near-fault ground motion

The South Iceland Lowland is an active seismic zone. In May 2008 a magnitude 6.3 ( M w) earthquake struck the area. The 370-m-long base-isolated Oseyrar Bridge situated close to the epicentre was hit by strong ground motion. Concrete stoppers at the top of all piers were badly smashed and all four wing walls of the abutments were severely damaged. The study showed that it is possible with numerical models to back-calculate the observed damage based on recorded ground motion in the area. The recorded loads were larger than prescribed by Eurocode 8 for the site even though the magnitude and location of the earthquake was as expected. The main reason was a near-fault pulse which is not covered in the code. The pulse dominated the bridge response and is therefore important to consider. Finally, an improved design is presented that could have prevented the damage.

Earthquakes, stress, and strain along an obliquely divergent plate boundary: Reykjanes Peninsula, southwest Iceland

We investigate the seismicity and the state of stress along the obliquely divergent Reykjanes Peninsula plate boundary and compare the directions of stress from inversion of earthquake focal mechanisms with the directions of strain rate from GPS data. The seismicity on the peninsula since early instrumental recordings in 1926 shows a systematic change from primarily earthquake swarms in the west to main shock–aftershock sequences in the east. The largest earthquakes on the Reykjanes Peninsula typically occur by right‐lateral slip on N‐S faults and reach magnitude 6 on the eastern part of the peninsula. During 1997–2006 most earthquakes on the Reykjanes Peninsula were located in two areas, Fagradalsfjall and Krísuvík on the central part of the peninsula, as recorded by the South Iceland Lowland (SIL) seismic network. The state of stress estimated by inversion of microearthquake focal mechanisms from the SIL catalogue is mainly oblique strike slip, with a tendency toward a normal stress state. Mapping the directions of the least compressive horizontal stress (Shmin) shows an average Shmin direction of N(120±6)°E and a remarkable agreement with the directions of greatest extensional strain rate (Hmax) derived from GPS velocities during 2000–2006. The agreement between the directions of stress at depth and strain rate observed at the surface indicate that the earthquakes are primarily driven by plate motion.

Usage of an early warning and information system Web-site for real-time seismicity in Iceland

Iceland has been subjected to destructive earthquakes and volcanic eruptions throughout history. Such events are often preceded by changes in earthquake activity over varying timescales. Although most seismicity is confined to micro-earthquakes, large earthquakes have occurred within populated regions. Following the most recent hazardous earthquakes in 2000, the Icelandic Meteorological Office (IMO) developed an early warning and information system (EWIS) Web-site for viewing near-real-time seismicity in Iceland. Here we assess Web-site usage data in relation to earthquake activity, as recorded by the South Iceland Lowland (SIL) seismic network. Between March 2005 and May 2006 the SIL seismic network recorded 12,583 earthquakes. During this period, the EWIS Web-site logged a daily median of 91 visits. The largest onshore event (ML 4.2) struck 20 km from Reykjavik on 06 March 2006 and was followed by an immediate, upsurge in usage resulting in a total of 1,173 unique visits to the Web-site. The greatest cluster of large (≥ML 3) events occurred 300 km offshore from Reykjavik in May 2005. Within this swarm, 9 earthquakes ≥ML 3 were detected on 11 May 2005, resulting in the release of a media bulletin by IMO. During the swarm, and following the media bulletin, the EWIS Web-site logged 1,234 unique visits gradually throughout the day. In summary, the data reveal a spatial and temporal relationship between Web-site usage and earthquake activity. The EWIS Web-site is accessed immediately after the occurrence of a local earthquake, whereas distant, unfelt earthquakes generate gradual interest prompted by media bulletins and, possibly, other contributing factors. We conclude that the Internet is a useful tool for displaying seismic information in near-real-time, which has the capacity to help increase public awareness of natural hazards.

Habits of a glacier-covered volcano: seismicity patterns and velocity structure of Katla volcano, Iceland

AbstractThe Katla volcano, overlain by the Mýrdalsjökull glacier, is one of the most active and hazardous volcanoes in Iceland. Earthquakes show anomalous magnitude-frequency behaviour and mainly occur in two distinct areas: within the oval caldera and around Goðabunga, a bulge on its western flank. The seismicity differs between the areas; earthquakes in Goðabunga are low frequency and shallow whereas those beneath the caldera occur at greater depths and are volcano-tectonic. The seismicity shows seasonal variations but the rates peak at different times in the two areas. A snow budget model, which gives an estimate of the glacial loading, shows good correlation with seismic activity on an annual scale. Data recorded by the permanent network South Iceland Lowland (SIL), as well as by a temporary network, are used to invert for a 3D seismic velocity model underneath Eyjafjallajökull, Goðabunga and the Katla caldera. The tomography resolves a 15 km wide, aseismic, high-velocity structure at a depth of more than 4 km between the Eyjafjallajökull volcano in the west and the Katla volcano in the east. Anomalously low velocities are observed beneath the Katla caldera, which is interpreted as being a significantly fractured area of anomalously high temperature.

An Application of Relative Moment Tensor Inversion to Aftershocks of the June 1998 Hengill Earthquake in Southwest Iceland

Using spectral amplitudes from the South Iceland Lowland (SIL) seismic network, we conduct a relative moment tensor inversion (RMTI) on aftershocks of the June 1998 M-w 5: 4 event that occurred a ...

The seismic velocity structure north of Iceland from joint inversion of local earthquake data

The tectonics of North Iceland is dominated by interaction of the Iceland hot spot and the mid-oceanic Kolbeinsey Ridge. Transform movement along the transition zone, often called Tjornes Fracture Zone, and the seismicity it generates has been documented in the past. This study uses the seismicity data of the permanent South Iceland Lowland (SIL) network to quantify velocity structure from travel time inversion. The SIL seismic dataset is capable of illuminating parts of the region in a 3D seismic velocity inversion, primarily between 7 and 12 km depth, with less resolution elsewhere because of the sparse setup of the monitoring network. The problem has been analysed in 1, 2 and 3 dimensions and evaluated with 4 different inversion tools. The study reports a correlation of a seismic velocity anomaly in compressional wave velocity v p and shear wave velocity v s with the Husavik-Flatey fault and a further subsurface lineament stretching between the islands of Flatey and Grimsey. Finally, our results support a decrease of crustal thickness between the mainland and the island of Grimsey.

Precise hypocentre relocation of microearthquakes in a high-temperature geothermal field: the Torfajökull central volcano, Iceland

The Torfaj¨okull volcanic system is one of approximately 30 active volcanoes comprising the neovolcanic zones of Iceland. The central volcano of the system is the largest silicic centre in Iceland with a caldera of approximately 12 km diameter. Its high-temperature geothermal system is one of the most powerful in Iceland. Torfaj¨okull is a source of persistent seismicity, where both high- and low-frequency earthquakes occur. To study this microseismicity in detail, a temporary array of 20 broad-band seismic stations was deployed between 2002 June and November. These temporary stations were embedded in the permanent South Iceland Lowland (SIL) network and data from nine adjacent SIL stationswere included in this study.Aminimum one-dimensional (1-D) velocity model with station corrections was computed for earthquake relocation by inverting manually picked P- and S-wave arrival times from events occurring in the Torfaj¨okull volcanic centre and its surroundings. High-frequency earthquakes from the Torfaj¨okull volcanic centre were then relocated calculating a non-linear, probabilistic solution to the earthquake location problem. Subsequently, we correlated the waveforms of these 121 events (∼2000 observations) to define linked events, calculated the relative traveltime difference between event pairs and solved for the hypocentral separation between these events. The resulting high-resolution pattern shows a tighter clustering in epicentre and focal depth when compared with original locations. Earthquakes are mainly located beneath the caldera with hypocentres between 1 and 6 km depth and lie almost exclusively within the geothermal system. A sharp cut-off in seismicity at 3 km suggests either that there is a marked temperature increase or that this is a structural boundary. No seismic activity was observed in the fissure swarms to the northeast (NE) and southwest (SW) of the volcanic centre.

Correlation of Microearthquake Body-Wave Spectral Amplitudes

For applications such as stress tensor inversion and the calculation of composite focal mechanisms, it is of great importance to be able to identify earthquakes with similar focal mechanisms. Inspired by the notion that earthquake focal mechanisms can be calculated by using spectral amplitudes and P -wave polarities, we propose a focal-mechanism assessment scheme based on the correlation of body-wave spectral amplitudes. The spectral amplitudes used are the low-frequency spectral amplitude asymptotes of the direct P, SV , and SH waves. We calculate correlation coefficients using all common spectral amplitudes for pairs of events. The correlation coefficients are used as the basis for grouping the events, groups that will contain closely located events with very similar focal mechanisms. We discuss three applications of the spectral amplitude correlation and grouping technique (SAG) by using microearthquake data from the South Iceland Lowland network in the Olfus area. First, using 26 very well-correlated events, we calculate a composite focal mechanism that has one nodal plane that agrees well with the events9 common fault plane from high-accuracy relative relocation. Second, the SAG scheme is used to address the problem of assessing the redundancy of focal mechanisms for stress tensor inversion, enabling a more realistic calculation of the confidence limits and reducing inversion computing time. Finally, by correlating and grouping a sequence of 2943 events before a magnitude M L 5.1 earthquake, we observe significant temporal changes in the grouping pattern before the large earthquake. This suggests a change from seismicity dominated by clusters of events with very similar focal mechanisms to a regime dominated by solitary, nonsimilar events.

Body wave moment tensor inversion of local earthquakes: an application to the South Iceland Seismic Zone

The focal mechanisms and scalar seismic moments of four local earthquakes are determined using a seismic moment inversion method for frequencies larger than those of dominant oceanic noise and lower than the corner frequency. The earthquakes have epicentral distances ranging from 9 to 42 km and were recorded by the South Iceland Lowland (SIL) seismic network. Synthetic seismograms are computed using the frequency–wavenumber integration technique for an elastic horizontally layered medium. Moment tensor inversion is then carried out using windows designed around P and S waveforms on individual components and at individual seismic stations. The waveforms within each window are then inverted in the time domain simultaneously on all components and at all available seismic stations. The trace elements of the seismic moment tensor are subject to the deviatoric constraint (zero trace element). The results of the inversion are classified as a sum of double couple and CLVD sources, and then compared with the results from the automatic amplitude spectra method routinely used in the SIL network. The comparison shows fairly good agreement between the two approaches, considering that the former uses both amplitude and phase information and the latter uses only amplitude information. The first three events studied have moment magnitudes in the range 2.3 ≤ Mw ≤ 3.1 and comprise two foreshocks and an associated main shock. The weighted least squares inversions for these events show dominant strike slip faulting with scalar seismic moments of the order of 0.285 × 1013 ≤ M0 ≤ 4.966 × 1013 N m. The fourth event is characterized by a moment magnitude Mw = 2.2 and scalar seismic moment M0 = 0.243 × 1013 N m. This event also shows a significant strike slip component.

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

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.

Strain changes for the 1987 Vatnafjöll earthquake in south Iceland and possible magmatic triggering

On May 25, 1987, at 11:31, a M = 5.8 earthquake occurred at the southern end of Vatnafjöll volcanic ridge in south Iceland. This is the largest event in the south Iceland lowland since a M = 7.0 earthquake in 1912 which was located approximately 15 km to the west of the Vatnafjöll earthquake. Vatnafjöll is located at the junction of the south Iceland seismic zone, a left lateral transform zone, and the eastern volcanic zone which is a zone of rifting and volcanism. In May and June 1987, several foreshocks and aftershocks were recorded on the local seismic network as well as the mainshock. A clear coseismic step associated with the mainshock was observed at all operating stations of a volumetric strainmeter network in southern Iceland. Steps associated with some foreshocks and aftershocks were also observed at the closest strain stations. Slow strain changes, before and after the mainshock, lasting a few days, were also observed. Forward modeling of the coseismic strainmeter signals of the mainshock suggests a double couple solution where the slip is mostly right lateral strike slip on a subvertical plane with a northerly strike. The solution has a good fit to observations and is in good agreement with interpretation of seismometer data. This solution indicates a stress field similar to that in the south Iceland seismic zone. The slow strain changes, which start about 10 min after the first foreshock, may indicate magma involvement in the process. Changes, associated with an intrusion and pressure release, may affect the strain field and possibly trigger the mainshock. The strainmeter records open up a new view of the seismic strain event as a combination of seismic strain release and a slower process of magma intrusion.

Seismic attenuation in the crust at the mid-Atlantic plate boundary in south-west Iceland

SUMMARY Very little seismic attenuation occurs in the mid to lower crust of south-west Iceland. The lowest path-averaged quality factor for a wave turning in the mid to lower crust (12-20 km) is QP = 110 for P waves and Q5 = 250 for S waves, with most of the data having higher values, typically QP = 200-300 and Qr = 400-600. Attenuation estimates based on a 1 -D, layered inversion give correspondingly high values, Qp > 800 and Q, = 800-2000. These Q values are inconsistent with thermal models that predict a broad (100 km wide) region of above-solidus temperatures centred on the volcanic zones. The observed attenuation implies an upper limit for mid to lower crustal temperature in the 700-775 C range (assuming a gabbroic lithology). Much higher attenuation (Qp = 60, Qr = 100) occurs in the uppermost 4 km of crust. This is most likely apparent attenuation caused by strong near-surface seismic heterogeneity, resulting from fissures, faults and extreme changes in porosity (up to 20-30 per cent). The quality factor of the near-surface layer varies regionally, and is lower in volcanic zones than in either the Reykjanes Peninsula or the South Iceland Lowland.

A demonstration of the joint use of P wave polarization and travel‐time data in tomographic inversion: Crustal velocity structure near the South Iceland Lowland Network

A joint inversion of both P‐wave travel‐time and polarization direction data is performed on a data set from the South Iceland Lowland network. The root mean square reduction of the polarization and travel‐time residuals are 72% and 76%, respectively. Furthermore, travel‐time residuals drop 45% even when they were given no weight in the inversion, indicating that the polarization measurements are related to the same regional‐scale velocity heterogeneities thought to cause the travel‐time anomalies. The results show two crustal low‐velocity zones trending NE across the study area, a direction that is sub‐parallel to the strike of the Western Volcanic Zone in south Iceland.

Earthquake prediction research in the South Iceland seismic zone and the SIL project

The South Iceland Lowland (SIL) project, started in 1988, is a concerted effort of the Nordic countries toward earthquake prediction research in the South Iceland seismic zone (SISZ), where most destructive earthquakes in the history of iceland have occurred. The zone has many characteristics of a transform zone and takes up east-west relative motion between two offset branches of the mid-Atlantic rift zone; i.e., the eastern volcanic zone of Iceland and the Reykjanes Ridge. Earthquakes in this area reach the magnitude of roughly 7 and tend to culminate in sequences with duration from a few days to a few years. These events are associated with right-lateral faulting on N-S striking faults arranged side-by-side along the E-W trending zone. Thus, apparently the general left-lateral transform motion is accommodated by counterclockwise rotation of the fault blocks. The seismogenetic crust is 10 to 45 km thick and is underlain by partially molten mantle material. Earthquake swarms, slowquakes, and strain episodes are commonly observed in the zone and in its proximity. The SIL project started with the construction of a seismic network for near-real time data acquisition. The network has been collecting data since the beginning of 1990, and it has a nearly complete record down to magnitude 0 earthquakes inside the area. Besides epicenter location and magnitudes the system automatically computes fault plane solutions and dynamic source parameters. First results indicate good consistency between expected regional deviatoric stresses and stress release in microearthquakes as recorded and evaluated in the SIL system.