Bjarni Bessason

Automatic detection of avalanches and debris flows by seismic methods

The road along the Oshli ´ð hillside in the West Fjords region of Iceland is one of the most hazardous roads in Iceland due to avalanches, rockfalls and debris flows. The road has little traffic, but nevertheless traffic accidents caused by the severe conditions at the site are common. A number of avalanche tracks are found on the hillside. In some of these tracks, avalanches occur more frequently than in others. When there is an avalanche threat, avalanches generally flow over many tracks in a short time. Monitoring vibrations in the tracks with the highest avalanche frequency can indicate when avalanches start flowing down the hillside in a snowstorm, and avalanche hazard can then be declared with the specific site indicated. The same methodology can be used for rockfalls and debris flows, which are strongly affected by weather conditions and typically occur in clusters. Based on this knowledge, a research project was initiated in February 1996 with the objective of developing an automatic system based on seismic measurements to detect and analyze avalanches on the Oshli ´ð hillside and to instantly

Site amplification in lava rock on soft sediments

Seismicity and volcanic activity in Iceland are related to the Mid-Atlantic plate boundary that crosses the island. Due to volcanic activity, different sea levels through the ages and glacial drift, the geology in Iceland is quite complex at many sites. In June 2000, two major earthquakes of magnitude 6.6 (Mw) and 6.5 (Mw) occurred in southern Iceland. Ground motion from these main shocks and a number of aftershocks were recorded at the Icelandic strong motion network operated by the University of Iceland. At one of the instrumented sites considerable amplification was recorded on lava-rock overlying alluvial deposits. This site fits poorly in the soil classification systems of most earthquake codes. In this paper the recorded data is analyzed with different methods and the results are compared with the results of a one-dimensional site response analysis. The different methods produce the same characteristic of soil amplification at the site. The findings of this study have important implications for design criteria for sites with similar geology. q 2002 Elsevier Science Ltd. All rights reserved.

Recorded and numerical strong motion response of a base-isolated bridge

Since 1983, 12 Icelandic bridges have been base isolated for seismic protection. Lead-rubber bearings have been used in all the cases. The Thjorsa River Bridge, built in 1950 and retrofitted with base isolation in 1991, is instrumented by strong-motion accelerometers. The bridge has one 83-m-long main span and two 12-m-long approach spans. Only the main span, a steel arch truss with concrete deck, is base isolated. In June 2000, two major earthquakes of magnitude 6.6 and 6.5 occurred in South Iceland; the epicenter was close to the Thjorsa River Bridge. In the first earthquake, a peak ground acceleration of 0.53 g was recorded at the bridge site, and in the second earthquake, a peak ground acceleration of 0.84 g was recorded. The Thjorsa River Bridge survived the earthquakes without any serious damage and was open for traffic immediately after the earthquakes.

Vibration criteria for metrology laboratories

Establishment of appropriate vibration criteria is essential when designing vibration-sensitive metrology laboratories. Boundary values that are too severe may lead to unnecessarily high construction costs, whereas limits that are too broad may result in degradation of the performance of measurement equipment. The Norwegian Metrology and Accreditation Service (Justervesenet) inaugurated a new facility early in 1997. The facility will allow measurements of mass, density, dimensional, force, volume, optical, pressure, temperature and electrical quantities. Vibration control is of concern in most of the laboratories. Vibration criteria have been defined in terms of frequency-dependent peak values. In this paper, these criteria are described and the most conservative criterion is compared with other known vibration criteria for standard laboratories and high-technology facilities. The vibration criteria considered have different formulations and cannot be compared directly. They are therefore compared with regard to three different kinds of idealized vibration excitation, that is, transient, harmonic-motion and broad-band noise. The comparison shows that the most conservative Justervesenet vibration criterion is stricter with respect to high-frequency vibrations than are the others, but it is less strict for low-frequency vibrations.

Probabilistic Earthquake Damage Curves for Low-Rise Buildings Based on Field Data

All buildings in Iceland are registered in an official database that contains detailed information about them, and insurance against natural disasters is obligatory. When a destructive earthquake occurs, all damage is reported, and the repair and replacement cost for every affected building is evaluated. In May 2008, a shallow earthquake of magnitude Mw = 6.3 struck in South Iceland. A great deal of damage occurred, but fortunately, there was no loss of life. The recorded maximum PGA was 0.88 g. Detailed and complete information of all real estate property and the damage incurred, along with recorded strong-motion data and an area-specific attenuation model, have provided an opportunity to create probabilistic damage curves for the building stock in the affected area. The damage model obtained from the 2008 earthquake was tested and verified by using it to back-calculate the damage that occurred in the two South Iceland earthquakes of June 2000 (Mw = 6.5).

Evaluation of site vibrations for metrology laboratories

The Norwegian Metrology and Accreditation Service (Justervesenet) inaugurated a new facility early in 1997. The facility allows measurements of mass, density, dimension, force, volume, optics, pressure, temperature and electrical quantities. Vibration control is of concern in most of the laboratories. Vibration criteria have been defined by frequency-dependent peak values. Vibrations affecting the laboratories can be classified as originating in either internal or external vibration sources. Internal vibrations are generated by human activity, mechanical systems and equipment inside the building, while external vibration sources represent all sources outside the building. In order to map and estimate the vibration level at the site and inside the new facility, an extensive measurement and analysis programme was carried out. This programme included four main activities, that is, vibration monitoring at the site for 37 days, controlled measurements of vibrations from diverse internal vibration sources, earthquake hazard analysis and, finally, analysis of footstep-induced vibrations. Based on the results from this programme, the principal layout of the building was formulated, and countermeasures were designed to fulfil the vibration criteria specified for the laboratories. The countermeasures were basically threefold, that is, active vibration isolation of the internal sources, constructional joints to cut wave transmission paths and passive vibration isolation of the foundations supporting the sensitive equipment.

Seismic Vulnerability of Icelandic Residential Buildings

The seismicity in Iceland is related to the Mid-Atlantic plate boundary which crosses the country from north to south. Since 1700, there have been 25 earthquakes of magnitude six or greater in the two major seismic zones in Iceland. For many of the historical earthquakes (older than 100 years), detailed written descriptions are available that describe the damage from farm to farm. For the most recent earthquakes, like the two south Iceland earthquakes of June 2000 and the south Iceland earthquake of May 2008, comprehensive building-by-building loss data exist. The loss data in these cases are split into a number of subcategories of structural and non structural damage. The building stock in south Iceland has changed significantly from 1700 to the present. For ages, vulnerable turf and stone houses dominated, but in the twentieth century, concrete buildings and timber buildings took over. Seismic codes were implemented in 1976 and have gradually improved the seismic capacity of present building stock. In this book chapter, an overview of the seismic performance of old and new Icelandic buildings is given. Observed and reported damage caused by three earthquake sequences and a single event are discussed; first, the damage caused by two earthquakes in August 1784 in south Iceland; then, by the 1896 earthquake sequence in south Iceland; then, the damage after a single event in 1934 in north Iceland; and finally, the loss data from the 2000 and 2008 earthquakes in south Iceland. The main focus is on the last three events for which most of the data exist.

Performance of Base Isolated Bridges in Recent South Iceland Earthquakes

Since 1983, 15 Icelandic bridges have been base isolated for seismic protection. Lead-rubber bearings have been used in all cases. Nine of these bridges are located in the South Iceland Lowland, which is an active seismic zone where earthquakes of magnitude seven can be expected. In June 2000 two Mw6.5 earthquakes struck in the area and in May 2008 a Mw6.3 event hit the area again. Four of the base isolated bridges were located in the near-fault area of these earthquakes with a fault-to-site distance of less than 15 km, and most likely three of them were subjected to strong low-frequency near-fault velocity pulses. None of the bridges collapsed or were severely damaged and all of them were open for traffic immediately after these events. Post-earthquake response analysis has indicated that the base isolation was important for the performance of the bridges.

Monitoring and Damage Detection of a 70-Year-Old Suspension Bridge – Ölfusá Bridge in Selfoss, Case Study

Olfusa Bridge in South Iceland is located in the town of Selfoss on the ring road Route 1 in Iceland, approximately 50 km from Reykjavik. It is a suspension bridge across the Olfusa glacial river and while being a very important link in the road transport network in Iceland, it also serves as a vital link for the community of Selfoss for both vehicles and pedestrians. Inspections of the main cables of the suspension bridge have indicated an inadequate level of safety due to corrosion, as well as significantly heavier traffic loads and increased self-weight due to rehabilitation of the bridge deck. A system identification of the bridge was conducted in 2012 and a permanent monitoring system was installed in 2014 with the aim of identifying changes in the structural performance of the bridge. The measurements have provided valuable information which are used in the operation of the bridge. The measured modal properties have been used to calibrate the FE models of the bridge, which are used for structural analysis and the structural reliability assessment of the bridge. The continuous monitoring of the cable forces gives the possibility of identifying changes in the structural response of the bridge in addition to the regular visual inspections.

Application of MASW in the South Iceland Seismic Zone

Multichannel Analysis of Surface Waves (MASW) is a seismic exploration method to evaluate shear wave velocity profiles of near-surface materials. MASW was applied at seven locations in or close to the South Iceland Seismic Zone, providing shear wave velocity profiles for the top-most 15–25 m. The profiles were utilized for seismic soil classification according to Eurocode 8. The results indicated that the sites that are characterized by sandy glaciofluvial, littoral or alluvial sediments fall into category C and the sites where the deposits are cemented to some degree belong to category B. Furthermore, the MASW measurements were used to evaluate the liquefaction potential at a site where liquefaction sand boils were observed during an Mw6.3 earthquake occurring in May 2008. The simplified procedure of assessing cyclic stress ratio to normalized shear wave velocity revealed that liquefaction had occurred down to 3–4 m depth, which is consistent with observations on site.