Questions and answers

A seismic event occurs when natural tension in the bedrock changes so rapidly that the rock mass breaks, i.e. fractures. When the shock wave hits the surface it is felt as a vibration.

Mining causes tension changes in the bedrock. These changes occur partly because cavities are created, and partly because of mining at greater depths. The tension in the bedrock is distributed differently around the mine, and the pressure can become so great that a seismic event occurs.

Magnitude is a measurement of the size of a seismic event. It describes how much energy has been released by a seismic event.

To find out exactly where the seismic events take place a scale adapted to the conditions is required. The Richter Scale is designed to measure natural earth tremors at great distance, it measures a different frequency and is therefore too inaccurate for the smaller tremors that occur during mining.

Currently it is not possible to predict a single event in time, place and magnitude. Predicting that an area probably will have more seismic activity is what we are able to do. Increased seismic activity sometimes results in larger seismic events starting to occur. This is why we are currently working to close down areas of production as a preventative measure. We also work in detail to manage how the different production areas should be mined in order to minimise the risk of increased seismic activity.

Vibration can affect a house. You can not say at what magnitude. Impact is dependent on magnitude, distance from the seismic event, the quality of the bedrock and the layer of earth. The effect can best be described by the actual vibration the house is exposed to. There is also a function from how the house is founded and constructed.

No, the seismic events caused by mining will not be that large. LKAB never mines ore under areas where people live or stay.

At Malmberget the largest magnitude on the local scale was 2.9. In Kiruna the largest event that has occurred was 3.0.

When a seismic event occurs tremors are generated which spread in the bedrock. Geophones that are placed around the mine register these tremors and the locality and magnitude can then be calculated. The seismic system is currently made up of approximately 200 geophones each in Malmberget and Kiruna. The system expands at more or less the same pace as the mines go deeper.

It depends on the geophone coverage. Close to the production the accuracy is approximately 20 metres. Outside, as for example in the hanging wall the accuracy is estimated as 50-100 metres. The accuracy depends on how far from the geophones the incident occurs, the quality of the bedrock, size and type of occurrence.

Magnitude is calculated in a logarithmic scale. Scales of magnitude were originally designed to measure earth tremors, when a different type of instrument placed at a greater distance from the incident was in use. A requirement to measure smaller incidents at shorter distances became necessary in the development of mining seismology. Because of this the scale of magnitude was scaled down. This is why the magnitude scale can be negative.

LKAB partly uses the information as a part of our knowledge building for each area to allow better planning of our operation so as to minimise activity. It also acts as a tool to follow-up the development of cavities and movements in the hanging wall. In the short term we close down the area when we can see that the activity is increasing in a specific area.

LKAB measures and reports vibrations on the ground for seismic events in millimetres per second. The value shown is called Peak Particle Velocity, and is the greatest value that occurs regardless of the direction of the vibrations. Earlier vibrations were reported in the vertical, i.e. upright, since the instruments followed a Swedish standard that stipulated that only vertical vibrations should be measured. The instruments have been replaced, and from February 2015 LKAB reports in Peak Particle Velocity.