Seismic events occur when the natural tension in the bedrock changes. The greatest rock stress is horizontal – lateral – rather than vertical, i.e. top-to-bottom as we might expect.
When iron ore mining in the underground mines continues deeper and 'cuts off' the bedrock, the stress must find new ways around and under the extracted ore. Stress concentrations are formed, and if the stress is greater than the strength of the rock mass, the rock will give, i.e. fracture. Seismic activity is the rapid fracture of a rock mass that releases a lot of energy. The majority of such events near LKAB mines – sometimes several hundred to a thousand a day – are so minor that they cannot be felt and are only measurable with sensitive instruments. Sometimes though, energy builds up and is released in larger tremors that are felt in the communities closest to the mines.