Seismicity and volcanic activity in Iceland are related to the Mid-Atlantic plate boundary thatcrosses the island. Due to the volcanic activity with lava flow and ash fall, different sea levelsthrough the ages, glacial drift and deposits of glacial streams, the geology in Iceland is quitecomplex at many sites. This can affect the site-specific ground motion during an earthquake.The Earthquake Engineering Research Centre, University of Iceland, operates the strongmotion network in Iceland. At one of the instrumented site, the Thjorsa Bridge, the geology isquite different on each side of an approximately 80 m wide river canyon. On the east side,there is dolerite rock, which can be considered bedrock conditions. On the west side, there is 8to 10 m thick lava rock on 18 to 20 m thick alluvial deposits. In June 2000, two majorearthquakes of magnitude 6.6 (Mw) and 6.5 (Mw) occurred in South Iceland with theirepicentres close to the Thjorsa Bridge. Recorded time histories at the Thjorsa Bridge site fromthe two main earthquakes and a number of aftershocks have shown that there is a considerableamplification of the ground motion on the lava rock, compared with the ground motion on thebedrock. This effect is of great importance for design criteria on sites with similar geology.
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