Speaker: Sigurjon Jonsson
On the western part of the Arabic Peninsula, along the Red Sea, are extensive lava provinces (called harrats) that have developed during the past 30 million years along with the opening of the Red Sea. A few volcanic eruptions during the past centuries, including the 1256 AD Al Madinah eruption, show that these harrats are still active, although the provinces are seismically inactive most of the time. One of these areas, Harrat Lunayyir reawakened in April-May 2009 with an intensive seismic swarm and surface faulting, peaking on 17-20 May with six magnitude 4.6-5.7 earthquakes. The activity prompted the Saudi civil protection authorities to evacuate more than 30000 people from the area. While the earthquake activity significantly decreased after 20 May, it continued throughout June and July with a few earthquakes as large as magnitude ~4, before quieting down in August. Much of what we have learned about the activity comes from interferometric satellite radar (InSAR) observations and from analysis of the seismic data collected by a broadband seismic network that was installed soon after the earthquake swarm started in April. In this presentations I discuss the InSAR observations of the area that show a large-spatial-scale (40km x 40km) approx. east-west extension of over 1m took place as well as broad uplift amounting to over 40cm. This deformation well explained with a near-vertical ~10-km-long dike intrusion that almost made it to the surface and caused extensive graben-bounding normal faulting. I will show how the InSAR observations and seismic data helped unfolding the sequence of events during the intrusion and what broader implications the results have for the understanding of the tectonics in the region.