Thousands of earthquakes detected in Harrat Lunayyir

Over 30,000 earthquakes hit northwest Saudi Arabia between April and June 2009, most however were only detectable using sensitive equipment. The earthquakes were localized to a lava field in an area known as Harrat Lunayyir (locally called Harrat Al-Shaqah), prompting the Saudi Arabian government to evacuate 40,000 residents from the area.

Researchers from the Saudi Geological Survey (SGS) joined forces with their counterparts from the US Geological Survey (USGS) and several universities to study the seismic data to analyse the earthquake swarm and published their report in Nature Geoscience.

The team concluded that the many small earthquakes were the result of a lava dyke, formed when magma surged upward but, instead of forming a magma chamber, spread in a thin vertical sheet over a large area. The magma pressure then suppressed large earthquakes, leading to thousands of smaller tremors rather than the normal earthquake pattern of a single high magnitude event followed by smaller aftershocks. The researchers concluded that the crustal stress in the affected region is caused by the flow of hot material in the asthenosphere, which is earth's upper mantle, away from the Red Sea.

After reviewing the data, the scientists determined that imminent danger has passed making it safe for the return of evacuees. However, they suggest the SGS should continue to monitor tectonic and magma activity in the region, while expanding its hazard assessment and communication capabilities. The Harrat fields still remain active, posing a continuous threat to the expanding urban population in the region.

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