This is the first time scientists have been able to generate and observe deep and intermediate focus earthquakes in the laboratory, recreating the exact pressure and temperature conditions of the deep earth. Their results have helped elucidate the origin of some of the largest and most violent earthquakes to occur on earth.
Intermediate and deep earthquakes occur between 70 and 650 km depth in the earth and until now their cause has proven enigmatic. They are triggered by various chemical reactions which occur in down-going material below subduction zones (most of the Pacific coastline, for example) and can be extremely large; the largest deep earthquake ever recorded was at magnitude 8.3 and occurred 600 km below Bolivia in 1994.
The odd thing about deep earthquakes is that they shouldn't happen. Earthquakes require brittle failure and frictional slipping, and at depth rocks deform plastically not by brittle processes. Dr David Dobson and colleagues brought together a unique collection of skills and techniques in the Mineral Ice and Rock Physics Laboratory at UCL to study the dehydration reactions that can occur down to 300 km depth below subduction zones. Their results show that these dehydration reactions can cause a phenomenon known as "dehydration embrittlement" which produces these deep earthquakes where they shouldn't occur.
The scientists used a multianvil press to compress rocks similar to those found deep in the earth to pressures and temperatures of extreme magnitude, recreating the conditions hundreds of kilometres below the earth's subduction zones. Their results suggest that earthquakes can be generated to at least 210 km depth by dehydration reactions in old oceanic crust caused by it slowly heating up as it is subducted. Dr David Dobson who led this study at UCL said: "Understanding the processes behind deep earthquakes is vital in helping scientists understand how plate motions are related to the convection in the deep Earth which drives plate tectonics."
"Deep and intermediate focus earthquakes are an important and mysterious class of earthquakes, Understanding these deep earthquakes could be the key to unlocking the remaining secrets of plate tectonics".
Further information and interviews: David Burrett Reid, UCL Media Relations, 44-207-679-1618.
Notes to Editors: Simulation of Subduction Zone Seismicity by Dehydration of Serpentine (David P. Dobson, Philip G. Meridith, Stephan A. Boon) is published in the 15th November 2002 issue of Science.
Journal
Science