30.01.2014|Potsdam:The relative motion of tectonic plates is generally accompanied by earthquakes, slow slip or creeping. On mature divergent plate boundaries however, it is magma intrusions that compensates for the missing volume due to crustal splitting. In rifting episodes, magma from crustal chambers propagates through the rock and creates vertical sheet-like intrusions, called dikes. This process is thought to release periodically extensional strain, analogously to how earthquake mainshock-aftershocks sequences release the shear strain accumulated during tectonic plate motion.

Two major rifting episodes have been recorded since modern monitoring techniques are available: the 1975-1984 Krafla (Iceland) and the 2005-2010 Manda-Hararo (Ethiopia) dike sequences. For these episodes, estimates of the intruded magma volumes are available from published inversions of crustal deformation data. Scientists from GFZ German Research Centrer Geosciences and University of California Santa Cruz have now demonstrated that the statistical properties of dike intrusions during rifting have many similarities with earthquake mainshock-aftershock sequences.

The distribution of the frequency of earthquakes relative to their size is known to follow from local to global scale a power law, the Gutenberg-Richter relation. The decay of aftershocks with time after a large earthquake is known to follow the Omori law. These statistical laws for earthquakes are the basis for hazard evaluation and the physical mechanisms behind them are the object of wide interest and debate. The authors of this study find that the dimensions of dike intrusions obey a power law analogous to the Gutenberg-Richter relation and the long-term release of geodetic moment is governed by a relationship consistent with the Omori law.

However, size matters in this case and makes the difference: the availability of magma affects the timing of subsequent dike intrusions.  After large volume intrusions the time intervals are longer, which is in contrast to aftershock sequences where interevent times are shorter after large events.

Luigi Passarelli, Eleonora Rivalta and Ashley Shuler: “Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes”; Scientific Reports, 4, Article number:3886, doi:10.1038/srep03886

http://www.nature.com/srep/2014/140127/srep03886/full/srep03886.html