Remote Triggering of Deep Earthquakes

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Deep earthquakes are enigmatic because at very high pressures friction should stop rocks from rupturing. So we find in most of the Earth's mantle there are no earthquakes and rocks deform plastically. However in subducting slab regions there are significant numbers of really quite large earthquakes occurring down to almost 700 km depth - these are deep earthquakes. In order for seismicity at this depth we need a mechanism which will allow stresses to build up to a critical level and then produce very rapid strains, sufficiently fast for seismic waves to be produced and recorded at the Earth's surface. Laboratory experiments have shown that phase transformation from the main upper mantle mineral olivine to its high-pressure polymorphs can provide this kind of stress critical process, known as anticrack faulting. But anticrack faulting does not occur all the way down to 700 km depth and indeed the largest deep earthquakes do not occur in a region where metastable olivine persists.One possibility is that faults produced in the transforming region then cause stresses to increase in regions outside transformation and the rupture can slowly propagate from the anticrack faulting region into the surrounding slab. This model is known as remote triggering, and is popular among seismologists who study deep earthquakes, but it has yet to be proven demonstrated in experimental simulations of faulting processes. This study will investigate ways in which anticrack fulting can cause larger earthquakes away from the region of transformation. In studying this remote triggering process we will develop new tests for faulting mechanisms which will be applicable to both the deep Earth and also to monitoring man-made seismicity such as might occur during resource abstruction and carbon sequestration.