Abstract
The trench-parallel distribution of aftershocks generated after a major subduction-related earthquake does not conform to a fractal or normal distribution as expected but does appear to be spatially constrained by fractures on the subducting oceanic crust. Newly developed and very detailed 3D models of subducting plates (slabs) are combined with high-resolution topographic and bathymetric models of the crustal upper-plate and subducting lower-plate respectively to show how irregularities such as transform faults, spreading centers, fossil subduction zones and vertical tears form structural barriers to the spatial distribution of subduction zone aftershocks. When correctly constructed, slab models can now be used to better forecast the lateral (along-arc) extent of damaging aftershock swarms following large magnitude subduction-related earthquakes. Two earthquakes, (magnitude MW 7.8 in Vanuatu, 7th October 2009 (UTC) and magnitude MW 8.8 in southern Chile, 27th February 2010 (UTC)) were followed by hundreds of aftershocks, with many recording magnitudes over MW 6.5. Closer examination of these aftershocks and their spatial distribution indicate that they were unexpectedly confined to discrete sections of the subducting plate. It is revealed here that lithospheric-scale structures including fossil oceanic transform faults represent crustal and lithosphere-scale fractures in the crust that terminates the lateral spread of aftershocks. This contrasts with the predicted decrease in aftershock size and number with increasing distance from the epicenter referred to as the Omori (power law) distribution. This hypothesis addresses the paradigm that oceanic crust is a single, semi-continuous and uniform section of crust where the lateral distribution of earthquakes is described in terms of uniform rock mechanics. Additional research on the MW 9.0 earthquake in Japan (11th March 2011 (UTC)) and the MW 8.0 earthquake in the eastern Solomon Islands (6th February 2013 (UTC)) supports the aforementioned hypothesis. Using new techniques to help define the location of sub-plate boundaries, it is now possible to generate aftershock probability maps for some of the most seismically active subduction margins around the world.
Highlights
The impact of a large magnitude earthquakes (MW > 7.0) and associated aftershocks can be devastating, for example, the 2011 Japan earthquake and Tsunami (Norio et al, 2011) or the Mw 7.0 earthquake in Haiti, 2010
This paper presents detailed evidence from two locations where large earthquakes have recently occurred: in the northern New Hebrides Trench near Vanuatu and near the city of Concepción in Southern Chile
The earthquakes have been selected from subduction zones around the world in order to test the hypothesis in different subduction systems
Summary
The impact of a large magnitude earthquakes (MW > 7.0) and associated aftershocks can be devastating, for example, the 2011 Japan earthquake and Tsunami (Norio et al, 2011) or the Mw 7.0 earthquake in Haiti, 2010 Resent research by Sparkes et al (2010) has shown that the northern limit of rupture propagation after the Mw 8.8 earthquake on the west coast of Chile (Maule, February, 2010) was terminated by the presence of the Juan Fernandez Ridge which is a bathymetric high on the subducting Nazca Oceanic Plate.
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