Supplementary Note S2: Relocation of NEIC hypocentre

The location of the mainshock nucleation point is a vital constituent of our study's conclusion. We therefore need to analyse the robustness of the NEIC hypocentre (http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2010tfan.php). 
For aftershocks that were recorded in both the International Maule Aftershock Dataset and the USGS global catalogue, we compare the locations from the NEIC catalogue with the 3D relocations (Figure S8). We then calculate an average relocation vector for different regions of the rupture area. We then relocate the NEIC location for the mainshock with an average relocation vector for the offshore region, in the centre of the rupture zone. We find that the original NEIC epicentral location moves by only ~15 km further offshore; therefore, we believe that both the original location and our correlation of the nucleation point with the velocity/gravity anomaly are robust features.