Large scale lithospheric detachment of the downgoing Cocos plate: The 8 September 2017 earthquake (Mw 8.2)

On 8 September 2017, a great earthquake (Mw 8.2) took place in the Mexican subduction zone in the Tehuantepec gap, where no large subduction earthquakes have taken place since 1902. However, the 8 September earthquake did not occur on the contact between the Cocos and North American plates. The centroidal hypocentral depths reported by different agencies, including the National Seismological Service, range from 45 to 47.4 km, placing it immediately beneath the down-dip limit of the interplate locked zone. The source mechanism reflects down-dip tensional faulting. The inversion of the fault process shows a rupture that initiated at the bottom of the lithosphere and propagated upward, breaking through the entire subducted lithosphere. An unusually long, complex and copious aftershock sequence followed the main event. The relocated aftershocks, in the first 20 days following the main event, delineate a 160-km-long fault, sub-parallel to the oceanic trench, immediately beneath the interplate contact. The aftershocks also concentrate in secondary intraslab faults ∼50 km down-dip from the mainshock rupture, revealing that pervasive tensional stresses are present within the subducted plate. This deformation pattern suggests a large-scale tensional regime in the slab, apparently induced by its own gravitational weight that pulls it away from the strongly coupled interplate contact. Other large intraplate earthquakes in the vicinity, in 1931 (Mw 7.8) and 1999 (Mw 7.5), also reflect this detachment of the downgoing Cocos plate that sinks into the mantle under its own gravitational weight. The slab detachment suggests that the up-dip segment of the plate interface near the Isthmus of Tehuantepec is locked and possibly primed for a megathrust earthquake, such as the Mw 8.6 that took place in 1787, on an adjacent segment of the Mexican subduction zone.