Earth Seminar 25 February - Professor Gordon Lister

Gravity drives Great Earthquakes

Abstract

This presentation discusses the role of gravity in driving Great Earthquakes, with data from the 2004 Great Sumatran Earthquake, and from the 2011 Tohoku-oki Great Earthquake. We show the Sumatran segments of the 2004 megathrust event was subject to compression in a direction near to orthogonal with the margin trend, consistent with effect of relative movement of the adjacent tectonic plates. In contrast, the crust above the Andaman Sea segments was subject to margin-orthogonal extension, consistent with motion towards the gravitational potential well accumulated due to prior lateral (westward) rollback of the subducting edge of the northward moving Indian plate. The story is quite different for the 2011 Tohoku-oki earthquake, however. Here, lineament-bounded extensional channels mark segments of the East Japan megathrust with different geodynamic behaviour to that of  adjacent compressional segments. This pattern implies movement in the extensional channel driven by seaward gravitational collapse of the Japanese crust, requiring the rupture to have offered negligible resistance. The upward migration of fluids and magma would explain the prominent volcanic lineaments. Fluid activity would also have reduced effective stress on the overlying megathrust, or produced lubricating mineralogy as the megathrust slowly unlocked in the decade preceding catastrophic failure. The consistent landward-dip of normal faults at the trenchward-end of the extensional channel suggests an array of tilt-blocks linking to a detachment beneath a slowly slumping slab sheet, with a strike dimension comparable to the width of the extensional channel. Again, nevertheless although for different geodynamic reasons, gravity has driven a Great Earthquake.

Biography

Prof. Gordon Lister from the Research School of Earth Sciences, Australian National University, PhD (ANU), BSc (UQ) is a structural geologist who has worked in several different parts of Alpine Tethys, including the Alps, the Cyclades, Greece, NW Himalaya, SE Asia and the SW Pacific, while maintaining a strong interest in Precambrian geology. However, in this talk he shows a different side of his research, as part of the satellites, seismometers and mass spectrometers initiative, looking at the seismotectonics of the most destructive of all earthquakes - those that take place on the killer megathrusts adjacent to subducting oceanic lithosphere.