How Shallowing Slab Dip Could Produce Extensional Upper Plate Earthquakes after a Megathrust Earthquake?
Abstract
The Tohoku-oki earthquake was one of the strongest earthquakes ever recorded. Up to 60 meters of lateral motion of the sloping seafloor resulted in a tsunami that exceeded predictions and caused one of the costliest natural disasters in history. It was also the first time that extensional aftershocks were observed in the upper plate above the megathrust, and they were located over a region as wide as 230km. Inspired by these findings, researchers found similar upper plate extensional earthquakes after reexamining seismic data from past earthquakes that had also produced large tsunamis.
Such extensional aftershocks are difficult to explain in terms of standard subduction models that assume the dip of the subducting plate remains constant with time. However, geological evidence from Japan indicates that the dip angle of the subducting plate have diminished in the last few Ma. We hypothesize that a reduction in the dip angle of the subducting plate can cause upper plate extensional earthquakes by adding extensional bending stress to the upper plate. During an inter-seismic period, the interface is 'locked' causing regional compression that prevents the release of extensional energy. Relief of compressional stresses during a megathrust event can trigger the release of any accumulated extensional energy caused by increased bending, explaining why extensional earthquakes were observed after some megathrust events. Numerical models were used to test our hypothesis. First, we modeled long term subduction with a nearly constant dip angle. Then, we imposed 'mantle wind' to change the dip angle of the subducting plate in correspondence with dip angle changes constrained by geological evidence from Japan. Eventually, we modeled multiple seismic cycles of imposed periodic megathrust weakening events. We simulated magathrust earthquakes on a timescale of seconds to years and showed that a broad region of extensional earthquakes in the upper plate could follow such events. Extensional upper plate earthquakes are result for cases with a steady reduction of the slab dip angle. Long-term reduction in the dip angle produces uplift of the upper plate, similar to that inferred from borehole stratigraphic data for the region west of the Japan Trench.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T41G0377B
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITYDE: 7240 Subduction zones;
- SEISMOLOGYDE: 8123 Dynamics: seismotectonics;
- TECTONOPHYSICSDE: 8170 Subduction zone processes;
- TECTONOPHYSICS