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Long Distance GNSS Wobbles Preceded Massive Subduction Zone Earthquakes


Researchers: Jonathan R. Bedford, Zhiguo Deng, Onno Oncken, and Bernd Schurr, GFZ German Research Centre for Geosciences; Marcos Moreno, Universidad de Concepcion; Timm John, Freie Universitat Berlin; Juan Carlos Báez, University of Chile; and Michael Bevis, Ohio State University

Written by Linda Rowan
19 May 2020


Summary

GNSS observations show surface horizontal displacements of 4 to 8 millimeters wobbling back and forth for more than five months and over thousands of kilometers before massive subduction zone earthquakes in Japan and Chile. Looking back at past GNSS observations and using an analysis to separate tectonic motion from other features shows a potential precursory signal of these destructive earthquakes and enhances our understanding of Earth processes along major plate boundaries. Detailed modeling of the dense geodetic data for the Japan earthquake suggests a fluid-triggered slab pull helping to initiate a massive event.


Observations

Subduction zone earthquakes are caused by the collision of tectonic plates at a plate boundary, where the denser plate is driven beneath the less dense plate. When the plates shift suddenly and over a large distance, the deformation at the plate boundary can generate a magnitude 8 or greater earthquake. The 2010 Maule Chile, moment magnitude 8.8 and the 2011 Tohoku-oki, Japan moment magnitude 9 earthquakes are recent examples of very energetic and damaging subduction zone earthquakes.

Terrestrial GNSS networks track the motions of the plates along subduction zones. Geodetic observations have measured large earthquakes to seismically imperceptible slow slip along subduction zones. Geodetic networks are exquisite for following surface motions of active tectonic regions and many have been operating for decades. Japan has a very dense network of GNSS stations called GNSS Earth Observation Network System (GEONET) that has been critical for observing many seismic events. Chile has a GNSS network that can track surface motions, as do many other countries in South America. The South American networks are not as dense as the one in Japan, but they can observe surface motions related to tectonic activity.

Here the authors developed an approach to find small surface motions associated with subduction zone tectonics using observations from over one thousand GNSS stations in Japan and over 40 stations in South America. The technique is called Greedy Automatic Signal Decomposition (GrAtSiD) and it isolates tectonically related surface motions that occur between earthquakes.


Results

Horizontal surface wobbles of 4 to 8 millimeters that trend perpendicular to strike of the eventual earthquake, mainly in an east to west direction, precede the 2010 Maule earthquake by 7 months. There is some downward vertical motion seen in Argentinian GNSS stations to the east of the subduction zone. Horizontal surface wobbles of 4 to 8 millimeters that trend perpendicular to the strike of the eventual earthquake in Japan, are found about 5 months before the 2011 Tohoku-oki earthquake. A part of the wobble starts about 1000 kilometers away near Taiwan and propagates to Japan. Modeling of the observations before the Tohoku-oki event suggests that rapid densification of the subducting slab causes a greater slab pull that led to the extension of the slab. It is suggested that together, these effects cause the surface wobbles and the foreshocks before the Tohoku-oki earthquake. The observations and modeling help to explain subduction zone processes and may help to identify precursors to large subduction zone earthquakes.

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Last modified: 2020-05-19  17:04:28  America/Denver