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Post-seismic deformation due to pore-pressure transients observed by InSAR

Sigurjon Jonsson(1) and Grimur Bjornsson(2)

(1) Harvard University, 20 Oxford Street, Cambridge, MA 02138, United States
(2) Iceland GeoSurvey, Grensasvegur 9, 108 Reykjavik, Iceland

Abstract

Large earthquakes alter the stress in the surrounding crust leading to triggered quakes and aftershocks. Several time-dependent post-seismic processes can further alter the stress state and change the tendency for triggered events. Such processes include (1) afterslip on the mainshock rupture, (2) viscous flow of the lower crust and upper mantle, and (3) pore-fluid flow caused by the coseismic pore-pressure changes. Although it has proven difficult in many cases to distinguish between these post-seismic mechanisms, we present an example from South Iceland where geodetic measurements and modeling calculations can help us to do so.

Satellite radar interferometric (InSAR) observations of post-seismic deformation after the two June 2000 Mw6.5 earthquakes in South Iceland are consistent with simple models of poro-elastic rebound during the first few weeks after the earthquakes, but inconsistent with both afterslip models and visco-elastic relaxation. Interferograms from subsequent periods show that the duration of the deformation transient was 2 months. Afterslip is not a plausible explanation of the fringe pattern, as it would demand left-lateral back-slip as well as vertical slip, neither favored by Coulomb stress changes on the faults. Visco- elastic relaxation is also not plausible for two reasons: (1) the two month duration of the transient deformation would require very low viscosity (~10^17 Pa s), and (2) the observed deformation is too localized near the faults to be explained by lower crustal or mantle processes.

Our poro-elastic interpretation is further supported by water level changes observed in numerous boreholes throughout the epicentral region. The sign of the coseismic water level changes exhibits a quadrantal pattern consistent with the predicted undrained response. The post-seismic water level recoveries have a spatial distribution consistent with the InSAR observations. While the post-seismic behavior varies from well to well, the duration of the water level recovery of most wells is 1-2 months, consistent with the two month long deformation signal observed in the interferograms.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry