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An estimate of post-seismic gravity change caused by the 1960 Chile earthquake and comparison with GRACE gravity fields Y. Tanaka 1, 2, V. Klemann 2, K. Fleming 2 and Z. Martinec 2 1 Geographical Survey Institute of Japan 2 GFZ Potsdam

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Contents Post-seismic deformation due to the 1960 Chile earthquake A new method to calculate post-seismic gravity changes including ‘slab effects’ Comparison with current secular gravity variations observed by GRACE Discussion and conclusions ‘Is a post-seismic relaxation over decadal time scales detectable by GRACE?’

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The ongoing post-seismic deformation caused by the 1960 Chile earthquake A decadal characteristic time is observed. Modeling studies using GPS and tide-gauge data indicate viscoelastic relaxation mechanism. e.g. Piersanti (1999), Lorenzo-Martin (2006) Tide-gauge station 25 yrs. event (Mw=9.5) (Barrient et al., 1992) South America fault 1,000 km

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A spectral finite-element approach (Martinec, 2000; Dahlen, 1972) enables these effects to be considered simultaneously. Semi-analytical approaches A new method to compute post- seismic gravity changes sphericity and self-gravitation strong lateral heterogeneities in the viscoelastic structure like a slab Fully numerical approaches complex geometry and heterogeneities approximated self-grav. effects arising from non-global modeling

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The fault model and the viscoelastic structure for the forward modeling We use the result of an inversion of GPS displacement data ( Lorenzo-Martin et al., 2006 ) A 2-D structure and incompressibility is assumed. 10 20 Pa s in the asthenosphere 10 23 Pa s in the slab, 10 30 Pa s in the lithosphere

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The predicted current inter- seismic deformation rates Both models agree with GPS horizontal rate data (Klotz et al., 2001). The differences in the vertical deformation are detectable with terrestrial measurements (GPS, AG…) Solid: without slab Dotted: with slab dip strike vert. grav. Eastward positive Northward positive No cut-off

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The effects of the slab on the lower- degree gravity potential fields Internal displacements for the first 50 years With slab 110 km Without slab The slab decreases the amplitude by 50% (0.2 0.1mm/yr). with slab A cut-off harmonic degree, j max =32 w/o slab relaxation hindered dominant stress

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mm/yr CSR The observed secular variations in the geoid height changes over South America GFZ Least-square fitting to all of the Level 2 data (2002-2007) Surrounding two strong signals due to the hydrological effects and ice-mass changes ( Ramillien et al., ‘06; Rignot et al., ‘06 ) apparent signals spread over the fault

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Comparison between the profiles along the dip direction (a) Raw (j max =32) (b) 400 km Gaussian post-seismic CSR GFZ The expected post-seismic signal is comparable with differences between results from two analysis centers.

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The observed secular variations in the geoid height changes (GIA corrected) The expected post-seismic signal is still comparable with differences between results from two analysis centers, after GIA signals are corrected. GIA model based on Klemann et al. (2007); Ivins & James (2004) -0.3 -0.4 -0.2 mm/yr CSR (400 km Gauss.) GFZ (400 km Gauss.)

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Comparisons between the profiles along the dip direction The GIA signal correction removes the long-wavelength offset, but the differences are still comparable. Uncertainties in modeling the hydrological effects will also mask the post-seismic signal. (c) GIA corrected post-seismic CSR GFZ (a) Raw (j max =32) (b) 400 km Gaussian

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Discussion and Conclusions The expected lower-degree post-seismic geoid height change due to the 1960 Chile event is 0.1 mm/yr when including the slab. Detecting the post-seismic signal and the effects of the slab is possible by GPS and AG, but very difficult by GRACE at present. Better constraints on the viscoelastic parameters vertical deformation data For events in other subduction zones with a lower viscosity inferred, expected rates will increase, which may be detected by GRACE.

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Effects due to compressibility on the post-seismic gravity change incompressible compressible [microgal/yr] 1-D spherically symmetric earth model (PREM), Tanaka et al. (2006) The amplitude is smaller for the compressible model when excluding a slab. j max =32 Blue color is positive!

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The differences in the internal maximum stress

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Comparison in secular gravity anomaly Theory GRACE

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