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

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?’

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

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

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.  Pa s in the asthenosphere Pa s in the slab, Pa s in the lithosphere

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

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

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 ( )  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

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.

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) mm/yr CSR (400 km Gauss.) GFZ (400 km Gauss.)

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

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.

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!

The differences in the internal maximum stress

Comparison in secular gravity anomaly Theory GRACE