An estimate of post-seismic gravity change caused by the 1960 Chile earthquake and comparison

with GRACE gravity fields

Y. Tanaka1, 2, V. Klemann2,

K. Fleming2 and Z. Martinec2

1 Geographical Survey Institute of Japan2 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

112 4 G

Iug r00

),(,, jmYu

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.

1020 Pa s in the asthenosphere

1023 Pa s in the slab, 1030 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 slabDotted: 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% (0.20.1mm/yr).with slab

•A cut-off harmonic degree, jmax=32

w/o slab

relaxation hindereddominant 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 (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

Comparison between the profiles along the dip direction

(a) Raw (jmax=32) (b) 400 km Gaussian

post-seismic

CSRGFZ

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)

-0.3

-0.4

-0.2

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 (jmax=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

incompressiblecompressible

[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.

jmax=32

Blue color is positive!

The differences in the internal maximum stress

Comparison in secular gravity anomaly

Theory

GRACE