an estimate of post-seismic gravity change caused by the 1960 chile earthquake and comparison with...
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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!
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