High-Harmonic Geoid Signatures due to Glacial Isostatic Adjustment, Subduction and Seismic Deformation

L.L.A. Vermeersen(1), H. Schotman(1), M.-W. Jansen(1), R. Riva(1) and R. Sabadini(2)

(1) DEOS, Fac. Aerospace Engineering, Delft University of Technology, Kluyverweg 1, NL-2629 HS Delft, The Netherlands, (2) Fac. Earth Sciences, University of Milan,Via L. Cicognara 7, I-20129 Milan, Italy

1 ABSTRACT

GOCE is expected to increase our knowledge of the higher spherical harmonics of the quasi-static geoid, with "higher" being in the range of about harmonic degree 50 (half-wavelength 400 km) to harmonic degree 250 (half- wavelength 80 km). One of the major challenges in interpreting these high-harmonic (regional-scale) geoid signatures in GOCE solutions will be to discriminate between various solid-earth contributions. Here, emphasis will be placed on three major contributors: remaining deviations from isostasy due to late-Pleistocene ice ages; shallow upper mantle subduction of oceanic lithosphere; and accumulated deformation due to sequences of large earthquakes. However, there are many more possible high-harmonic (shallow) solid-earth contributions, including uncertainties related to isostasy of a chemically and stratigraphically heterogeneous crust and lithosphere; tectonic processes like mounting building, continental plateau and oceanic basin formation; and high-harmonic signatures related to shallow mantle density variations and mantle-based processes as plumes. Discrimination between all these various causes might be accomplished by combining the geoid signal with other (space-)geodetic observables, geological data, seismic models and by 2-D pattern matching.

2 INTRODUCTION

The interpretation of GOCE geoid and gravity anomaly maps in terms of structure and dynamics of the Earth is neither simple nor straightforward. Observed geoid and gravity anomalies do not carry any direct information about their geodynamical cause or causes, and at the same time they do not pinpoint the depth or depths at which the observed signatures originate. Interpretation will thus depend on assumptions and on combining gravity data with other geodetic, oceanographic, glaciological, seismic, geological, etc., observations. Of the many solid-earth related processes that can induce significant high-harmonic (regional) geoid signatures, only a few will be treated in this contribution: glacial isostatic adjustment due to post-glacial rebound and concomitant sea-level variations, subduction of ocean lithosphere into the Earth’s mantle, and quasi-static deformation of the Earth’s crust after earthquakes.

3 GLACIAL ISOSTATIC ADJUSTMENT (GIA)

Glacial Isostatic Adjustment (GIA) of the solid Earth due to the rise and fall of Pleistocene Ice-Age cycles has created geoid and gravity anomalies, although the separation between GIA-induced contributions and those induced by plate tectonic and mantle convection is not always obvious. For example, it is now widely acknowledged that the deep geoid low above Canada is partly due to non-GIA induced lithosphere and mantle heterogeneities and partly due to GIA (e.g., [1]). In [2] it has been shown that the inclusion of a crustal low viscosity zone in earth models for GIA simulations induce high-harmonic patch-like features in the geoid with typical wavelengths of 50 – 1,000 km and magnitudes on the cm to m level. Further simulations [3] have shown that for a geoid accuracy of 1 cm with a spatial resolution of 100 km or better these GIA-induced high-harmonic geoid patterns should be detectable by GOCE up to about degree 150 with a half-wavelength of about 130 km at the equator. An example of the influence of variations in viscosity of the crustal low-viscosity zone is presented in Fig. 1.

______Proc. Second International GOCE User Workshop “GOCE, The Geoid and Oceanography”, ESA-ESRIN, Frascati, Italy, 8-10 March 2004 (ESA SP-569, June 2004)

Fig. 1. Geoid anomaly perturbations due to a low-viscosity crustal zone: sensitivity to viscosity variations.

The three panels of Fig. 1 show differential geoid anomaly perturbations above Canada for three viscosity values of a low-viscosity crustal zone between 20 and 40 depth: 1018, 1019 and 1020 Pa s, from top to bottom, respectively. The low-viscosity crustal zone is embedded in a 115 km thick elastic lithosphere. The earth model is radially stratified, spherical, self-gravitating, having a linear Maxwell rheology for the mantle with an upper mantle viscosity of 1020 Pa s and a lower mantle viscosity that is 10 times larger. The core is assumed to be inviscid. As Pleistocene ice loading model the ICE-3G model [4] is used, extended by a linear ice load growth phase of 90,000 years and by 7 complete glacial pre-cycles of 100,000 years each. The geoid anomalies are differential in the sense that the panels show the difference between the geoid calculated with and with the low-viscosity crustal zone. It is clear from Fig.1 that GOCE should be able to discern most of the differences visible between the three panels. A similar kind of study with respect to ice load variations is given in [5].

4 GIA AND SUBDUCTION IN THE ADRIATIC

An example of a region where two geodynamical processes might contribute to high-harmonic geoid signatures is the Adriatic coast of Italy.

Fig. 2. Map of sea-level changes in the Adriatic in mm/yr over the past 2,000 year derived from Roman artefacts [6].

From Aquileia in the north to Apulia in the south, a concave pattern of relative sea-level rise becomes discernible in Fig. 2. Simulations of relative sea-level change induced by subduction of oceanic lithosphere into the mantle under the Italian peninsula show a pattern of high relative sea-level rise in the north falling to low relative sea-level rise in the south, as is depicted in the top panel of Fig. 3.

Fig. 3. Simulated sea-level changes for mantle subduction (top), GIA (middle) and GIA + subduction (below). The opposite pattern, of low relative sea-level rise in the north and high relative sea-level rise in the south, is visible in the GIA-simulation of the middle panel of Fig. 3. It is clear that neither the process of subduction (top panel) nor GIA (middle panel) can explain the observed concave pattern of Fig. 2, but the combination of the two processes does, as is shown in the lower panel of Fig. 3 [6]. This example shows that 1-D or 2-D pattern matching might offer a possibility to discriminate between various geodynamical contributors in interpreting observed geoid anomalies.

5 SEISMIC DEFORMATION

Quasi-static deformation of the Earth after an earthquake will result in regional geoid and gravity changes that might be detectable by GOCE if during its mission a huge or a sequence of strong earthquakes would occur. An example of the induced co- and postseismic geoid anomaly perturbations after the 1964 Alaska earthquake illustrates this.

Fig. 4. Coseismic (top) and postseismic (1964 – 2004) geoid anomalies following the 1964 Alaska earthquake. The Good Friday magnitude 9.2 earthquake ruptured at about 30 km depth under northern Prince William Sound and extended horizontally for about 800 km, more or less parallel to the Aleutian trench. A crustal area of about 200,000 km2 was deformed with maximum vertical displacements up to 10 m. The top panel of Fig. 4 shows the predicted coseismic geoid anomalies associated with the main faulting event of this 1964 Alaska earthquake. Over an area of more than 500 x 100 km the geoid anomaly perturbations are larger than 10 cm, implying that if such an earthquake would occur during GOCE’s lifetime it could be detected. A simulation of the geoid variations associated with postseismic deformation over the past 40 years is shown in the lower panel of Fig. 4. The spherical earth model used in the simulation consisted of a 120 km thick elastic lithosphere, an upper mantle with a viscosity of 5 x 1020 Pa s and a lower mantle that is 10 times more viscous. The lower panel of Fig. 4 indicates induced geoid anomaly perturbations of about 1 cm over the same area, due to upper- mantle relaxation. This would be below the detection level of GOCE, although this magnitude could be considerably larger if crustal low-viscosity zones would be present in the region these values could be much larger.

6 CONCLUSIONS

Only a limited amount of (solid-earth) geodynamical processes that contribute to geoid and gravity signatures has been treated here, and it remains to be seen in how far one will be able to discriminate between the various processes once GOCE data become available. Apart from combining GOCE data with seismic, geodetic, geological, etc., data sets, it might prove fruitful to correlate1-D and 2-D patterns in geoid and gravity from GOCE observations with those expected from the various geodynamics processes as seen in numerical simulations. The case of the sea-level variations at the Adriatic coast of Italy, induced by at least two processes, subduction of ocean lithosphere and glacial isostatic adjustment, illustrates this. However, in this respect one should keep in mind that the deep geoid low over Canada, once attributed to GIA alone, is presently considered to be a combined effect of isostasy deviations due to GIA and to mantle convection.

7 REFERENCES

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