D43 First version of software implementation of model for VO-hydrological coupling – NVI

A study has been conducted of how to best discriminate between magmatic and hydrologic origin of surface deformation resulting from pressure sources in the crust. The preferred method is to use a combination of geodetic and gravimetric observations. A temporal change in gravity, Dg, can be expressed as:

The first term is the free-air gravity effect, the elevation change, Dz, scaled with the free-air gravity gradient at the surface of the Earth (308.6 mGal/m). The second term is due to change in ground water level,. The porosity of the crust is and is the density of groundwater. The term is due to density changes in the crust caused by deformation. For an isotropic deformation source (e.g., a sphere) in an elastic halfspace this effect is zero. The final term is the residual gravity change, caused by mass change associated with the deformation. If successfully estimated, it provides important constraints on deformation processes, in particular at volcanoes and geothermal areas. In the case of a Mogi point source of pressure, then this residual gravity change relates to the mass change of the source, DM, by:

where G is the Universal gravitational constant, d is the depth to the source, and r is the horizontal distance from the Mogi source to the observation site where the residual gravity change is measured. On the other hand, inversion of geodetic data (e.g., InSAR and GPS) for deformation sources gives the volume of the responsible source. Comparison of the volume estimate and the mass estimate then gives the “apparent density” of the material inflow to (or outflow from) the deformation source. Careful interpretation, considering compressibility of magma if magma chambers are involved, can discriminate between a magmatic or hydrologic origin of the deformation source. Realization of uncertainties involved is important.

Implementation of a VO-hydrological coupling model consists then of:

1. Collection of all available geodetic data on crustal deformation in a study area.

2. Interpretation of deformation data in the terms of a deformation source. Estimate uncertainties.

3. Evaluation of the volume of the deformation source from the deformation model.

4. Collection of data on temporal gravity change in the study area. Requires high-precision repeated gravity measurements especially dedicated to capture temporal changes.

5. Correct gravity data for fluctuations in water-table during the measurement period.

6. Infer mass change associated with the deformation.

7. Consider the compressibility effect of magma, if inflow/outflow of magma from a magma source is suspected.

8. Use “apparent density” to constrain the origin of the deformation in terms of magmatic or hydrothermal process.

This is the procedure that has been applied to a test area in Iceland, in a project lead by coworkers of NVI and IMO. The group leading the effort is the Volcano Dynamics Group, Open University, UK. A manuscript with results from the Askja volcano in Iceland has been submitted (Elske van Dalfsen et al., 2004)

Significant effort has also been made in studying EQ-hydrological coupling models. This work has been conducted in cooperation with Stanford and Harvard Universities, USA, as well as NEA. Poroelastic deformation has been documented as a process responsible for deformation in the South Iceland Seismic Zone associated with the June 2000 earthquakes. Modeling of this process and application to the 2000 earthquakes in Iceland are the topic of a paper published by Jonsson et al. (2003). The results have also been given in a series of presentations (see abstracts by Jonsson et al., 2003).