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Analytical Joint inversion of the 1982-1999 surface deformations and gravity changes at Long Valley caldera

Mehdi Nikkhoo, & Eleonora Rivalta

Submitted September 11, 2022, SCEC Contribution #12005, 2022 SCEC Annual Meeting Poster #184

Pressurization of magma chambers, due to magmatic intrusions or magma degassing, may lead to surface displacements and gravity changes, which can be measured by geodetic instruments. The gravity changes are caused by 1) the net mass of the intrusion, and 2) the deformation-induced density changes of the country rocks. The deformation effects consist of contributions due to the density changes at the surface and chamber-rock boundary caused by the surface displacements and chamber volume change, respectively, and density changes due to volumetric strain throughout the country rocks. In order to constrain the intrusion mass from gravity data, the deformation-induced gravity changes need to be constrained. The contributions associated with the Earth’s surface can be determined from vertical displacements; however, the contributions associated with the chamber volume change and country rock strains can only be inferred from a deformation model. We propose that the deformation-induced gravity changes, if properly formulated in the inversion, can provide better constraints on the deformation source parameters including the mass change. In such a formulation, due to the coupling between deformation-induced gravity changes and deformation, gravity changes may even substitute vertical or horizontal displacements if these are missing. We show this by applying the Nikkhoo&Rivalta (2022) model for pressurized triaxial chambers to the 1982-1999 deformation and gravity changes at Long Valley caldera. The available observations besides gravity change are the horizontal distances (EDM) and vertical displacements (leveling). As a proof of concept for our idea we perform a joint inversion involving the gravity changes and horizontal distances only. We then show that the source parameters are in good agreement with joint inversions involving all datasets. We show that the predicted vertical displacements from our inversion compare well with the observed ones. Recalling that, if the source shape is unknown, the source depth can be constrained only if both horizontal and vertical displacements are available in inversions, our results imply that the gravity data provide the information, to the extent that the data uncertainties allow, that the vertical displacements could provide. We discuss the implications of our method for future deformation-gravity joint inversions at volcanoes and propose that this method may potentially have applications to earthquakes and fault zones.

Key Words
Gravity change modelling, Volcano deformation, Joint deformation-gravity inversions

Citation
Nikkhoo, M., & Rivalta, E. (2022, 09). Analytical Joint inversion of the 1982-1999 surface deformations and gravity changes at Long Valley caldera. Poster Presentation at 2022 SCEC Annual Meeting.


Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)