HR: 14:55h
AN: V52E-06    [PDF]
TI: Distributed Source Model for Deformation Caused by Magma Reservoirs
AU: * Mann, D
EM: doerte@pangea.stanford.edu
AF: Dept. of Geophysics, Stanford University, 397 Panama Mall, Stanford, CA 94305 United States
AU: Segall, P
AF: Dept. of Geophysics, Stanford University, 397 Panama Mall, Stanford, CA 94305 United States
AB: Volume and pressure changes in magma reservoirs cause inflation or subsidence at the surface. The changes can be caused by accumulation, withdrawal or crystallization of magma. Deformation models are used to determine parameters like location, depth, extent and shape of the reservoir. In most models today, overly simple predefined source geometries are used. These may not only give an unrealistic image of the source shape, but also mislead when interpreting the underlying deformation processes. We present a model of distributed deformation sources that inverts for the volume strain in discrete subsurface units. The deformation sources consist of centers of dilatation and double forces. Solutions are sought that minimize a residual norm weighted by the data covariance and a regularizing functional that requires the source volume to be spatially compact. Compactness is realized by penalizing locations in the model space proportional to their distance from a target location. This compactness criterion distinguishes the modeling from the commonly used approach of spatial smoothing which would smear the source out over an unrealistically large volume, implying a magma body too dispersed to be viable over long time periods. To interpret the estimated distribution of point sources in terms of a physical model, we seek closed surfaces surrounding the sources on which the normal stress is constant, and the shear stress vanishes. These surfaces then represent an equivalent magma pressure increase or decrease in a magma chamber of uniform pressure. To define the absolute size of a magma body, additional constraints are needed. The modeling algorithm is applied to deformation data from Long Valley caldera between 1985 and 1999, which experienced nearly 1 meter of uplift during that time period. We find an inflation source with a center depth of about 8 km, and with a vertical extent of about 4 km. The total volume increase over the 14 year time period is 0.06 km3. We compare our results with those obtained from simple spherical and ellipsoidal models.
DE: 8145 Physics of magma and magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting