Crustal and Upper Mantle Structure from Joint Inversion of Body Wave and Gravity Data
LOS ALAMOS NATIONAL LAB NM
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We present a preliminary model of the three-dimensional seismic structure of the Iran region obtained via simultaneous, joint inversion of body wave travel time and gravity observations. The body wave data set is derived from location calibration efforts and includes a large 1000 events subset of GT5-level events. The arrival time data sets for these events include many readings of direct crustal P and S phases, as well as regional Pn and Sn and teleseismic phases. These data have been groomed to identify and remove outlier readings empirical reading errors are estimated for most arrivals from multiple event relocation analysis. We use both free-air and Bouguer gravity anomalies derived from the global gravity model of the GRACE satellite mission. The gravity data provide information on shallow density variations with short spatial wavelengths, and deeper density structures with longer spatial wavelengths. To increase the usefulness of the gravity data, we apply high-pass filtering, yielding gravity anomalies that possess higher resolving power for crustal and lithospheric structures. To integrate both data sets into a single inversion a functional relationship between seismic velocities and density is required. We first use a combination of two empirical relations one most appropriate for sedimentary rocks, and a linear Birchs Law that is more applicable to basement rocks. These relationships, however, assume a constant Poisson ratio throughout the inversion. We therefore also apply a method that allows the Poisson ratio to vary by mapping densities to VP and VS independently for different earth materials. Final results of the simultaneous inversion will help us to better understand the crustal and lithospheric structures associated with early-stage continental collision. Such models also provide an important starting model for computationally more expensive and time-consuming fully 3D waveform inversions.
- Geology, Geochemistry and Mineralogy