Research

Together with collaborators Max Rudolph at Portland State University and Carolina Lithgow-Bertelloni at University College London, we used a novel inversion framework to detect the presence of an increase in viscosity in the Earth’s mid mantle. As Science magazine put it: A mysterious mid-mantle slowdown The viscosity of Earth’s deep interior plays a key role […]

Using data from the EarthScope Transportable Array, we observed and modeled conversions of S waves to P waves across velocity contrasts within the lithosphere across the Western United States. You can read more about our findings in Lekic and Fischer, 2013 and Hopper et al. 2014, but this cartoon provides a nice summary:

Conversions of teleseismic shear (S) waves to compressional (P) waves across velocity interfaces (such as the Moho or the lithosphere-asthenosphere boundary) allow us to map out variations in plate thickness across Southern California. Anomalously thin lithosphere is observed beneath rifted regions, such as the Salton Trough. The stretching and break-up of lithosphere through a process

When cluster analysis is used to classify the profiles of shear velocity (Vs) into N groups whose members are similar, the geographic extents of these groups mirrors the surface expression of tectonics. When N = 2 (panel a), one cluster corresponds to oceanic regions and the other to continental ones. As N increases, ocean basins

Map of lateral variations in the velocity of seismic shear waves (S-waves) at a depth of 125 km. Warm colors denote slower wavespeeds, and are thought to indicate warmer temperatures and/or the presence of melt or water. Cool colors denote faster wavespeeds, and indicate cool regions, such as the mantle beneath the ancient, stable portions

Preferred model (solid line) of frequency dependence of attenuation within the absorption band compared with constraints from laboratory studies (hachured region) and the frequency-independent assumption (dashed line). In our model, α is approximately 0.3 at periods shorter than 200 s, decreasing to 0.1 in the period range 300–800 s, and becoming negative (−0.4) at periods longer than

Isotropic shear wavespeed variations in the upper mantle from the SEMum model [Lekic and Romanowicz, 2008]. The model is developed using the spectral element method to perform full waveform modeling. Note the structural complexity in the uppermost 100-200 km which must be corrected for in order to ensure robust scattered wave imaging. The slow velocity