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WHO WE ARE & WHAT WE DO

The Global Seismology Research Group resides at UC Berkeley, within the Department of Earth and Planetary Science and the Berkeley Seismological Laboratory. Our research focuses on structure and dynamics of the deep earth, from the crust to the inner core. We tackle theoretical wave propagation problems in complex 3D media, including forward modeling and full waveform tomographic inversion (FWI) for elastic as well as anelastic structure. In order to better understand the chemical and thermal state of the mantle and the processes operating therein, we seek to apply the latest findings of the mineral physics community within the context of our seismic probing. We also study earthquake source mechanisms and scaling laws, as well as global seismic moment release and its relation to plate tectonics. One of our recent research directions concerns the Earth's "hum" and the insights it brings to ocean/atmosphere/solid earth interactions. Another one is "Box Tomography", i.e. how to perform FWI of remote target regions in the deep earth while keeping the computational cost manageable. Our research is supported through a variety of sources, mostly through grants from NSF.

Group photo June 2022 at Berkeley, from left to right, Li-Wei Chen, Yuancong Gu, Chao Lyu, Sylvain BRisson, Mrinal Darsun, Utpal Kumar, Federico Munch, Barbara Romaowicz, Dan Frost

Group photo May 2022 at Berkeley, on the occasion of Li-Wei's graduation. From left to right, Sylvain Bresson, Yuncong Gu, Chao Lyu, Mrinal Dursun, Federico Munch, Li-Wei Chen, Heng-Yi Su, Barbara Romanowicz, Utpal Kumar, Claire Doody (from Richard Allen's group)

Group photo May 2015 at Berkeley, from left to right, Kaiqing Yuan, Alizée DUBOIS, Barbara Romanowicz, Heidi Fuqua, Laura Salmi, Anais Ibourichene, Thomas Bodin, Dorian Soergel, Corinna Roy

Group photo Dec 2013 - after the AGU meeting in San Francisco. From left to right: Zhao (Allen) Zheng, Huaiyu Yuan, Pierre Clouzet, Scott French, Barbara Romanowicz, Florian Rickers, Marco Calo, Thomas Bodin, Joanne Adam, Sergi Ventosa

RECENT HIGHLIGHTS

2021: Dynamic history of the inner core constrained by seismic anisotropy: The inner core is seismically anisotropic with P-waves travelling along the rotation axis slightly faster than in the plane of the equator. This anisotropy is known to be stronger in the west then in the east and has previously been parameterised as being separated into distinct, sharply-bounded hemispheres. Using new data, in combination with 20 years of observations from the Berkeley catalogue, we found instead that the strength of anisotropy smoothly increases with depth into the inner core, and that the strongest anisotropy is offset into the western hemisphere. We built a combined geodynamic and mineralogical model of the inner core that suggests that the observed pattern of anisotropy results from growth of the inner core that is faster at the equator than at the poles, and faster in the east than in the west. Our model has implications for the viscosity, age, and heat flux of the inner core, and thus the geodynamo. This work was published in Nature Geoscience in 2021."

2018: Building from our 2014/2015 whole mantle radial anisotropy model SEMUCB_WM1 (see below), we have developed a 3D global shear attenuation model SEMUCB_UMQ, the first one to be based on accurate forward wavefield computations using the Spectral Element Method. The construction of this model by full waveform inversion of 3-component long period waveforms (including fundamental mode and overtone surface waves) followed an iterative approach where the 3D elastic and anelastic structure were alternatively updated. Find abstract, codes to read and plot the model, and related papers here

2017: Post-doc Kai Yuan used Sdiff waveforms to investigate the root of the Iceland mantle plume and detected the presence of a mega-Ultra Low Velocity Zone (ULVZ), about 900 km in horizontal extent. Because it was possible to illuminate this ULVZ from several different directions, we documented that the ULVZ is, to first order, quite circular at its base, suggesting that it may be manifesting the presence of partial melt (likely enriched in iron). This work was published as Yuan and Romanowicz (2017, Science). This was the 3rd mega-ULVZ found at the root of a large mantle plume, and suggests it might be worthwhile to look for large ULVZ's beneath other "fat plumes".

2015: We have now extended this waveform imaging methodology to the WHOLE mantle. The technical details of the resulting whole mantle global shear velocity model (SEMUCB_WM1, sometimes known as SEMUCB-WM1) can be found in French and Romanowicz (2014, Geophys. J. Int.). This model shows several interesting features. In particular it clearly shows the presence of fat "mantle plumes" extending from the vicinity of the core-mantle boundary through the lower mantle under those hotspot volcanoes that are located over the large low shear velocity provinces (LLSVPs), two major antipodal structures well documented to cover equatorial regions under the Pacific ocean and under Africa. These plumes must be much wider than the classical thermal plumes expected in a purely thermally driven convecting mantle and are likely of thermo-chemical nature, drawing material of different composition from the base of the mantle, and rising through a very sluggish background lower mantle. For more details, see our paper recently published in Nature (French and Romanowicz, 2015, Nature). Find abstract, codes to read and plot the model, and related papers HERE

2013: Applying a new waveform imaging methodology that takes advantage of accurate numerical seismic wavefield computations, Barbara Romanowicz's group has constructed a global shear velocity model in the upper mantle that reveals the presence of low velocity channels at the base of the oceanic asthenosphere. In a paper recently published in Science (http://www.sciencemag.org/content/342/6155/227.full), graduate student Scott French, former graduate student Vedran Lekic (now assistant professor at the University of Maryland) and Barbara Romanowicz show that these quasi-periodic finger-like structures of wavelength ~2000 km, stretch parallel to the direction of absolute plate motion for thousands of kilometers. Below 400 km depth, velocity structure is organized into fewer, undulating but vertically coherent, low-velocity plume-like features, which appear rooted in the lower mantle. This suggests the presence of a dynamic interplay between plate-driven flow in the low-velocity zone, and active influx of low-rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer. Hotspots are not the direct consequence of plumes impinging on the lithosphere. Find abstract, codes to read and plot the model, and related papers HERE

NEW CHANGES

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