Götz Bokelmann: Research

Laboratory of Tectonophysics
University Montpellier, France

Current Research Projects:

Plate-Mantle Interaction and Forces that Drive the Plates >>Papers. Although the concept of plate tectonics is universally accepted, it is incomplete in that there is no agreement on the mechanism that drives the plates. It has been suggested that they are driven from the side, that is pulled by subducting slabs and pushed by negative buoyancy forces from subsidence of oceanic plates ("ridge-push" force). Another view is that plate motion is driven by mantle convection. The motion of the plates can be explained by either of these concepts. Seismic anisotropy, on the other hand, records deformation at depth, and may in principle determine the sense of relative motion between plates and deeper mantle. We suggest that the pattern of deformation in the deep continental roots under the shield areas gives important insights into the question of driving forces, since they represent mechanical obstacles in the relative motion between plates and deeper mantle. Seismic anisotropy may allow to determine the sense and also the level of shear stress at the base of the plates. The technique is so far based on the analysis of teleseismic P-wave delays. One of the results of this study is that the deeper mantle helps to drive the North American plate. These and related questions were discussed in a union session at the fall 2000 AGU meeting. Recently Peter Bird confirmed the view that the mantle under North America helps to drive plate motion. We are now studying deep deformation under the European/Eurasian continent in the framework of the project SEADOME ("Seismic anisotropy, plate-driving forces, and the motion of Europe"). We hope to address the question of which role the Earth's mantle plays for the motion of the European plate. Does the Earth's mantle under Europe help the motion of the plate, or does it rather resist it? The answer may not necessarily be the same as for the North American plate.

Mechanics of Plate Boundary Systems >>Papers. Some important questions in the context of plate boundary deformation are: How is plate boundary deformation accomodated at depth? What is the role of the lower crust in crustal deformation (detachment, block rotation, depth extension of faults)? Does it decouple the upper crust from the mantle? Do crust and mantle share a coherent deformation? How are driving forces for crustal deformation applied, e.g., do they act from the side or from below? Is the lower crust mechanically weak or strong? And how does the weakness/strength of the lower crust relate to the weakness of major faults in the upper crust? Is their long-term strength as low as the heat flow paradox suggests? And how are they loaded before there is a large earthquake? What role do crustal fluids play in this? We have developed a new tool for studying stress at depth, which allows us to address these questions? It uses a proxy for the shear traction on horizontal planes (e.g., the free surface has very small shear traction). In Northern California, the surface and also the base of the seismogenic crust show up with low shear traction. This suggests that the lower crust is mechanically weak, or that perhaps it relaxes with time. Interestingly, this behaviour occurs on both locked and creeping sections in Northern California. This technique also allows important insight into depth-dependent properties of fault zones.

Time Dependent Earth Properties >>Papers. The shape of Earth's surface varies over geological time scales and similarly does the structure of Earth's interior. But are there measurable changes also on time scales of the duration of a scientific project, say within 1-3 years? Our primary measurements are seismic waves, and for waves that propagate along the same paths at different times one can measure temporal changes in the medium. We have striking observational evidence that the medium seen by seismic waves is altered during large earthquakes, e.g. after the 1989 Loma Prieta earthquake in Central California (medium change over the 6 years following the event) and it slowly recovers in the following years. Such temporal changes are primary indicators for the dynamics of fault zone behaviour, e.g. fluid migration in the vicinity of the earthquake rupture, stress propagation, and perhaps fault corrugation. If we can measure temporal changes very accurately, we may use the technique to measure the buildup of mechanical stress at seismogenic depths, that is where the earthquakes occur, and to measure fault zone corrugation in situ. Such techniques can also be used for monitoring oil and gas reservoirs. Beside natural sources we also use man-made sources, which run continuously and emit waves that are measured to distances of up to several hundred km. CHECK OUT OUR SPLITLAB PAGE! http://www.gm.univ-montp2.fr/splitting/

For previous research interests (at Stanford University) click here .

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