IMAGING OF CRUSTAL SEISMIC ANISOTROPY: HOW WE MEASURE IT AND WHAT DOES IT MEAN?

Processes in the deeper crust play an important role in continental deformation, and whether new structures are created or pre-existing ones are rearranged, these processes produce features that can be revealed by seismic waves. In all tectonic regimes, ductile flow or deep shear zones in the middle to lower crust are thought to accommodate brittle faulting in the upper crust. Numerical models support the idea that crustal flow must occur during the formation of high continental plateaus, as well as during their collapse. Petrological studies highlight a range of large-scale mixing processes between the crust and mantle—including foundering of the lower crust and underplating of subducted continental margins—that may play major roles in the evolution of the crust. Despite the widespread acceptance of these ideas, many details of the relationship between crustal deformation processes and its seismic imprints remain unclear. This situation can be attributed to a number of factors, including the heterogeneity of the crust, the poorly constrained rheology, the rarity of lower crustal seismicity, the difficulty of imaging the lower crust, and the scarcity of surface exposures of the lower crust. Recent developments in seismic imaging of crustal anisotropy hold promise to improve this problem. A diverse set of seismic waves exist that can offer unique imaging of subsurface anisotropy. These waves differ by source (earthquakes, explosions), vertical/horizontal ray path (local, regional, teleseismic earthquakes), resolving ability (0.1 to several km), and phases (P-, S-, and converted-body waves, and surface waves). A major hindrance in this effort, however, remains our incomplete understanding of the possible causes, scales, and relationships of structural fabrics to seismic anisotropy in the crust. Clearer communications among seismologists, mineral physicists, and structural and metamorphic geologists will be required to advance this effort. In this talk, I will review the basics of the seismic techniques, highlight the major assumptions and limitations, present several examples of crustal seismic anisotropy studies, contrast this to the mantle proxy (anisotropy=flow or strain), and conclude with a list of questions where interdisciplinary discussions are needed to answer the question, “what does it mean?”