UNDERSTANDING CRUSTAL COMPLEXITY IN SEISMIC IMAGES

Among the problems that arise in presenting seismic images of the Earth's crust to general scientific audiences, to students, or to the public are conveying how the seismic method works, the scales involved, and the nature of geological and seismological models and observational proxies. Seismic waves behave oddly to human senses and experience. We see distinct objects along straight lines of sight. We don't often recognize refraction or diffraction phenomena in everyday life. They are the norm in seismology. All seismic waves refract in the Earth, and higher frequency seismic waves interact with heterogeneities about their own size along much of their transit in the crust, with the result that reflection images can be exceedingly messy. Although the crust appears to have variations on all scales, in the seismic band the minimum size of a discernible object is about that of a large building. Since much of our understanding of the Earth's crust results from synthesizing more tangible, usually smaller scale, geologic data with seismic images, there is a natural scale mismatch between field observations and the images. Both the geologist and the seismologist are presented with an overabundance of information that we force into intellectual models. The geologist's information from an outcrop can cover the scientific bases but is limited to the exposure at the surface, whereas the seismologist's information is of a single type, but can span the crust. What we choose to interpret in the data is driven by our models of what the Earth should be like, and in seismology, by the computational mathematics available for analysis. Even the most objective seismic images are inherently interpretational when viewed in a geologic context. Seismic velocity and impedance are imperfect proxies we use to ascribe geologic meaning: High and low velocities in the mantle represent colder and hotter rocks, in the crust they represent different lithologies (unless part of the rock has melted or has water in it), reflections of a certain type are faults, of another are intrusions, and so on. Despite these difficulties, seismic imaging and geological inference illustrate many of the basic tenets of science, making it a potentially rich pedagogical subject, as well as illustrating that modern science is a collaborative, cross-disciplinary endeavor.