Earthquake distributions get you most of the way there, e.g., compare the locations of earthquake with that of plate boundaries. More recently, definition and identification of individual plates (along with understanding directions of motion) have been aided by a variety of geodetic data, chiefly GPS (e.g., this map showing velocity vectors as determined by individual permanent GPS stations). With these type of data, you can relatively quickly begin to identify areas that are "torsionally rigid", i.e., their motion can be described by a single rotation (plate motions, becuase they're occurring on a sphere are best described as rotations which we can define with an Euler Pole and an angular rotation rate and direction about that pole) with deformation (as signified by earthquakes) localized along their edges and with limited internal deformation, i.e., plates.

In detail though, if you look at either the distribution of earthquakes or the GPS velocity vectors, you'll see that some plate boundaries appear much more messy than others. While some boundaries (like the majority of mid-ocean ridges) are relatively distinct and effectively are represented by a single fault, many others tend to be better thought of as relatively wide zones (and this is exactly how people who study these processes describe them, i.e., plate boundary zones). Good examples of areas better described as plate boundary zones are regions like the Himalaya or much of the western United States. In these cases, we tend to pick a single large mapped structure (e.g., in portions of the the western US, the San Andreas fault) to define as the formal plate boundary, but deformation related to the plate boundary extends well beyond this single structure.

At subduction zones, can the effect of the (presumably) lower temperature and so higher density of the subducted plate on seismic waves be used to map the boundary?

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CrustalTrudger comes through, as usual.

I would add only that I had the opportunity to participate in several seafloor mapping projects using side-scan sonar, and we mapped out some of these plate boundaries on the East Pacific Rise, Gorda Ridge, and Juan da Fuca Ridge. Actually "flying" the instrument through these canyons and mountains really brought them to life, and in great detail.

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Seismic tomography, which exploits changes in seismic wave speed as a function of temperature and other properties to "image" the interior structure of the lithosphere and mantle, can help fill in some details of the geometry of plate boundaries with depth, but I'm not aware of an application where it's been used to reveal the location of a plate boundary we didn't already know about through other means.

Thank you.