After yesterday's post on a new feature I found on the USGS earthquakes site, reader Tony Edger asks, "After exploring the USGS website and elsewhere without much success, I am hoping you might steer me to a description of how to read a seismicity cross section. " He was referring to these images:

So here's how this works: the top image is a map. It gives you a "bird's eye" perspective on earthquake locations at the subduction zone near Samoa. It shows you the epicenters (location on the earth's surface above a quake's actual location, called its "focus" or "hypocenter") of many earthquakes, along with Tuesday's big quake, shown with a star. The thick red line is the position of the trench, a bathymetric expression of the subduction zone. The epicenters are color-coded for their depth. Orange and yellow are shallow; green and blue are medium depth; and purple and red are the deepest. Notice that they make a sort of "rainbow" pattern, with the shallowest quakes in the east, and the deepest quakes in the west. This is "looking down" on the subducting slab: it's like we're able to "see" the subducting slab as it descends into the mantle.

The lower image is the cross-section. It gives you a "gopher's eye" perspective on the same data. A cross section is drawn along the line A-A' on the map. This is conceptually slicing the Earth open along that line, then removing half, and looking sideways at the remaining half. Note that the A-A' line is now along the top of the figure, representing the surface of the earth. Along the horizontal axis is horizontal distance, measured in kilometers. Along the vertical axis is depth, also measured in kilometers. The two axes are not drawn to exactly the same scale, but pretty close. In other words, 100 km of horizontal distance is approximately equal to 100 km of vertical distance (depth). The same data are plotted, or at least the subset of the map's data which happen to fall on that particular line, A-A'.

With this new perspective, a side-view, what do we see? Well, there's the star, which shows the depth of the quake that triggered all this discussion, and a whole bunch of other (historical) earthquakes. Now, instead of the epicenter being plotted, we're getting a more robust sense of the hypocenter (or focus). Note that the earthquakes are being generated in a big swath, starting at the surface in the northeast, and dipping down deeper and deeper to the southwest. This line of seismic activity reflects the jerking passage of the subducted slab of oceanic lithosphere. As it plunges down, it generates lots of shaking. This zone of seismicity was first described (independently) by two scientists, Kiyoo Wadati and Hugo Benioff: in their honor, it is referred to as the Wadati-Benioff zone. (Wikipedia has more) Their realization is our gain: we can "see" the subducted plate diving at an angle of 30 to 40 degrees. That's what's so cool about this:

Something that no human will ever directly observe is "visible" to us because we can pinpoint the three-dimensional location of thousands of earthquakes. These bumps and jolts reveal the position of the bumper and jolter: the subducting plate!

Labels: analogies, art, earthquakes, plate tectonics, websites