The Crust-Mantle Boundary

As an earthquake wave rises towards the surface of the Earth, it can encounter sharp changes in the speed of the waves. Such sharp changes can cause new waves to be created. For instance, at the boundary between the crust and mantle (called the Moho), a P-wave will often produce a new S-wave. Because S-waves travel more slowly than the P-waves, they arrive later at the surface. Just as you can count the number of seconds between seeing lightning and hearing thunder to know how far away a lightning strike was, seismologists can measure the time between the P-wave and the new S-wave and know how deep the boundary was that produced the S-wave. The size of the S-wave tells how different the rocks are on either side of the boundary.

Image of the crust-mantle boundary from seismology

Seismologists working in Yosemite have observed these S-waves and made images like that above. Red stripes are boundaries where the deeper rock has a faster wavespeed; blue stripes are where the deeper rock has a slower wavespeed. Colors are stronger where the S-waves were bigger. The blue triangles are seismometers. For many mountains, the Moho is deeper under higher mountains, but not here in Yosemite. The bright red stripe at about 35 km (21 miles) depth shows the Moho under the high part of Yosemite National Park, while to the west, under the foothills, the Moho gets fainter and deeper.