Upper mantle velocity structure in the different regions of the Earth is studied in detail to a depth of 660 km by analysis of P- and S-wave travel-time data of the teleseismic and local earthquakes. P- and S-wave velocities of this depth interval is the realm of regional travel-time tomography that includes follow procedures:
- Install an array of seismographs on a region;
- Record as many earthquakes as possible (usually at teleseismic distances: epicentral distance larger than 30⁰ );
- Measure the arrival times of P- (and S-) waves across the array;
- Subtract the times predicted using a radial reference model;
- Invert these residual times to build an image of the lateral variations of seismic velocities from the surface to a maximum depth that depends upon the aperture of the array.
There are two major seismic discontinuities in the mantle, at depths of about 410 km and 660 km. They are a result of solid-state transitions of the major minerals of mantle rocks.
A P-wave can convert into an S-wave at either discontinuity (410 km and 660 km) on its way up to a seismic station. When both P410s and P660s are observed, the time difference between the two arrivals provides a good measure of the thickness of the transition zone in the region beneath the station, with a lateral resolution better than a couple of hundred kilometers, depending on the azimuthal distribution of earthquakes used in the stack.