Seismic studies of the lithosphere and asthenosphere over the last quadrennium have taken full advantage of recent advances in sensor and computer technology with concomitant increase in data volume, dynamic range, and resolution. The IRIS (Incorporated Research Institutions for Seismology) consortium has implemented first order improvements in portable array seismic studies of the crust and lithosphere (PASSCAL) and in the global seismic network (GSN). PASSCAL came of age with the development of a national pool of standardized, field-rugged seismic equipment used in experiments worldwide. The GSN now numbers 62 broad-band seismic stations which, along with GEOSCOPE (the French global network 22 stations) provides key data on global variations in the lithosphere and asthenosphere. This report covers some of the recent results from PASSCAL and other portable seismic array experiments, as well as analyses of regional network and global data, that pertain to the lithosphere and asthenosphere.
The debate continues as to what exactly are the lithosphere and asthenosphere. Seismologists tend to equate the lithosphere to the crust and the high velocity lid below it, and the asthenosphere to the low velocity zone (LVZ) seen beneath the oceans and in some places under the continents, though not well developed beneath cratons. I adopt that definition in this discussion.