Seismic velocity structure of the continental lithosphere from controlled source data

Abstract

The purpose of this chapter is to provide a summary of the seismic velocity structure of the continental lithosphere, i.e., the crust and uppermost mantle. We define the crust as the outer layer of the Earth that is separated from the underlying mantle by the Mohorovičić discontinuity (Moho). We adopted the usual convention of defining the seismic Moho as the level in the Earth where the seismic compressional-wave ( P -wave) velocity increases rapidly or gradually to a value greater than or equal to 7.6 km sec −1 (Steinhart, 1967), defined in the data by the so-called “ Pn ” phase ( P-n ormal). Here we use the term uppermost mantle to refer to the 50–200+ km thick lithospheric mantle that forms the root of the continents and that is attached to the crust (i.e., moves with the continental plates). This summary has been preceded by 90 y of intense scientific activity. Mohorovičić (1910) was the first to publish an estimate for crustal thickness (54 km near Zagreb, Croatia), and to describe the seismically defined boundary between the crust and mantle that now bears his name (often shortened to “the Moho”). The fact that the oceanic crust is significantly thinner than continental crust (5 km versus about 40 km) was documented 40 y later (e.g., Hersey et al., 1952). Numerous later studies demonstrated that the continental crust varies in thickness from about 15 km to greater than 70 km beneath the Tibet plateau. Jarchow and Thompson (1989) provide a useful summary of early crustal studies, and Table 1 provides additional references. We emphasize results from active-source seismic refraction profiles that provide detailed P -wave velocity information. The shear-wave ( S -wave) structure of the crust and uppermost mantle, as determined by surface waves, is discussed, for example, by Ekström et al. (1997) and Ritzwoller et al. (1998).