Seismic tomography and anisotropy: studies of intraplate seismic zones
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Both body and surface wave tomography has been applied to some significant intraplate seismic zones to explore the cause of intraplate earthquakes. Pn tomography in the Central and Eastern United States reveals that the major intraplate seismic zones are all near the edges of high-velocity anomalies in the upper mantle. It indicates that the origin of intraplate earthquakes could be related to the rheological boundaries around rigid lithospheric roots where stress may accumulate. A joint teleseismic and local P tomographic study shows that the New Madrid Seismic Zone is associated with a local, NE-SW trending low-velocity anomaly in the lower crust and upper mantle, suggestive of a weak zone caused by local deformation due to rheological contrast. Rayleigh wave tomography in the northeastern Tibetan Plateau discovers a lithospheric velocity boundary that separates the low-velocity northeastern Tibet and the high-velocity Ordos and Sichuan blocks. Again, the velocity contrast zone is also linked to significant intraplate earthquakes.