Seismic tomography and anisotropy: Studies of intraplate seismic zones.

摘要

Unlike interplate earthquakes which can be readily explained by relative motions at plate boundaries, intraplate earthquakes within stable continents have puzzled scientists for decades. To understand the velocity and anisotropy structures beneath the intraplate seismic zones and further constrain their cause, both body and surface wave tomography has been applied to some of the most significant intraplate seismic zones in the world.;Pn tomography in the Central and Eastern United States reveals that the major intraplate seismic zones at New Madrid, Charleston, East Tennessee, and New England 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. This idea is further supported by a Pn anisotropy model that shows large anisotropy (presumably local deformation) surrounding these high-velocity blocks.;A joint teleseismic and local P tomographic study has been performed to explore the enigmatic intraplate earthquakes in the New Madrid seismic zone. The results show that the seismically active zone is associated with a local, NE-SW trending low-velocity anomaly in the lower crust and upper mantle. Combining the result from Pn tomography, the low-velocity lithosphere is suggestive of a weak zone caused by local deformation due to rheological contrast. This weak source in the lithosphere may shift stress upwards when loaded, thus leading to repeated crustal earthquakes.;The northeastern Tibetan Plateau and North China are other areas where numerous intraplate earthquakes are located. Significant intraplate earthquakes have also occurred at the edges of the Ordos Plateaus and the Sichuan Basin that border the eastern Tibetan Plateau. Rayleigh wave tomography in this area demonstrates the presence of a low-velocity, presumably weak lithosphere beneath the northeastern Tibetan Plateau and a high-velocity, presumably rigid lithosphere beneath the Ordos and Sichuan blocks. Again, the earthquake locations are linked to the low-velocity lithosphere that surrounds rigid lithospheric roots, suggesting that the lithospheric rheology contrast may lead to intraplate seismicity. Moreover, the velocity and anisotropy models imply that an east-southeastward deformation field down to the asthenosphere exists in the study area. This deformation field is possibly associated with the escape tectonics and may also contribute to the occurrence of the intraplate earthquakes.