Eurasian fundamental mode surface wave phase velocities and their relationship with tectonic structures

Abstract

We automatically analyzed 32,000 fundamental mode Love and Rayleigh wave signals with earthquake‐station paths traversing Eurasia and Indonesia and obtained robust average phase velocity measurements between 20 s and 170 s periods along 4389 Love and 4020 Rayleigh paths. These were inverted to give phase velocity maps at 14 fixed periods. Resolution tests suggest that features with diameter >750 km and >500 km are resolved over most of Eurasia and central/SE Asia respectively. Low‐period Love waves image areas with thick sedimentary cover as low‐velocity zones, and almost all periods image mountainous regions since these have thick crust and hence low average lithospheric shear velocity. At long periods, both Love and Rayleigh waves define high phase velocity zones across shield and cratonic areas reflecting their deep lithospheric roots. We observe significant along‐strike heterogeneity in the Zagros fold belt and Tien Shan‐Altai system. Taking sections across Eurasian phase velocity space allows us to make approximate interpretations in terms of shear velocity structure directly. For example, the Red River and East Vietnam Boundary faults are traced on their eastern side by low velocities which extend at depth into Indonesia. We relate this to mantle upwelling associated with early Eocene rotation of Indochina and reversal of the sense of shear across the Red River fault post‐20 Ma. We observe dipping subduction of the Mediterranean beneath the Aegean, of the Philippine Sea beneath Indonesia, and of the Indian shield beneath Tibet. We also image a fossil subducted plate beneath NE Borneo which we associate with subduction of the proto‐South China Sea between 50 Ma and 15 Ma.