Crust‐Mantle Velocity Structure of S Wave and Dynamic Process Beneath Burma Arc and Its Adjacent Regions

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AbstractThe Rayleigh dispersion data of 530 selected paths crossing Burma Arc and its adjacent regions are determined by applying the MF‐FTAN (Matched‐Filter Frequency‐Time Analysis) technique to long period data recorded at SHIO, CHTO, KMI and LSA stations. The periods of the Rayleigh dispersion are in the range of 10.45~105.03 s. On the basis of this, a method of grid dispersion inversion is proposed to extract pure‐path dispersion in each 1° × 1° grid from mixed‐path dispersion. Then the S wave velocity structure to a depth of 200 km is inverted from pure‐path dispersion in each grid, and finally the 3‐D velocity structure of S wave is reconstructed beneath Burma Arc and its adjacent regions. The result shows that taking Sagain fault as the boundary, the velocity of crust on the west side of the fault is higher than that on the east side of the fault. The lithosphere thickness in the Indo‐Burma areas is in the range of 110~130 km and the S wave velocity at the top of upper mantle is 4.3~4.4 km/s beneath the Indo‐Burma regions. However there is a lower‐velocity column of uprising mantle about 150~200 km wide beneath Yunnan‐Burma‐Thailand Block on the east side of Burma Arc. The lithosphere thickness is 70~80 km and the velocity of S wave at the top of the upper mantle is 4.1~4.2 km/s in the area. On the other hand, the S wave velocity structure shows an NS (north‐south) variation, this feature corresponds well to the distribution of earthquake sources, strikes of the faults and volcanoes.