Extent of the Pacific Low Velocity Province
Constrained by the Vietnam Broadband Seismograph Array

 

Figure: (a) Observed ScS-S and sScS-sS travel time residuals projected at the bouncing points of ScS or sScS.  (b) Synthetic ScS-S and sScS-sS travel time residuals computed for our preferred model.



We present evidence showing the extent of the low-velocity region in the lowermost mantle beneath the western Pacific.  We analyzed S, sS, ScS-S, and sScS-sS travel times observed by the Vietnam broadband seismograph array deployed as part of the Ocean Hemisphere Project.  The abrupt changes in ScS-S and sScS-sS travel times suggest that the western geographical boundary of the low-velocity region is located around 140^\circ E and is sharp (more than 4 % velocity contrast within 200 km; Figure). The dependency of S and sS travel time anomalies of epicentral distances suggests that the strong low-velocity region is confined to within 400 km from the CMB (core-mantle boundary).  The existence of lateral heterogeneities with a 100 km scale inside the low-velocity region is also suggested by the abrupt changes in S and ScS waveforms.

<Reference>

• Takeuchi, N., Morita, Y., Xuyen, N.D. & Zung, N.Q., 2008. Extent of the low-velocity region in the lowermost mantle beneath the western Pacific detected by the Vietnamese broadband seismograph array, Geophys. Res. Lett., 35, L05307, doi:10.1029/2008GL033197.