Fig. 1

Distribution of the reef islands in the Xisha area (after Chen, 1978). Red stars are the seismic stations on islands, in which the Yongxing Station lies on the Xuande Atoll and the Chenhang Station lies on the Yongle Atoll; Black star represents the Gaojianshi reef"

Fig. 2

Well logs of the Xiyong-1 and Xichen-1 drillings (based on Wei et al, 2007; Ma et al, 2010)"

Fig. 3

Ray paths of the converted phases in receiver function (a) and the travel times (b). The ray slowness in the modeling is 0.07, related to earthquakes from epicenters of 50º"

Fig. 4

Theoretical receiver function and stacking results of the model with sedimentary layer covered. (a): crustal structure model; (b): theoretical receiver function; (c): H-k stacking results."

Fig. 5

The converted and multiple phases from the sedimentary basement in the Chenhang Station (a) and Yongxing Station (b) and the picked travel times (c). Pp: direct P wave; Ps: converted phases from basement; PpPs and PpSs+PsPs: multiple phases"

Tab. 1

Travel times of the converted phases from the Chenhang Station and the Yongxing Station"

Fig. 6

The estimated thickness and velocity ratio of the Chenhang Station (a) and the Yongxing Station (b). The stars represent the thickness and velocity ratio that correspond to the minimum root-mean-square error; gray color scale shows the root-mean-square error distribution"

Fig. 7

Influences on the root-mean-square error (a), estimated k (b), and H (c)"