Figures 3 and 4 show all the P-wave velocity models across the KPR and demonstrate the variation along the ridge axis from north to south. We split the KPR into five regions shown in Fig. 1 and describe in detail representative profiles for each region. We loosely divide our crustal models into three parts (upper, middle, and lower crust) based on their P-wave velocities, velocity gradients, and thickness proportions following Calvert (2011) and Nishizawa et al. (2014) and will describe their characteristics below.
Region 1
We show the Vp model for KPr4 (Fig. 5), which is representative of the northernmost region of the KPR where the bathymetric highs are very shallow and wide. The shallowest portion along KPr4 corresponds to Komahashi-Daini Seamount.
Tomographic inversion misfit is less than 50 ms, and the checkerboard test result and ray diagram (Fig. 5 bottom) show the model shallower than 15 km is reliable. The Vp model of Komahashi-Daini Seamount has a thick middle crust with a thickness of around 7 km and Vp of 6–6.5 km/s. Since the region below 15 km depth is less resolvable, and the crustal thickness beneath the seamount is 18 km, the Vp at the bottom of lower crust of 7.1 km/s and Pn velocity of 7.6 km/s were estimated by two-dimensional ray tracing. The transition region between the Shikoku Basin and the KPR is characterized by thin crust less than 5 km thick excluding the topmost sediment layer and a high Pn (uppermost mantle) velocity of 8.1–8.2 km/s (Fig. 5). On the other hand, a small bathymetric high exists to the west of Komahashi-Daini Seamount at a distance of 200 km, and it has a slightly thicker curst with no thick middle crust layer. A thick sediment layer with Vp smaller than 3.5 km/s and with a maximum thickness of around 3 km exists between Komahashi-Daini Seamount and the small bathymetric high. Deeper Moho at the southwestern end of the Vp model indicates a subducting bathymetric high beneath the forearc of the Nansei-Shoto island arc.
The total thicknesses of the KPR crusts are appreciably greater than those of the Shikoku Basin for all the lines of KPr4, SPr10, KPr6, KPr8, and SPr7 in Region 1 (Fig. 3). Particularly thick middle crust was found also for SPr10 and KPr6, and slightly thin middle crusts are estimated for KPr8 and SPr7 where the widths of the ridge topography are narrower than those of the northern lines.
It is notable that thin crust occurs at the boundary of the KPR and Shikoku Basin on all lines and is less than 3 km thick on line KPr8 (Fig. 3). Pn velocities in this area are higher than 8.0 km/s, which are significantly higher than those of the Shikoku Basin. To the west of the KPR, the upper sedimentary layer is relatively thick.
Region 2
Four lines DAr4, KPr11, KPr12, and KPr31 were located to cross the region where the KPR lies close to several seamounts and areas of shallow seafloor the west of the KPR.
We show the result for KPr11 in Fig. 6 as a representative in this region. KPr11 was positioned around 10 km south of DAr4 which connects to the Daito Ridge to the west. The Vp model beneath the KPR for KPr11 shows a total crustal thickness of around 13 km, a middle crust thickness of about 5 km, Vp at the bottom of lower crust of 7.2 km/s, and Pn velocity of 7.6 km/s. Slightly shallow Moho at a distance of 140 km is required to explain observed PmP arrivals. The eastern transition zone from the KPR to the Shikoku Basin is characterized by abrupt crustal thinning to less than 5 km. At the western transition, around 20-km-thick KPR crust thins to 10 km beneath the Minami-Daito Basin. We detected a very fast Pn velocity of 8.3 km/s at this transition.
Similar Vp models were obtained for DAr4, KPr12, and KPr31 (Fig. 3). The boundary between the KPR and the Shikoku Basin is again characterized by very thin crust of 3 to 5 km and Pn velocities of 8.0–8.1 km, which are different from values found generally in the Shikoku Basin. On the other hand, the Minami-Daito Basin has thick crust of around 10 km and high Pn velocity of 8.1–8.3 km/s, which is notably different from a typical oceanic crust. The crustal thickness in the western transition to the Minami-Daito Basin is not thinner than that of the Minami-Daito Basin, in general.
Region 3
Region 3 corresponds the area where the eastern edge of the KPR bounds the deepest part of the Shikoku Basin and the KPR topography is slightly deeper and narrower than in other regions. We shot five lines including KPr13, SPr11, KPr14, KPr32, and KPr15 in this region and compiled each Vp model in Fig. 3.
We show the Vp model for KPr13 in Fig. 7. The crustal thickness beneath the KPR is about 10 km with a middle crust of 2 km. Vp at the bottom of the lower crust and the Pn velocity are 7.2 and 8.2 km/s, respectively. The Pn velocity beneath the KPR is very high, over 8.2 km/s, much different from other lines. Since this line has a thin crust of less than 10 km as a whole, the checkerboard test result in Fig. 7 bottom shows good recovery. We could confirm the high Pn velocity and thin crust not only by the tomographic inversion but also by 2-D forward ray-tracing method.
Common characteristics among the five lines are follows: Crustal thickness below the KPR is more than 10 km, but less than 5 km beneath the eastern transition into the Shikoku Basin. To the west of the KPR, the crustal thicknesses of the Minami-Daito Basin on lines KPr13, SPr11, and KPr14 and of the Mangetsu Basin on lines KPr32 and KPr15 are larger than in the Shikoku Basin. The crust of the Minami-Daito Basin is slightly thinner in Region 3 than in Region 2. Uppermost mantle velocities for the lines other than KPr13 beneath the KPR were estimated to be 7.6–7.8 km/s, while Pn velocities beneath the Minami-Daito and Mangetsu Basins are 8.2 and 8.0 km/s, respectively.
Region 4
Region 4 corresponds to be the central part of the KPR where the KPR is basically bounded by the Parece Vela Basin (PVB) to the east and by the West Philippine Basin to the west. There are 11 seismic lines across the KPR from 22°N to 15°N, and the Vp models are shown in Fig. 4. Since the previous paper by Nishizawa et al. (2007) has already reported velocity models for KPr19, Kpr20, Spr5, and KPr26 (white lines in Fig. 1 right), we will highlight the result for KPr24 in Region 4 (Fig. 8). The seafloor topography to the west of the KPR is flat, 5000 m deep, but a rather rugged seafloor fabric with N-S strike characterizes PVB to the east of the KPR. As shown in the checkerboard test result (Fig. 8 bottom), it is difficult to determine deeper structure beneath the KPR from the tomographic inversion. Thus, we inferred velocities in the lower crust and uppermost mantle by trial-and-error fitting of observed distant travel times using the forward modeling. Vp model for KPr24 has a middle crust with thickness of 4.5 km and a total crustal thickness of 14 km. The P-wave velocities at the base of the crust and in the uppermost mantle are estimated to be 7.2 and 7.7 km/s, respectively.
We observed large amplitude reflection signals at far offsets in several OBS records on KPr24. Figure 9 shows an example of the record section obtained at OBS 43 in the PVB. The signals in red ellipses in Fig. 9a were mapped to a black curve with white arrows at depth of 25 km in Fig. 9c using the travel time mapping method by Fujie et al. (2006). The other black curves correspond to the mapping results from reflection signals recorded by other OBSs. Although similar signals with large amplitudes at far offsets were also observed on lines KPr21 and KPr22, their appearances, that is, their travel times and offsets from the KPR are different. Furthermore, such signals were not observed on line KPr25.
KPr17 is located from the southeastward extension of the Oki-Daito Ridge to the PVB in the east (Fig. 1). The Vp model (Fig. 10) is distinctive compared with other KPR models; that is, the total crustal thickness beneath the KPR is only around 8 km including the thick top sedimentary layer with Vp less than 4 km/s and with a thickness of about 3 km. The thin total crustal thickness is due to an anomalously thin lower curst. However, the KPR Pn velocity of 7.8 km/s is well constrained as shown in the checkerboard result (Fig. 10 bottom), which is similar to other KPR models. Large amplitude reflection signals were recorded on many record sections over offsets of around 80 km along KPr17. Assuming a Vp below the Moho of roughly 8.0 km/s, the travel times of these signals can be explained by reflectors at 10–14 km below the Moho under the Oki-Daito Ridge and KPR at distances of 40–160 km and of 200–250 km. These reflectors are much deeper compared with those below KPr24.
The KPR Vp models in Region 4 (Fig. 4) differ from line to line, but there are some common features: thicker crust and especially middle crust than in the adjacent basins, Pn velocities less than 8.0 km/s, thin crust and high Pn velocities over 8.0 km/s at the eastern boundary with the PVB.
Region 5
Lines KPr40 and KPr41 are situated to the south of the intersection between the CBF Rise and KPR, and the southernmost profiles in this study. On line KPr40, the Vp model beneath KPR reveals that the middle crust is about 5 km thick and the Vp at the bottom of the lower crust is 7.2 km/s. The total crustal thickness is 15 km, and Pn velocity is 7.8 km/s (Fig. 11). The model for KPr41 shows slightly thinner crust and slower Vp at the bottom of the crust compared with the KPr40 (Figs. 4, 11).