410-km discontinuity

Although the 410 km discontinuity is a dominant feature in most 1-dimensional Earth models, only a few events in this study show reflections from this discontinuity. Of the 124 events studied, only 13 events show a reflection from the depth range from 330 km to 430 km. These events are summarized in Table A.9 and displayed in Figure 6.8. The figure shows the reflector depths for the same part of the central and northwestern Pacific as Figures 6.1 and 6.5 but for a depth of 330 - 430 km.

Figure 6.8: Reflector depths for the depth range of the 410 km discontinuity (330 km - 430 km). Depths are mapped in the same way as in Figure 6.5.

Figure 6.9: Vertical cross section for the depths of the reflections at the 410 km discontinuity. The same profile as in figure 6.2 is used.

The depth cross section along the dashed line is displayed in Figure 6.9. The mean depth of the reflection points is 404.0$\pm$ 16.1 km. The coverage with reflection points from this depth range is not continuous. The reflections form two groups: One is near the subduction zone at the beginning of the profile. The other one is located at the tip of the Hawaii-Emperor seamount chain, close to the end of the profile. Between these two groups a region of $\sim$3600 km along the profile does not show any reflections near depths of 400 km, although this region shows surface reflection points for the same dataset (compare Figure 4.7). Different reasons for this gap in the 410 reflections are discussed in chapter 7.
Except for the two reflection points closest to the subduction zone the reflections show very similar depths. These two points show very shallow depths of the reflector. The shallowest indicates a reflector depth of 330 km beneath the Sea of Okhotsk, showing a depth of the 410 at this point $\sim$80 km shallower than the mean depth. This topography amplitude is larger than reported by others (Flanagan and Shearer, 1999; Neele and Snieder, 1992). The shallow depth can be the result of the interaction of the cold subducting slab with the phase transformation at 410 km depth. This scenario is discussed in detail in chapter 7.
Other possible causes for this shallow depth could be an asymmetrical reflection at the dipping slab, or unusual velocities beneath the Sea of Okhotsk.
The reflector shows some small scale topography. A cross-section across the Hawaiian Islands is shown in Figure 6.10. The profile runs perpendicular to the volcanic chain from southwest to northeast. The position of the Hawaiian Islands is marked by the dashed line at a profile length of $\sim$880 km. The reflections southwest of the Islands are $\sim$30 km shallower than in the northeast. The subsidence of the reflector occurs within 160 km along the profile. Within the next 400 km the reflector rises again from $\sim$430 km to $\sim$410 km. In this region the reflector depths show a strong variation perpendicular to the strike of the profile. Sometimes, the discontinuity seems to show a strong three-dimensional structure, complicating the interpretation of the reflector depth in the vertical cross sections.

Figure 6.10: Vertical cross section for the depths of the reflections at the 410 km discontinuity along a profile perpendicular to the Hawaiian Chain. The profile covers the 5 reflections at the tip of the Hawaiian Chain. The strike of the Hawaii-Emperor Seamount Chain is marked by the dashed line. Note the jump in the depth of the reflector when the profile crosses the volcanic Islands.

This detail of the topography of the 410 and geodynamic implications are discussed in the next section.