Lehmann discontinuity

The Lehmann discontinuity has been detected at depths ranging from 150 km - 250 km. In this study, 24 events show PP precursors from this depth range. These events are summarized in Table A.8 and displayed in Figure 6.5. This figure is similar to Figure 6.1, but for a depth range of 150 km to 250 km. The reflector depths along the profile, indicated by the dashed line, are shown in Figure 6.6. Most of the reflection points are near the subduction zone at the beginning of the profile.

Figure 6.5: Reflector depths for the depth range of the Lehmann discontinuity (150 - 250 km). The depths of the reflector are displayed as columns at the location of the geometrical PP reflection point. The dashed line indicates the profile as in Figure 6.1.

Figure 6.6: Vertical cross section for the depths of the Lehmann discontinuity. The location of the profile is marked in Figure 6.5 as a dashed line. The depth at the geometrical reflection point of the reflector is projected onto the profile. The profile stretches from the Sea of Okhotsk to the Hawaiian Islands. The mean depth of the reflector and the errors are marked as solid and dashed lines, respectively.

Nevertheless, the reflection points are distributed across the whole profile. The concentration of P$^{210}$P reflection points near the subduction zone is partly due to the fact that most of the events studied show surface reflection points in this area. In Figure 6.6 the mean depths of all reflections shown is 200 km, as indicated by the solid line. The error is $\pm$20 km indicated by the dashed lines. Comparing Figure 6.5 and 6.6, the reflector shows a dip from the Hawaiian Islands to the Sea of Okhotsk. In this direction, the depths increase from $\sim$200 km to $\sim$220 km. Although the data show a lot of scatter, this general trend is apparent.
Some of the reflections are very shallow. The scale of Figure 6.2 and Figure 6.5 overlaps by 20 km. The reflections at a depth range from 130 km to 190 km are displayed in Figure 6.7.

Figure 6.7: Vertical cross section for the intermediate depths from 110 km to 200 km. These reflector depths do not fit into the interpretation of the Hales, Gutenberg and Lehmann discontinuities. The location of the profile is marked in Figure 6.5 as a dashed line. The depth at the geometrical reflection point of the reflector is projected onto the profile. The profile stretches from the Sea of Okhotsk to the Hawaiian Islands.

In this diagram, the deepest reflections from Figure 6.2 and the shallowest reflections from Figure 6.6 are summarized. An obvious single reflector is not visible in this graph. A polarity study for these events like the one discussed in chapter 6.1.1 shows, that reflections with depths larger than 160 km show a positive and with depths less than 160 km show a negative polarity relative to PP. Therefore, these points possibly show the upper and lower boundary of a very narrow lamellae within the low velocity zone beginning at the Gutenberg discontinuity as discussed above. The depth correlation of the lower and upper boundary support this interpretation, but the few datapoints prevent any final conclusion.
The possibilities of an influence of source location, source depth or source parameter have been tested, but these parameters can neither explain the additional phases (e.g. due to depth phases of PP), nor the correlation of the positive and the negative reflectors. A PP reverberation (PPp$_d$p) within the crust beneath the array does not result in a phase with a travel time comparable to the phases discussed here. A reverberation of the precursor phase (P$^d$Pp$_{d'}$P) cannot produce an arrival with an amplitude detectable by the fk analysis. Both methods were tested using gauss-beam ray tracing.
A localized reflector in the upper mantle, as discussed by Castle and Creager (1999), is unlikely due to the wide extension of this unusually shallow Lehmann reflections.
A final conclusion on the nature of this lamellae cannot be reached. All Lehmann reflections show a positive polarity relative to PP, indicating a velocity jump from fast material below the discontinuity and slower material above it. This velocity change is in agreement with most models proposed for the L.