MANtle transition zone topography from ss precursors
SS precursors are the underside reflections at 410 and 660 km discontinuities. They are sensitive to the structure beneath the bounce points. We use SS precursors to map out the topography of transition zone discontinuity topography so we can study the thermal and chemical heterogeneity in the mantle.
The topography maps of 410 and 660 km discontinuities are mapped out using SS precursors. The transition zone thickness are the difference between 410 depth and 660 depth. The transition zone is thicker in most suduction zones (e.g. South America, Sumatra and Japan) and thinner beneath mantle plumes (e.g. Hawaii, Greenland and NW America). Here is a bounce point map of our SS precursor data set. This up-to-date data set has good global coverage and also azimuthal coverage in several locations.
Here is the stacking result of the whole data set by epicentral distance. We can see the two SS precursors clearly in the stacks and also interfering phases such as topside reflections and ScS reverberations.
Seismic anisotropy in The mantle transition zone
Wadsleyite and ringwoodite has certain amount of single crystal anisotropy at the pressure and temperature conditions of mantle transition zone. Seismic anisotropy can be produced in the transition zone if the minerals are aligned by mantle flows like subducting slabs or mantle plumes. We use SS precursors to detect and quantify the magnitude and orientations of seismic anisotropy in the transition zone and upper mantle.
The geometry of central Pacific bin is plotted on top of the MTZ topography map. It is located above the Hawaii hotspot. The black lines are the SS bounce points and their orientations represent the azimuths of bounce points. The arrows and numbers on the edge of globe denotes the azimuths of different directions. The pink circles are earthquakes and green triangles are stations. The MTZ thickness is plotted as background color: red denotes a thinned MTZ and blue denotes a thickened MTZ.
We also look through five subduction zones and stack all the data falling into these regions by the relative azimuth compared to the flow direction. The travel time of S410S and S660S both have a fast direction perpendicular to the subduction flow direction which is consistent with the polarization direction of SS precursors. However, the differential travel times between S410S and S660S are not significantly different at different azimuth. It indicates that subducting slab is causing seismic anisotropy in the upper mantle rather than the mantle transition zone.