Transition Zone Structures beneath Hawaii Imaged with SS precursors
Thursday, 18 December 2014
High-resolution seismic images of upper mantle transition zone structures are important for our understanding of the thermal, chemical, and geodynamical evolution of the Earth. In this study, we used precursors of the teleseismic SS phase to image upper mantle transition zones beneath Hawaii. The teleseismic SS phase bounces off the free surface roughly at the middle point between the earthquake epicenter and the recording station, thus providing crucial data coverage of areas where neither natural earthquakes nor passive receivers are available. Its precursors, e.g. S410S and S660S, reflect at deeper interfaces, and therefore arrive before the main SS phase. Differential travel times between the SS phase and its precursors are used for mapping transition zone structures.
Due to relatively small amplitudes of SS precursors, a large number of seismograms are usually stacked to enhance their signal to noise ratio (SNR). Traditionally seismograms which share a common bouncepoint are stacked, using a technique known as common middle point stacking (CMP). Random noise is suppressed and the small amplitude precursor signals should become visible. However, if there is strong contamination from other seismic phases in the precursor window, e.g. near receiver reflections such as Ss410s and Ss660s, it will be difficult to identify SS precursors. Here, we applied radon transform filtering to remove multiples both before and after stacking. Application of this technique to our new SS precursor dataset shows that not only is the seismic noise level greatly reduced, but also the resolution of upper mantle structures is significantly improved. With the noise-reduced seismic dataset, we also compared seismic imaging results using both CMP stacking method and generalized radon transform (GRT) method. Except for some subtle and tractable differences, these two methods generally give consistent results.
We observe strong and laterally continuous upper mantle reflectors near 410 and 660 km. We also detect local splitting of the “520”. Above the transition zone, there are strong reflectors at ~220 km and ~310 km. Thinning of the mantle transition zone is observed to the southeast of the Hawaiian Islands, suggesting a tilted mantle plume in the upper mantle.