P'P' and PP precursor studies of the mantle transition zone

Seismology provides us with a variety of powerful techniques that we can use to probe the Earth's deep structure. Precursors to seismic phases are a seismological tool commonly used for studying the mantle transition zone. Precursors to P'P'df (also known as PKIKPPKIKP) are particularly valuable as they are relatively high frequency (~1Hz) waves that approach mantle discontinuities with a small angle of incidence, making them highly sensitive to sharp, narrow transitions. P'P' precursors also have relatively small Fresnel zones, allowing them to resolve much smaller scale structures than precursors to longer period waves like PP. Previous P'P' studies (Xu et al., 2003) have detected a large amplitude reflection from the 660km discontinuity and a much smaller signal from the 410km discontinuity, indicating that the 660km transition is relatively narrow (<2km) while the 410km transition is broader. The findings of P'P' precursor studies contrast with those of PP precursor studies at both long (~25s, Deuss et al., 2006) and short periods (~1s, Rost & Weber, 2002.). PP precursor studies detect a reflection from the 410km discontinuity on a global scale, but rarely observe a simple reflection from the 660km discontinuity. In longer period PP precursor studies a combination of single and double reflectors at depths of between 640 and 720 kilometers have been detected. The complexity of observations of the 660km discontinuity in PP precursors has been attributed to additional phase transitions in the non-olivine components of the mantle, particularly garnet. By analyzing both P'P' and PP precursor data at long and short periods, and in different tectonic settings, we are able to improve our understanding of the contrasting observations of the mantle transition zone in the two precursor types. Each data type has a unique sensitivity which allows us to discern different, but complementary, information about the mantle. Comparing the P'P' and PP precursor observations of the mantle transition zone with synthetic predictions, receiver function studies and seismic tomography, and combining these results with work from mineral physics, allows us to further constrain the properties of this complex region of the Earth, particularly focusing on the nature of the 660km discontinuity.