Browsing by Subject "Transition zone"
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Item Processing innovations to improve PP precursor analysis and increase frequency content of studies in the Mid-Pacific(2012-08) Duncan, Gregory; Gurrola, Harold; Yoshinobu, Aaron S.; Zhou, Hua-weiPP wave data is gathered for bouncepoints primarily in the Pacific Ocean using earthquakes of magnitude 6.2 or greater occurring along the Mariana/Izu-Bonin subduction region, as well as Tonga and the Indonesia plate. The data is composed of midpoints that fall in the Mid-Pacific with a particular concentration under Hawaii. Data is recorded in seven different seismic arrays in the United States but is mainly from the Transportable Array (TA) and USArray (US) stations. The data underwent rigorous cleaning before final analysis. On top of the normal aspects, such as rotating and cutting of data, we utilized a beaming technique on both sides. On the receiver end, it’s simply called beamforming; on the source end it is called simultaneous iterative deconvolution (SID). This cleaning technique is utilized to see how well PP waves can be used in long offset mantle discontinuity studies. Of particular interest to our study are the 410, 520, and 660 discontinuities, which react differently to thermal anomalies such as subducting slabs or hotspots. The dataset had very good ray coverage around Hawaii but dispersed a bit on the outskirts of the dataset, particularly in the southern region. The double-beaming technique is applied to the PP data. Frequency content is increased to as high as 1 Hz while still getting legitimate results. SID was able to get frequencies as high as 4 Hz. 1 Hz is significantly higher than frequencies typically used in these types of studies, which is around .1 Hz. Though frequency was attempted as high as 8 Hz, it was found that PP waves had too little of these upper frequencies. The high frequencies were wiped out during waterlevel deconvolution, a method used to help stabilize the data by filling spectral nulls in frequency with white noise. The improved PP method allows interpretation of both cubes around Hawaii as well as a long line that intersects the majority of the data. Overall, the Tahiti and Hawaii hotspots are analyzed, as well as the subduction zone at the Southern Explorer Ridge (SEXP). It is found that in the Tahiti region, the 410 and 520 both deepen significantly due to the high thermal anomalies associated with hotspots. Hawaii, however, does not display these deepened horizons. This implies thermal anomalies associated with the Tahiti hotspot could be thermally stronger than the Hawaii hotspot. A double 520 can be seen in regions around Hawaii in datasets that have not been ocean corrected. Discontinuities seem stronger in non-ocean-corrected datasets overall, probably due to receiver function familiarity. Farther north, we anticipate a deepening of the 410 associated with the data line moving to continental crust, but it is difficult to decipher. We also look for a rise in this region for the subducting slab, but it is difficult to see due to resolution on the extremities of the dataset. The 410 may rise at this point, but it could possibly be an inaccurate interpretation.Item Shear velocity structure and mineralogy of the transition zone beneath the East Pacific Rise(2013-05) Wang, Yang, M.S. in Geological Sciences; Grand, Stephen P.; Lin, Jung-Fu; Lavier, LucModels of seismic velocity as a function of depth through the upper mantle provide some of the strongest constraints on the mineralogy and composition of the mantle. Although receiver function studies have provided new information on the depths of upper mantle discontinuities they do not provide as much information on seismic gradients and velocities. The waveforms and travel times of upper mantle turning waves provide the strongest constraints on vertical variations in upper mantle velocity although in the past they suffered from the lack of dense profiles of data sampling a single part of the upper mantle that would minimize effects of 3D variations in velocity. Here we model three dense profiles of triplicated upper mantle broadband S and SS waves recorded by US-Array, Canadian and NARS-Baja stations located in western North America. Earthquakes along the East Pacific Rise were recorded along profiles within 5°back azimuth windows and with stations at a maximum of .5°separation. The distance range covered is from 30° to 60° and thus the waves sample the mantle from the lithosphere to depths near 1000 km. The data were inverted using a conjugate gradient algorithm that utilizes the reflectivity synthetic technique. The results show a much smaller gradient within the transition zone than the PREM model with larger jumps in velocity at the 410 km and 660 km depth discontinuities. These results are consistent with velocities predicted for a pyrolite composition mantle transition zone. Compositional models with lower olivine content, such as piclogite, are not consistent with our seismic model.