Browsing by Subject "PP precursor data"
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Item Investigation of the upper mantle beneath the Hawaiian Island chain using PP-precursors(2013-08) Rogers, Kenneth D.; Gurrola, Harold; Nagihara, Seiichi; Karlsson, Haraldur R.The Hawaiian hotspot is of great geological significance, but data collection in the area can be challenging due to the water depth around these islands. By using PP bounce point data, with receivers in mainland United States, we analyze the area with a greater wealth of data than possible using data collected locally. The increased amount of data, in addition to new beamforming and iterative deconvolution techniques, has increased the frequency content in PP precursor data, from around the traditional 0.01 Hz to above 5 Hz, enabling us to image to shallower depths and thinner layers than previously possible. Profiles of stacked PP precursors across the island chain were produced along perpendicular lines. Data were stacked in bins 1˚ along the profiles and 4˚ perpendicular to the profile (parallel to the island chain). An additional profile was produced some 10˚ away from the island chain as a control group. The control group shows pairs of high- and low-velocity horizons in the mantle. These may be the base and top of shear zones. These horizons are strongly disrupted near the Hawaiian Island chain. In the lithosphere, low velocity zones are more abundant to the south of the island chain but are less common on the north side. If these indicate melt, the low velocity zones may be blocked by the Islands, which are sinking into the lithosphere. As this study and other recent work imply the hot spot is more active to the southwest of the island chain than to the north, the island chain itself may be causing the crust to warp downward into the mantle and could act as a dam to melt migrating to the north. Furthermore, we believe that the island’s weight downwarping the lithosphere causes a crack to propagate out past the youngest island, which also acts as a dam that keeps most of the melt to the southwest of the island chain.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.