Modeling oceanic crust by waveform modeling of PP bounce point data with application to the Nazca Plate
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Teleseismic waves have been used to investigate mantle and crustal structure beneath recording stations and near the source of the earthquake, but investigations of structure at the turning and bounce points have only focused on the mantle. The focus of this thesis is to model oceanic crust to determine the velocities and thicknesses of layers 1, 2A, 2B, and 3 using teleseismic P-to-P bounce points. We used recordings of PP data that record structure as precursors to invert for models of typical oceanic crust. The inversion algorithm used raytracing to produce delta function synthetics convolved with PP source function, which was estimated by beam forming data from the same events as the ocean data but with bounce points on land, as the forward model. The inversion technique then employs a genetic algorithm to minimize misfits between the synthetics and observed seismograms. Before applying this method to real data, we produced a reference synthetic based on a velocity model of a hypothetical oceanic crust and tested the inversion algorithm by modeling this synthetic. Modeling of the synthetics suggests that the inversion will be most sensitive to the sediment layer 1 and extrusive layer 2A. This PP-precursor inversion was then applied to real seismic data from an area approximately 400 km south of a region off the coast of Lima, Peru. We inverted for the thickness of layers 1, 2A, 2B, and 3. We also inverted for the P-wave velocity of each of these layers and the mantle. Results from seven nearby seismic refraction profiles were available for comparison to our results. Although the refraction results were not in the same region, they are located on crust of similar ages, lithology, and regional settings. The layer thicknesses we found are in the range of 0.22 to 0.39 km for sediment layer 1, 1.46 to 1.92 km for layer 2A, 1.96 to 2.46 km for layer 2B, and 6.12 to 6.91 km for layer 3. P-wave velocity ranges are 2.0 to 2.2, 3.8 to 4.5, 5.6 to 6.1, 6.0 to 6.1, and 8.47 to 8.5 km/s for layers 1, 2A, 2B, 3, and the mantle half space, respectively. The velocities we found are similar to those modeled by the nearby refraction study, but our estimates of the thickness of layer 2A are much larger than those found by the refraction work. The refraction profiles were shot 400 km to the north and our study area was very close to the Nazca Ridge, a volcanic hotspot seamount chain. Other studies found that layer 1 and 2A are typically thicker than in “generic” abyssal oceanic crust. We estimated a larger than likely thickness for layer 3, but testing with synthetics indicated that we would be most sensitive to the thickness of the top two layers.