Description: We analyse data from seismic stations surrounding the Alboran Sea between Spain and North Africa to constrain variations of the lithosphere–asthenosphere boundary (LAB) in the region. The technique used is the receiver function technique, which uses S-to-P converted teleseismic waves at the LAB below the seismic stations. We confirm previous data suggesting a shallow (60–90 km) LAB beneath the Iberian Peninsula and we observe a similarly shallow LAB beneath the Alboran Sea where the lithosphere becomes progressively thinner towards the east. A deeper LAB (90–100 km) is observed beneath the Betics, the south of Portugal and Morocco. The structure of the LAB in the entire region does not seem to show any indication of subduction related features. We also observe good P receiver function signals from the seismic discontinuities at 410 and 660 km depth which do not indicate any upper-mantle anomaly beneath the entire region. This is in agreement with the sparse seismic activity in the mantle transition zone suggesting the presence of only weak and regionally confined anomalies.

Description: Seismological models of upper mantle structure provide important constraints on the Earth"s convection system. Resolving the details of the upper mantle discontinuities is important for modelling the composition of the mantle and for understanding the effect that the discontinuities may have on mantle convection. Recently, numerous permanent and temporary seismic stations and networks have been set up around the world. It is possible to get the seismic records for the research needs from data management systems like IRIS, GEOFON, GEOSCOPE, FREESIA, etc. The use of seismograms collected from a large number of stations and earthquakes around the world enable us to study the global and the regional structure of the Earth. In this work, the receiver function technique (e.g. Owens et. Al., 1995) is applied to study the upper mantle structure in the northwest Pacific subduction zone and in the Hawaiian hotspot area. In the northwest Pacific, the Pacific plate is subducted into the upper mantle to more than 600 km depth, indicated by seismicity. In Hawaii, the volcanic edifice of the Hawaiian Islands and seamounts are believed to result from the passage of the oceanic lithosphere over a stationary mantle hotspot (Wilson, 1963; Morgan, 1971; Morgan et. al., 1995). In both regions the upper mantle structure is affected by the cold and warm materials, respectively. To study the extension of the temperature anomaly is important for understanding the Earth"s convection system. The olivine component of the mantle material is intensively studied in laboratories (e.g. Ito and Takahashi, 1989; Irifune, 1987). With increasing temperature and pressure, the olivine crystal undergoes a series of phase transformations which will result in a variation of the seismic structure. The effect of the temperature anomaly on the main upper mantle discontinuities will be discussed in chapter 2. Recently, the receiver function technique is increasingly applied to investigate the upper mantle discontinuities. To isolate the upper mantle conversion phases, newly developed moveout correction and migration methods are applied to separately distributed seismic stations as well as station arrays. The receiver function method used in this study will be introduced in chapter 3. In chapter 4 and 5, receiver function studies in the area of the northwest Pacific subduction zone and the Hawaiian mantle plume are presented. Regional tectonic background and the previous seismological works in these two areas will be first introduced in each chapter, and followed by description of data, processing steps, results and interpretations. In chapter 6, I will summarize the observations of the 410 and 660 topography in the northwest Pacific subduction zone and in the area around the Hawaiian mantle plume.

Description: The volcanism responsible for creating the chain of the Hawaiian islands and seamounts is believed to mark the passage of the oceanic lithosphere over a mantle plume1,2. In this picture hot material rises from great depth within a fixed narrow conduit to the surface, penetrating the moving lithosphere3. Although a number of models describe possible plume–lithosphere interactions4, seismic imaging techniques have not had sufficient resolution to distinguish between them. Here we apply the S-wave ‘receiver function’ technique to data of three permanent seismic broadband stations on the Hawaiian islands, to map the thickness of the underlying lithosphere. We find that under Big Island the lithosphere is 100–110 km thick, as expected for an oceanic plate 90–100 million years old that is not modified by a plume. But the lithosphere thins gradually along the island chain to about 50–60 km below Kauai. The width of the thinning is about 300 km. In this zone, well within the larger-scale topographic swell, we infer that the rejuvenation model5 (where the plume thins the lithosphere) is operative; however, the largerscale topographic swell is probably supported dynamically.

Description: We employ P to S converted waveforms to investigate effects of the hot mantle plume on seismic discontinuities of the crust and upper mantle. We observe the Moho at depths between 13 and 17 km, regionally covered by a strong shallow intracrustal converted phase. Coherent phases on the transverse component indicate either dipping interfaces, 3-D heterogeneities or lower crustal anisotropy. We find anomalies related to discontinuities in the upper mantle down to the transition zone evidently related to the hot mantle plume. Lithospheric thinning is confirmed in greater detail than previously reported by Li et al., and we determine the dimensions of the low-velocity zone within the asthenosphere with greater accuracy. Our study mainly focuses on the temperature-pressure dependent discontinuities of the upper mantle transition zone. Effects of the hot diapir on the depths of mineral phase transitions are verified at both major interfaces at 410 and 660 km. We determine a plume radius of about 200 km at the 660 km discontinuity with a core zone of about 120 km radius. The plume conduit is located southwest of Big Island. A conduit tilted in the northeast direction is required in the upper mantle to explain the observations. The determined positions of deflections of the discontinuities support the hypothesis of decoupled upper and lower mantle convection.