Investigating deep and shallow earth structures using different seismological approaches

Investigating the elastic structure at multiple scales of the Earth is crucial for understanding the composition and dynamics of the Earth. Different seismological approaches to extract the information contained in seismic waveforms are presented here. The work consists of three topics: (I) applications of source stacking followed by cross-correlation, (II) modeling seismic anisotropy in the lower mantle, and (III), temporal variation of near-surface seismic velocity and anisotropy in Hokkaido.I. Accurate synthetic seismic wavefields can now be computed in 3D earth models using the spectral element method, which helps improve resolution in full-waveform global tomography. However, computational costs are still a challenge. These costs can be reduced by implementing a source stacking method (Capdeville et al. 2003), in which multiple earthquake sources are simultaneously triggered in only one teleseismic SEM simulation. One drawback of this approach is the perceived loss of resolution at depth, in particular because high-amplitude fundamental mode surface waves dominate the summed waveforms, without the possibility of windowing and weighting as in conventional waveform tomography. This can be addressed by redefining the cost function and computing the cross-correlation wavefield between pairs of stations before each inversion iteration. While the Green’s function between the two stations is not reconstructed as well as in the case of ambient noise tomography, where sources are distributed more uniformly around the globe, this is not a drawback, since the same processing is applied to the 3D synthetics and to the data, and the source parameters are known to a good approximation. By doing so, time windows with large energy arrivals corresponding to fundamental mode surface waves can be separated. Also, applying a weighting scheme can bring out the contribution of overtones and body waves and also can balance the contributions of frequently sampled paths. The approach is computationally very efficient and can help address such questions as model resolution in the presence of noise, and trade-offs between different physical parameters (anisotropy, attenuation, crustal structure etc..) that would be computationally very costly to address adequately when using conventional full waveform tomography based on single-event wavefield computations./nII. By assuming a geodynamical scenario of slab subduction and considering the seismic anisotropy in D” is dominated by crystal preferred orientation, the textural evolution and re- resultant elastic properties can be computed, which may help to explain the seismic anisotropy observations (e.g., McNamara et al. 2002, 2003; Wenk et al. 2011). In previous work, Cottaar et al. (2014) calculated the seismic anisotropy produced in a single mineral system, comparing bridgmanite (MgSiO3 perovskite, Pv) to magnesium post-perovskite (pPv), based on elastic properties and slip systems determined from laboratory experiments and theoretical ab-initio computations. More realistic situations are considered in this study: (a) the polycrystal plasticity model (Lebensohn and Tom ́e 1993) which consists Pv/pPv, cubic calcium perovskite (CaSiO3, CaPv), and cubic periclase (MgO), (b) the forward and reverse Pv-pPv phase transitions during the slabs’ subduction and the subsequent upwelling, (c) partial melting in the deepest portions of the slab at the base of upwelling. To validate and compare the results with seismological observations, the spatial distribution of radial anisotropy described by the parameter ξ = (VSH/VSV)2, and shear wave splitting (SWS) directions and strengths are extracted. III. Here an earthquake-based method (Chen et al. 2017) is applied through interferometry of earthquake coda waves to investigate the temporal changes of elastic properties by using borehole-surface station pairs. The technique provides comprehensive observations not only of S-wave velocity but, additionally, of P-wave and S-wave azimuthal anisotropy between two sensors. The 15-year temporal variations of elastic properties are monitored at the KiK- net station IBUH03 in southern Hokkaido, which experienced the 2003 Mw 8.3 Tokachi-Oki (PGA= ∼ 350cm/s2) and the 2018 Mw 6.7 Hokkaido Eastern Iburi (PGA= ∼ 500cm/s2) earthquakes. The preliminary results show sudden reductions of VSiso and VP after the two major earthquakes considered, and these were subsequently recovered with short-term and long-term recovery rates. Changes in the fast S-wave polarization direction and strength of anisotropy were also detected during the coseismic periods. Furthermore, a clear seasonal trend of elastic parameters can be observed. The results are compared with temporal variations in equivalent water thickness, precipitation, and temperature. A Hudson-Crampin anisotropic effective model is implemented to understand the seasonal crack behavior. The modeling result indicates that crack density and the aspect ratio could play an important role in our observations.

Advisor: Romanowicz, Barbara