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In August of 1985 the crustal structure underlying the Kenya rift valley was investigated by long range explosion seismology. The experiment (KRISP85) consisted of two seismic lines in the central sector of the rift, one along the axis (140 km) and the other across it (50 km). Interpretation of the data, including time-term analysis and ray tracing has yielded the following information. The thickness of rift infill varies from about 6 km below Lake Naivasha to about 2 km and 1.5 km below Lake Magadi and Lake Bogoria respectively. The underlying material has a P-wave velocity of 6.05 +/- 0.03 km/s which suggests the rift is underlain by Precambrian metamorphic basement. A localised high velocity zone identified to the east of Nakuru may be associated with basic intrusive material. The P-wave velocity increases discontinuously to 6.45 +/- 0.05 km/s at a depth of 12.5 +/- 1-0 km. This depth is similar to that inferred for the brittle-ductile transition zone from a study of local seismicity in the Lake Bogoria region. A high P-wave velocity layer (7.1 +/- 0.15 km/s) occurs at 22 +/- 2 km depth which might be associated with a sill-like basic intrusion in the lower crust. An upper mantle velocity of 7.5 +/- 0.2 km/s (unreversed) is reached at a depth of 34.0 +/- 2.0 km. This implies that only moderate crustal attenuation has occurred beneath the central sector of the rift. No evidence was obtained for the existence of an "axial intrusion" reaching to shallow levels below the rift and causing crustal separation as suggested by previous studies. Relative residuals determined for 46 teleseismic events recorded by a 15 station, small aperture seismic array in the vicinity of Lake Bogoria indicate considerable lateral heterogeneity in the upper crust. An Aki inversion of the relative residuals has revealed the existence of two distinct low velocity zones which may be associated with magma chambers.

Moveout velocities sampled frequently along seismic horizons on a selection of seismic lines are used to derive interval velocities in an 'inversion' algorithm developed from work published by Hubral. This inversion is based on zero-offset raytrace modelling in a simplistic local ground model. The 'Hubral algorithm' is incorporated into a database which allows spatial smoothing of velocities. The spatial consistency of derived interval velocities can then be assessed by reference to mis-ties at line intersections, while interval velocities from well data can be used to check their validity. These principles have been used to derive interval velocities both from real data and from 'synthetic' data generated by common mid-point raytracing over schematic ground models. The latter study reveals that the procedure performs well if the local subsurface sampled by the CMP gather conforms approximately to the simplistic ground model assumed by the Hubral algorithm. The method is unsuitable in areas of faulting and interval velocity heterogeneity, and may yield spurious results over fold axes. Application of the procedure to real data indicates that it is generally desirable to smooth both moveout velocities before inversion and interval velocities after inversion. Comparison with well information shows that interval velocities derived by the Hubral algorithm are consistently higher than those measured from calibrated velocity logs. This observation is disturbing, since the derived interval velocities require a correction if they are to be used for depth conversions, but the discrepancy cannot be explained by ray theoretical considerations. No advantage appears to be gained by the 'layer-by-layer' mode of inversion over the 'direct' inversion, despite the greater potential for error propagation anticipated in the latter. Further work on different data sets is required to justify general use of the layer-by-layer mode of inversion.

The ultimate goal of seismology is to estimate the timing, magnitude and potential spatial extent of future seismic events along pre-existing faults. Based on the rate-state friction law, several theoretical physical earthquake models have been proposed towards this goal. Tectonic loading rate and frictional properties of faults are required in these models. Modern geodetic observations, e.g. GPS and InSAR, have provided unprecedented near-field observations following large earthquakes. In theory, according to the frictional rate and state asperity earthquake model, velocity-weakening regions holding seismic motions on faults should be separated with velocity-strengthening regions within which faults slip only aseismically. However, early afterslip following the 2011 MW 9.1 Tohoku-Oki earthquake revealed from GPS measurements was largely overlaid on the historical rupture zones, which challenged the velocity weakening asperity model. Therefore, the performance of the laboratory based friction law in the natural events needs further investigation, and the factors that may affect the estimates of slip models through geodetic modelling should also be discussed systematically. In this thesis, several moderate-strong events were investigated in order to address this important issue. The best-fit co- and post-seismic slip models following the 2009 MW 6.3 Haixi, Qinghai thrust-slip earthquake determined by InSAR deformation time-series suggest that the maximum afterslip is concentrated in the same area as the coseismic slip model, which is similar to the patterns observed in the 2011 Japan earthquake. In this case, complex geometric asperity may play a vital role in the coseismic nucleation and postseismic faulting. The major early afterslip after the 2011 MW 7.1 Van mainshock, which was revealed by one COSMO-SkyMed postseismic interferogram, is found just above the coseismic slip pattern. In this event, a postseismic modelling that did not allow slip across the coseismic asperity was also tested, suggesting that the slip model without slip in the asperities can explain the postseismic observations as well as the afterslip model without constraints on slip in the asperities. In the 2011 MW 9.1 Tohoku-Oki earthquake, a joint inversion with the GRACE coseismic gravity changes and inland coseismic GPS observations was conducted to re-investigate the coseismic slip model of the mainshock. A comparison of slip models from these different datasets suggests that significant variations of slip models can be observed, particularly the locations of the maximum slips. The joint slip model shows that the maximum slip of ~42 m appears near the seafloor surface close to the Japan Trench. Meanwhile, the accumulative afterslip patterns (slip >2 m) determined in previous studies appear in spatial correlation with the Coulomb stress changes generated using the joint slip model. As a strike-slip faulting event, the 2011 MW 6.8 Yushu earthquake was also investigated through co- and post-seismic modelling with more SAR data than was used in previous study. Best slip models suggest that the major afterslip is concentrated in shallow parts of the faults and between the two major coseismic slip patterns, suggesting that the performance of the rate and state frictional asperity model is appropriate in this event. Other postseismic physical mechanisms, pore-elastic rebound and viscoelastic relaxation have also been examined, which cannot significantly affect the estimate of the shallow afterslip model in this study. It is believed that the shallow afterslip predominantly controlled the postseismic behaviour after the mainshock in this case. In comparison to another 21 earthquakes investigated using geodetic data from other studies, complementary spatial extents between co- and post-seismic slip models can be identified. The 2009 MW 6.3 Qinghai earthquake is an exceptional case, in which the faulting behaviours might be dominated by the fault structure (e.g. fault bending). In conclusion, the major contributions from this thesis include: 1) the friction law gives a first order fit in most of natural events examined in this thesis; 2) geometric asperities may play an important role in faulting during earthquake cycles; 3) significant uncertainties in co- and post-seismic slip models can appreciably bias the estimation of fault frictional properties; 4) new insights derived from each earthquake regarding their fault structures and complex faulting behaviours have been observed in this thesis; and (5) a novel package for geodetic earthquake modelling has been developed, which can handle multiple datasets including InSAR, GPS and land/space based gravity changes.

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