From May 1987 to April 1988, a 9 station, seismic network was operated near the town of João Cãmara (5033'S, 35°51'W) In Rio Grande do Norte state, Brazil. The network was installed 6 months after the occurrence of a mb =5.1 earthquake. This thesis is concerned with the study of mfcroearthquakes recorded by this network, which is In the Precambrian Borborema Province of northeast Brazilian shield. The study revealed a remarkably well-defined distribution of seismicity associated with a simple fault structure. Hypocentre location was done using the HYP071 program. The results of the analysis show that the fault is divided into two main segments with the same N370E strike. The north segment is dipping 76°NW. The south segment is more nearly vertical, dipping about 82 1NW and composed of three or more closely spaced almost parallel faults. A small gap and a low seismicity zone were found to separate the north and south segments. No events deeper than 9 km were found In the entire region studied. Examination of the epicentral map and corresponding vertical cross sections also revealed that events are not random but are clustered. Right lateral strike slip with a small normal component was inferred as the fault mechanism from composite fault plane solutions. Off-fault events distant from the main fault were also observed, their pattern showing a classical example explained by the 'Model of Das & Scholz as a consequence of off-fault shear stress Increase after the occurrence of the main earthquake. Magnitude determinations for the events recorded by the telemetric network have been performed by developing a specific earthquake magnitude scale based on the duration of the observed seismic signal. A maximum likelihood estimate of b-value shows no significant variation during the recording period. - Shear wave splitting was observed In the J.Camara region in all the selected events recorded by the three component station JCAZ. The polarization direction of the first split shear-wave arrival for those events lay In the north-south direction which does not agree with the premise of extensive dilatancy anisotropy for a maximum compressive stress In the east-west direction as Inferred by the fault plane solutions.

The Zambezi Delta Basin is situated in central Mozambique at the present day Zambezi River mouth and extends from onshore to the offshore in a NW-SE direction into the waters of the Mozambique Channel over an estimated area of about 350,000 square kilometres both on- and offshore to the 2500m isobath. The analyses of approximately 21,700km of 2D seismic reflexion profiles and a well log suite for nine exploration wells supplied by the state oil company of Mozambique (ENH) has enabled an improved seismic stratigraphic model of the late Cretaceous and Tertiary evolution of the Zambezi Delta Basin to be made. It is demonstrated in this work that sedimentation alternated along a SWNE zone between two main depocentres separated by the Beira Basement High during their Tertiary development. These depocentres are the Zambezi Delta southwestern depocentre which is bounded to the southeast by the Beira Basement High and the East African Rift Active Extension in the northeast also bounded to the southwest by the Beira Basement High. The latter is suspected to be an active E-W graben structure. This finding is supported by seismological data studied here. Generally, periods of sedimentation in one depocentre mean a period of nondeposition and erosion in the other depocentre with sediment bypass, reworking and redeposition in the deeper parts of the basin (not covered by seismic data used in this study). Moreover sedimention is controlled by onshore tectonics controlling sediment sources onshore with tectonics, eustatic sea level variations and sediment load controlling basin subsidence and accommodation space in the basin offshore. Sedimentation generally was restricted to small areas which with time become widespread in the depocentres and at times cover the whole basin. This was the case during the early stages of major depositional cycles. Lithofacics relationships are very complex in the basin due to the sedimentary architecture produced by the interplay between sea level variation, basin subsidence and sediment supply. The resulting along-strike variability mapped in the Zambezi Delta Basin in this work has the potential not only to impact upon existing sequence stratigraphic models for deltaic settings but also on existing interpretation methods, for example the implications for the recognition of depositional sequences and "systems tracts". There is evidence of listric growth faulting of Middle Eocene age affecting turbidite deposits in the southwestern part of the basin while long wavelength folding observed in the deeper part of the sedimentary succession raises questions over the possibilities of evaporites or mobile shales underneath the sediment pile. Deltaic sedimentation switched from one depocentre to the other via distributary channels producing marked along-strike variability in the NE-SW direction. Generally sedimentation was more active in the southwestern part of the basin in late Cretaceous until late Middle Miocene when deposition moved into channel deposition in the northeastern part of the basin. Prom early late Miocene to Recent times various basinward prograding, southwestward-migrating depositional cycles with relatively high sedimentation and basin subsidence rates have characterized the evolution of the basin. Gravity and magnetic data supplied by GETECH (Leeds University) are used to derive crustal structure and to estimate the depth to magnetic basement beneath the basin. The Free-Air gravity model achieved in this study is a variant of the usual model achieved when crustal thinning and accumulated sediments extend seaward from the coastal area at a rifted passive continental margin. In view of the fact that oceanic magnetic anomalies lie 300km away to the south, thin crust beneath the Zambezi Delta Basin sediments is thought to be continental in origin and stretched immediately prior to breakup in middle Jurassic times.

Full-waveform inversion (FWI) is a powerful nonlinear tool for quantitative estimation of high-resolution high-fidelity models of subsurface seismic parameters, typically P-wave velocity. A solution is obtained via a series of iterative local linearised updates to a start model, requiring this model to lie within the basin of attraction of the solution space's global minimum. The consideration of seismic anisotropy during FWI is vital, as it holds influence over both the kinematics and dynamics of seismic waveforms. If not appropriately taken into account, then inadequacies in the anisotropy model are likely to manifest as significant error in the recovered velocity model. Conventionally, anisotropic FWI either employs an a priori anisotropy model, held fixed during FWI, or uses a local inversion scheme to recover anisotropy as part of FWI; both of these methods can be problematic. Constructing an anisotropy model prior to FWI often involves intensive (and hence expensive) iterative procedures. On the other hand, introducing multiple parameters to FWI itself increases the complexity of what is already an underdetermined problem. As an alternative I propose here a novel approach referred to as combined FWI. This uses a global inversion for long-wavelength acoustic anisotropy, involving no start model, while simultaneously updating P-wave velocity using mono-parameter local FWI. Combined FWI is then followed by multi-parameter local FWI to recover the detailed final model. To validate the combined FWI scheme, I evaluate its performance with several 2D synthetic datasets, and apply it to a full 3D field dataset. The synthetic results establish the combined FWI, as part of a two-stage workflow, as more accurate than an equivalent conventional workflow. The solution obtained from the field data reconciles well with in situ borehole measurements. Although combined FWI includes a global inversion, I demonstrate that it is nonetheless affordable and commercially practical for 3D field data.

Volcanoes generate a variety of seismic signals. One specific type, the so-called low frequency (LF) event, has proven to be crucial for understanding the internal dynamics of the volcanic system. While many endeavours have concentrated on the nature and cause of the seismic coda, the actual trigger mechanism of these events is still poorly understood. Several conceptual source models have been developed, ranging from magma-water interaction, stick-slip motion of magma plugs, magma flow instabilities, repeated release of gas-ash mixtures into open cracks, magma wagging, to brittle fracturing of magma. All but one trigger model, namely brittle failure of magma in the glass transition as response to the upwards movement of magma, fail to explain all observed characteristics of LF volcanic seismicity. Here, a spatially extended source, the ring fault structure, is developed to mimic the proposed source mechanism. The extended LF source is modelled as an arrangement of 8, 16 and 32 double couples (DCs) approximating a 30 m, 50 m and 70 m wide circular ring fault bounding the circumference of the volcanic conduit. Due to (partial) destructive interference, P-wave amplitudes of a ring fault structure are greatly reduced compared to single double couples and compensated linear vector dipoles (CLVDs), by 350% and 470%, respectively. It is shown here that these seismic amplitude differences may result in the underestimation of average slip and thus magma ascent rate by a factor of up to 3.5 when using an over simplified point source. To resolve the driving forces of LFs, synthetic seismograms representing both point and spatially extended sources were inverted for the apparent physical source mechanism using moment tensor inversion techniques (MTI). If original input parameters were unknown, MTI results of the ring fault would indicate a combination of 63% isotropic and 37% CLVD components. The proposed moment tensor strongly resembles that of a real CLVD case. The results of this study give evidence that slip along the conduit walls yields the same MTI results as a subhorizontal tensile crack, and the importance of knowledge about the source nature becomes eminently significant. Spatially extended source geometries describe an alternative to point dislocation sources. Additionally to the ring fault structure, this study provides a catalogue of further complex source scenarios involving new spatially extended sources, such as slip along a dike, conduit segments, two simultaneously acting ring fault structures, and helix-like flow patterns. P-wave amplitudes and waveforms vary largely with source geometry, stressing that source geometry is key for source interpretations and thus it is not sufficient to assume a point source nature of the processes involved to generated the observed seismicity.

Unsatisfactory performance of pile supported structures in liquefiable areas (ranging from tilting/settlement to complete collapse) is still observed after most major earthquakes. As a result, further research is required in this subject. This thesis therefore aims to study the response of pile supported structures during seismic liquefaction. The ground liquefies progressively in a top down fashion when the soil transform from solid material to liquid-like material. This is referred to as transient behaviour (from no-liquefaction to full liquefaction state) and is particularly focused in this work. In practice, piles are usually analysed as laterally loaded beams using Beam on Nonlinear Winkler Foundation model where earthquake loading is applied in a pseudo-static way. Therefore, this study reviews methods of analysis of laterally loaded pile. Six different field case records were analysed using different approaches and the results were compared. Large scale shake table experiments were also conducted consisting of four pile models (two single piles and two pile groups of 2×2) placed in a rigid soil container with energy absorbing boundaries. Redhill-110 sand was used and earthquake motions were applied to liquefy the soil. It was observed that the bending moment along the piles changed with the progression of liquefaction and the maximum bending moment occurred in the transient phase. It was also observed that the time taken to reach liquefaction may affect the amplification of the bending moment. Design of piles requires soil parameters and as a result, a series of multi-stage soil element tests were carried out on four different types of sands; Redhill-110 sand, Japanese silica sand No. 8, Assam sand, and Ganga sand where the sands were first liquefied and then tests were carried out to obtain stress-strain of liquefied sand (post-liquefaction). The results showed that the post liquefaction behaviour of sand depends on the soil relative density. Furthermore, the results from the Redhill-110 sand were used to back analyse the shake table test results. Finally, a method has been proposed to incorporate transient behaviour of pile in liquefiable soils, based on an assessment of the estimated dynamics amplification factors in the shake table tests. Keywords: Dynamic soil-pile interaction, Liquefaction, Shake table test, multi-stage soil element test, transient dynamics, dynamic amplification factors.

No description available.

The problem of inferring the velocity field is central to exploration seismology. Conventional velocity analysis is based on the hypothesis that reflection traveltime is a hyperbolic function of the distance between the source and the receiver. This is basis upon which interval velocities are obtained using Dix's equation for a horizontally layered model and Shah's equation for a dipping layered structure. However, in laterally inhomogeneous areas, traveltimes do not follow hyperbolae, hence, hyperbola based velocity estimation techniques fail in such areas. Although many sophisticated techniques, such as tomography, migration and model based velocity analysis can be used to obtain accurate velocity fields from seismic data in such areas, these methods are very computationally expensive. In this thesis, a simple, quicker and accurate velocity estimation technique is proposed. This technique does not abandon conventional velocity analysis, but gives further processing to stacking velocity data provided by conventional techniques. The new technique is based on the hypothesis that stacking slowness variations due to lateral interval slowness anomalies can be represented by the outputs of a linear impulse response system. The inputs of the system are the interval slowness anomalies. The system is space invariant for a horizontally layered model, but is space variant for a dipping layered model. A pre-determined background model is required to compute the linear system. Since the linear system is space invariant for a horizontally layered model and space variant for a dipping layered model, there are two schemes for velocity estimation for these two cases. In horizontal geology, the relationship between stacking slowness variations and interval slowness anomalies can be expressed by a set of linear equations in the wavenumber domain. The singular value decomposition method is used to solve the set of linear equations to obtain interval slowness anomalies from stacking slowness variations. In dipping geology, the relationship between stacking slowness variations and interval slowness anomalies cannot be written as a set of linear equations in the wavenumber domain. Interval slownesses must in this case be derived in the least square sense. Basis functions are introduced to construct interval slowness anomalies. Once the interval slowness anomalies have been estimated, interval slownesses (and hence velocities) are obtained by adding these to the background interval slownesses. Finally, the sensitivity of the linear system to data errors and model errors is investigated through a series of synthetic examples, the applications of these velocity estimation techniques and suggestions for further studies of the linear system are discussed

No description available.

No description available.

No description available.