Local, regional and teleseismic earthquakes recorded at seismic stations in northern Kenya have been analysed to determine the seismicity in this region and the crustal and upper mantle structure beneath an array on the north-eastern flank of the Kenya dome. A total of 389 microearthquakes, occurring during the period 17th Jan. to 26th Aug., 1981, have been located using a simple half space velocity model. The seismic activity is mainly confined to the Kenya rift and a zone approximately 150km to the east of the main rift. The northward extent of the activity suggests that the active section of the rift continues beneath Lake Turkana. Apparent velocities of local and regional earthquakes recorded at the Ngurunit array have been interpreted in terms of crustal velocity structure. A two layered crustal velocity model has been derived. The velocity of the upper crustal layer is considered to increase linearly with depth, according to the function V = 5.8 + 0.0245z. The velocity of the lower crustal layer is 6.5 0.2 km/s and the intermediate crustal boundary is placed at 24km. The variation of apparent velocity with azimuth of Moho arrivals suggests an eastwardly dipping Moho of 7 beneath the array. The depth of the Moho directly beneath the array is derived to be 46km and the velocity of sub-Moho material is 8.3 0.2 km/s. Delay times and slowness measurements of teleseismic P wave arrivals indicate the presence of anomalously low P wave velocity material in the upper mantle. Three-dimensional ray tracing modeling suggests a thickening of the anomalous body to the north and east of the array, such that the top surface of the body reaches to within 60-80km of the ground surface. The thickening may be associated with rift structures beneath Lake Turkana and Quaternary volcanic activity observed on the eastern flank of the Kenya dome.

A seismic refraction profile was undertaken to investigate the source of an aeromagnetic anomaly located above the Widmerpool Gulf, East Midlands. Ten shots were fired into 51 stations at c. 1.5km spacing in a 70km profile during 41 days recording. The refraction data were processed using standard techniques to improve the data quality. A new filtering technique, known as Correlated Adaptive Noise Cancellation was tested on synthetic data and successfully applied to controlled source and quarry blast data. Study of strong motion data reveals that the previous method of site calibration is invalid. A new calibration technique, known as the Scaled Amplitude method is presented to provide safer charge size estimation. Raytrace modelling of the refraction data and two dimensional gravity interpretation confirms the presence of the Widmerpool Gulf but no support is found for the postulated intrusion. Two dimensional magnetic interpretation revealed that the aeromagnetic anomaly could be modelled with a Carboniferous igneous source. A Lower Palaeozoic refractor with a velocity of 6.0 km/s is identified at a maximum depth of c. 2.85km beneath the Widmerpool Gulf. Carboniferous and post-Carboniferous sediments within the gulf have velocities between 2.6-5.5 km/s with a strong vertical gradient. At the gulf margins, a refractor with a constant velocity of 5.2 km/s is identified as Dinantian limestone. A low velocity layer of proposed unaltered Lower Palaeozoics is identified beneath the limestone at the eastern edge of the Derbyshire Dome. The existence and areal extent of this layer are also determined from seismic reflection data. Image analysis of potential field data, presents a model identifying 3 structural provinces, the Midlands Microcraton, the Welsh and English Caledonides and a central region of complex linears. This model is used to explain the distribution of basement rocks determined from seismic and gravity profiles.

The Derbyshire Dome is thought to have been a stable uplifted area since at least Lower Carboniferous times. This project is principally concerned with four 30km seismic refraction lines which crossed the limestone outcrop of Derbyshire and N. Staffordshire in order to investigate the Dome's upper crustal structure, using quarry blasts as seismic sources. A time-term analysis of refracted arrival data defined basement structure more complicated than implied by the surface geology. The interpretation of these data was complicated by high (5.6-5.8km/s) velocity refractions from dolomitic horizons within the limestone sequence; the mean overburden velocity was determined to be about 5.2 km/s. The Dome could be divided into two pre-Carboniferous geological units separated approximately by the line of the NNW trending Bonsall Fault. To the north a broadly domal refractor of velocity 5.5-5.55km/s was mapped, and thought to correlate with both the shallow pre-Carboniferous volcanics encountered by the Woo Dale borehole and"the Ordovician shales encountered by the Eyam borehole below 1.8km of limestone. This refractor accordingly deepens beneath the Carboniferous sedimentary basins flanking the Dome. To the south of the Bonsall Fault zone, the Carboniferous was found to be underlain by a refractor of velocity 5.63-5.7km/s, thought to be of Precambrian material similar to the rocks of Charnwood Forest, Leicestershire, some 40km south. By analysing later arrivals, this refractor has been mapped to the north of the Bonsall Fault at a depth of 2.5-3.5km. The shallower Lower Palaeozoic refractor is thought to be no more than 500m thick, and underlain by lower velocity, possibly Cambrian, material. This interpretation is consistent with the Bouguer anomaly map of the region, and sheds light on the structural control of Carboniferous sedimentation. The basement fault dividing the two pre-Carboniferous units is thought to have been active during the Dinantian as the northern unit tilted eastwards.

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A theory for predicting the dynamic response of earth structures subject to earthquake forces as presented herein. The proposed theory essentially introduces a stress limit into the viscoelastic shear-slice or shear-beam analogy and thus, in essence, the structure is modelled by a multidegree of freedom system which responds in an elastic-viscoplastic manner. The stress limit may be varied with both the magnitude of displacement and the number of stress cycles so that, in practice, a complex stress-strain relationship can be considered in the analysis. The method allows the dynamic displacements, velocities, and accelerations within the structure to be calculated at any time, t, when the base is subjected to a known acceleration. If magnitude of displacements is considered the criterion for earthquake design, then the proposed method of analysis gives a direct measure of these displacements. The analysis is applied to a number of earth structures using a base acceleration corresponding to the 0-10 secs. of El Centro, 1940 earthquake. Displacements, velocities and accelerations are calculated at discreet time intervals. Results are compared with those obtained from a viscoelastic dynamic analysis and it is found that plastic action causes larger displacements and smaller accelerations. The location of plastic yield is dependent on the strength properties of the material. For material of uniform strength, yield will occur essentially at the base of the structure, while for ideal frictional material, yield will take place throughout the structure but will be greatest at the top. Lateral force or seismic,coefficients from viscoelastic and elastic-viscoplastic are compared. Results suggest that the common practice of allowing for plastic-action by assuming some high viscous damping factor such as 20% of critical in a viscoelastic analysis is unlikely to give correct results. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

archives@tulane.edu / The crustal state-of-stress is influenced by crustal heterogeneity and anisotropy, regional tectonics, gravitational loading of topographic relief, and subsurface magma pressure. The local stress field dictates the location and orientation of magmatic intrusions and faulting, which are important to understand tectonic processes and to inform hazard studies. I analyze earthquakes and their corresponding source mechanisms, complemented by analytical and numerical stress modeling, to interpret state-of-stress in rifting and magmatic regions. I investigate how brittle deformation is accommodated in early-stage rifting of cratonic lithosphere via seismic moment tensors in regions that are considered magma-rich and magma-poor. I also study the caldera stress change associated with an eruption. My research can contribute to our understanding of the controlling mechanisms in an extensional regime and in volcanic regions, and contribute to earthquake and volcanic hazard assessment in magmatic and rift zones in Africa, and worldwide. / 1 / Sarah Jaye Oliva

This dissertation presents an analysis of the spatiotemporal variations of strong ground shaking and style of faulting at regional scales in the Chilean subduction zone. These properties are part of the two main components of seismic hazard: ground motions and seismic source characterization. Results are interpreted to be related to the tectonic dynamics and heterogeneities in the region. The ground motion component was evaluated using residual analysis between observed and predicted earthquake strong ground motions. By analyzing strong motions of interplate earthquakes along the Chilean subduction zone (from the northern border to the tectonic triple junction in the south), we corroborate the better predictive performance of locally-derived ground motion models over globally-derived models. The results presented here show a stronger short-period radiation generated by deeper interplate earthquakes in north and central Chile, which is not currently considered in local ground motion models. We interpret this depth-dependent radiation pattern as a result of frictional variations on the plate interface. Additionally, it is shown that not every aftershock sequence following a large interplate earthquake exhibits predominantly weaker short-period radiation, which could be dependent on whether there was precursory activity before the mainshock. This work also outlines the need of developing local ground motion models for crustal earthquakes, since ground motion observations from the Mw 5.7 Pica earthquake (a crustal reverse-oblique event in northern Chile with a large stress drop) are significantly larger than predictions from current global models, particularly at short periods. The seismic source component was assessed for crustal earthquakes in northern Chile. Clear regionalization is found in the spatial patterns of style of faulting and the tectonic stress field. The coastal region exhibits a clear margin-parallel compressional regime shown by the mostly reverse and strike-slip earthquakes in this area, while the analyzed portion of the Andean Precordillera shows a strike-slip regime with a compressional direction nearly parallel to the plate convergence direction. We interpret these two tectonic regimes as a result of the concave shape of the subduction margin and the effect of local topography, respectively. Our results show that these regimes do not show any temporal change during the ∼10 year analysis period, and likely have remained stationary for the last 10 Ma. Although the earthquake recurrence relations presented in this work show that crustal earthquakes happen less frequently and at smaller magnitudes than interplate and intraplate earthquakes in northern Chile, crustal earthquakes still pose an important hazard, with the possibility of occurrence of more earthquakes with damaging-level ground accelerations, such as the Pica earthquake. / Graduate

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