Download or read book Treatise on Seismicity and Velocity Structure of the Northern Mississippi Embayment written by Akramalsadat Mostafanejad and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Seismicity of the New Madrid seismic zone, velocity structure of the northern Mississippi Embayment and extent of lithospheric discontinuities have been investigated in this work. Spatial variations of seismic b-value in the NMSZ have been determined. A region of high b-value (b1̃.8) in the northern segment of the Reelfoot fault has been found which reflects higher frequency in the occurrence rate of small earthquakes. This is attributed to fault creep in a medium with high silica content, high pore fluid pressure and extensive velocity heterogeneity. In another study, power spectral ratios of horizontal to vertical (H/V) and vertical to horizontal (V/H) components of teleseismic P-waves recorded by broadband seismic stations inside the Mississippi embayment have been examined to produce a 3-D average velocity map of the sediments. Fundamental resonance frequencies for S-wave reverberations in the northern Mississippi Embayment are about 0.2 to 0.4 Hz. Detailed linear gradient velocity models for Vp and Vs have also been calculated using teleseismic vertical and radial transfer functions at 60 broadband stations inside the Mississippi embayment and simultaneously inverting for velocity parameters. This led to a detailed 3-D linear gradient Vp and Vs model of the sediments for the first time. P-wave velocity starts at about 1.0 km/s near the surface and increases with depth to 3.5 to 4 km/s in deeper parts of the embayment. S-wave velocities vary from 0.3km/s to 1.6 km/s in deeper sections to the southwest. This detailed velocity structure of the unconsolidated sediments is developed so that it could be used to downward continue the transfer functions to the depth of 5 km to remove the effects of observed amplification, reverberation and large P-S conversions that occurs at the sediment-bedrock interface. The resulting waveforms are depth migrated to image the lateral and vertical changes of major reflectors with depth. The Moho boundary is observed with high resolution at depths of 45 to 55 km in the study area.

Download or read book A SEISMOLOGIC STUDY OF THE NORTHERN MISSISSIPPI EMBAYMENT written by Yang Yang and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Part 1: Crustal structure in the New Madrid Seismic Zone (NMSZ) is investigated through a detailed study of explosion data obtained from the Embayment Seismic Excitation Experiment. The data show a distinct anisotropy in distance attenuation for both P and S waves in the range from 0 to 200km distance. Waves that propagate northward from the 1,134kg Marked Tree, Arkansas, explosion attenuate quickly with distance until a range of about 100km from the source where high-amplitude, high-phase velocity critical reflections from the boundary between the middle crust and rift pillow structure produce high amplitude waves. Propagation southward from the 2,268kg Mooring, Tennessee blast shows less distance attenuation compared to northward propagation. Reflections from the middle crust-lower crust boundary occur but do not significantly increase in amplitude with distance and travel with slower apparent phase velocity than observed for the northward propagation data set. A smooth velocity model is developed using a stabilized Weichert-Herglotz travel time inversion using first arrival travel times. Then an inversion using the travel time of both direct and middle crustal reflected waves is developed to obtain a 2D inhomogeneous-layered isotropic crustal model. The result reveals that there is a significant southwest dip to the top of the middle crust interface in the vicinity of the NMSZ, consistent with previously inferred changes in the thickness of the rift pillow model. This 2D feature characterizes the local wave propagation along the Reelfoot Rift and demonstrates the need for an improvement of the current Central United States velocity model.Part 2: Obtaining reliable empirical Greens functions (EGFs) from ambient noise by seismic interferometry requires homogenously distributed noise sources. However, it is difficult to attain this condition since ambient noise data usually contains highly correlated signals from earthquakes or other transient sources from human activities. Removing these transient signals is one of the most essential steps in the whole data processing flow to obtain EGFs. We propose to use a denoising method based on the continuous wavelet transform to achieve this goal. The noise level is estimated in the wavelet domain for each scale by determing the 99% confidence level of the empirical probability density function of the noise wavelet coefficients. The correlated signals are then removed by an efficient soft thresholding method. The same denoising algorithm is also applied to remove the noise in the final stacked cross-correlogram. A complete data processing workflow is provided with the overall data processing procedure divided into four stages: (1) single station data preparation, (2) removal of earthquakes and other transient signals in the seismic record, (3) spectrum whitening, cross-correlation and temporal stacking, and (4) remove the noise in the stacked cross-correlogram to deliver the final EGF. The whole process is automated to make it accessible for large datasets. Synthetic data constructed with a recorded earthquake and recorded ambient noise is used to test the denoising method. We then apply the new processing workflow to data recorded by the USArray Transportable Array stations near the New Madrid Seismic Zone where many seismic events and transient signals are observed. We compare the EGFs calculated from our workflow with commonly used time domain normalization method and our results show improved signal-to-noise ratios. The new workflow can deliever reliable EGFs for further studies.Part 3: We incorporate seismic ambient noise data recorded by different temporary and permanent broadband stations around the northern Mississippi Embayment from 1990 to 2018 to develop a crustal shear wave velocity (Vs) model for this area with full waveform ambient noise tomography. Empirical Greens functions at periods between 8 and 40s for all the possible pairs of stations are extracted by using a new seismic ambient noise data processing flow based on the continuous wavelet transform. Synthetic waveforms are then calculated through a heterogeneous Earth model using a GPU-enabled collocated finite-difference code. The cross-correlation time shifts between the synthetic waveforms and the extracted empirical Greens functions are used to construct the velocity updated kernel by using the adjoint method. Starting from the Central United States Velocity Model, the shear wave velocity model is then iteratively updated with the Vs kernel calculated in each iteration. Checkerboard tests show that perturbations in the top 30km of the crust are well recovered but amplitude recovery ability gradually decreases for deeper structure. We find that velocity lows characterize the Reelfoot Rift Graben and Rough Creek Graben separated by a high velocity crust. High velocity anomalies are observed under the Ozark Uplift and Paducah Gravity Lineament. A low velocity area previously interpreted as the Missouri Batholith is observed between them. A massive high velocity body in the southeast Mississippi Embayment is observed and is explained by the faulting as well as partly mafic intrusion. The Ouachita-Appalachian Thrust Front is clearly observed with a thinner crustal layer underneath. The rift pillow is well observed in the final tomography model along the Reelfoot Rift in the lower crust. The final inverted velocity model is consistent with local geological features and can be used for other seismological studies such as earthquake source determination and earthquake hazard assessment..

Download or read book Crustal Structure of the Northern Mississippi Embayment and Reelfoot Rift from Seismic Refraction reflection Profiling written by William James Braile and published by . This book was released on 1993 with total page 378 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Relocation of Microearthquakes in the Mississippi Embayment written by Mary Catherine Andrews and published by . This book was released on 1985 with total page 138 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Understanding Seismic Velocity Structure and Its Time varying Process Beneath the Mississippi Embayment Through Ambient Noise Analysis written by Chunyu Liu and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: We apply ambient noise analysis to image shear wave velocity from near surface to uppermost mantle beneath the Mississippi embayment, and investigate the crustal response to climatological loadings. To further understand the generation mechanism of microseisms, we explore the azimuthal distribution of the signal-to-noise ratio and amplitude difference of crustal surface waves and estimate possible source locations in the ocean through back- projections.A shear wave velocity model with 0.5 0.5 resolution for the crust and uppermost mantle has been determined. We take advantage of the dense coverage and long-term deployments of 277 3-component broadband stations installed from 1990 to 2018 to image the shear wave velocity. Rayleigh group velocity dispersion curves extracted from ambient noise are inverted to obtain shear wave velocity at 5, 12, 24, and 43 km. We find that low velocity features characterize the Reelfoot Graben, Rough Creek Graben, Black Warrior basin, and southern Mississippi embayment in the upper 5 km of crust. High velocity features characterize the Ozark plateau, Ouachita mountains and Nashville dome. From 5 to 12 km, a low velocity anomaly is associated with the Missouri batholith. From 12 to 24 km, high velocity features characterize the Reelfoot-Rough Creek graben, and along the Appalachian-Ouachita thrust front. From 24 to 43 km, high velocity anomalies are commonly observed in the Mississippi embayment, and spatially correlated with the crustal thickness.Cross-correlation of the ambient seismic field is also used to estimate seasonal seismic velocity variations and to determine the underlying physical mechanisms. We process continuously recorded broadband data from 53 stations in 2014 to obtain daily and yearly cross- correlations and measure the Rayleigh wave phase velocity change over 4 frequency bands, 0.3-1, 0.5-1.2, 0.7-1.5, and 1-2 Hz. We then calculate the correlation coefficients between the velocity variations and the precipitation, water table fluctuation, temperature, atmospheric pressure and wind speed to find which external variable correlates most strongly with the observed changes. We observe high t/t (a proxy for velocity variation), the slowest velocity relative to annual average, from May to July and low t/t in September/October, and find the t/t variations correlate primarily with water table fluctuation. The correlation coefficients between water table fluctuation and t/t are independent of the interstation distance and frequency, but high coefficients are observed more often in the 0.3-1 Hz than 1-2 Hz band probably because high-frequency coherent signals attenuate faster than low-frequency ones. The t/t variations lag behind the water table fluctuation by about 20 days, which suggests the velocity changes can be attributed to the pore pressure diffusion effect. The maximum t/t variations decrease with frequency from 0.03% at 0.3-1 Hz to 0.02% at 1-2 Hz, and the differences between them might be related to different local sources or incident angles. The seasonal variations of t/t are azimuthally independent, and a large increase of noise amplitude only introduces a small increase to the t/t variation. The maximum t/t variations non-linearly decrease with the distance, which could be associated with the attenuation of coherent noise. At close distances, the maximum t/t holds a wide range of values, which is likely related to local structure. At larger distances, velocity variations sample a larger region so that it stabilizes to a more uniform value. We find that the observed changes in wave speed are in agreement with the prediction of a poroelastic model.The source distribution of ambient noise is of fundamental importance to understanding the generation mechanism of microseisms. Cross-correlations of ambient seismic noise from 277 broadband stations with at least 1-month recording between 1990 and 2018 are used to estimate source locations of primary and secondary microseisms inside the Mississippi embayment. We investigate source locations by analyzing the azimuthal distribution of the signal-to-noise ratio (SNR) and amplitude difference of crustal surface wave arrivals and by 2D F-K analysis. We also use 84 stations with continuous 1-year recording to explore seasonal variations of SNRs of the surface wave, which could be used to locate active sources in different seasons. We observe that (1) four azimuths could be identified in the azimuthal distribution of SNRs and reflect four different energy sources. Two energy sources are active in the Pacific and Atlantic ocean of northern hemisphere during winter and two relatively weak sources are active near Australia and South America in the southern hemisphere during summer. (2) Primary microseisms originate along the coastlines of southern Australia, Canada and Alaska, Newfoundland, and northeast South America. (3) Secondary microseisms could be generated in the deep water of northern and southern Pacific ocean, along coastlines of Canada and Alaska associated with reflections, and in the deep water of south of Greenland. (4) Azimuthal distribution of SNRs of sediment surface waves observed at 1-5s is negatively correlated with the geometry of the edge of the Mississippi embayment. The sediment surface waves could be induced by the basin-edge..

Download or read book Location And Characteristic Features of the Quaternary Deformation in the Mississippi Embayment from High resolution Seismic Reflection Data written by Lei Guo and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The New Madrid Seismic Zone (NMSZ), central US. is a controversial midcontinental region where the occurrence of large magnitude historical and prehistorical earthquakes and the high level of instrumental seismicity, clash with the lack of evident surface and subsurface deformation and with slow geodetic rates. To unravel this apparent paradox locally and to contribute to the understanding of intraplate seismicity globally, I integrate new high-resolution seismic reflection data acquired as part of the Mississippi River Project, data acquired for the Meeman-Shelby Fault study, and existing seismic reflection data, to investigate the long-term deformation in the northern Mississippi Embayment. In particular this study focuses on the identification and characterization of the structures where Quaternary deformation is accommodated in the northern Mississippi Embayment, on the comparison of the newly discovered structures with those that are seismically active today, and on the analysis of the distribution of the Quaternary deformation with respect to the NMSZ. The results show that Quaternary deformation has been accommodated along adjacent faults additional to the NMSZ fault system, and that the NMSZ faults are virtually undistinguishable from quiescent faults, except for the instrumental seismicity. The distribution of Quaternary deformation extends beyond the footprint of the high-velocity lower crustal layer (i.e. & ldquo;mafic rift pillow & rdquo;), invoked by several geodynamic models as the possible feature capable of concentrating stress in this region and responsible for the seismicity in the central US. The long-term Quaternary deformation appears to be accommodated along pre-existing features predominantly associated with the Reelfoot Rift structure.

Download or read book Treatise on Geophysics Volume 6 written by Anthony B Watts and published by Elsevier. This book was released on 2010-05-13 with total page 627 pages. Available in PDF, EPUB and Kindle. Book excerpt: Treatise on Geophysics: Crust and Lithosphere Dynamics, Volume 6, provides a comprehensive review of the state of knowledge on crust and lithosphere dynamics, which is defined as the study of how the outermost layers of the Earth respond to loads that are emplaced on, within, and below it and its implications for plate mechanics and mantle flow. The book begins with a chapter on plate kinematics, which shows how new observations and methodologies have improved the resolution of relative and absolute plate motions. This is followed by studies of plate mechanics, focusing on the long-term rheology of the plates and response of the plates to both relatively short-term and long-term loads. The book also includes chapters that examine the evidence from surface heat flow, borehole breakouts, and magma models for the thermal and mechanical structure of the lithosphere; the deformation of the lithosphere in extensional and compressional settings. The final two chapters deal with the structural styles of faulting in the shallow brittle part of the lithosphere, the brittle-ductile transition, and the shear zone in the ductile part of the lithosphere; and how developments in plate mechanics have impacted our understanding of geological processes. Self-contained volume starts with an overview of the subject then explores each topic with in depth detail Extensive reference lists and cross references with other volumes to facilitate further research Full-color figures and tables support the text and aid in understanding Content suited for both the expert and non-expert

Download or read book Neotectonics of the Northern Mississippi Embayment written by W. John Nelson and published by . This book was released on 1999 with total page 42 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book General Geology of the Mississippi Embayment written by Elliott Morse Cushing and published by . This book was released on 1964 with total page 36 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book A Crustal Structure Study of the Northern Mississippi Embayment written by Geological Survey (U.S.) and published by . This book was released on 1982 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Seismic reflection profiles in the northern Mississippi Embayment written by Robert Morrison Hamilton and published by . This book was released on 1979 with total page 7 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book A Crustal Structure Study of the North Mississippi Embayment written by C. B. Austin and published by . This book was released on 1982 with total page 10 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book A Joint Local Regional and Teleseismic Tomography Study and Shear Wave Splitting Beneath the Mississippi Embayment and New Madrid Seismic Zone written by Cecilia Anyango Nyamwandha and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Part 1: We have determined detailed crust and upper mantle 3-D P wave and S wave velocity models to a depth of 400 km for the Mississippi Embayment (ME) and the New Madrid Seismic Zone (NMSZ). This study incorporates data from three networks; the New Madrid Seismic Network (CNMSN) operated by the Center for Earthquake Research and Information (CERI), the Earthscope Transportable Array (TA), and the Flex Array (FA) Northern Embayment Lithospheric Experiment (NELE) project stations to aid in constructing the most detailed velocity images of the NMSZ to date. For the crust we observe a separation of velocity highs from velocity lows along the axis of the Mississippi Valley Graben (MVG). For the upper mantle, we image a significant low velocity anomaly of ̃-3% to -5% at depths of 100 - 250 km. A high velocity anomaly of ̃+3% to +4% is observed at depths of 80 - 160 km and it occurs along the sides and top of the low velocity anomaly. The Vp and Vs solutions in the upper mantle show a remarkable similarity both in shape and anomaly magnitude. We propose that the observed low velocity features in the upper mantle are as a result of various tectonic activities in the area, which could result in: 1) Rejuvenated/primitive mantle, 2) Elevated temperatures, and 3) Increased fluid content. A combination of elevated temperatures and increased fluid content reduce P wave velocity (Vp) whereas the three effects combined significantly reduce S wave velocity (Vs). The high velocity anomalies observed are associated with mafic rocks emplaced in the lithosphere beneath the ME during initial rifting in the early Paleozoic and/or remnants of the depleted, lower portion of the lithosphere. Part 2: Using teleseismic SKS phases recorded with the Northern Embayment Lithospheric Experiment (NELE), and the USArray Transportable Array (TA), we apply the SplitLab processing environment to measure shear wave splitting within and outside the Mississippi Embayment (ME) for the period 2011 - 2015. Our dataset consists of 4̃570 individual splitting measurements from 112 earthquakes. Stations outside of the ME exhibit significant shear wave splitting, with average delay times at individual stations between ̃0.4 sec and ̃1.5 sec. To the northeast and east of the ME, nearly all observed fast directions are approximately oriented NE-SW, in agreement with the absolute plate motion (APM) predicted by HS3-Nuvel-1A. The homogeneity of the fast directions in this region could be indicative of active flow in the asthenosphere beneath these stations. A counter-clockwise rotation in the splitting pattern is observed moving through the study area from northeast to northwest. Inside the ME, there is a complex pattern of anisotropy with large but systematic deviations ranging from ̃0.7 sec to ̃2.5 sec. This region of large deviations also coincides with imaged low P- and S-wave velocity anomalies in the upper mantle. The splitting complexity could be due to a contribution from 'frozen in' lithospheric fabrics formed during past orogenic events and additional fabric due to a northeast to southwest (NE-SW) asthenospheric flow as a result of the sinking of the Farallon slab in the lower mantle beneath the Central and Eastern United States (CEUS).

Download or read book Neotectonics of the Northern Mississippi Embayment written by W. John Nelson and published by . This book was released on 1999 with total page 50 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book High resolution Upper Crustal P and S Velocity Structures in the Upper Mississippi Embayment written by Fuchun Gao and published by . This book was released on 1999 with total page 276 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Time Variations in Energy Released Through Earthquakes in the Mississippi Embayment Region written by Kathleen M. Power and published by . This book was released on 1980 with total page 154 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book A Crustal Structure Study of the Mississippi Embayment written by C. Bradford Austin and published by . This book was released on 1978 with total page 116 pages. Available in PDF, EPUB and Kindle. Book excerpt: