A joint local, regional and teleseismic tomography study and shear wave splitting beneath the Mississippi Embayment and New Madrid seismic zone
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Seismic Tomography
Seismic zone
Geophysical Imaging
Shear wave splitting
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The tectonic setting and the earthquake hazards of the New Madrid seismic zone in the northern Mississippi embayment have been the subject of intensive study the past several years as part of the Earthquake Hazard Reduction Program of the USGS (U.S. Geological Survey). These efforts have improved our knowledge of the geologic history, general tectonic setting, and patterns of modern seismicity (see McKeown and Pakiser, 1982) in the New Madrid seismic zone. Identifying and characterizing the specific structures that may be responsible for the seismicity is the focus of continuing research. Unfortunately, these structures are buried by the veneer of poorly consolidated Upper Cretaceous and Tertiary sediments that overlie competent rocks in the embayment. In such a geologic setting, seismic-reflection profiling is one of the most valuable techniques available to obtain useful information on the distribution of faults, and on the local and regional structure. Three major seismic-reflection programs have been conducted by the USGS in the New Madrid seismic zone. The first program consisted of 32 km of conventional Vibroseis profiling designed to investigate the subsurface structure associated with scarps and lineaments in northwestern Tennessee (Zoback, 1979). A second, more extensive Vibroseis program collected about 250 km of data from all parts of the New Madrid seismic zone in Missouri, Arkansas, and Tennessee (Hamilton and Zoback, 1979, 1982; Zoback and others, 1980). The profiles presented here are part of the third program that collected about 240 km of high-resolution seismic-reflection data from a boat along the Mississippi River between Osceola, Ark., and Wickliffe, Ky. (fig. 1). The data for profiles A, B, C, and D were collected between river miles 839-1/2 and 850-1/2 from near the Interstate-155 bridge to upstream of Caruthersville, Mo. (fig. 2). Profiles on this part of the river are important for three reasons: (1) they connect many of the land-based profiles on either side of the river, (2) they are near the northeast termination of a linear, 120km-long, northeast-southwest zone of seismicity that extends from northeast Arkansas to Caruthersville, Mo. (Stauder, 1982; fig. 1), and (3) they cross the southwesterly projection of the Cottonwood Grove fault (fig. 1), a fault having a substantial amount of vertical Cenozoic offset (Zoback and others, 1980).
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Abstract Detailed, upper mantle P and S wave velocity ( V p and V s ) models are developed for the northern Mississippi Embayment (ME), a major physiographic feature in the Central United States (U.S.) and the location of the active New Madrid Seismic Zone (NMSZ). This study incorporates local earthquake and teleseismic data from the New Madrid Seismic Network, the Earthscope Transportable Array, and the FlexArray Northern Embayment Lithospheric Experiment stations. The V p and V s solutions contain anomalies with similar magnitudes and spatial distributions. High velocities are present in the lower crust beneath the NMSZ. A pronounced low‐velocity anomaly of ~ −3%–−5% is imaged at depths of 100–250 km. High‐velocity anomalies of ~ +3%–+4% are observed at depths of 80–160 km and are located along the sides and top of the low‐velocity anomaly. The low‐velocity anomaly is attributed to the presence of hot fluids upwelling from a flat slab segment stalled in the transition zone below the Central U.S.; the thinned and weakened ME lithosphere, still at slightly higher temperatures from the passage of the Bermuda hotspot in mid‐Cretaceous, provides an optimal pathway for the ascent of the fluids. The observed high‐velocity anomalies are attributed to the presence of mafic rocks emplaced beneath the ME during initial rifting in the early Paleozoic and to remnants of the depleted, lower portion of the lithosphere.
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