scholarly journals Double layer anisotropy beneath the New Madrid seismic zone and adjacent areas: insights from teleseismic shear wave splitting

2014 ◽  
Vol 3 (1) ◽  
pp. 3 ◽  
Author(s):  
Moikwathai Dax Moidaki
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Small Scale ◽  
New Madrid Seismic Zone ◽  
Seismic Zone ◽  
Wave Splitting ◽  
Anisotropic Layers ◽  
The Individual ◽  
Orogenic Events ◽  
New Madrid

A total of 93 well-defined PKS, 54 SKKS, and 126 SKS shear-wave splitting parameters are determined at 25 broadband seismic stations in an approximately 1000 by 1000 km<sup>2</sup> area centered at the New Madrid seismic zone (NMSZ) in order to test the existence of two anisotropic layers and to map the direction and strength of mantle fabrics. The individual splitting parameters suggest a significant and systematic spatial and azimuthal variation in the splitting parameters. The azimuthal variations at most stations can be explained as the results of present SW ward asthenospheric flow and NNE trending lithospheric fabrics formed during past orogenic events. In the NMSZ, rift-parallel fast directions (potentially related to a long-rift flow) and rift-orthogonal fast directions from small-scale mantle convection are not observed. In addition, reduction in splitting times as a result of vertical asthenospheric flow is not observed.

Shear‐wave splitting in Quaternary sediments: Neotectonic implications in the central New Madrid seismic zone

Geophysics ◽  
1996 ◽  
Vol 61 (6) ◽  
pp. 1871-1882 ◽  
Author(s):  
James B. Harris
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Azimuthal Anisotropy ◽  
Data Matrix ◽  
New Madrid Seismic Zone ◽  
Seismic Zone ◽  
Earthquake Hazards ◽  
Near Surface ◽  
Neotectonic Deformation ◽  
Wave Splitting

Determining the extent and location of surface/near‐surface structural deformation in the New Madrid seismic zone (NMSZ) is very important for evaluating earthquake hazards. A shallow shear‐wave splitting experiment, located near the crest of the Lake County uplift (LCU) in the central NMSZ, shows the presence of near‐surface azimuthal anisotropy believed to be associated with neotectonic deformation. A shallow four‐component data set, recorded using a hammer and mass source, displayed abundant shallow reflection energy on records made with orthogonal source‐receiver orientations, an indicator of shear‐wave splitting. Following rotation of the data matrix by 40°, the [Formula: see text] and [Formula: see text] sections (principal components of the data matrix) were aligned with the natural coordinate system at orientations of N35°W and N55°E, respectively. A dynamic mis‐tie of 8 ms at a two‐way traveltime of 375 ms produced an average azimuthal anisotropy of ≈2% between the target reflector (top of Quaternary gravel at a depth of 35 m) and the surface. Based on the shear‐wave polarization data, two explanations for the azimuthal anisotropy in the study area are (1) fractures/cracks aligned in response to near‐surface tensional stress produced by uplift of the LCU, and (2) faults/fractures oriented parallel to the Kentucky Bend scarp, a recently identified surface deformation feature believed to be associated with contemporary seismicity in the central NMSZ. In addition to increased seismic resolution by the use of shear‐wave methods in unconsolidated, water‐saturated sediments, measurement of near‐surface directional polarizations, produced by shear‐wave splitting, may provide valuable information for identifying neotectonic deformation and evaluating associated earthquake hazards.

Shear-wave splitting of SK(K)S-phases beneath the Upper Rhine Graben area: Indications for laterally and vertically varying seismic anisotropy

2021 ◽  
Author(s):  
Yvonne Fröhlich ◽  
Michael Grund ◽  
Joachim R. R. Ritter
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Symmetry Axis ◽  
Shear Wave Splitting ◽  
Small Scale ◽  
Upper Rhine Graben ◽  
Event Analysis ◽  
Horizontal Symmetry ◽  
Wave Splitting ◽  
Upper Rhine

&lt;p&gt;The observed backazimuthal variations in the shear-wave splitting of core-refracted shear waves (SK(K)S-phases) at the Black Forest Observatory (BFO, SW Germany) indicate small-scale lateral and (partly) vertical variations of the elastic anisotropy in the upper mantle. However, most of the existing seismic anisotropy studies and models in the Upper Rhine Graben (URG) area are based on short-term recordings and thus suffer from a limited backazimuthal coverage and averaging over a wide or the whole backazimuth range. Hence, to find and delimit basic anisotropy regimes, also with respect to the connection to geological and tectonic processes, we carried out further SK(K)S splitting measurements at permanent (BFO, WLS, STU, ECH) and semi-permanent (TMO44, TMO07) broadband seismological recording stations.&lt;/p&gt;&lt;p&gt;To achieve a sufficient backazimuthal coverage and to be able to resolve and account appropriately for complex anisotropy, we analysed long-term recordings (partly &gt; 20 yrs.). This was done manually using the MATLAB-program SplitLab (single-event analysis) together with the plugin StackSplit (multi-event analysis). The two splitting parameters, the fast polarization direction &lt;em&gt;&amp;#934;&lt;/em&gt; given relative to north and the delay time &lt;em&gt;&amp;#948;t&lt;/em&gt; accumulated between the two quasi shear waves, were determined by applying both the rotation-correlation method and the minimum-energy method for comparison. Structural anisotropy models with one layer with horizontal or tilted symmetry axis and with two layers with horizontal symmetry axes (assuming transvers isotropy with the fast axis being parallel to the symmetry axis) were tested to explain the shear-wave splitting observations, including lateral variations around a recording site.&lt;/p&gt;&lt;p&gt;The determined anisotropy is placed in the upper mantle due to the duration of the delay times (&gt; 0.3 s) and missing discrepancies between SKS- and SKKS-phases (so not hints for significant lowermost mantle contributions). The spatial distribution and the lateral and backazimuthal variations of the measured (apparent) splitting parameters confirm that the anisotropy in the mantle beneath the URG area varies on small-scale laterally and partly vertically: On the east side of the URG, from the Moldanubian Zone (BFO, STU, ECH) to the Saxothuringian Zone (TMO44, TMO07) a tendency from two layers with horizontal symmetry axes to one layer is suggested. In the Moldanubian Zone, between the east side (STU, BFO) and the west side (ECH) of the URG, a change of the fast polarisation directions of the anisotropy models with two layers with horizontal symmetry axes is observed. Inconsistent measured apparent splitting parameters and the observation of numerous null measurements, especially below the URG may be at least partly related to scattering of the seismic wavefield or a modification of the mantle material.&lt;/p&gt;

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