Christine A. Powell

A seismotectonic model for the 300-kilometer-long eastern Tennessee seismic zone

Ten years of monitoring microearthquakes with a regional seismic network has revealed the presence of a well-defined, linear zone of seismic activity in eastern Tennessee. This zone produced the second highest release of seismic strain energy in the United States east of the Rocky Mountains during the last decade, when normalized by crustal area. The data indicate that seismicity produced by regional, intraplate stresses is now concentrating near the boundary between relatively strong and weak basement crustal blocks.

Polarization tomography for P wave velocity structure in southern California

We develop a tomographic inversion method that uses teleseismic P wave polarization data to obtain velocity structure. Polarization inversion has some intrinsic advantages over travel time inversion: It is not influenced by source location and origin time errors; it is not sensitive to deep mantle velocity structure and can be used iteratively to improve the tomographic result. Polarization inversion is more sensitive to near-station velocity structure and to velocity gradient and is complementary to travel time inversion in this sense. The method is applied to California Institute of Technology-U.S. Geological Servey southern California array data. The result is generally consistent with previous work and also reveals that the high-velocity feature beneath the Transverse Ranges is bounded between 40 and 200 km depths and possibly has a second small piece at about 300 km depth. The slow velocity anomaly under the Salt on Trough is limited to shallow depths, less than about 60 km.

Three‐dimensional S wave velocity structure and Vp/Vs ratios in the New Madrid Seismic Zone

A three-dimensional S wave velocity model for the New Madrid Seismic Zone (NMSZ) has been developed using nonlinear travel time tomography. The inversion utilized 5544 S wave arrival times from 720 earthquakes recorded by digital, three-component stations deployed in the NMSZ over the time period 1989 through 1992. We imaged S wave velocity anomalies ranging from −5% to +8% relative to the starting one-dimensional velocity model. Lowest S wave velocities are found south of Ridgley, Tennessee, in an area characterized by a high earthquake swarm rate and shallower than normal hypocenters. Two centers of higher than average S wave velocity are located west of the Mississippi river, north of Caruthersville, Missouri. The S wave model is similar to a P wave velocity model generated using the same earthquake data set. The similarity in ray coverage in both the P and S wave solutions allowed calculation of Vp/Vs ratios. Most of the seismicity in the NW trending central arm of the NMSZ is associated with normal Vp/Vs values that border regions with high Vp/Vs. North of Ridgely, high Vp/Vs values are associated with higher than average compressional and shear wave velocities and are interpreted to be due to mafic intrusions along the axis and edges of the Reelfoot rift. SE of Ridgley, the end of the central arm coincides with high Vp/Vs values that are due to a significant shear wave low-velocity zone and are interpreted to be the result of highly fractured and fluid saturated crust.

Three‐dimensional P wave velocity structure in the New Madrid seismic zone

A three-dimensional P wave velocity model for the New Madrid seismic zone (NMSZ) has been developed using a nonlinear travel time tomography method. The inversion involved 709 earthquakes recorded by digital, three-component Portable Array for Numerical Data Acquisition stations deployed in the NMSZ over the time period 1989 through 1992. Analysis of ray coverage and inversion of a synthetic data set showed that the model has high resolution to a depth of 11 km. Low-velocity anomalies correlate with the prominent northwest and northeast trending arms of seismicity. Lowest-velocity anomalies (−8%) occur at the intersection of these arms and are associated with shallow seismicity and a high swarm rate. These low-velocity regions are interpreted to be the result of increased fluid pressure associated with structurally disrupted rocks beneath the Blytheville-Pascola arch complex. Small regions of high velocity, limited to the upper few km of the crust, correlate with gravity and magnetic anomalies and are interpreted to be igneous intrusions. High-velocity anomalies also parallel the edges of the Reelfoot rift margin and probably are associated with crystalline Precambrian rocks that form the margin of the Reelfoot rift graben. Most earthquakes are associated with low-velocity regions and avoid regions of high velocity.

Joint hypocenter-velocity inversion for the eastern Tennessee seismic zone

A joint hypocenter-velocity inversion for the eastern Tennessee seismic zone (ETSZ) has resolved velocity features in basement rock below detached Appalachian thrust sheets. P and S wave arrival times from 492 earthquakes have been inverted for one-(1-D) and three-dimensional (3-D) velocity models to midcrustal depths. The 3-D P and S wave velocity solutions are computed independly and are very similar. In relation to the 1-D model, velocity anomalies range from −8% to +16% in the first layer (upper 5 km) and between ±7% in deeper layers. Prominent velocity anomalies parallel the seismic zone and are consistent from layer to layer. The most persistent anomaly is a low-velocity region that borders the seismic zone to the northwest and is flanked on either side by regions of anomalously high velocity. The New York-Alabama (NY-AL) magnetic lineament coincides with or lies close to the southeast boundary of the prominent velocity low in both the P and S wave velocity images. The spatial coincidence between velocity, gravity, and magnetic gradients suggests that major discontinuities are present in the basement. Relocation in the 3-D velocity model reduced the number of very deep earthquakes (below 20 km) and further accentuated differences in seismogenic properties on either side of the NY-AL lineament. After relocation, most earthquakes occur in a vertically bounded region roughly 30 km wide extending from 4 to 22 km in depth. Most earthquakes occur in regions characterized by either average velocity or small velocity anomalies.

Relative array analysis of the southern California lithosphere

Lateral velocity structure in the lithosphere beneath southern California is investigated using relative array analysis. In this form of analysis, P wave polarization values measured by a subset of receivers (subarray) are compared to values measured by the entire set of receivers (array). The effects of lateral velocity structure beneath the subarray can be distinguished from the effects of average velocity structure beneath the entire array. Lateral velocity gradients can be mapped as the subarray window is moved across the network. The method is insensitive to effects of near-source and lower mantle heterogeneity, does not lose resolution near the periphery of the array, and utilizes a locally derived ray geometry. Lateral velocity variations beneath the California Institute of Technology-United States Geological Survey network detected by relative array analysis can be compared directly to velocity anomalies found by inversion of travel time residuals. Two regions near the edge of the tomographic image developed by Humphreys and Clayton are investigated; both regions are experiencing active tectonics. Relative array analysis suggests that relatively high velocity may be present beneath the westernmost Transverse Ranges. This feature extends to a depth of roughly 80 km and may be related to the high velocity upper mantle anomaly imaged beneath the central and western Transverse Ranges. Relatively low velocity is associated with the crust and uppermost mantle south of the Salton Sea. This feature is probably related to hot, ascending mantle but is located east of the surface expression of the spreading axis in the Imperial Valley.

Magnetic susceptibility of rocks in the Nutbush Creek ductile shear zone, North Carolina

Abstract Applicability of anisotropy of magnetic susceptibility (AMS) as a strain indicator is investigated for the Falls lineated gneiss, a strongly lineated, weakly foliated tectonite lying within the Nutbush Creek fault zone (NCFZ). The NCFZ is a major Alleghanian ductile shear zone within the southern Appalachian Piedmont and is characterized along most of its length by a pronounced, linear trend on aeromagnetic maps. Sampling was conducted along a traverse near Raleigh N.C., that extended through the lineated gneiss, across the eastern boundary of the NCFZ and into adjacent, supposedly unsheared Raleigh-belt gneiss. Petrofabric element orientations are similar within the Falls lineated gneiss and the Raleigh-belt gneiss but foliation is better developed than lineation in the Raleigh-belt gneiss. Mineral lineation parallels the strike of the NCFZ. Results of the AMS analysis are remarkably consistent along the entire traverse. Mean susceptibilty ellipsoids are uniformly prolate and display little variation in spatial orientation. In both the Falls lineated gneiss and the Raleigh-belt gneiss, mean maximum susceptibility axes are oriented along the trend of mineral lineation, and mean minimum susceptibility axes coincide with poles to foliation. Magnetic fabric in both lithologies is dominated by the alignment of magnetite grains in the direction of silicate-mineral lineation; magnetic lineation is better developed than magnetic foliation even in Raleigh-belt rocks. Similarity of the magnetic fabrics in the lineated gneiss and the Raleigh-belt gneiss suggests that both fabrics were produced by the same deformational process. The possibility that the Raleigh-belt rocks are actually part of the NCFZ is suggested by alignment of petrofabric elements along the strike of the fault zone. The presently mapped eastern boundary of the NCFZ which was defined on the basis of lithologie and mesoscopic rock fabric differences, may separate two distinct units within the fault zone. AMS results for Raleigh-belt rocks located north of the study area suggest that deformation associated with the fault zone may extend several kilometers east of its presently mapped boundary at certain locations. AMS results provide insight into the size of the strain field associated with the NCFZ but cannot be used to estimate either the strain history or the magnitude of strain experienced by rocks within the fault zone.