Gordana Vlahovic

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.

Crustal velocity structure associated with the eastern Tennessee seismic zone: Vp and Vs images based upon local earthquake tomography

We present three-dimensional P and S wave velocity models for the active eastern Tennessee seismic zone (ETSZ) using arrival time data from more than 1000 local earthquakes. A nonlinear tomography method is used that involves sequential inversion for model and hypocenter parameters. We image several velocity anomalies that persist through most of the inversion volume. Some anomalies support the presence of known features such as an ancient rift zone in northern Tennessee. Other anomalies reveal the presence of basement features that can be correlated with regional gravity and magnetic anomalies. We image a narrow, NE-SW trending, steeply dipping zone of low velocities that extends to a depth of at least 24 km and is associated with the vertical projection of the prominent New York-Alabama magnetic lineament. The low-velocity zone may have an apparent dip to the SE at depths exceeding 15 km. The low-velocity zone is interpreted as a major basement fault juxtaposing Granite-Rhyolite basement to the NW from Grenville southern Appalachian basement to the SE. Relocated hypocenters align in near-vertical segments suggesting reactivation of a distributed zone of deformation associated with a major strike-slip fault. We suggest that the ETSZ represents reactivation of an ancient shear zone established during formation of the super continent Rodinia.

Geospatial Education at North Carolina Central University: An HBCU Perspective

North Carolina Central University (NCCU) is recognized as one of the top institutions of baccalaureate origin for African-American geography doctorate recipients and is one of only two Historically Black Colleges or Universities (HBCUs) in the southeast to offer a degree in Geography (McKee and Dixon 2004). In an effort to increase the quality of geospatial education, the Geospatial Research, Innovative Teaching and Service (GRITS) Center was established in 2006 at NCCU. The GRITS Center is a hub for Geographical Information Science (GIS) curriculum development, faculty and professional workshops, internship programs, outreach efforts and diverse partnerships in the geospatial arena—and as a result, opens doors to the professional world for our students. This paper describes activities and programs that can serve as a template for the development of geospatial programs at HBCUs and other schools of similar size striving to add new disciplines such as GIS to their repertoire.