Robert B. Hawman

Associate Professor
B.S., Dickinson College
M.S., Penn State University
Ph.D., Princeton University
Postdoctoral research associate, University of Wyoming, 1986-1990

Teaching Awards

J. Hatten Howard, III Honors Teaching Award, April 27, 2005

Courses

Honors 2080H: Honors Science
Geol 1250: Physical Geology
Geol 4600/6600: Solid Earth Geophysics
Geol 4620/6620: Exploration Geophysics
Geol 8030: Advanced Topics in Geophysics
Geol 8250: Plate Tectonics
Geol 8600: Topics in Seismology

Research Interests

Ongoing field studies in the Southern Appalachians include wide-angle seismic reflection profiling of the crust and upper mantle using quarry blasts, shallow reflection profiling across major fault zones and barrier islands, and monitoring of regional seismicity.

Wide-Angle Studies of the Crust and Upper Mantle

Blue Ridge Mountains

We recently completed a series of wide-angle seismic experiments in the Blue Ridge Mountains of North Carolina (Hawman, 2008; Baker and Hawman, 2006; Hawman et al., 2003).  The principal question here is whether this part of the Appalachian orogen, site of the highest peaks east of the Mississippi, has a mountain root.  The wide-angle experiments supplement existing COCORP and ADCOH profiles by taking advantage of elevated reflection coefficients near the critical angle.  The strategy was to deploy small-aperture arrays over a wide range of source-receiver distances (5-200 km) to constrain P-wave and S-wave velocities while keeping receiver spacings small enough (200 meters) to provide unaliased recordings of wide-angle reflections for migration.

Preliminary velocity models suggest an average crustal P-wave velocity of 6.5-6.6 km/s and a crustal thickness of 50-55 km (Hawman, 2008).  Migration of shot gathers suggests that crustal thickness increases from about 38 km beneath the Carolina Terrane (Hawman, 1996) to 47-51 km along the southeastern flank of the Blue Ridge Province in North and South Carolina.  Analysis of receiver functions computed for USNSN broadband stations GOGA (Carolina Terrane/Inner Piedmont boundary) and MYNC (Blue Ridge) shows a similar variation in crustal thickness (Baker and Hawman, 2006; 2007).  Migrated sections from within the Blue Ridge suggest that crustal thickness varies from 46 to 55 km.  The minimum value correlates with the Asheville  Basin, suggesting that topography may be supported by Airy-type crustal roots.  These results differ from previous models that show a flatter, shallower Moho, but are consistent with regional gravity data (Hawman, 1996) and with the occurrence of crustal roots imaged by profiles crossing other Paleozoic orogens such as the Ural Mountains.

The experiments have been carried out with a portable array of twenty digital, three-component seismic recorders, using timed quarry blasts as seismic sources. Quarry blasts can be very useful because they generate a significant amount of shear-wave energy, but the extended source signatures produced by ripple firing can greatly complicate the interpretation of records. A fair amount of effort, therefore, has been devoted to the investigation of various techniques for deconvolving non-minimum-phase signals (Hawman, 2004). We have had some success using a combination of minimum-entropy deconvolution with spectral whitening. We have also developed an alternative procedure for migrating common-source gathers (Hawman, 2004; 2008). This has been particularly useful for generating single-fold images from data recorded with isolated, short-aperture arrays.

Elberton Granite, Inner Piedmont, and Carolina Terrane

Other recent field experiments include a pilot study using instantaneous blasts at dimension-stone quarries within the Elberton Granite of northeast Georgia (Khalifa and Hawman, 2005a,b). The principal goal of this study was to image the base of the intrusion; field gathers for several of the blasts show prominent reflections that migrate to depths between 2-4 km, possibly marking a layered complex at the base of the granite. Field recordings within the Inner Piedmont using larger ripple-fired blasts show prominent reflections from the master decollement, lower crust, and Moho (Khalifa et al., 2001). An earlier, reversed wide-angle reflection/refraction profile within the Carolina Terrane, along the crest of the Appalachian Gravity High, reveals average crustal P-wave velocities between 6.5 and 6.6 km/s and Moho depths between 37 and 39 km (Hawman, 1996).

Eastern Tennessee

We have also conducted pilot studies in the Eastern Tennessee seismic zone, in cooperation with investigators at Virginia Polytechnic Institute and the University of North Carolina at Chapel Hill (Hawman et al., 2001). Although sparsely sampled, preliminary migrated sections suggest the presence of several highly reflective structures, including a concentration of reflectors at a depth of about 25 km, close to the maximum reported depth for earthquakes in the region. The long-term goal of this work is to construct localized models of P and S velocity structure and reflectivity that should help us to better understand the factors responsible for seismicity within this region.

Shallow Seismic Reflection Profiling

We have used a 24-channel system for continuous, ”roll-along“ CMP profiling over several major fault zones within crystalline terranes. Profiles in the Carolina Terrane of northeast Georgia (Clippard and Hawman, 1995) were shot over several mafic/ultramafic complexes to determine their subsurface geometry. Profiles (total length: about 1 km) recorded with 100-Hz geophones at 1-m intervals and very light taps with a sledge hammer show strong reflectors close to the time predicted by field mapping, supporting the interpretation of the ultramafics as klippen in thrust-fault contact with surrounding rocks. Profiles within the Brevard Zone (total length: 1 km) were shot within a few kilometers of ADCOH Lines 1 and 2 in South Carolina (Hawman et al., 2000). In spite of strong attenuation and statics effects associated with a zone of severe chemical weathering, the profiles show events that correlate with projections of mapped lithologic contacts. Coherent reflections have also been observed in shallow profiles recorded over an exposed mylonitic shear zone in the Ruby Mountain metamorphic core complex of northeast Nevada (Hawman and Ahmed, 1995).

We have also used the shallow system with a modified shotgun source to obtain profiles across Sapelo Island, a Pleistocene/Holocene barrier island off the coast of Georgia (Adesida et al., 2000). Shotgun blanks generated energy sufficient to image features to depths of about 270 m. The CMP stacked sections show several erosional surfaces within Miocene, Oligocene, and Eocene sequences.

Selected Publications and Abstracts

Hawman, R. B., Crustal thickness variations across the Blue Ridge Mountains, southern Appalachians: An alternative procedure for migrating wide-angle reflection data, Bull. Seism. Soc. Am., in press for February 2008.

Baker, M.S., and R.B. Hawman, 2007, Structure of the crust and upper mantle in the southern Appalachians from receiver function analysis (abs.), Geological Society of America, SE Section, 56thAnnual Meeting, Abstracts with Programs, 39, 21, Savannah, GA, March 2007.

Baker, M.S., and R.B. Hawman, 2006, Combined wide-angle reflection and receiver function studies of the crust and upper mantle beneath the Carolina Terrane and Blue Ridge Provinces, southern Appalachians (abs.), Eighteenth Annual IRIS Workshop, Tucson, Arizona, June 8-10, 2006.

Khalifa, M.O., and R.B. Hawman, 2005a, Wide-angle seismic imaging of the Elberton Granite, Georgia: A pilot study using instantaneous blasts at dimension-stone quarries, Geophysics, 70, B67-B72.

Khalifa, M.O., and R.B. Hawman, 2005b, Speculations regarding the subsurface geometry of the Elberton granite from sparse wide-angle reflection data, Southeastern Geology, 43, 193-214.

Hawman, R.B., 2004, Using delay-fired quarry blasts to image the crust: A comparison of methods for deconvolving mixed-delay source wavelets, Bull. Seism. Soc. Am., 94, 1476-1491.

Hawman, R.B., M.O. Khalifa, J.A. Kucinskis, and J.E. Clippard, 2003, Using quarry blasts to image the crust: Deconvolution and migration of wide-angle data, 12th International Workshop on Seismic Imaging Techniques, International Association of Seismology and Physics of the Earth's Interior, Blacksburg, Va., extended abstract.

Khalifa, M.O., J.A. Kucinskis, J.E. Clippard, and R.B. Hawman, 2001, Wide-angle reflection profiling of the Elberton Granite and deep structure of the Inner Piedmont, Southern Appalachians, using instantaneous and delay-fired quarry blasts (abs.), EOS Trans. Am. Geophys. Union, 82, S271.

Hawman, R.B., M.C. Chapman, C.A. Powell, J.E. Clippard, and H.O. Ahmed, 2001, Wide-angle reflection profiling with quarry blasts in the Eastern Tennessee seismic zone, Seismol. Res. Lett., 72, 108-122.

Adesida, A.O., J.E. Clippard, and R.B. Hawman, 2000, The stratigraphic framework of Sapelo Island, Georgia : A shallow seismic reflection study (abs.), Geol. Soc. Am., SE Section.

Hawman, R.B., C.L. Prosser, and J.E. Clippard, 2000, Shallow seismic reflection profiling over the Brevard Zone, South Carolina, Geophysics, 65, 1388-1401.

Hawman, R.B., J.E. Clippard, H.O. Ahmed, and C.L. Prosser, 1998, Source comparison and statics estimation for shallow reflection profiles across the Brevard Zone and Carolina Terrane, southern Appalachians (abs.), EOS Trans. Am. Geophys. Union, 79, 217.

Hawman, R.B., 1996, Wide-angle, three-component seismic reflection profiling of the crust beneath the East Coast Gravity High, southern Appalachians, using quarry blasts, J. Geophys. Res., 101, 13,933-13,945.

Hennet, C.G., R.B. Hawman, and R.A. Phinney, 1995, Slant stacks of refraction data from Maine : Effects of lateral variations in velocity structure, Bull. Seism. Soc. Am., 85, 1541-1559.

Clippard, J.E. and R.B. Hawman, 1995, Shallow seismic reflection profiling over an ultramafic complex in the Carolina Terrane, northeast Georgia, South Carolina Geology, 38, 79-94.

Hawman, R.B. and A.O. Ahmed, 1995, Shallow seismic reflection profiling over a mylonitic shear zone, Ruby Mountains - East Humboldt Range metamorphic core complex, NE Nevada, Geophys. Res. Lett., 22, 1545-1548.

Long, L.T., A. Kocaoglu, R.B. Hawman, and P. Gore, 1994, The Norris Lake earthquake swarm of June through September, 1993: Preliminary findings, Seism. Res. Lett., 65, 171-174.

Gohl, K., R.B. Hawman, and S.B. Smithson, 1993, Wide-angle reflection studies of the crust and Moho beneath the Archean gneiss terrane of southern Minnesota, Geophys. Res. Lett., 20, 619-622.

Colburn, R.H., and R.B. Hawman, 1992, Inversion of deep crustal refraction data from the Great Valley, California, Bull. Seism. Soc. Am., 82, 2224-2247.

Hawman, R.B., and R.A. Phinney, 1992, Structure of the crust beneath the Great Valley and Allegheny Plateau of eastern Pennsylvania, Part 1: Comparison of linear inversion methods for sparse wide-angle reflection data, J. Geophys. Res., 97, 371-391.

Hawman, R.B., and R.A. Phinney, 1992, Structure of the crust beneath the Great Valley and Allegheny Plateau of eastern Pennsylvania, Part 2: Gravity modeling and migration of wide-angle reflection data, J. Geophys. Res., 97, 393-415.

Hawman, R.B., and R.A. Phinney, 1991, Analysis of sparse wide-angle reflection data in the tau-p domain, Bull. Seism. Soc. Am., 81, 202-221.

Hawman, R.B., R.H. Colburn, D.A. Walker, and S.B. Smithson, 1990, Processing and inversion of refraction and wide-angle reflection data from the 1986 Nevada PASSCAL experiment, J. Geophys. Res., 95, 4657-4691.

Other Interests

Georgia Water Resources