Gao, S. S., K. Liu, R. Stern, G. R. Keller, J. Hogan, J. Pulliam, and E. Y. Anthony, Characteristics of mantle fabrics beneath the south-central United States: Constraints from shear-wave splitting measurements, Geosphere, 4, 411-417, 2008, 7 citations, doi:10.1130/GES00159.1, #1972 
New shear-wave splitting measurements at permanent broadband seismic stations in the south-central United States reveal the orientation and degree of polarization of mantle fabrics, and provide constraints on models for the formation of these fabrics. For stations on the stable North American craton, correspondence between observed polarization direction of the fast wave and the trend of Proterozoic and Paleozoic structures associated with rifts and orogenic belts implies a lithospheric origin for the observed anisotropy. The largest splitting times (up to 1.6 s) are observed at stations located in the ocean-continent transition zone, in which the fast directions are parallel to the Gulf of Mexico continental margin. The parallelism and the geometry of the keel of the craton beneath the study area suggest that asthenospheric flow around the keel of the North American craton, lithospheric fabrics developed during Mesozoic rifting, or a combination of these factors are responsible for the observed anisotropy on stations above the transitional crust.
Gangopadhyay, A., J. Pulliam, and M. K. Sen, Waveform modeling of teleseismic S, SP, SsPmP, and shear-coupled PL waves for crust- and upper-mantle velocity structure beneath Africa, Geophys. J. Int., 170, 1210-1226, 2007, 2 citations, doi:10.1111/j.1365-246X.2007.03470.x, #1911
We describe a waveform modelling technique and demonstrate its application to determine the crust- and upper-mantle velocity structure beneath Africa. Our technique uses a parallelized reflectivity method to compute synthetic seismograms and fits the observed waveforms by a global optimization technique based on a Very Fast Simulated Annealing (VFSA). We match the S, Sp, SsPmP and shear-coupled PL phases in seismograms of deep (200â800 km), moderate-to-large magnitude (5.5â7.0) earthquakes recorded teleseismically at permanent broad-band seismic stations in Africa. Using our technique we produce P- and S-wave velocity models of crust and upper mantle beneath Africa. Additionally, our use of the shear-coupled PL phase, wherever observed, improves the constraints for lower crust- and upper-mantle velocity structure beneath the corresponding seismic stations. Our technique retains the advantages of receiver function methods, uses a different part of the seismogram, is sensitive to both P- and S-wave velocities directly, and obtains helpful constraints in model parameters in the vicinity of the Moho. The resulting range of crustal thicknesses beneath Africa (21â46 km) indicates that the crust is thicker in south Africa, thinner in east Africa and intermediate in north and west Africa. Crustal P- (4.7â8 km s−1) and S-wave velocities (2.5â4.7 km s−1) obtained in this study show that in some parts of the models, these are slower in east Africa and faster in north, west and south Africa. Anomalous crustal low-velocity zones are also observed in the models for seismic stations in the cratonic regions of north, west and south Africa. Overall, the results of our study are consistent with earlier models and regional tectonics of Africa.
Gangopadhyay, A., J. Pulliam, and M. K. Sen, Modeling of teleseismic waveforms for crust and upper mantle velocity structure, in National Conference on Modern Trends in Geophysical Sciences and Techniques, Indian School of Mines, Jharkhand, India, 2007, #1977
Pulliam, J., M. K. Sen, and A. Gangopadhyay, Waveform modeling of teleseismic converted waves for crust and upper mantle structure beneath Canada and China, in Proc. 29th Monitoring Res. Rev: Ground-Based Nuclear Explosion Monitoring Technologies, Denver, CO, 1-22, 2007, #1975
Pulliam, J., M. K. Sen, and A. Gangopadhyay, Determination of crust and upper mantle structure beneath Africa using a global optimization based waveform modeling technique, Proc. 28th Seis. Res. Rev., 196-208, 2006, #1891
Huerta-Lopez, C. I., K. H. Stokoe, J. Pulliam, C. Valle-Molina, and J. M. Roesset, Modelling of seafloor soft marine sediments and spectral characteristics of earthquakes recorded on the Gulf of Mexico, J. Offshore Mechanics and Arctic Eng., 127, 59-67, 2005, doi:10.1115/1.1854696, #1790
In situ evaluation of the response of seafloor sediments to passive dynamic loads, as well as spectral analyses of earthquakes are presented in this investigation. The overall goal of this work was to develop a cost-effective method of characterizing offshore geotechnical sites in deep water. The generic approach was to place an ocean bottom seismograph on the seafloor and record ambient noise and distant earthquakes over periods of a month or more. Horizontal-to-vertical (H/V) spectral ratios are used to characterize the local sediment response in terms of the distribution of ground motions with their respective resonant frequencies. Both ambient noise and distant earthquakes are used as generators of passive dynamic loads. One-dimensional (1D) wave propagation modeling using the stiffness matrix method is used to estimate sediment properties (mainly shear stiffness, density, and material damping) and theoretical amplification factors of the shallow sediment layers. The objectives in this study were fourfold: First, to characterize the spectral characteristics of earthquake signals recorded in the seafloor at an experimental site in the Gulf of Mexico (GOM); second, to characterize the local site effect produced by shallow marine sediments at the GOM experimental site; third, to characterize the site in terms of its physical properties (layering and sediment properties); and fourth, to estimate the transfer functions of the top 50 m (164 ft) of soil and of each layer in the discrete soil model. The resulting sediment properties fall well within the expected range, indicating the potential of the proposed exploration approach for characterizing deep-water sites.
Pulliam, J., and M. K. Sen, Assessing uncertainties in waveform modeling of the crust and upper mantle, Proc. 27th Seis. Res. Rev.: Ground-Based Nuclear Explosion Monitoring Technologies, Palm Springs, CA, 152-160, 2005, #1852
Pulliam, J., Waveform modeling of the crust and upper mantle using S, Sp, SSPmP, and shear-coupled PL waves for improved event location, focal depth determination, and uncertainty estimation, Proc. 26th Seismic Res. Rev., Trends in Nuclear Explosion Monitoring, Orlando, FL, I, 142-152, 2004, #1791
Huerta-Lopez, C. I., J. Pulliam, and Y. Nakamura, In situ evaluation of shear-wave velocities in seafloor sediments with a broadband ocean-bottom seismograph, Bull. Seismol. Soc. Amer., 93, 139-151, 2003, 5 citations, doi:10.1785/0120010276, #1585
We present an in situ evaluation of the response of seafloor sediments to passive dynamic loads. Horizontal-to-vertical (H/V) spectral ratios are used to characterize local sediment response, and 1D wave propagation modeling is used to estimate soil properties and theoretical amplification factors of shallow sediment layers. Horizontal amplitudes increased by an order of magnitude at 0.35 Hz and by at least 2 orders of magnitude at 1.9 Hz relative to the vertical amplitude. A 50-m-thick soil system parameterized as three solid layers resting over a half-space with a water layer at the top produces theoretical H/V spectral ratios that are largely consistent with the observed H/V spectral ratios. Our modeling results were consistent between earthquake and background noise records. Modeling H/V spectral ratios of noise data recorded by a three-component broadband ocean-bottom seismograph (BBOBS) offers a fast and inexpensive method for site investigation in deep water with the potential of in situ seafloor sediment characterization, as well as local site effect studies for foundations (30-100 m) and pipelines (2-5 m) in deep water. One need not supply an active source or wait for an appropriate earthquake, and the BBOBS is small, inexpensive, and autonomous once deployed.
Pulliam, J., Y. Nakamura, C. I. Huerta-Lopez, and B. Yates, Field test of an inexpensive, small broadband ocean-bottom seismograph, Bull. Seismol. Soc. Amer., 93, 152-171, 2003, 4 citations, doi:10.1785/0120010275, #1586
We conducted tests of a three-component broadband ocean-bottom seismograph (OBS), including side-by-side comparisons with the broadband Global Seismic Network/U.S. National Seismic Network station HKT at Hockley, Texas, and a 28-day deployment in the Gulf of Mexico. Our goals were to evaluate seismometer performance and determine whether our seafloor deployment strategy allows useful earthquake data to be collected. The seismometer generally performed well, but showed unexpectedly high intrinsic noise at frequencies above 1 Hz and produced occasional spikes that were confined to a single component at a given time. We identified 32 earthquakes from the Gulf of Mexico data; only five additional events were observed at the nearby, highly sensitive station HKT on land. While noise levels were higher throughout the 0.01-20 Hz frequency band in the Gulf of Mexico compared with HKT, site amplification effects in the gulf were only significant at frequencies above 1 Hz and power in most earthquake signals peaked at frequencies below 0.1 Hz, nearly coinciding with a minimum in background noise. We found the seismometer and OBS package encouraging for far-regional and teleseismic studies, given the low cost of the OBS and inexpensive means for deployment and recovery, but less encouraging for detecting and locating small-magnitude local and near-regional events.
Pulliam, J., and M. K. Sen, Assessing uncertainties in waveform modeling of the crust and upper mantle, in Proc. 25th Seismic Res, Rev: Nuclear Explosion Monitoring: Building the Knowledge Base, Tucson, AZ, 2003, #1976
Nuth, V., J. Pulliam, and C. R. Wilson, Migration of radar altimeter waveform data, Geophys. Res. Lett., 29, 1493, 2002, doi:10.1029/2001GL014015, #1584
Wingham [1993] demonstrated that migration of radar waveforms appears to improve the images of ice sheet topography. We examine whether this qualitative improvement might lead to quantitative benefits in measuring ice sheet elevation change. We find, instead, that estimates based on migrated waveforms at cross-over points are degraded relative to unmigrated waveforms. Lack of cross-track data, which would allow true 3D migration, and migration ânoiseâ related to the Kirchoff algorithm are likely contributors to the degradation. Additionally, lack of phase information in radar waveforms degrades the result. This problem is not encountered in seismic applications of migration and is a limitation of the current generation of radar altimeter data. We have not investigated whether other quantitative benefits may be derived from migrating altimeter waveforms. For example, improved resolution of smaller scale topography in migrated data may lead to better quantitative understanding of ice sheet dynamics.
Huerta-Lopez, C. I., J. Pulliam, Y. Nakamura, and K. H. Stokoe, Soft sediment characterization from passive motion measurements on the seafloor, Proc. 2001 Int. Conf. Offshore Tech. Res., Houston, TX, 32-51, 2001, #1567
Huerta-Lopez, C. I., J. Pulliam, Y. Nakamura, and B. Yates, Modeling amplification effects of marine sedimentary layers via horizontal/vertical spectral ratios, Proc. 71st Ann. Mtg. SEG, San Antonio, Texas, MC 2.8, v. 1, 825-828, 2001, #1581
Pulliam, J., (Book review): Advances in Seismic Event Location, edited by N. Rabinowitz and C. H. Thurber, Eos, Trans. Amer. Geophys. Un., 82, 228, 2001, doi:10.1029/01EO00126, #1576
Pulliam, J., M. K. Sen, C. Frohlich, and S. P. Grand, Crustal Structure from waveform inversion of shear-coupled PL, in Proceedings of the 23rd Annual Seismic Research Symposium on Monitoring a Comprehensive Test Ban Treaty, Jackson, WY, 110-119, 2001, #1582
Pulliam, J., C. Frohlich, and B. Phillips, Single station event locations: Epicentral distance, bearing, and focal depth, Proc. 22nd Annual Seismic Research Symposium, II, 311-319, 2000, #1519
Frohlich, C., and J. Pulliam, Single-station location of seismic events: A review and a plea for more research, Phys. Earth Planet. Int., 113, 277-291, 1999, 3 citations, doi:10.1016/S0031-9201(99)00055-2, #1405
Traveltime-based location methods are ineffective for locating regional seismic events which are recorded by few stations; however, the Comprehensive Test Ban Treaty (CTBT) makes it imperative that we monitor sparsely observed events. This paper reviews the current status of single-station location (SSL) methods: the location of seismic events using records from one three-component broadband digital station. We argue here that SSL methods may ultimately provide the most effective and economical means for monitoring small-magnitude (M3) seismic events over most of the earth. Potentially, station-event distance and focal depth may be determined accurately by utilizing broadband three-component data and matching observed and synthetic waveforms; station-event azimuth is determinable from the polarization of the P-wave arrival and P coda. However, more research is required before SSL methods become a practical alternative to traveltime-based methods. Currently, the most significant problem is that regional seismograms are strongly dependent on regional crustal structure. Thus, future research efforts should focus both on determining appropriate crustal models in specific regions, and also on improved methods for dealing with regional variations in crustal structure. A second important problem concerns location errors; with SSL they may involve 180ð errors in station-event azimuth or other highly non-Gaussian `blunders'.
Pulliam, J., C. Frohlich, and R. B. Schlottmann, Accuracy and uncertainty in single-station event location, Proc. 21st Seismic Research Symposium: Technologies for Monitoring the Comprehensive Nuclear-Test-Ban Treaty, Vol. I, Las Vegas, NV, 589-598, 1999, #1440
Pulliam, J., (Book review): The Earth's Mantle: Composition, Structure, and Evolution, edited by I. Jackson, Eos, Trans. Amer. Geophys. Un., 80, 127, 1999, doi:10.1029/99EO00088, #1459
Luhurbudi, E. C., J. Pulliam, J. A. Austin, S. Saustrup, and P. L. Stoffa, Removal of diurnal tidal effects from an ultra-high resolution 3-D marine seismic survey on the continental shelf offshore New Jersey, Geophysics, 63, 1036-1040, 1998, 2 citations, doi:10:1190/1.1444381, #1277
An ultra-high-resolution 3-D, single-channel seismic survey was performed off the coast of New Jersey in 1993 to study the late Quaternary history of sedimentation on the northwest Atlantic continental margin (see Davies et al., 1992) as a part of the Office of Naval Research STRATAFORM initiative (Nittrouer and Kravitz, 1995). Three different sets of profiles were acquired (Figure 1), but only the set with highest spatial density is discussed here. A single ten-element receiver recorded 300 ms of data for every shot during the survey, which covers a total area of 0.6 km (north-south) Ã 7.75 km (east-west) (see Table 1). The deep-towed HuntecTM source (deployed at ~30 m depth) produced frequencies of 500 to 3500 Hz; a band-pass filter with corner frequencies at 1000 and 3500 Hz was applied during preprocessing
Pulliam, J., C. Frohlich, and S. P. Grand, Factors controlling single-station location, Proc., 19th Annual Seismic Research Symposium, edited by M. J. Shore, R. S. Jih, A. Dainty, and J. Erwin, 272-280, 1998, #1298
Pulliam, J., and C. Frohlich, Case studies of event location with a single station, Proc., 20th Annual Seismic Research Symposium, edited by J. Fantroy, D. Heatley, J. Warren, F. Chavez, and C. Meade, 275-285, 1998, #1390
Pulliam, J., and M. K. Sen, Seismic anisotropy in the core-mantle transition zone, Geophys. J. Int., 135, 113-128, 1998, 32 citations, doi:10.1046/j.1365-246X.1998.00612.x, #1391
Split S waves observed at Hockley, Texas from events in the TongaâFiji region of the southwest Pacific show predominantly vertically polarized shear-wave (SV ) energy arriving earlier than horizontally polarized (SH) energy for rays propagating horizontally through D". After corrections are made for the effects of upper-mantle anisotropy beneath Hockley, a time lag of 1.5 to 2.0 s remains for the furthest events (93.9°â100.6° ), while the time lags of the nearer observations (90.5°â92.9° ) nearly disappear. At closer distances, the S waves from these same events do not penetrate as deeply into the lower mantle, and are not split. These observations suggest that a patch of D" beneath the central Pacific is anisotropic, while the mantle immediately above the patch is isotropic. The thickness of the anisotropic zone appears to be of the order of 100â200 km.
Observations of shear-wave splitting have previously been made for paths that traverse D" under the Caribbean and under Alaska. SH leads SV , the reverse of the Hockley observations, but in these areas the fact that SV leads SH in the HKT data shown here suggests a different sort of anisotropy under the central Pacific from that under Alaska and the Caribbean. The case of SH travelling faster than SV is consistent with transverse isotropy with a vertical axis of symmetry (VTI) and does not require variations with azimuth. The case of SV leading SH is consistent with transverse isotropy with a horizontal axis of symmetry (HTI), an azimuthally anisotropic medium, and with a VTI medium formed by a hexagonal crystal. Given that (Mg,Fe)SiO3 perovskite appears unlikely to form anisotropic fabrics on a large scale, the presence of anisotropy may point to chemical heterogeneity in the lowermost mantle, possibly due to mantleâcore interactions.
Pulliam, J., and R. Snieder, Ray perturbation theory, dynamic ray tracing and the determination of Fresnel zones, Geophys. J. Int., 135, 463-469, 1998, 9 citations, doi:10.1046/j.1365-246X.1998.00667.x, #1392
Ray perturbation theory and dynamic ray tracing both describe the behaviour of seismic rays near a reference ray. The equations of dynamic ray tracing relate changes in position and direction of a ray in the vicinity of a reference ray (paraxial ray) to changes in the reference ray's initial conditions. Ray perturbation theory relates changes in paraxial rays to changes in the slowness field or the reference ray's endpoints. Dynamic ray tracing is performed in ray-centred coordinates, whereas ray perturbation theory may be performed in either ray-centred coordinates or in an external reference frame. Both methods derive from asymptotic ray theory, but their developments have proceeded along largely independent paths. In this paper we explore the relationship between dynamic ray tracing and ray perturbation theory and outline an efficient scheme, based on ray perturbation theory, to compute approximate Fresnel zones in inhomogeneous media.
Pulliam, J., Sensitivity kernals for travel time tomography with band-limited seismic data, Proc., SEG 67th Ann. Meeting, Dallas, TX, 1881-1884, 1997, #1365
Pulliam, J., J. A. Austin, E. C. Luhurbudi, S. Saustrup, and P. L. Stoffa, An ultrahigh resolution 3-D survey of the shallow subsurface on the continental shelf of New Jersey, Leading Edge, 15, 839-845, 1996, doi:10.1190/1.1437378, #1218
Regional seismic surveys have identified a wedge of late Quaternary sediment extending 150 km south from the Hudson River apron along the edge of the continental shelf off New Jersey. The bottom of the sediment wedge is defined by a prominent reflector that is assumed to be an erosional surface carved during a lowstand of sea level, probably corresponding to the Wisconsin Maximum glaciation (about 18 000 years ago). An extremely high resolution 3-D seismic reflection survey of the southern part of the wedge (Figure 1) was carried out in 1993 as part of the STRATAFORM (STRATA FORMation on Margins) initiative, funded by the U.S. Office of Naval Research.
Pulliam, J., P. L. Stoffa, E. C. Luhurbudi, S. Saustrup, and J. A. Austin, 3-D depth migration of an ultra high resolution seismic survey on New Jersey's continental shelf, Proc., SEG 66th Ann. Meeting, Denver, CO, 847-850, 1996, #1367
Pulliam, J., and R. Snieder, Fast, efficient calculation of rays and travel times with ray perturbation theory, J. Acoustical Soc. Amer., 99, 383-391, 1996, 12 citations, doi:10.1121/1.414550, #1376
A ray perturbation formulation for the calculation of rays and travel times in isotropic inhomogeneous media is presented. The ray perturbation theory employed is of first order for the ray deflection and of second order for the travel time. The initial slowness model is parametrized in terms of triangular cells; values are assigned initially to grid nodes and the slowness gradient is assumed to be constant between nodes. The assumption of a constant slowness gradient within a cell leads to a simplification of the ray perturbation equations and a straightforward analytic solution for ray segments in the cells. Imposing boundary conditions that require continuity at cell interfaces leads to a separate tridiagonal system of equations for each component of the ray-path location vector, which produces an extremely efficient algorithm. The accuracy and speed of this scheme with a 2-D synthetic crosswell experiment is evaluated. The computation times for the calculations described in this paper depend only on the number of nodes that influence each ray, not the total number of nodes parametrizing the model, so the method promises an even greater increase in speed for 3-D applications.
Pulliam, J., and R. Snieder, Rays and traveltimes in heterogeneous media from ray perturbation theory, in Proc., SEG 65th Ann. Meeting, Houston, TX, 1262-1265, 1995, #1375
Vasco, D. W., L. R. Johnson, and J. Pulliam, Lateral variations in mantle velocity structure and discontinuities determined from P, PP, S, SS and SS-SdS travel time residuals, J. Geophys. Res., 100, 24037-24059, 1995, 31 citations, #1973
On the basis of P, PP, S, SS arrival times and SS - S 410 S, SS - S 660 S differential times, we construct models of mantle P and S velocity structure and boundary topography of the 410-km and 660-km discontinuities. Events from the catalog of the International Seismological Centre (ISC) are relocated relative to the International Association of Seismology and Physics of the Earth's Interior 1991 (IASP91) velocity model using both P and S arrival times. The arrival times are corrected for ellipticity and the PP and SS residuals are corrected for the topography at the bounce point. The cap-averaged PP - P and SS - S differential time residuals, plotted at the PP and SS surface reflection points, form broad coherent patterns. The geographic distribution of the cap averaged residuals agrees quite well with PP - P and SS-S differential time residuals derived from long period Global Digital Seismograph Network (GDSN) data. A robust lp inversion scheme is used to infer global mantle structure. Synthetic tests indicate that for regions well sampled by SS - S 410 S and SS - S 660 S differential times, the velocity estimates are not seriously contaminated by the topography of the 410- and 660-km discontinuities. However, estimates of boundary deflections may be influenced by extensive P and S velocity variations of 3 % or greater. We find the 410-km discontinuity to be depressed by as much as 24 km beneath North America. Conversely, the discontinuity is deflected upward underneath Eurasia. In some regions the topography of the 660-km discontinuity is quite distinct from that of the 410-km discontinuity, but the two appear to be positively correlated. A series of depressions are found at several intersections of the 660-km discontinuity with known subduction zones. The elevated topography in the 410-km discontinuity beneath Europe is underlain by a trough in the 660-km discontinuity. A number of subduction zones are characterized by a thinning of the transition zone. Negative P and S velocity anomalies, underlying back-arc basins and tectonically active continental regions, encircle the Pacific. Where they are resolved, the stable continental cratons are systematically positive velocity features that extend below 200 km. With the inclusion of PP and SS travel time residuals we are better able to constrain midmantle structure. Most notably, in the depth range 35â660 km beneath the Northwest Pacific we observe high P velocity. Where they are resolved, mid-ocean ridges are most clearly imaged as low velocity features in the S model. The northern portion of the Mid-Atlantic Ridge is underlain by negative S velocity anomalies. In the Pacific, the East Pacific Rise is an extensive low S velocity anomaly.