Jay Pulliam

Lateral variations in mantle velocity structure and discontinuities determined from P, PP, S, SS, and SS — SdS travel time residuals

On the basis of P, PP, S, SS arrival times and SS - S410S, SS - S660S 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 Earths 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 - S410S and SS - S660S 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 35–660 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.

Characteristics of Mantle Fabrics beneath the South-Central United States: Constraints from Shear-Wave Splitting Measurements

New shear-wave splitting measurements at permanent broadband seismic stations in the south-central United States reveal the orien- tation 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, correspon- dence between observed polarization direc- tion of the fast wave and the trend of Protero- zoic and Paleozoic structures associated with rifts and orogenic belts implies a lithospheric origin for the observed anisotropy. The larg- est splitting times (up to 1.6 s) are observed at stations located in the ocean-continent transi- tion zone, in which the fast directions are par- allel 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 fl ow around the keel of the North American craton, lithospheric fab- rics developed during Mesozoic rifting, or a combination of these factors are responsible for the observed anisotropy on stations above the transitional crust.

An ultrahigh resolution 3-D survey of the shallow subsurface on the continental shelf of New Jersey

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.

Fast, efficient calculation of rays and travel times with ray perturbation theory

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 t...

In Situ Evaluation of Shear-Wave Velocities in Seafloor Sediments with a Broadband Ocean-Bottom Seismograph

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. Manuscript received 23 October 2001.

Field Test of an Inexpensive, Small Broadband Ocean-Bottom Seismograph

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. Manuscript received 23 October 2001.

Removal of diurnal tidal effects from an ultra‐high‐resolution 3-D marine seismic survey on the continental shelf offshore New Jersey

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) × 7.75 km (east‐west) (see Table 1). The deep‐towed Huntec™ 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.

Advances in Seismic Event Location

Advances in Seismic Event Location is a collection of nine papers presented at a workshop held in the Dead Sea region in January 1998. The motivating influence of the Comprehensive Test Ban Treaty (CTBT) is a common theme throughout the book, but a broad perspective on basic Earth science research is also maintained. Although the book is comprised of papers presented at a single conference, it comprehensively discusses relevant research, including a broad review of classical, derivative-based location methods using one-dimensional Earth models through modern innovations that incorporate three-dimensional models. Also covered are global optimization algorithms, different types of data, probabilistic approaches to earthquake location that convey uncertainties more accurately, optimal network configuration, and prospects for automation, including refining automatic phase detection and an artificial intelligence approach that includes pattern recognition.

Spectral characteristics of earthquakes recorded on the gulf of México seafloor and modeling of soft marine sediments

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. Horizontal-tovertical (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. 1-D 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.

Modeling amplification effects of marine sedimentary layers via horizontal/vertical spectral ratios

Summary We studied the amplification effects of the shallow sediment layer at a site in the Gulf of Mexico during a test of an autonomous ocean bottom seismograph that incorporated a three component broad band seismometer (0.033-50 Hz). We characterized the site response in terms of horizontal to vertical (H/V) spectral ratios (Nakamura, 1989) and performed 1-D wave propagation modeling using a modified Thomson (1950) and Haskell (1951) propagation matrix method. This method, known as the ‘stiffness matrix method’ was developed by Kausel and Roesset (1981) to estimate soil properties and the theoretical amplification factor of shallow soil layers. Relative to the vertical spectral amplitudes, we observed that in the range of 0.35-2.5 Hz the horizontal spectral amplitudes increased by an order of magnitude at 0.35 Hz and by at least two orders of magnitude at 1.9 Hz, which was an indication of the local site effect. The theoretical H/V spectral ratio modeled with a 1-D 50 m soil system is largely consistent with the experimental H/V spectral ratio that exhibits a clear peak at 1.9 Hz. The site effect appears to be less significant at frequencies below 1 Hz.