Institute for Geophysics

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Since its founding in 1972, the University of Texas Institute for Geophysics (UTIG) is a world leader in expeditionary-scale geophysical research, conducting research in five broad themes: climate, energy, marine geosciences, seismology and tectonophysics, and planetary and polar geophysics. UTIG is home to more than 50 research scientists and postdocs — research entrepreneurs — providing a broadband of expertise that can do everything from conducting scientific ocean drilling to leading airborne radar studies of ice sheets. UTIG scientists supplement their fieldwork with computer analysis, modeling, and laboratory work. Whether collecting seismic data, responding to natural disasters, or searching space for signs of life, UTIG is there.

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    Field Measurement of Penetrator Seismic Coupling in Sediments and Volcanic Rocks (Final Report)
    (Institute for Geophysics, 1979) Nakamura, Yosio; Latham, Gary V.; Frohlich, Clifford A.; Blanchard, Maxwell B.; Murphy, James P.
    Field experiments were conducted to determine experimentally how well a seismometer installed using a penetrator would be coupled to the ground. A dry-lake bed and a lava bed were chosen as test sites to represent geo- logical environments of two widely different material properties. At each site, two half-scale penetrators were fired into the ground, a three- component geophone assembly was mounted to the aft end of each penetrator, and dummy penetrators were fired at various distances to generate seismic signals. These signals were detected by the penetrator-mounted geophone assembly and by a reference geophone assembly buried or anchored to surface rock about 1-m from the penetrator. The recorded signals were digitized, and cross-spectral analyses were performed to compare the observed signals in terms of power spectral density ratio, coherence, and phase difference. Most of the energy in the recorded signals was in the frequency range of 3 to 30 Hz. The analyses indicate that seismometers deployed by penetrators will be as well coupled to the ground as are seismometers installed by conventional methods for the frequency range of interest in earthquake seismology, although some minor differences were observed at frequencies near the upper limit of the frequency band.
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    Late Pleistocene fluvial-deltaic deposition, Texas coastal plain and shelf
    (1979) Winker, Charles David; Baker, Victor R.
    Deposition on the Texas coastal plain and shelf during the last Pleistocene glacial cycle has been interpreted from topographic and bathymetric maps, from borehole data including results of a detailed drilling study in Brazoria County, and from offshore sparker-profiles. The stratigraphic unit deposited during the last glacial cycle is bounded above by a largely undissected topographic surface (Beaumont), and below by a buried paleosol of stiff gray clay that correlates offshore with a persistent seismic reflector. The lower Texas coastal plain is essentially a clay-rich alluvial plain made up of coalescing low-gradient fans. The Beaumont alluvial plain onlaps an older surface (Lissie) which was tilted seaward prior to Beaumont deposition; the Lissie in turn onlaps remnants of older surfaces. During Beaumont deposition, each major coastal river deposited a branching network of meanderbelt sand-bodies by repeated avulsions. Borehole data for a meanderbelt of the ancestral Brazos River indicate that the channel was 5 to 7 m deep but that substantially greater sand thicknesses developed by stacking of point-bar sequences during fluvial aggradation. Downdip transition of fluvial deposits into deltaic and paralic sediments is inferred from shell beds, strike-oriented sand bodies, beach ridges, changes in clay color, and clinoform reflectors on sparker profiles. The updip limit of marine influence is delineated by the distribution of shell beds; the downdip limit of deltaic pro gradation is indicated by a paleobathymetric break in slope. Strike-oriented sands (Ingleside), including barrier islands and strandplains, developed contemporaneously with Beaumont fluvial aggradation. Sand thicknesses and multiple levels of sand suggest that beach-shoreface sequences are multistoried, similar to the fluvial sands. The thickest and widest strike sands formed in bights between the larger, more prominent deltaic systems. In response to falling sea level, deltas prograded from the Ingleside shoreline to the shelf edge. Sparker profiles show that deltaic thicknesses and offshore slopes increased gradually during progradation, then rapidly near the shelf edge, where deltaic sequences became stacked or imbricated. Major growth faulting and salt flowage near the shelf break were associated with the thickest deposits. Between the large Colorado and Rio Grande delta systems, reefs grew near the shelf edge. Late Pleistocene sea-level fluctuations resulted in three depositional phases: an aggradational phase (ca. 120,000 B.P.) during late rise and stillstand, dominated by fluvial and strike systems; a progradational phase (100,000 to 20,000 B.P.) during a gradual fall, dominated by deltaic systems, and a rapid transgressive phase (20,000 to 4000 B.P.). The Texas coast is now in another aggradational phase. Average rates of late Pleistocene sediment influx were similar to historic rates, and show a decrease in sediment production toward the arid southwest. Post-depositional deformation of the Beaumont and Lissie alluvial plains and Ingleside shoreline can be explained largely as an isostatic response to sedimentary loading
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    Passive Seismic Experiment, Long Period Event Catalog, Final Version (1969 Day 202 - 1977 Day 273, ALSEP Stations 11, 12, 13, 14, 15, and 16)
    (Institute for Geophysics, 1981) Nakamura, Yosio; Latham, Gary V.; Dorman, H. James; Harris, J.E.
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    Pueblo Viejo-Quixal Seismograph Network, 13 February 1979 through 10 March 1981 (Technical Report #1)
    (Institute for Geophysics, 1981) Matumoto, Tosimatu; Kim, Jung Joon|Rabb, Lewis
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    Final Report: Offshore Alaska Seismic Movements Program
    (Institute for Geophysics, 1982) Frohlich, Cliff; Nakamura, Yosio
    This report summarizes the results of 18 deployments of the Texas strong motion ocean bottom seismograph (SM-OBS) which took place between July 1980 and July 1982 offshore of Alaska. From these deployments, 12 instruments were recovered, and 10 recorded data. Some of the instruments were successfully recovered after more than 240 days, longer than any previous OBS deployment that we know of. Two of the instruments apparently recorded data from four different earthquakes. The recorded ground accelerations for these events were in the 10cm/sec2 to 20 cm/sec2 range. Unfortunately, only one earthquake of magnitude larger than 6.0 occurred near the SM-OBS instruments in the July 1980 - July 1982 period, and this was not recorded by the instruments. The main effort of the program was on the development of this new technology. Countless difficult problems encountered during the program were solved, but a few problems remained resulting in some instrumental malfunctions which affected all of the OBS deployments and prevented us from obtaining more data or data of uniformly high quality during this period. This report also evaluates some of these problems and suggests ways to avoid these problems in future SM-OBS programs. Considering the high potential for serious earthquake damage in the offshore Alaska region, it is imperative that the effort to obtain SM-OBS data continues.
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    Reprocessing of Multichannel Seismic Data off Guatemala for an IPOD Transect (Final Report to Joint Oceanographic Institutions, Inc.)
    (Institute for Geophysics, 1982) Norton, Ian; von Huene, Roland; Ladd, John
    Slope deposits drilled during Leg 67 were detailed in redisplayed seismic records after the leg. These deposits are of significantly lower seismic velocity and probably lower density than the underlying basement to indicate a contact between rocks of differing consolidation and not a continuous sedimentary sequence. The slope deposits cover basement terranes of 3 different topographies. The shelf edge is an arch whose seaward flank forms a steep (up to 15°) upper slope. The mid-slope area has a rugged topography covered by thick slope deposits. The lower slope is relatively smooth except where broken locally by benches. The upper and middle slope areas are associated with strong magnetic anomalies and rare landward dipping reflections truncated by the rough surface. We explain the rough mid-slope topography by subareal erosion succeeding the first uplift of this area in the Paleocene and prior to major subsidence in the early Miocene. The present slope deposits then covered the trench landward slope, perhaps coincident with the increased arc volcanism indicated by ash layers, and thus the present period of subduction. The subducted ocean crust has a distinct linear topography of hundreds of meters relief that seems to disappear beneath the landward slope of the trench along with most of the ocean basin and trench sediment. The almost passive assimilation of oceanic material without significant accretion in the late Neogene argues for significant decoupling at the front of the subduction zone. A base of gas hydrate reflections can be identified in many of the redisplayed seismic records off Guatemala. Base of hydrate reflections are most common where slope deposits are thick and the reflections have not been identified in the underlying acoustic basement. This is consistant with the geochemical evidence that gas hydrate has its source in the organic rich slope sediment. The hydrate depth and temperature measurements in drill holes indicate a temperature gradient similar to that measured across the Japan Trench.
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    Earth Strain Measurements with the Transportable Laser Ranging System: Field Techniques and Planning (Final Report, NASA Contract NAS 5-25897)
    (Institute for Geophysics, 1982) Nakamura, Yosio; Dorman, H. James; Cahill, Thomas
    We have conducted a feasibility study to examine the potential of the Transportable Laser Ranging System (TLRS) for monitoring the ground deformation around satellite ranging stations and other geodetic control points. Emphasis has been placed on testing the usefulness of the relative lateration technique. The temporal variation of the ratio of the length of each survey line to the mean length of all survey lines in a given area is directly related to the mean shear strain rate for the area. The data from a series of experimental measurements taken over the Los Angeles basin from a TLRS station at Mt. Wilson show that such ratios can be determined to an accuracy of one part in 107 with a measurement program lasting for three days and without using any corrections for variations in atmospheric conditions. A numerical experiment using a set of hypothetical data indicates that reasonable estimates of the present shear strain rate and the direction of the principal axes in southern California can be deduced from such measurements over an interval of one to two years. Thus, the relative lateration from the TLRS appears to be a very economical way to monitor ground deformations, although there has: been no opportunity yet to measure the actual ground strain by reoccupying the Mt. Wilson site.
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    Seismic structural analysis of deformation in the southern Mexican Ridges
    (1982-05) Pew, Elliott; Muehlberger, William R.
    The southernmost region of the Mexican Ridges extends from Bryant's gap near 22.5 N latitude to the Campeche Knolls near 19.0 N latitude. Analysis of 23,030 kilometers of sparker and CDP seismic data from six surveys reveals the existence of two separate areas of folding, Zones 4a and 4b. In the Zone 4a foldbelt symmetrical folds form a gentle salient which parallels the curved outline of Isla de Tuxpan. Structural relief often in excess of 500 meters is reflected by similar bathymetric relief. Fold wavelengths average 10-12 kilometers. A detachment or decollement is interpreted in a thick Upper Cretaceous to Lower Tertiary pelagic shale sequence by the existence of relatively undeformed reflectors below this interval. The 3 to 3.5 kilometer thick allochthonous sheet has experienced approximately 1% shortening and a maximum displacement of 1 to 2 kilometers. The Zone 4a foldbelt appears to be a massive gravity slide. Folded Plio-Pleistocene strata establish the youth of these folds. A large deep-rooted structure of questionable origin is observed on GLG 22. This structure, exhibiting roughly 1500 meters of bathymetric relief, acts as a foreland buttress against which the gliding allochthonous mass deforms. The tightly appressed thrust-faulted folds up dip from the buttress exhibit anomalously short wavelengths. While no folding is observed directly down dip from the buttress, folding is observed 30 to 50 kilometers basinward of this structure just a few kilometers to the south. The boundary separating Zones 4a and 4b is a linear feature oriented transversely to regional strike and may be a tear fault. Reflections at depth are not continuous across this feature. The Zone 4b foldbelt lies directly down dip from the Veracruz Basin. Structural relief commonly doubling that observed in Zone 4a is rarely expressed as bathymetric relief. Individual folds are asymmetric, having gently dipping landward flanks and either steeply dipping or growth-faulted seaward flanks. Fold cores appear to contain diapiric material. Fold growth due to gravity sliding began in the Middle Miocene. Subsequent loading by a thick Middle-Upper Miocene section gradually halted downslope movement and initiated flowage of plastic substrata from beneath loaded synclinal troughs into anticlinal cores. This deformation has continued to the present in some folds.
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    Wave Propagation Studies of the Central Mediterranean Sea Using Ocean Bottom Seismometers (Report for Office of Naval Research Contract N00014-77-C-0606. Modification #P00007)
    (Institute for Geophysics, 1983) O'Brien, William P., Jr.; Chatterjee, Subir
    Ocean bottom seismometers (OBS) were deployed in the Mediterranean for two refraction surveys shot with underwater sound signal (SUS) charges. The digital data were analyzed to determine 1) the attenuation features, signal/noise (S/N) characteristics and frequency content of water waves and body waves, and 2) the crustal structure of the test areas. The attenua­ tion of water-wave signals was fairly uniform within the passband of the OBS (10-31 HZ) and was greater in deep water than in shallower water, and body waves were much more strongly attenuated than water waves. The S/N ratios were much larger for the SUS shots detonated at 91 m depth than for those detonated at 244 m depth. The body-wave data indicated the presence of a layer with P-wave velocity of 3.8 km/sec about 0.8 km below mean sea level in one test area. Probably this is a Miocene evaporite sequence.
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    Development of an Advanced Ocean-Bottom Sensor System, ONR Contract N00014-77-C-C0606 (Final Technical Report, period covered 15 August 1977 - 31 December 1982)
    (Institute for Geophysics, 1983) Nakamura, Yosio
    A low-cost, light-weight, long-life ocean-bottom sensor system has been developed. It incorporates three microprocessors, which control data acquisition, intermediate processing, and recording, all in digital form. The system has been used successfully in several seismic field experiments, which include detection of natural earthquakes, seismic refraction surveys and investigation of acoustic wave propagation.