Jerry A. Carter

Leg 67: The Deep Sea Drilling Project Mid-America Trench transect off Guatemala

Drill cores from a transect of the Mid-America Trench off Guatemala were obtained at three sites on the oceanic Cocos plate, and at four sites on the continental Caribbean plate. An ocean sub-bottom seismometer was successfully emplaced in the deepest hole in the trench landward slope where it was left to record data after departure of the drill ship. Drilling on the Cocos plate recovered a basal chalk sequence deposited during early and mid-Miocene time, a short interval of abyssal red clay, and an upper sequence of late Miocene and younger sediment deposited within an area influenced by a terrigenous source. In the trench, a mud and sand fill less than 400,000 yr old overlies the oceanic sequence. The entire section shows no evidence of compressive deformation even at holes drilled against the trench9s landward slope. In contrast, the section cored on the trench9s landward slope 3 km from the trench axis is affected by tectonism. The section contains a Cretaceous to Pliocene claystone sequence, broken by hiatuses but in a normal stratigraphic succession that is capped by Pliocene to Quaternary hemipelagic slope deposits. Seismic records show that the section overlies probable igneous oceanic crust from which it is separated by a few hundred metres. That thickness of undrilled section is insufficient to accommodate the potential offscraped volume of oceanic sediment carried into the trench during Neogene plate convergence. At the estimated 10 cm/yr rate of convergence, much of the oceanic sediment must have been subducted rather than tectonically accreted to the Guatemalan margin. Current models for convergent margin tectonics do not satisfactorily explain the surprising occurrence of Cretaceous to Miocene mudstone at the base of this trench slope. The recovery of gas hydrates prevented drilling to some landward-dipping reflections presumed to be imbricate thrust slices at two sites near the middle of the trench landward slope.

Accommodating lateral velocity changes in Kirchhoff migration by means of Fermat’s principle

When velocity varies laterally as well as with depth, an exact Kirchhoff depth migration requires that rays be traced from each depth point in the section to each source/receiver location. Because such a procedure is prohibitively expensive, Kirchhoff migration is usually carried out by using a velocity function that depends only on depth. This paper introduces a new method, based on Fermat’s principle, which is a compromise between these two extremes. The slowness (reciprocal velocity) function is written as the sum of two functions, the first of which is large and depends only on depth, while the other is small and varies both with depth and position along the line. Raypaths are traced for the first slowness function and are used to calculate migration curves. For each depth point these same raypaths are used to calculate traveltime perturbations due to the laterally varying part of the slowness. The traveltime perturbations are added to the migration curve to obtain an approximation to the exact migrat...

Shallow water sediment properties derived from high-frequency shear and interface waves

Low-frequency sound propagation in shallow water environments is not restricted to the water column but also involves the subbottom. Thus, as well as being important for geophysical description of the seabed, subbottom velocity/attenuation structure is essential input for predictive propagation models. To estimate this structure, bottom-mounted sources and receivers were used to make measurements of shear and compressional wave propagation in shallow water sediments of the continental shelf, usually where boreholes and high-resolution reflection profiles give substantial supporting geologic information about the subsurface. This colocation provides an opportunity to compare seismically determined estimates of physical properties of the seabed with the “ground truth” properties. Measurements were made in 1986 with source/detector offsets up to 200 m producing shear wave velocity versus depth profiles of the upper 30–50 m of the seabed (and P wave profiles to lesser depths). Measurements in 1988 were made with smaller source devices designed to emphasize higher frequencies and recorded by an array of 30 sensors spaced at 1-m intervals to improve spatial sampling and resolution of shallow structure. These investigations with shear waves have shown that significant lateral and vertical variations in the physical properties of the shallow seabed are common and are principally created by erosional and depositional processes associated with glacial cycles and sea level oscillations during the Quaternary. When the seabed structure is relatively uniform over the length of the profiles, the shear wave fields are well ordered, and the matching of the data with full waveform synthetics has been successful, producing velocity/attenuation models consistent with the subsurface lithology indicated by coring results. Both body waves and interface waves have been modeled for velocity/attenuation as a function of depth with the aid of synthetic seismograms and other analytical techniques. Some results give strong evidence of anisotropy and lateral heterogeneity in shear velocity of the upper 5–10 m of sediments and of extremely high velocity gradients in the topmost 1–2 m, possibly exceeding 30 s−1.

Relative stress variations as determined by b-values from earthquakes in circum-pacific subduction zones

Abstract Assuming a relation of “b” to stress state, the possibility of globe-wide stress variation and transmission was investigated. The NOAA earthquake data file served to determine the temporal change in “b” of log N = a − bM from 1963 to 1975. Periods of six to eight years are observed in the b-values (stress pattern) for most circum-Pacific areas (South America, Tonga, Kermadec, New Hebrides, Kamchatka and Eastern Aleutians). In the Kurils, fore- and aftershock sequences of large earthquakes seem to mask any characteristic global pattern that might exist. These sequences exhibit low b-values (high stress) through the time of foreshocks and early stages of aftershocks, followed by rapid increase in b-values (decrease in stress). Use of a worldwide earthquake data file clearly yields less resolution of the temporal “b” variation than the use of local network studies published by other authors. Incidental to the study, 1124 earthquakes of the NOAA data file yield the Ms − mb relations: Ms = 1.16mb − 0.835 for 4.5 ⩽ mb ⩽ 6 and: log10Ms = 0.1432mb − 0.0629 formb > 6 with correlative coefficients of 0.994 and 0.992 respectively.

A summary of Deep Sea Drilling Project Leg 67 shipboard results from the Mid-America Trench transect off Guatemala

Summary The Middle America Trench off Guatemala was transected by 24-channel seismic reflection surveys, seismic-refraction surveys, and drilling with the Glomar Challenger. The drilling was done at three sites on the oceanic Cocos plate and four sites on the Caribbean plate. These plates converge at about 10 cm yr−1. At all drill sites sediment of upper Miocene to Quaternary age is almost entirely hemipelagic mud with interbedded thin volcanic ash, except in the trench where mud and fine sand turbidites less than 400 000 yr old are ponded. However, the underlying rocks are very different. On the oceanic Cocos plate a basal chalk sequence of lower and middle Miocene age is overlain by a thin section of abyssal clay. At a site only 3 km landward of the trench axis where drilling penetrated the slope deposits we recovered a Cretaceous to lower Miocene claystone sequence resting on a section containing igneous rock of continental affinity. A large net subduction of sediment along with ocean crust has occurred during the present (Miocene-Quaternary) episode of subduction and perhaps parts of the continental framework have been subducted as well. However, no current model satisfactorily explains the surprising occurrence of Cretaceous-Miocene claystone at the foot of the trench slope.

Analysis of Ocean-Subbottom Seismograph (OSS) Data

Data from OSS-IV, a deep-sea borehole seismic instrument installed approximately 750 kilometers southeast of the Central Kuril Islands, have been analyzed for signal directionality and the importance of scattering. Azimuths from instrument to source for a series of small SUS charge explosions determined by polarization analysis agree well with those obtained by navigation and travel time ranging. Synthetics generated for receivers at the ocean bottom and at the OSS location for the SUS line demonstrate the differences between the two sensor locations. Comparison with the data is good except for the large amplitude water wave arrivals at small offsets in the radial data. Noise from a passing ship is used to evaluate the ability of the polarization method to track continuous moving sources.

REFRACTION MEASUREMENT OF SHEAR WAVE ANISOTROPY IN SHALLOW MARINE SEDIMENTS AND IMPLICATIONS FOR REFLECTION PROCESSING

In 1986, an on-bottom shear wave source and a multi-component on-bottom receiver with a hydrophone were used by Woods Hole Oceanographic Institution and Rondout Associates, Inc. to record a short (< 200m) refraction profile near the United States Geological Survey’s AMCOR drill hole 6011, off the coast of New Jersey. Synthetic seismograms match amplitudes and travel times for all components only if a transversely isotropic model is used. The data require a ~10-m-thick layer with ~16% shear wave anisotropy, overlying a 40-m-thick isotropic region. This model agrees with drill core data showing 10m of silty clay overlying 40111 of sand. If a conventional reflection profile were recorded, the interval velocity would be overestimated by about 84% for SV-waves in the silty clay.