Purdy G. Michael

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G. Michael

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  • Technical Report
    The crustal structure of the Kane fracture zone from seismic refraction studies
    (Woods Hole Oceanographic Institution, 1980-12) Detrick, Robert S. ; Purdy, G. Michael
    A detailed seismic refraction experiment was carried out across the Kane Fracture Zone near 24°N, 44°W using explosive and air gun sound sources and eight ocean bottom hydrophone receivers. The shooting lines and receive rs formed a 'T' configuration across the fracture zone, with two receivers located about SO km apart in the fracture zone trough and the remaining six receivers positioned 25-30 km apart on either side of the fracture zone. The crustal thicknesses and velocities observed at the receivers located north and south of the Kane Fracture Zone fall within the range of those typically observed for normal oceanic crust. There is no convincing evidence for signficantly different crustal thicknesses or upper mantle velocities on either side of the fracture zone despite a 10-m.y. age difference. Anomalously thin crust is present beneath the Kane Fracture Zone trough with total crustal thicknesses of only 2-3 km, about half the thickness of normal oceanic crust. This crust is also characterized seismically by low compressional wave velocities (~4.0 km/s) at shallow depths and the absence of a normal layer 3 refractor. This anomalous crust extends over a width of a t least 10 km. Dense, high-velocity mantle type material may also exist at shallow depths beneath the adjacent Kane Fracture Zone ridge. Results from other geological and geophysical studies of fracture zones suggest that this type of crustal structure may by typical of many Atlantic fracture zones. We propose that the anomalously thin crust found within these fracture zones is a primary feature caused by the accretion of a thinner volcanic and plutonic layer within the fracture zone. This anomalous crust, which probably is restricted to a zone no wider than a typical transform fault valley (~10 km) in most cases, is inferred to consist of a few hundred meters of extrusive basalts and dikes overlying about 2 km of gabbro and metagabbro, possibly interbedded with ultramafics. This anomalously thin crustal section may be extensively fractured and brecciated at shallow levels by faulting in the active transform domain. A relatively narrow zone of thin crust within fracture zones can ex plain a number of geological and geophysical characteristics of fracture zones including the depth of the transform fault valley and the exposure of deep crustal and upper mantle rocks in the walls of fracture zones.
  • Technical Report
    A simple ocean bottom hydrophone with 200 megabyte data capacity
    (Woods Hole Oceanographic Institution, 1993-06) Peal, Kenneth R. ; Purdy, G. Michael ; Koelsch, Donald E. ; Wooding, Frank B.
    The Woods Hole Ocean Bottom Hydrophone instrument records the digitized output of a single hydrophone sensor at rates between 250 and 1200 samples per second with a dynamic range of 98 dB and can be deployed at depths to 600 meters. The unit's 200 megabyte disk recorder allows operation for periods up to 5 days. Designed for typical marine seismic refraction operations the unit is reliable and simple to deploy and recover. A detailed description is provided of the instrument design and application including transfer function, clock accuracy, data format, sample data and power requirements.
  • Technical Report
    A geophysical survey within the Mesozoic magnitic anomaly sequence south of Bermuda
    (Woods Hole Oceanographic Institution, 1979-12) Purdy, G. Michael ; Rohr, Kristin Marie Michener
    This geophysical survey of an approximate 1° square covers Mesozoic magnetic anomalies M0, M2, and M4 south of Bermuda. Bathymetry, magnetics, seismic reflection profiling, and seismic refraction data are presented. The isochron trend within the survey area at magnetic anomaly M4 time is 025°. Two left lateral fracture zones exist: the southern fracture zone has an offset of <10 km at M4 time and 33 km at M0 time. The northern fracture zone has an offset of 37 km at M4 time and 26 km at M0 time. These changes in offset are accounted for by asymmetric spreading, an 11° change in trend of anomaly M0 relative to M4, and by M0 time, growth of a small right lateral fracture zone. Seismic refraction data provide poor control on the shallow crustal structure but suggest the presence of significant lateral inhomogeneities within layer 2.
  • Technical Report
    Microearthquake activity on the Orozco Fracture Zone : preliminary results from Project ROSE
    (Woods Hole Oceanographic Institution, 1981-09) Ewing, John I. ; Purdy, G. Michael
    We present preliminary hypocenter determinations for 52 earthquakes recorded by a large multiinstitutional network of ocean bottom seismometers and ocean bottom hydrophones in the Orozco Fracture Zone in the eastern Pacific during late February to mid-March 1979. The network was deployed as pan of the Rivera Ocean Seismic Experiment, also known as Project ROSE. The Orozco Fracture Zone is physiographically complex, and the pattern of microeanhquake hypocenters at least partly reflects this complexity. All of the well-located epicenters lie within the active transform fault segment of the fracture zone. About lialf of the recorded earthquakes were aligned along a narrow trough that extends eastward from the northern rise crest intersection in the approximate direction of the Cocos-Pacific relative plate motion; these events appear to be characterized by strike-slip faulting. The second major group of activity occurred in the central portion of the transform fault; the microearthquakes in this group do not display a preferred alignment parallel to the direction of spreading, and several are not obviously associated with distinct topographic features. Hypocentral depth was well resolved for many of the earthquakes reported here. Nominal depths range from 0 to 17 km below the seafloor.
  • Technical Report
    The Woods Hole Oceanographic Institution digital ocean bottom hydrophone instrument : technical report
    (Woods Hole Oceanographic Institution, 1982-06) Koelsch, Donald E. ; Peal, Kenneth R. ; Purdy, G. Michael
    This report describes the design and capabilities of a new ocean bottom hydrophone instrument. The instrument is microprocessor controlled and records digitally on a commercially available cartridge tape recorder with a formatted capacity of 16.7 megabytes. It can operate at sampling intervals between 80 and 8500 Hz and has a dynamic range of 120dB. Both the hardware and software are designed to provide the maximum flexibility in operation allowing either preprogrammed or event detect operation for either short deployment high sampling rate experiments or extended deployment low data rate applications. The microprocessor and recording electronics are capable of handling four data channels and thus the existing recording package is suitable for the ocean bottom seismometer application (or similar} with little or no modification.
  • Technical Report
    A seismic refraction experiment in the central Banda Sea
    (Woods Hole Oceanographic Institution, 1979-02) Purdy, G. Michael ; Detrick, Robert S.
    A seismic refraction experiment in the central Banda Sea is interpreted by using both slope intercept and delay time function methods. The crustal structure is shown to be oceanic, with velocities (4.97, 6.47, 7.18, and 7.97 km/s) typical of oceanic layers 2, 3A, and 3B and the mantle. Individual layer thicknesses va ry systematica lly along the line, though the range of thicknesses observed for layers 2 ( 1.5-2.0 km) and 3A (2.0-3.5 km) falls well within the range observed for normal oceanic crust. Layer 3B is unusually thick (2.5-4.6 km), the result being slightl y greater than normal depths-to Moho of9-IO km below the sea floor. Shear head waves from layers 3A and 3B are identified on two receivers. In both cases, shear wave conversion occurred at the sediment/layer 2 interface. The observed shear wave velocities and intercepts indicate a Poisson's ratio of 0.25-0.28 in layer 3 and ~0.33 in layer 2. These and earlier results from the southern Banda basin indicate that the entire Banda Sea is underlain by oceanic type crust.