Webb Spahr C.

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Webb
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Spahr C.
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  • Article
    Microearthquake patterns following the 1998 eruption of Axial Volcano, Juan de Fuca Ridge : mechanical relaxation and thermal strain
    (American Geophysical Union, 2004-01-14) Sohn, Robert A. ; Barclay, Andrew H. ; Webb, Spahr C.
    Ocean bottom seismic networks deployed following the 1998 eruption of Axial seamount reveal an evolving pattern of microearthquake activity associated with subsurface magmatism and thermal strain. Seismicity rates decay steadily over 15 months of observation (February 8, 1998, to April 30, 1999), consistent with a trend toward thermal and mechanical equilibrium in the shallow crust after the magmatic event. Immediately after the eruption, seismicity rates were high for about 60 days in the southeast corner of the caldera where lava flows from the 1998 eruption were mapped. A small burst of seismic activity was observed on the southeast shoulder of the volcano from 100 to 150 days after the eruption. These events, which are characterized by slip on nearly vertical faults in the shallow crust, extend about 6 km from the southeast corner of the caldera and overlie a mid-crustal low-velocity zone. After this episode, seismicity rates remain low until the end of the observation period, 455 days after the eruption. Shallow (~0.7 km depth) events, consistent with thermal contraction and volume changes of ~2 × 10−3 m3 in ~5 m3 sources, are observed in individual clusters beneath hydrothermal vents within the 1998 lava flow at the southeast edge of the caldera. Microearthquakes observed during the last 70 days of observation are distributed around the central caldera, most likely representing small amounts of subsidence on caldera faults during the final stages of equilibration following melt withdrawal associated with the 1998 eruption.
  • Technical Report
    Report of a workshop on technical approaches to construction of a seafloor geomagnetic observatory
    (Woods Hole Oceanographic Institution, 1995-09) Chave, Alan D. ; Green, Arthur W. ; Filloux, Jean H. ; Law, Lawrie K. ; Petitt, Robert A. ; Rasson, Jean L. ; Schultz, Adam ; Spiess, Fred N. ; Tarits, Pascal ; Tivey, Maurice A. ; Webb, Spahr C.
    This report considers the technical issues on sensors, data recording and transmission, control and timing, power, and packaging associated with constricting a seafloor geomagnetic observatory. Existing technologies either already in use for oceanographic purposes or adapted from terrestral geomagnetic observatories could be applied to measure the vector magnetic field components and absolute intensity with minimal development. The major technical challenge arises in measuring absolute direction on the seafloor because terrestral techniques are not transferrable to the deep ocean. Two solutions to this problem were identified. The first requires the development of an instrument which measures the instantaneous declination and inclination of the magnetic field relative to a north-seeking gyroscope and the local vertical. The second is a straightforward extension of a precision acoustic method for determining absolute position on the seafloor.
  • Article
    Fine-scale seismic structure of the shallow volcanic crust on the East Pacific Rise at 9°50′N
    (American Geophysical Union, 2004-12-09) Sohn, Robert A. ; Webb, Spahr C. ; Hildebrand, John A.
    We use a combination of body wave and interface wave observations from an on-bottom seismic refraction survey to constrain the fine-scale seismic structure of the upper crust in a ∼3 × 3 km field area centered on the East Pacific Rise at 9°50′N. We detonated 18 explosive shots (18 sources) in a circular pattern (1.5 km radius) on the rise axis and recorded seismic arrivals with eight ocean bottom seismometers (eight receivers). We observed 30–40 Hz compressional body waves from all shots (144 P waves) and 1–3 Hz Stoneley (interface) waves on a subset of source-receiver pairs (58 interface waves). Using a station correction inversion, we find that roughly half of the variance in the P wave first-arrival times results from lateral variations in the thickness of the surface low-velocity layer (SLVL), a layer of extremely porous lava and basalt breccia with an average P wave velocity of 2.2 km s−1. The SLVL thickness increases from <20 m along the axial summit trough (AST) to ∼120 m at near-axis lava depocenters, which are not symmetric about the rise axis. Depocenters are located ∼0.5 km to the west and ∼1.5 km to the east of the rise axis. Tomographic inversion of the Stoneley wave first arrivals reveals that shear velocities in the SLVL covary with the layer thickness, exhibiting a similar asymmetric pattern, with shear velocities increasing from ∼320 m s−1 near the AST to ∼520 m s−1 at the near-axis depocenters. Our analysis demonstrates that the seismic characteristics of the extrusive layer near the rise axis are related primarily to volcanic features and processes. The thickness and velocity of the SLVL are low on the axis and within channel networks that deliver lava flows away from the axis and then increase rapidly at the distal ends of the channels where the lavas are deposited. We find that azimuthal anisotropy exerts only a weak influence on our P wave first-arrival times, which we model as weak (4%) seismic azimuthal anisotropy in the upper dikes with a fast axis oriented N23°–32°W. We find no evidence for seismic azimuthal anisotropy in the extrusive layer.
  • Article
    Field measurements of sonic boom penetration into the ocean
    (Acoustical Society of America, 2000-06) Sohn, Robert A. ; Vernon, Frank L. ; Hildebrand, John A. ; Webb, Spahr C.
    Six sonic booms, generated by F-4 aircraft under steady flight at a range of altitudes (610–6100 m) and Mach numbers (1.07–1.26), were measured just above the air/sea interface, and at five depths in the water column. The measurements were made with a vertical hydrophone array suspended from a small spar buoy at the sea surface, and telemetered to a nearby research vessel. The sonic boom pressure amplitude decays exponentially with depth, and the signal fades into the ambient noise field by 30–50 m, depending on the strength of the boom at the sea surface. Low-frequency components of the boom waveform penetrate significantly deeper than high frequencies. Frequencies greater than 20 Hz are difficult to observe at depths greater than about 10 m. Underwater sonic boom pressure measurements exhibit excellent agreement with predictions from analytical theory, despite the assumption of a flat air/sea interface. Significant scattering of the sonic boom signal by the rough ocean surface is not detected. Real ocean conditions appear to exert a negligible effect on the penetration of sonic booms into the ocean unless steady vehicle speeds exceed Mach 3, when the boom incidence angle is sufficient to cause scattering on realistic open ocean surfaces.