Seismic reflection and Sea Beam bathymetric data plus submarine geological measurements define a ramp anticline at the deformation front of the central Oregon subduction zone. At its northem termination the ramp anticline is deeply incised by a large 500-m-deep submarine canyon and cut by a probable backthrust.To the south along the strike of the fold, a smaller submarine canyon shallowly erodes the anticline, and backthrusting is not apparent in the submersible observations. Two Alvin dives along a transect through the southem canyon show active fluid vents demarked by biological communities at the frontal thrust and at the breached crest of the anticline. Along a northern transect, encompassing the large submarine canyon, 10 Alvin dives indicated no venting on the frontal thrust, limited venting in the canyon, but numerous biological communities along a scarp interpreted as the surface trace of the backthrust. These observations suggest a scenario of vent and structuralgeomorphic development consisting of (1) frontal thrust faulting and associated venting, facilitated by high fluid pressure; (2) erosion of the oversteepened seaward flank of the ramp anticline assisted by seepage forces and leading to fluid flow out of stratigraphically controlled conduits in the limbs of the overthrust deposits; (3) locking of the frontal thrust due to dewatering or a local decrease in wedge taper associated with development of the large canyon, leading to failure along the backthrust; and (4) redirection of fluid flow by the backthrust. Thus, within < 0.3 m.y., deformation of the relatively permeable sediments of the Oregon margin results in stratigraphically controlled flow being partially captured by faults. The initial series of Alvin dives in 1984 first discovered chemosynthetic biological communities and associated authigenic carbonate deposits at a convergent margin [Suess et al., 1985; Kulm et al., 1986]. Other components of the Alvin program have examined the chemistry of fluids from the vents [Suess and Whiticar, 1989]; the role of expulsed fluid in global geochemical cycles and diagenesis [Hah and Sues& 1989], the association of carbonate deposits with fluid venting processes [Kulm and Suess, this issue], and the relationship of venting to large scale geologic structure [Lewis and Cochrane, this issue].

A model for the high-frequency backscatter angular response of gassy sediments is proposed. For the interface backscatter contribution we adopted the model developed by Jackson et al. [J. Acoust. Soc. Am. 79, 1410-1422 (1986)], but added modifications to accommodate gas bubbles. The model parameters that are affected by gas content are the density ratio, the sound speed ratio, and the loss parameter. For the volume backscatter contribution we developed a model based on the presence and distribution of gas in the sediment. We treat the bubbles as individual discrete scatterers that sum to the total bubble contribution. This total bubble contribution is then added to the volume contribution of other scatters. The presence of gas affects both the interface and the volume contribution of the backscatter angular response in a complex way that is dependent on both grain size and water depth. The backscatter response of fine-grained gassy sediments is dominated by the volume contribution while that of coarser-grained gassy sediments is affected by both volume and interface contributions. In deep water the interface backscatter is only slightly affected by the presence of gas while the volume scattering is strongly affected. In shallow water the interface backscatter is severely reduced in the presence of gas while the volume backscatter is only slightly increased. Multibeam data acquired offshore northern California at 95 kHz provides raw measurements for the backscatter as a function of grazing angle. These raw backscatter measurements are then reduced to scattering strength for comparison with the results of the proposed model. The analysis of core samples at various locations provides local measurements of physical properties and gas content in the sediments that, when compared to the model, show general agreement.

There are many seafloor features on and near the base of the Sigsbee Escarpment that were formed as the result of mass gravity flows. Accordingly, a multidisciplinary team conducted an evaluation of these features as part of an overall study of geohazards. The goals of these evaluations were to:catalog the varieties of mass gravity flows that have occurred in the past,understand the causes of these events,characterize the kinematics (e.g., speeds, dimensions) of these flows, andportray the relative magnitudes of the spatially dependent exposures to possible future events. To accomplish these goals, there was extensive use of previously collected seismic exploration data that included seafloor bathymetric images and shallow reflection amplitudes. Maps of mass gravity flow features, sources, and flow paths were developed. These were used to plan a comprehensive program of bottom surveying with an autonomous underwater vehicle (AUV). The fieldwork also included a program of navigated deep piston-cores and box cores, along with video observations from an ROV. One goal of the field program was to acquire adequate data to characterize seafloor mass gravity flow features and deposits. These were used as inputs and "targets" for numerical modeling to simulate the features. The approach is that properly detailed mathematical representations of the physics of mass gravity flows are used to "backcast" the behaviors and flow kinematics of the events that caused the observed features. This provides model calibration, which allows the model to be applied in a prognostic mode. Sediments were described and analyzed for grain size, erodibility and age. Geophysical data were reduced and archived in a GIS system. Numerical models of debris flow, mudflows and turbidity currents were applied. The results show that several types and scales of mass gravity flows (debris flows and turbidity currents) have been active all along the portion of the Sigsbee Escarpment covered by the surveys. The scale and form of these features are localized and predictable. Representative flow kinematics were determined for each of the flow types using the results of the fieldwork and modeling. Activity of these features diminished markedly over the past 10,000 years as sea level rose following the last ice age. Introduction One component of a multi-faceted examination of potential geohazards in the Atlantis and Mad Dog prospect areas along and below the Sigsbee Escarpment has been directed at understanding mass gravity flow activity. In principle, an understanding of past activity is a necessary step in developing the capability to forecast future activity. The goals of the evaluation were to:catalog the varieties of mass gravity flows that have occurred in the past,understand the causes of these events,characterize the kinematics (e.g., speeds, dimensions) of these flows, andportray the relative magnitudes of the spatially dependent exposures to possible future events.

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