Northern Cascadia Research Articles

Sedimentary underplating at the Cascadia mantle-wedge corner revealed by seismic imaging

Earth’s largest earthquakes occur at the boundary between subducting oceanic crust and the overriding plate, yet the position of the plate boundaries remains uncertain. Analysis of zones of low seismic wave velocities beneath the northern Cascadia subduction zone implies that the plate boundary here may be deeper than previously thought.

Modeling sulfate reduction in methane hydrate-bearing continental margin sediments: Does a sulfate-methane transition require anaerobic oxidation of methane?

The sulfate-methane transition (SMT), a biogeochemical zone where sulfate and methane are metabolized, is commonly observed at shallow depths (1–30 mbsf) in methane-bearing marine sediments. Two processes consume sulfate at and above the SMT, anaerobic oxidation of methane (AOM) and organoclastic sulfate reduction (OSR). Differentiating the relative contribution of each process is critical to estimate methane flux into the SMT, which, in turn, is necessary to predict deeper occurrences of gas hydrates in continental margin sediments. To evaluate the relative importance of these two sulfate reduction pathways, we developed a diagenetic model to compute the pore water concentrations of sulfate, methane, and dissolved inorganic carbon (DIC). By separately tracking DIC containing 12C and 13C, the model also computes δ13C-DIC values. The model reproduces common observations from methane-rich sediments: a well-defined SMT with no methane above and no sulfate below and a δ13C-DIC minimum at the SMT. The model also highlights the role of upward diffusing 13C-enriched DIC in contributing to the carbon isotope mass balance of DIC. A combination of OSR and AOM, each consuming similar amounts of sulfate, matches observations from Site U1325 (Integrated Ocean Drilling Program Expedition 311, northern Cascadia margin). Without AOM, methane diffuses above the SMT, which contradicts existing field data. The modeling results are generalized with a dimensional analysis to the range of SMT depths and sedimentation rates typical of continental margins. The modeling shows that AOM must be active to establish an SMT wherein methane is quantitatively consumed and the δ13C-DIC minimum occurs. The presence of an SMT generally requires active AOM.

Episodic tremors and slip in Cascadia in the framework of the Frenkel-Kontorova model

Received 27 August 2010; revised 20 October 2010; accepted 28 October 2010; published 13 January 2011. [1] The seismic moment for regular earthquakes is proportional to the cube of rupture time. A second class of phenomena, collectively called slow earthquakes, has very different scaling. We propose a model, inspired from the phenomenology of dislocation dynamics in crystals, that is consistent with the scaling relations observed in the Cascadia episodic tremor and slip (ETS) events. Two fundamental features of ETS are periodicity and migration. In the northern Cascadia subduction zone, ETS events appear every 14.5 months or so. During these events, tremors migrate along‐strike with a velocity of 10 km/day and simultaneously zip back and forth in the relative plate‐ motion direction with at ypical velocity of 50 km/h. Our model predicts the formation of a sequence of slip pulses on the boundary of the plates, which describes the major features of fault dynamics, including periodicity and the migration pattern of tremors. Citation: Gershenzon, N. I., G. Bambakidis, E. Hauser, A. Ghosh, and K. C. Creager (2011), Episodic tremors and slip in Cascadia in the framework of the Frenkel‐Kontorova model, Geophys. Res. Lett., 38, L01309,

Capillary controls on methane hydrate distribution and fracturing in advective systems

[1] We simulate 1-D, steady, advective flow through a layered porous medium to investigate how capillary controls on solubility including the Gibbs-Thomson effect in fine-grained sediments affect methane hydrate distribution in marine sediments. We compute the increase in pore fluid pressure that results from hydrate occluding the pore space and allow fractures to form if the pore fluid pressure exceeds a fracture criterion. We apply this model to Hydrate Ridge and northern Cascadia, two field sites where hydrates have been observed preferentially filling cm-scale, coarse-grained layers. We find that at Hydrate Ridge, hydrate forms in the coarse-grained layers reaching saturation of 90%, creating fractures through intervening fine-grained layers after 2000 years. At northern Cascadia, hydrate forms preferentially in the coarse-grained layers but 2 × 105 years are required to develop the observed hydrate saturations (∼20%–60%), suggesting that hydrate formation rates may be enhanced by an additional source of methane such as in situ methanogenesis. We develop expressions to determine the combinations of sediment physical properties and methane supply rates that will result in hydrate-filled coarse-grained layers separated by hydrate-filled fine-grained layers, the conditions necessary to fracture the fine-grained layers, and the conditions that will lead to complete inhibition of hydrate formation as pore space is constricted. This work illustrates how sediment physical properties control hydrate distribution at the pore scale and how hydrate distribution affects fracturing behavior in marine sediments.

Introduction to special section on Phenomenology, Underlying Processes, and Hazard Implications of Aseismic Slip and Nonvolcanic tremor

This paper introduces the special section on the “phenomenology, underlying processes, and hazard implications of aseismic slip and nonvolcanic tremor” by highlighting key results of the studies published in it. Many of the results indicate that seismic and aseismic manifestations of slow slip reflect transient shear displacements on the plate interface, with the outstanding exception of northern Cascadia where tremor sources have been located on and above the plate interface (differing models of the plate interface there also need to be reconciled). Slow slip phenomena appear to result from propagating deformation that may develop with persistent gaps and segment boundaries. Results add to evidence that when tectonic deformation is relaxed via slow slip, most relaxation occurs aseismically but with seismic signals providing higher‐resolution proxies for the aseismic slip. Instead of two distinct slip modes as suggested previously, lines between “fast” and “slow” slip more appropriately may be described as blurry zones. Results reported also show that slow slip sources do not coincide with a specific temperature or metamorphic reaction. Their associations with zones of high conductivity and low shear to compressional wave velocity ratios corroborate source models involving pore fluid pressure buildup and release. These models and spatial anticorrelations between earthquake and tremor activity also corroborate a linkage between slow slip and frictional properties transitional between steady state and stick‐slip. Finally, this special section highlights the benefits of global and multidisciplinary studies, which demonstrate that slow phenomena are not confined to beneath the locked zone but exist in many settings.

An earthquake‐like magnitude‐frequency distribution of slow slip in northern Cascadia

Major episodic tremor and slip (ETS) events with Mw 6.4 to 6.7 repeat every 15 ± 2 months within the Cascadia subduction zone under the Olympic Peninsula. Although these major ETS events are observed to release strain, smaller “tremor swarms” without detectable geodetic deformation are more frequent. An automatic search from 2006–2009 reveals 20,000 five‐minute windows containing tremor which cluster in space and time into 96 tremor swarms. The 93 inter‐ETS tremor swarms account for 45% of the total duration of tremor detection during the last three ETS cycles. The number of tremor swarms, N, exceeding duration τ follow a power‐law distribution N ∝ τ−0.66. If duration is proportional to moment release, the slip inferred from these swarms follows a standard Gutenberg‐Richter logarithmic frequency‐magnitude relation, with the major ETS events and smaller inter‐ETS swarms lying on the same trend. This relationship implies that 1) inter‐ETS slip is fundamentally similar to the major events, just smaller and more frequent; and 2) despite fundamental differences in moment‐duration scaling, the slow slip magnitude‐frequency distribution is the same as normal earthquakes with a b‐value of 1.

Frontal ridge slope failure at the northern Cascadia margin: Margin-normal fault and gas hydrate control

On the northern Cascadia accretionary margin off Vancouver Island, Canada, there are numerous sedimentary slide features near the base of the slope, and significant amounts of gas hydrate found beneath the ridge suggest a possible connection. A 2-km-wide collapse structure with a 300-m-high headwall has been studied by multibeam bathymetry and a single-channel seismic reflection grid. Migrated seismic reflection lines on the frontal ridge image multiple 15–75-m-high seafloor scarps perpendicular to the margin that are the seafloor expressions of normal faults that cut deeply into the sediment section. Two of the largest faults are aligned with the sidewalls of the slide, indicating that the lateral extent of the collapse is fault controlled. The presence of marine gas hydrate beneath the ridge is based on a widespread hydrate bottom-simulating reflector (BSR), high velocities determined by ocean bottom seismograph data, and sediment core samples and downhole logs collected by the Integrated Ocean Drilling Program at Site U1326. The depth of the BSR at 255 (±15) m coincides closely with the estimated depth of the glide plane beneath the slide. This suggests that the base of the slope failure is related to the contrast between strong hydrate-cemented sediments above the BSR and underlying weak sediments containing free gas. Strong earthquake shaking on this convergent margin likely provides the trigger for the slide.

Estimating seismic moment magnitude (Mw) of tremor bursts in northern Cascadia: Implications for the “seismic efficiency” of episodic tremor and slip

We develop a method to estimate the seismic moments of deep non‐volcanic tremor bursts observed in northern Cascadia. For each tremor burst, the maximum amplitudes at individual stations within a time window ±5 s around the predicted arrivals of the S phase are measured and compared to the maximum S amplitudes measured from synthetic seismograms. The proposed method is thoroughly calibrated using 464 local earthquakes and the results show excellent consistency between the reported ML and the estimated Mw. We apply the method to northern Cascadia tremors and infer that most bursts have Mw∼1.0–1.7. The corresponding b value appears to be 1, consistent with that of ordinary earthquakes but over a narrower Mw range. Comparison of cumulative tremor Mw and the Mw estimated from the accompanying slow slip suggests that the “seismic efficiency” of the Episodic Tremor and Slip (ETS) is of the order of 0.1% or less.

Intraslab Stresses in the Cascadia Subduction Zone from Inversion of Earthquake Focal Mechanisms

At the Cascadia subduction zone, intraslab earthquakes occur mostly in the northern part of the margin and its southern end, the Mendocino triple junction (MTJ). We determine intraslab stress orientations by inverting earthquake focal me- chanisms and develop working hypotheses to explain the inferred intraslab stresses and observed seismicity. Our inversion results show that the subducting Juan de Fuca (JDF) slab in northern Cascadia is primarily under compression normal to the slab surface and tension in the downdip direction, most likely controlled by the net slab pull. An exception is a northernmost shallow region near the Nootka fault zone where the state of stress is dominated by nearly east-west tension. We hypothesize that the shear force on the Nootka fault zone and margin-parallel mantle resistance to slab motion induce the east-west tension in this region. Near the MTJ, stresses in the JDF plate are dominated by north-south compression down to about 20 km depth, consistent with a strong push by the Pacific plate from south of the Mendocino trans- form fault, but the deeper part of the slab shows downdip-tension, similar to northern Cascadia. Deviatoric stresses in the JDF slab appear to be very low, resulting in very low intraslab seismicity. In comparison with northern Cascadia, the stresses in most of southern Cascadia are even lower, resulting in nearly no intraslab seismicity.

Chemical and isotopic signature of bulk organic matter and hydrocarbon biomarkers within mid-slope accretionary sediments of the northern Cascadia margin gas hydrate system

The chemical and isotopic compositions of sedimentary organic matter (SOM) from two mid-slope sites of the northern Cascadia margin were investigated during Integrated Ocean Drilling Program (IODP) Expedition 311 to elucidate the organic matter origins and identify potential microbial contributions to SOM. Gas hydrate is present at both locations (IODP Sites U1327 and U1328), with distinct patterns of near-seafloor structural accumulations at the cold seep Site U1328 and deeper stratigraphic accumulations at the slope-basin Site U1327. Source characterization and evidence that some components of the organic matter have been diagenetically altered are determined from the concentrations and isotopic compositions of hydrocarbon biomarkers, total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS). The carbon isotopic compositions of TOC (δ 13C TOC = −26 to −22‰) and long-chain n-alkanes (C 27, C 29 and C 31, δ 13C = −34 to − 29‰) suggest the organic matter at both sites is a mixture of 1) terrestrial plants that employ the C3 photosynthetic pathway and 2) marine algae. In contrast, the δ 15N TN values of the bulk sediment (+ 4 to + 8‰) are consistent with a predominantly marine source, but these values most likely have been modified during microbial organic matter degradation. The δ 13C values of archaeal biomarker pentamethylicosane (PMI) (− 46.4‰) and bacterial-sourced hopenes, diploptene and hop-21-ene (− 40.9 to − 34.7‰) indicate a partial contribution from methane carbon or a chemoautotrophic pathway. Our multi-isotope and biomarker-based conclusions are consistent with previous studies, based only on the elemental composition of bulk sediments, that suggested a mixed marine-terrestrial organic matter origin for these mid-slope sites of the northern Cascadia margin.

Characterizing the thermal regime of cold vents at the northern Cascadia margin from bottom-simulating reflector distributions, heat-probe measurements and borehole temperature data

Several cold vents are observed at the northern Cascadia margin offshore Vancouver Island in a 10 km2 region around Integrated Ocean Drilling Program Expedition 311 Site U1328. All vents are linked to fault systems that provide pathways for upward migrating fluids and at three vents methane plumes were detected acoustically in the water column. Downhole temperature measurements at Site U1328 revealed a geothermal gradient of 0.056 ± 0.004°C/m. With the measured in situ pore-water salinities the base of methane hydrate stability is predicted at 218–245 meters below seafloor. Heat-probe measurements conducted across Site U1328 and other nearby vents showed an average thermal gradient of 0.054 ± 0.004°C/m. Assuming that the bottom-simulating reflector (BSR) marks the base of the gas hydrate stability zone variations in BSR depths were used to investigate the linkages between the base of the gas hydrate stability zone and fluid migration. Variations in BSR depth can be attributed to lithology-related velocity changes or variations of in situ pore-fluid compositions. Prominent BSR depressions and reduced heat flow are seen below topographic highs, but only a portion of the heat flow reduction can be due to topography-linked cooling. More than half of the reduction may be due to thrust faulting or to pore-water freshening. Distinct changes in BSR depth below seafloor are observed at all cold vents studied and some portion of the observed decrease in the BSR depth was attributed to fault-related upwelling of warmer fluids. The observed decrease in BSR depth below seafloor underneath the vents ranges between 7 and 24 m (equivalent to temperature shifts of 0.07–0.15°C).

Future Cascadia megathrust rupture delineated by episodic tremor and slip

A suite of 15 episodic tremor and slip events imaged between 1997 and 2008 along the northern Cascadia subduction zone suggests future coseismic rupture will extend to 25 km depth, or ∼60 km inland of the Pacific coast, rather than stopping offshore at 15 km depth. An ETS‐derived coupling profile accurately predicts GPS‐measured interseismic deformation of the overlying North American plate, as measured by ∼50 continuous GPS stations across western Washington State. When extrapolated over the 550‐year average recurrence interval of Cascadia megathrust events, the coupling model also replicates the pattern and amplitude of coseismic coastal subsidence inferred from previous megathrust earthquakes here. For only the Washington State segment of the Cascadia margin, this translates into an Mw = 8.9 earthquake, with significant moment release close to its metropolitan centers.

Northern Cascadia episodic tremor and slip: A decade of tremor observations from 1997 to 2007

We analyze continuous seismic and GPS records collected in the last decade (1997–2007) to establish the most comprehensive observational basis for northern Cascadia episodic tremor and slip (ETS) events. A simple “ETS scale” system, using a combination of a letter and a digit, is proposed to quantitatively characterize the spatial and temporal dimensions of ETS events. Clear correlation between GPS and tremor signals is observed for all A/B class episodes, but the GPS signature is less obvious for minor ones. Regular ETS recurrence can be established only for A/B class episodes in southern Vancouver Island. Halting and jumping are very common in ETS migration patterns, and along‐strike migration can happen in both directions. A prominent tremor gap is observed in midisland around 49.5°N. This gap coincides with the epicenters of the only two large earthquakes beneath Vancouver Island. ETS tremors also tend to occur in places where the local seismicity is relatively sparse. The tremor depth distribution shows a peak in the 25–35 km range where strong seismic reflectors (i.e., the E layer) are documented. Detailed waveform analysis confirms the existence of shallow tremors above the currently interpreted plate interface. Our results suggest that a significant portion of the tremor activity and perhaps associated shearing are taking place along well‐developed structures such as the E layer, while fewer tremor bursts are generated elsewhere in response to the induced stress variation throughout the source volume.

Deep low‐frequency earthquakes in tremor localize to the plate interface in multiple subduction zones

Deep tremor under Shikoku, Japan, consists primarily, and perhaps entirely, of swarms of low‐frequency earthquakes (LFEs) that occur as shear slip on the plate interface. Although tremor is observed at other plate boundaries, the lack of cataloged low‐frequency earthquakes has precluded a similar conclusion about tremor in those locales. We use a network autocorrelation approach to detect and locate LFEs within tremor recorded at three subduction zones characterized by different thermal structures and levels of interplate seismicity: southwest Japan, northern Cascadia, and Costa Rica. In each case we find that LFEs are the primary constituent of tremor and that they locate on the deep continuation of the plate boundary. This suggests that tremor in these regions shares a common mechanism and that temperature is not the primary control on such activity.

Seismic and geodetic constraints on Cascadia slow slip

Automatically detected and located tremor epicenters from episodic tremor and slip (ETS) episodes in northern Cascadia provide a high‐resolution map of Washington's slow slip region. Thousands of epicenters from the past four ETS events from 2004 to 2008 provide detailed map‐view constraints that correlate with geodetic estimates of the simultaneous slow slip. Each of these ETS events exhibits remarkable similarity in the timing and geographic distribution of tremor density and geodetically inferred slip. Analysis of the latest 15‐month inter‐ETS period also reveals ageodetic tremor activity similar both in duration and extent to ETS tremor. Epicenters from both ETS and inter‐ETS tremor are bounded between the 30‐ and 45‐km plate interface depth contours and locate approximately 75 km east of previous estimates of the locked portion of the subducting Juan de Fuca plate. Inter‐ETS tremor overlaps but is generally downdip of ETS tremor and does not yet correlate with geodetically observed slip, but this is likely because the slip is below current GPS detection levels. Based on the tremor and slip correlation and the tremor‐duration and slip magnitude relationship, we suggest that the well‐resolved, sharp updip edge of tremor epicenters reflects a change in plate interface coupling properties. The region updip of this boundary may accumulate stress with the potential for coseismic shear failure during a megathrust earthquake. Alternatively, plate convergence in this region could be accommodated by continuous slow slip with no detectable tremor or by slow slip events with sufficiently long recurrence intervals that none have been detected during the past 10 years of GPS observations.

Methane sources and production in the northern Cascadia margin gas hydrate system

The oceanographic and tectonic conditions of accretionary margins are well-suited for several potential processes governing methane generation, storage and release. To identify the relevant methane evolution pathways in the northern Cascadia accretionary margin, a four-site transect was drilled during Integrated Ocean Drilling Program Expedition 311. The δ 13C values of methane range from a minimum value of − 82.2‰ on an uplifted ridge of accreted sediment near the deformation front (Site U1326, 1829 mbsl, meters below sea level) to a maximum value of − 39.5‰ at the most landward location within an area of steep canyons near the shelf edge (Site U1329, 946 mbsl). An interpretation based solely on methane isotope values might conclude the 13C-enrichment of methane indicates a transition from microbially- to thermogenically-sourced methane. However, the co-existing CO 2 exhibits a similar trend of 13C-enrichment along the transect with values ranging from − 22.5‰ to +25.7‰. The magnitude of the carbon isotope separation between methane and CO 2 ( ε c = 63.8 ± 5.8) is consistent with isotope fractionation during microbially mediated carbonate reduction. These results, in conjunction with a transect-wide gaseous hydrocarbon content composed of > 99.8% (by volume) methane and uniform δD CH 4 values (− 172‰ ± 8) that are distinct from thermogenic methane at a seep located 60 km from the Expedition 311 transect, suggest microbial CO 2 reduction is the predominant methane source at all investigated sites. The magnitude of the intra-site downhole 13C-enrichment of CO 2 within the accreted ridge (Site U1326) and a slope basin nearest the deformation front (Site U1325, 2195 mbsl) is ~ 5‰. At the mid-slope site (Site U1327, 1304 mbsl) the downhole 13C-enrichment of the CO 2 is ~ 25‰ and increases to ~ 40‰ at the near-shelf edge Site U1329. This isotope fractionation pattern is indicative of more extensive diagenetic alteration at sites with greater 13C-enrichment. The magnitude of the 13C-enrichment of CO 2 correlates with decreasing sedimentation rates and a diminishing occurrence of stratigraphic gas hydrate. We suggest the decreasing sedimentation rates increase the exposure time of sedimentary organic matter to aerobic and anaerobic degradation, during burial, thereby reducing the availability of metabolizable organic matter available for methane production. This process is reflected in the occurrence and distribution of gas hydrate within the northern Cascadia margin accretionary prism. Our observations are relevant for evaluating methane production and the occurrence of stratigraphic gas hydrate within other convergent margins.

Slow slip predictions based on granite and gabbro friction data compared to GPS measurements in northern Cascadia

For episodic slow slip transients in subduction zones, a large uncertainty in comparing surface deformations predicted by forward modeling based on rate and state friction to GPS measurements lies in our limited knowledge of the frictional properties and fluid pore pressure along shallow subduction faults. In this study, we apply the laboratory rate and state friction data of granite and gabbro gouges under hydrothermal conditions to a Cascadia‐like 2‐D model to produce spontaneous aseismic transients, and we compare the resulting intertransient and transient surface deformations to GPS observations along the northern Cascadia margin. An inferred region along dip of elevated fluid pressure is constrained by seismological observations where available and by thermal and petrological models for the Cascadia and SW Japan subduction zones. For the assumed friction parameters a and a − b profiles, we search the model parameter space, by varying the level of effective normal stress , characteristic slip distance L in the source areas of transients, and the fault width under , to identify simulation cases that produce transients of total aseismic slip and recurrence interval similar to the observed 20–30 mm and 1–2 years, respectively, in northern Cascadia. Using a simple planar fault geometry and extrapolating the 2‐D fault slip to a 3‐D distribution, we find that the friction data for gabbro gouge, a better representation of the seafloor, fit GPS observations of transient deformation in northern Cascadia much better than do the granite data, which, for lack of a suitable alternative, have been the basis for most previous modeling.

Correction to “Correlation of tremor activity with tidal stress in the northern Cascadia subduction zone”

Correction to “Correlation of tremor activity with tidal stress in the northern Cascadia subduction zone”

Correlation of tremor activity with tidal stress in the northern Cascadia subduction zone

We analyze hourly data from five tremor episodes in the northern Cascadia subduction zone over the period 2003–2005 provided by the Tremor Activity Monitoring System (TAMS). All five tremor episodes correspond to slow slip events observed by GPS. Fourier decomposition is used to separate the hourly tremor counts for each episode into “long‐period” (0 < f < 0.8 cpd), “tidal” (0.8 < f < 2.2 cpd), and “short‐period” (f > 2.2 cpd) components. The tidal component of the observations is compared with theoretical stress variations at depths of 20, 30, and 40 km, with 40 km being the depth of the interpreted subduction thrust interface. The stress variations are predicted by a 2‐D ocean tide loading model combined with estimates of stress variations from Earth tides. We find that the shear stress in the thrust direction and the compressive normal stress on shallow dipping surfaces correlates with the data significantly better than the confining stress over the range of depths investigated. The relative amplitudes of tidal shear stress and compressive normal stress result in positive Coulomb stress favoring slip. Peak tremor activity occurs at times of maximum tidal shear stress in the thrust direction, which would assist slow slip and would suggest that tidal tremor and slip are colocated. The response of the tremor to tidal shear stress is roughly proportional to the mean activity level, controlled by tectonic conditions of stress and pore pressure. A significant, nontidal, daily variation in tremor activity of unknown origin is identified.

Comment on "Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone" by Chris Goldfinger, Kelly Grijalva, Roland Burgmann, Ann E. Morey, Joel E. Johnson, C. Hans Nelson, Julia Gutierrez-Pastor, Andrew Ericsson, Eugene Karabanov, Jason D. Chaytor, Jason Patton, and Eulalia Gracia

Goldfinger et al. (2008) conclude that in the northern San Andreas fault and Cascadia margins, Holocene turbidites (i.e., deposits of turbidity currents) were triggered by earthquakes. However, their conclusion falters because of (1) selective data sets that were used for establishing seismicity as the sole triggering mechanism of turbidites, (2) flawed sedimentological concepts and criteria that were applied for interpreting turbidites, and (3) faulty methodologies that were employed for correlating turbidites over long distances. This same turbidite (i.e., sedimentological) theme has also been published previously by Goldfinger et al. (2003a,b, 2006, 2007). As with the recent article in BSSA , their previous publications on the subject matter were published in journals that are concerned primarily with geophysical topics. As a result, critical sedimentological conclusions, which are an integral part of the articles by Goldfinger et al. , have not been subjected to a rigorous scrutiny. Therefore, I would like to comment on the following three issues. Along the Washington–Oregon–California margin, potential triggering mechanisms of sediment failures and related turbidity currents are (1) earthquake (Adams, 1990), (2) tectonic oversteepening (Greene et al. , 2006), (3) tsunami (Adams, 1990; Geist, 2005), (4) cyclone (Adams, 1990; Shanmugam, 2008a, fig. 4), (5) ebb tidal current (Puig et al. , 2003), (6) submarine volcanic activity (Davis et al. , 2002), (7) sediment loading (Adams, 1990), and (8) gas hydrates (Chapman et al. , 2004). The link between gas-hydrate decomposition and mass movements on the Norwegian margin was discussed by Mienert et al. (2005). In the Cascadia margin, a total of 40 cyclones along the Oregon coast were reported for the 1984–2006 period (Moritz and Moritz, 2006). A total of 24 tsunamis were reported along the Oregon coast for the 1868–1994 period (Table 1). Cyclones and tsunamis are powerful agents of sediment failures (Shanmugam, 2008a). On the outer continental shelf of the Gulf of Mexico, for example, the …