Offshore Northern California Research Articles

Improving Out of Network Earthquake Locations Using Prior Seismicity for Use in Earthquake Early Warning

ABSTRACTTimely alerts sent through earthquake early warning (EEW) programs allow those alerted to take protective actions to mitigate their risk from potentially damaging shaking. Over the past few years, ShakeAlert, the EEW program focused on the west coast of the contiguous United States, has grown, alerting communities within California, Oregon, and Washington about earthquakes where damaging shaking is expected. ShakeAlert uses a set of algorithms including the point-source algorithm, earthquake point-source integrated code (EPIC), to determine the location, magnitude, and origin time of potential earthquakes. Although EPIC produces low-latency and low error solutions for many events originating within the seismic network on land, numerous recent small earthquakes rupturing offshore of northern California have EPIC location solutions with high error (>50 km compared to USGS locations). Because most events are occurring offshore, there is a limited number of stations that can trigger and contribute information in a timely manner for use in EEW. To better constrain location solutions in this region, we propose to include information about contemporary past seismicity into EPIC’s grid-search algorithm through a Bayesian framework. This prior information layer downweights high error locations where EPIC’s proposed event location coincides with an area of low prior seismicity in preference for locations with a similar level of data fit that also have higher past seismicity. This addition to EPIC lowers the mean location error offshore northern California from 58 to 14 km.

A new approach to processing and imaging multibeam water column echosounder data: Application to a complex methane seep on the southern Cascadia margin

Acoustic echosounding systems are increasingly used to image water-column backscatter in addition to mapping the seafloor. We have imaged an acoustic flare generated by methane bubbles emanating from a vent sourced at 1840 m water depth offshore northern California using a shipboard Kongsberg EM122. Data include five transits over the flare and approximately 11 h of continuous observation when the ship held station. Shipboard observations showed a strong flare splitting into multiple smaller, intermittent flares at a water depth of 800–1200 m and pronounced temporal variability. We introduce a new approach to processing the data in which we correct the backscatter data for ship motion and bin the data into voxels with dimensions of 20 m in X and Y and 40 m in Z for a transit over the flare and into vertical slices with dimensions of 15 m in X and Z and 4 min in time when the ship was stationary. The processed data indicate that the signal is dominated by bubbles emanating from a source region with a diameter of approximately 40 m located on the southern edge of what is likely a ring of sources with a diameter of approximately 600 m. When the ship was stationary, we were able to track an individual pulse rising at a rate of 8–10 m/min. Our results illustrate the limitations of monitoring temporal variation in gas flux using multibeam echosounders because of the trade-off between imaging the entire flare by averaging over tens of minutes to hours and observing a slice through the flare to capture short-lived pulses of gas expulsion. Nevertheless, because echosounders are widely available, they can continue to provide valuable data on the spatial and temporal distribution of gas emissions on continental margins that can be used to frame hypotheses and plan more comprehensive follow-up experiments.

Source parameters estimation from broadband regional seismograms for earthquakes in the Aegean region and the Gorda plate

Seismic moment tensors are estimated for earthquakes offshore Northern California and Greece using inversion of regionally recorded broadband seismograms. This study includes inversion results for the strongest events that occurred inside the Gorda plate and near the Mendocino triple junction from 1991 to 2005. The regional results are in good agreement with obtained teleseismic results. We finally applied the moment tensor inversion methodology and validation mainly to moderate sized earthquakes, with magnitude greater than M~4.0, in the Aegean area. The focal mechanisms of HI earthquakes that occurred during the time period between June 2003 and March 2007 were estimated using this procedure.

Sub-decadal turbidite frequency during the early Holocene: Eel Fan, offshore northern California

Remotely operated and autonomous underwater vehicle technologies were used to image and sample exceptional deep sea outcrops where an ~100-m-thick section of turbidite beds is exposed on the headwalls of two giant submarine scours on Eel submarine fan, offshore northern California (USA). These outcrops provide a rare opportunity to connect young deep-sea turbidites with their feeder system. 14 C measurements reveal that from 12.8 ka to 7.9 ka, one turbidite was being emplaced on average every 7 yr. This emplacement rate is two to three orders of magnitude higher than observed for turbidites elsewhere along the Pacific margin of North America. The turbidites contain abundant wood and shallow-dwelling foraminifera, demonstrating an efficient connection between the Eel River source and the Eel Fan sink. Tur bidite recurrence intervals diminish fivefold to ~36 yr from 7.9 ka onward, reflecting sea-level rise and re-routing of Eel River sediments.

TheMw6.5 offshore Northern California earthquake of 10 January 2010: Ordinary stress drop on a high-strength fault

The 10 January 2010 Mw 6.5 earthquake offshore Northern California is one of the first intraplate earthquakes in oceanic lithosphere to be well captured by a GPS network. It presents an opportunity to evaluate rupture mechanics on a high-strength fault. Static inversion of the coseismic displacements shows that the slip peaks at the same depth as the expected strength envelope, where the differential stresses can be as high as 600 MPa. Laboratory experiments on peridotite predict dramatic dynamic weakening at these conditions. The observed ordinary stress drop, 2–20 MPa, may indicate that the lithosphere is much weaker than strength envelope predicts or that the failure mechanisms seen in the laboratory are not occurring during the rupture. The GPS observations show very little postseismic signal indicating that if a shear zone exists beneath the coseismic rupture, it operates at significantly greater stress levels than the coseismic stress change.

Evidence for superelevation, channel incision, and formation of cyclic steps by turbidity currents in Eel Canyon, California

We performed a multibeam survey of Eel Canyon, offshore northern California. The survey revealed a significant bend in the canyon that appears to be due to the oblique compressional tectonics of the region. A series of steps within a linear depression, similar to 280 m above the canyon floor, extends from the canyon rim at this bend to the subduction zone and a distinct fan-like topographic rise. We hypothesize that the linear depression is a distributary channel and the steps are cyclic-step bedforms created by turbidity currents. Our interpretation indicates that turbidity currents are able to run up and overspill the 280-m-high canyon wall, resulting in a partial avulsion of the canyon and the construction of a fan lobe that is offset from the canyon mouth. Simple hydraulic calculations show that turbidity currents generated in the canyon head from failure of 2-3 m of material would be capable of partially overflowing the canyon at this bend, assuming steady-uniform flow, full conversion of the failed mass into a turbidity current, and a range of friction coefficients. These estimates are consistent with analyses of sediment cores collected in the head of Eel Canyon, which suggest that 2-3 m of material fails on decadal time scales. Our calculations show that the overflowing parts of the currents would have large shear velocities (>10 cm/s) and supercritical Froude numbers, consistent with erosion of the distributary channel and formation of cyclic steps by turbidity currents.

Controls of tectonics and sediment source locations on along-strike variations in transgressive deposits on the northern California margin

We identify two surfaces in the shallow subsurface on the Eel River margin offshore northern California, a lowstand erosion surface, likely formed during the last glacial maximum, and an overlying surface likely formed during the most recent transgression of the shoreline. The lowstand erosion surface, which extends from the inner shelf to near the shelfbreak and from the Eel River to Trinidad Head (∼80 km), truncates underlying strata on the shelf. Above the surface, inferred transgressive coastal and estuarine sedimentary units separate it from the transgressive surface on the shelf. Early in the transgression, Eel River sediment was likely both transported down the Eel Canyon and dispersed on the slope, allowing transgressive coastal sediment from the smaller Mad River to accumulate in a recognizable deposit on the shelf. The location of coastal Mad River sediment accumulation was controlled by the location of the paleo-Mad River. Throughout the remainder of the transgression, dispersed sediment from the Eel River accumulated an average of 20 m of onlapping shelf deposits. The distribution and thickness of these transgressive marine units was strongly modified by northwest–southeast trending folds. Thick sediment packages accumulated over structural lows in the lowstand surface. The thinnest sediment accumulations (0–10 m) were deposited over structural highs along faults and uplifting anticlines. The Eel margin, an active margin with steep, high sediment-load streams, has developed a thick transgressive systems tract. On this margin sediment accumulates as rapidly as the processes of uplift and downwarp locally create and destroy accommodation space. Sequence stratigraphic models of tectonically active margins should account for variations in accommodation space along margins as well as across them.

Effect of the northward-migrating Mendocino triple junction on the Eel River forearc basin, California: Structural evolution

Offshore northern California, the Gorda plate is subducting obliquely beneath North America; the resulting complicated tectonic setting forms the southern end of the Cascadia subduction zone. The southern Cascadia subduction zone and overlying Eel River forearc basin lie just north of the unstable Mendocino triple junction. The Neogene strata of the Eel River basin record structural deformation caused by the underthrusting of the Gorda plate as well as deformation generated by northward migration and encroachment of the Mendocino triple junction. Three distinct deformation regimes are present in the Eel River forearc basin. (1) Along the western margin of the forearc basin and within the foreslope of the accretionary prism, thrust faults and anticlines record Pliocene– Pleistocene shortening caused by subduction of the Gorda plate. (2) The southern part of the basin rotated counterclockwise in the late Pleistocene, resulting in modern transpressional deformation offshore Humboldt Bay. The rotation and deformation are caused by north-south convergence across the boundary between the Pacific plate and the southernmost part of the forearc basin at the triple junction. (3) The northeastern margin of the Eel River basin is deformed by high-angle faults with a component of strike-slip motion that may represent the incipient northward propagation of the Pacific–North American transform system north of the triple junction.

The high-frequency backscattering angular response of gassy sediments: model/data comparison from the Eel River Margin, California.

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.

Molecular and isotopic analysis of anaerobic methane-oxidizing communities in marine sediments

Convergent lines of molecular, carbon-isotopic, and phylogenetic evidence have previously indicated (Hinrichs, K.-U., Hayes, J.M., Sylva, S.P., Brewer, P.G., DeLong, E.F., 1999. Methane-consuming archaebacteria in marine sediments. Nature 398, 802–805.) that archaea are involved in the anaerobic oxidation of methane in sediments from the Eel River Basin, offshore northern California. Now, further studies of those same sediments and of sediments from a methane seep in the Santa Barbara Basin have confirmed and extended those results. Mass spectrometric and chromatographic analyses of an authentic standard of sn-2-hydroxyarchaeol (hydroxylated at C-3 in the sn-2 phytanyl moiety) have confirmed our previous, tentative identification of this compound but shown that the previously examined product was the mono-TMS, rather than di-TMS, derivative. Further analyses of 13C-depleted lipids, appreciably more abundant in samples from the Santa Barbara Basin, have shown that the archaeal lipids are accompanied by two sets of products that are only slightly less depleted in 13C. These are additional glycerol ethers and fatty acids. The alkyl substituents in the ethers (mostly monoethers, with some diethers) are non-isoprenoidal. The carbon-number distributions and isotopic compositions of the alkyl substituents and of the fatty acids are similar, suggesting strongly that they are produced by the same organisms. Their structures, n-alkyl and methyl-branched n-alkyl, require a bacterial rather than archaeal source. The non-isoprenoidal glycerol ethers are novel constituents in marine sediments but have been previously reported in thermophilic, sulfate- and nitrate-reducing organisms which lie near the base of the rRNA-based phylogenetic tree. Based on previous observations that the anaerobic oxidation of methane involves a net transfer of electrons from methane to sulfate, it appears likely that the non-archaeal, 13C-depleted lipids are products of one or more previously unknown sulfate-reducing bacteria which grow syntrophically with the methane-utilizing archaea. Their products account for 50% of the fatty acids in the sample from the Santa Barbara Basin. At all methane-seep sites examined, the preservation of aquatic products is apparently enhanced because the methane-oxidizing consortium utilizes much of the sulfate that would otherwise be available for remineralization of materials from the water column.

Evolution of the Gorda Escarpment, San Andreas Fault and Mendocino triple junction from multichannel seismic data collected across the northern Vizcaino block, offshore northern California

The Gorda Escarpment is a north facing scarp immediately south of the Mendocino transform fault (the Gorda/Juan de Fuca‐Pacific plate boundary) between 126°W and the Mendocino triple junction. It elevates the seafloor at the northern edge of the Vizcaino block, part of the Pacific plate, ∼1.5 km above the seafloor of the Gorda/Juan de Fuca plate to the north. Stratigraphy interpreted from multichannel seismic data across and close to the Gorda Escarpment suggests that the escarpment is a relatively recent pop‐up feature caused by north‐south compression across the plate boundary. Close to 126°W, the Vizcaino block acoustic basement shallows and is overlain by sediments that thin north toward the Gorda Escarpment. These sediments are tilted south and truncated at the seafloor. By contrast, in a localized region at the eastern end of the Gorda Escarpment, close to the Mendocino triple junction, the top of acoustic basement dips north and is overlain by a 2‐km‐thick wedge of pre‐11 Ma sedimentary rocks that thickens north, toward the Gorda Escarpment. This wedge of sediments is restricted to the northeast corner of the Vizcaino block. Unless the wedge of sediments was a preexisting feature on the Vizcaino block before it was transferred from the North American to the Pacific plate, the strong spatial correlation between the sedimentary wedge and the triple junction suggests the entire Vizcaino block, with the San Andreas at its eastern boundary, has been part of the Pacific plate since significantly before 11 Ma.

Crustal structure of the northwestern Vizcaino block and Gorda Escarpment, offshore northern California, and implications for postsubduction deformation of a paleoaccretionary margin

The Vizcaino block is an anomalously shallow region of the western U.S. continental margin located southwest of the Mendocino triple junction. It originated as part of the accretionary prism of the North America plate and was transferred to the Pacific plate in the Miocene as the Pacific‐North America plate boundary migrated ∼130 km eastward, forming the Gorda Escarpment at its northern boundary. We present hybrid crustal models for the northwestern part of the Vizcaino block derived from marine seismic and gravity data. The velocity and density structure of the northwestern Vizcaino block are compatible with paleoaccretionary complex material similar to San Simeon/Patton terrane overlying oceanic crust or a mafic layer. The most significant result of our modeling is an abrupt increase in Moho dip from ∼5° to ∼20–30° beneath the western edge of the Oconostota ridge along the northwestern margin of the Vizcaino block. This Moho dip is steeper than observed anywhere along the Cascadia subduction zone, indicating postsubduction deformation. We suggest that the paleotrench was deformed by compression, which reactivated preexisting thrust faults in the upper crust and thickened the crust within this apparent weak zone. At least part of the deformation predates late Pliocene Pacific‐North America plate convergence and may result mainly from north‐south compression between the Pacific‐Juan de Fuca plates across the Mendocino transform fault. North‐south compression continues today and may dynamically support the uplifted northern margin of the Vizcaino block, although the primary locus of deformation shifted to the relatively weak Gorda plate sometime prior to 3 Ma.

Water segregation in the course of gas hydrate formation and accumulation in submarine gas-seepage fields

Gas hydrate accumulations associated with submarine gas-seepage fields have been detected in the Okhotsk Sea and the Gulf of Mexico. It is possible that gas hydrates known from the continental margin offshore northern California, Oregon and Nigeria are also connected with gas-seepage fields. Gas-hydrated sediments in the accumulations (1) occur at very low subbottom depths, sometimes directly at the sea bottom; (2) have specific structures, resembling cryogenic structures, caused by hydrates (subhorizontal lenticular-bedded and porphyraceous structures); (3) the sediments can be distinguished from overlying non-hydrated sediments by a higher water content; (4) an association of gas hydrates and calcium carbonate concretions occurs. The behaviour of gas and water during gas hydrate formation under bottom gas-seepage conditions is considered. Gas hydrate formation occurs at the upper part of the sedimentary cover, within the limits of the diffusion halo around the up-going flow of gas which saturates the pore water. Water migrating to the gas hydrate accumulation sites from adjacent sediments is involved in hydrate formation as well. The mechanism of gas transportation to the gas hydrate accumulation sites is diffusion. The way of water transport during hydrate accumulation is similar to ice segregation during permafrost formation. The water segregation is responsible for the formation of specific structure caused by hydrates and for higher than usual water content of hydrate-bearing sediments in submarine gas-seepage fields.

Observations of gas hydrates in marine sediments, offshore northern California

Biogenic gas hydrates were recovered in shallow cores (< 6 m deep) from the Eel River basin in offshore northern California between 40°38′ and 40°56′N. The gas hydrates contained primarily methane ( δ 13C = −57.6 to −69.1‰) and occurred as dispersed crystals, small (2–20 mm) nodules, and layered bands within the sediment. These hydrates, recovered in sediment at water depths between 510 and 642 m, coincide nearly, but not exactly, with areas showing bottom-simulating reflectors (BSRs) on seismic-reflection records. This study confirms indirect geophysical and geologic observations that gas hydrates are present north of the Mendocino Fracture Zone in sediment of the Eel River basin but probably are absent to the south in the Point Arena basin. This discovery extends the confirmed sites of gas hydrates in the eastern Pacific region beyond the Peruvian and Central American margins to the northern California margin.

Hydrocarbon geochemistry of hydrothermally generated petroleum from Escanaba trough, offshore Californi U.S.A.

In 1986, three samples of sulfide-rich sediments, impregnated with hydrothermally derived, asphaltic petroleum, were recovered in a dredge and by submersible from Escanaba Trough, the sediment-covered, southern end of the Gorda Ridge spreading axis, offshore northern California. The molecular distributions of hydrocarbons in the two pyrrhotite-rich samples recovered by submersible are similar and compare well the hydrocarbon composition of the first pyrrhotite-rich samples containing petroleum discovered at a 1985 dredge site about 30 km to the south of the site of the submersible dive. In contrast, the 1986 dredge sample, composed of a polymetallic assemblage of sulfides, containes petroleum in which the distribution of hydrocarbons indicates a slightly higher of maturity relative to the other samples. The observation that petroleum of variable composition occurs with metallic sulfides at two and probably more distinct site indicates that petroleum generation may be a common process in the hydrothermally active Escanaba Trough.

Hydrothermally Derived Petroleum: Examples from Guaymas Basin, Gulf of California, and Escanaba Trough, Northeast Pacific Ocean (1)

Petroleum is associated with sulfide-rich sediment at two active oceanic spreading axes in the northeast Pacific Ocean. In the Guaymas Basin, a spreading axis in the Gulf of California, petroleum having a wide range of compositions forms by hydrothermal alteration of organic matter in Quaternary sediment composed mainly of marine diatomaceous ooze and muddy turbidites. In Escanaba Trough, at the southern end of the Gorda Ridge spreading axis offshore northern California, petroleum is formed by hydrothermal processes acting on mainly terrigenous organic material in Quaternary turbiditic river-derived sediment. Comparisons of the distributions of hydrocarbons--n-alkanes, isoprenoids, terpanes, steranes, and aromatics--show that chemical differences among four petroleum samp es are such that two samples from Guaymas Basin can be distinguished from two samples from Escanaba Trough. Distinguishing characteristics resulting from differences in sources include n-alkane distributions and certain sterane ratios; distinguishing characteristics resulting from differences in thermal histories of the organic matter include hopane and sterane epimer ratios and various distributions of polycyclic aromatic hydrocarbons. These oils differ from conventionally derived petroleum in that they are admixtures of products generated over a wide range of thermal regimes, and their generation, expulsion, and migration occurred simultaneously over an instantaneous geological time period. The potential economic significance of hydrothermally derived petroleum is uncertain, but the act that petroleum can form at active oceanic spreading axes adds a new facet to understanding the processes of petroleum generation, expulsion, and migration.

Organic geothermometry of petroleum from Escanaba Trough, offshore northern California

We have measured the extent of hopane and sterane isomerization and of monoaromatic-steroid-hydrocarbon aromatization in a sample of hydrothermally derived petroleum from the Escanaba Trough, a sediment-covered, volcanically active ridge axis. The results, along with kinetic parameters, predict the possible time-temperature history of the petrolum-forming process for this sample. The extent of these reactions is consistent with petroleum formation by intense heating (about 300–350°C) if the time period of this heating was as short as about 100 yr. Such a time scale is reasonable for hydrothermal-discharge events associated with ridge-crest volcanism.

Exploration Potential of Offshore Northern California Basins: ABSTRACT

Exploration Potential of Offshore Northern California Basins: ABSTRACT

Thermogenic Hydrocarbon Gases in Unconsolidated Seafloor Deposits, Northern California Continental Margin: ABSTRACT

Modern unconsolidated sediment overlying a diapir-like structure in the Eel River Basin, offshore northern California, contains an unusual mixture of gas and gasoline-range hydrocarbons. Although concentrations of methane in a 2-m gravity core at a single sampling locality (water depth of 500 m) are in the same range as found in modern anoxic sediments, the concentrations of higher molecular weight hydrocarbon gases are anomalously high relative to background. For example, ethane, propane, isobutane, and n-butane are about 18, 5, 10, and 2 times, respectively, more abundant than the highest concentrations of these same hydrocarbons observed elsewhere in the same region. Associated with these high gas concentrations are anomalous contents of gasoline-range hydrocarbons. Mu h of the methane and part of the ethane appear to be derived from modern microbial processes. Most of the ethane and the higher hydrocarbons, however, probably have a thermogenic origin deep within the sediment of the Eel River Basin. These hydrocarbons may reach the surface through fractures in the diapir-like structure and seep into overlying unconsolidated sediments that are ponded locally within structural and bathymetric depressions on the surface of this structure. Thus, the hydrocarbons in these near-surface sediments may be derived from petroleum that has formed and accumulated in Tertiary sedimentary rocks of the Eel River Basin. End_of_Article - Last_Page 736------------

Oil and Gas Exploration Offshore Central and Northern California: ABSTRACT

Oil and Gas Exploration Offshore Central and Northern California: ABSTRACT