Deep-water turbidites as Holocene earthquake proxies: the Cascadia subduction zone and Northern San Andreas Fault systems

New stratigraphic evidence from the Cascadia margin demonstrates that 13 earthquakes ruptured the margin from Vancouver Island to at least the California border following the catastrophic eruption of Mount Mazama. These 13 events have occurred with an average repeat time of ?? 600 years since the first post-Mazama event ?? 7500 years ago. The youngest event ?? 300 years ago probably coincides with widespread evidence of coastal subsidence and tsunami inundation in buried marshes along the Cascadia coast. We can extend the Holocene record to at least 9850 years, during which 18 events correlate along the same region. The pattern of repeat times is consistent with the pattern observed at most (but not all) localities onshore, strengthening the contention that both were produced by plate-wide earthquakes. We also observe that the sequence of Holocene events in Cascadia may contain a repeating pattern, a tantalizing look at what may be the long-term behavior of a major fault system. Over the last ?? 7500 years, the pattern appears to have repeated at least three times, with the most recent A.D. 1700 event being the third of three events following a long interval of 845 years between events T4 and T5. This long interval is one that is also recognized in many of the coastal records, and may serve as an anchor point between the offshore and onshore records. Similar stratigraphic records are found in two piston cores and one box core from Noyo Channel, adjacent to the Northern San Andreas Fault, which show a cyclic record of turbidite beds, with thirty- one turbidite beds above a Holocene/.Pleistocene faunal «datum». Thus far, we have determined ages for 20 events including the uppermost 5 events from these cores. The uppermost event returns a «modern» age, which we interpret is likely the 1906 San Andreas earthquake. The penultimate event returns an intercept age of A.D. 1664 (2 ?? range 1505- 1822). The third event and fourth event are lumped together, as there is no hemipelagic sediment between them. The age of this event is A.D. 1524 (1445-1664), though we are not certain whether this event represents one event or two. The fifth event age is A.D. 1204 (1057-1319), and the sixth event age is A.D. 1049 (981-1188). These results are in relatively good agreement with the onshore work to date, which indicates an age for the penultimate event in the mid-1600 s, the most likely age for the third event of ?? 1500-1600, and a fourth event ?? 1300. We presently do not have the spatial sampling needed to test for synchroneity of events along the Northern San Andreas, and thus cannot determine with confidence that the observed turbidite record is earthquake generated. However, the good agreement in number of events between the onshore and offshore records suggests that, as in Cascadia, turbidite triggers other than earthquakes appear not to have added significantly to the turbidite record along the northernmost San Andreas margin during the last ?? 2000 years

Evolution of the Corvallis fault and implications for the Oregon coast range

The Corvallis fault is a 50 km long northeast-trending structure, part of which defines the boundary between the central Willamette Valley and the east-central Coast Range of Oregon. Previously the fault had been mapped as either a high-angle reverse or normal fault, with the east block down. New gravity data suggest that the main structure is a low-angle thrust, with early Eocene Siletz River Volcanics thrust southeastward over middle to late Eocene Tyee and Spencer sandstones. The thrust geometry is similar to that of the Laramide thrusts of the Rocky Mountain foreland. Gravity modeling produces a best-fit geometry with the thrust-plane dipping approximately 100 northwest. The surface geology is consistent with a fault-propagation fold geometry. Consistent dips averaging 20° in the hanging wall block suggest a ramp dipping at the same angle, somewhat steeper than the dip indicated by gravity modeling. Vertical separation is about 6.7 km, and if the ramp dip is the same as bedding dips in the Siletz River Volcanics, horizontal displacement is 13-15 km, assuming no other thrust faults repeat the Siletz River stratigraphy. The Corvallis thrust was active during the late Eocene, and was the eastern boundary of a tectonic highland in the Eocene forearc. The highland was a local source of material for the upper Yamhill and lower Spencer Formations, deposited in a partially restricted shallow shelf to neritic setting. Other late Eocene tectonic and volcanic highlands formed an archipelago in the position of the present Coast Range. In the middle Oligocene, the fault was intruded by gabbroic dikes during an intrusive episode that emplaced massive sheets of gabbro throughout the central Coast Range. A younger normal fault paralleling the original Corvallis thrust is interpreted to be the result of gravitational collapse of the tip of the thrust sheet, and has truncated the older structure. Numerous left offsets of the main fault trace along northwest-trending left-lateral faults are interpreted to be the result of clockwise rotations of western Oregon documented by paleomagnetics. Later reactivation of the Corvallis fault as a left-lateral strike-slip fault, indicated by horizontal slickenlines, is consistent with the present north-south compression in Oregon. The Corvallis fault may have continued minor intermittent activity into the late Quaternary

Linking Habitat and Benthic Invertebrate Species Distributions in Areas of Potential Renewable Energy Development

While the coastal waters of western North America hold great promise for wind and wave energy development, many concerns have been raised about the potential environmental impacts of the installation of these devices and their complex mooring systems. Here I focus on characterizing benthic habitats and biological communities in offshore sedimentary and reef habitats where wind and wave energy facilities could be located. While little is known about species-habitat relationships and community processes in the depths and substrate types targeted for offshore renewable energy installation, an understanding of the natural dynamics of these systems is of utmost importance if we hope to forecast changes that might be brought about by wind and wave development. Since May 2010 we have conducted surveys of benthic habitats from northern California to Washington using a variety of techniques, providing baseline data on habitats and species potentially affected by wind and wave development, identifying species-habitat relationships, and quantifying spatial and temporal trends in species abundances and distributions. The first step in identifying and evaluating benthic communities is sonar mapping to determine depth and substrate types. In summer 2010 and 2011 six new offshore sites were mapped by the Seafloor Mapping and Plate Tectonics Lab at OSU using high-resolution multi-beam sonar and acoustic backscatter. In addition to the backscatter, Shipek grabs were taken in soft-bottom areas to collect sediment samples, which were run through a laser particle size analyzer (LDPSA) to determine actual grain size. Mapping began at the federal jurisdiction line and extended 9 – 12 miles offshore. Oregon and California have undertaken extensive mapping of state waters, so many areas have been mapped inshore of these sites as well. In summer 2011 and 2012, we visited 8 sites (6 newly mapped sites, one previously mapped, and one unmapped site) to collect a total of 153 cores using a 0.1 m2 box-corer. A sub-sample of sediment was collected from the corer and analyzed using the LDPSA; the rest was sieved through 1 mm mesh and all infaunal organisms were counted and identified. At each box core sampling station, CTD casts were conducted to obtain physical data describing the overlying water column for further habitat characterization. Unique infaunal invertebrate assemblages were found in sedimentary habitats at each of the Pacific Northwest shelf sites. Thus for renewable energy siting, it does not appear that baseline surveys conducted at one site can necessarily serve as a proxy for distant sites. However, some general trends were detected. Significantly different invertebrate assemblages were found in different depth ranges with a break at approximately 80 to 90 m depth; deeper sites exhibited greater diversity. Shallower sites had greater spatial heterogeneity in infaunal invertebrate assemblages than deeper sites; thus as monitoring protocols are developed we recommend that shallower sites be sampled more extensively in order to adequately characterize those communities. Molluscs seemed to be the most responsive to substrate type, with different assemblages found in pure sand, slightly muddy sand, and mostly silt/clay. In addition to sampling of sedimentary habitat, we conducted limited surveys of offshore reef habitats. Although it is unlikely that devices would be installed in these areas, reefs may be crossed by electrical cables, and changes in sediment transport due to ocean energy extraction or alterations of flow around large device arrays could lead to community impacts. The aim of this study was to describe baseline relationships between macroinvertebrate communities and habitat features against which to measure potential future impacts and to develop tools to predict community compositions of unsampled areas in the region based on substrate features. To date we have analyzed submersible dive video from three sites conducted in the mid-1990s. In the summers of 2011 and 2012, we visited these previously surveyed sites with an ROV. Analysis of submersible and ROV surveys indicated that two major substratum groups held different macroinvertebrate assemblages: moderate to high-relief rocky habitats and low-relief fine sediment habitats. The majority of macroinvertebrate taxa were associated with high-relief rocks; these taxa were further differentiated between flat and ridge rock habitats. Low-relief fine sediment habitat was most often associated with motile invertebrates. Within this habitat it appeared that fine-sediment substrata mixed with mud, boulders, or gravel each yield unique macro-invertebrate associations versus those found on uniformly mud or sand substrata. Latitude also was correlated with variation in macroinvertebrate assemblages. A major challenge will be detecting effects of wind and wave energy installations above the inherent natural variability in these systems. Decadal scale shifts in the California Current affect this ecosystem, with warm regimes and associated declines in planktonic production resulting in degradation of benthic community. On shorter timescales El Niño events can cause major, short-term disturbances. Off the Oregon coast, summer hypoxia events can have dramatic effects on benthic communities, and ocean acidification is an increasing concern. Thus, evaluation of this ecosystem must be made in the context of seasonal and climatic trends. Prior to installation of device arrays, baseline sampling is usually required as part of the permitting process. However, one-time sampling will not capture the variability of the system in a given area, and developers and regulators typically are not able to make the investment (in time or money) to repeatedly survey an area before development. Funding agencies rarely support long-term monitoring studies. Thus, finding support for repeated field sampling across time and space is especially challenging. The biggest issue facing wind and wave energy developers in the environmental arena is the high level of uncertainty regarding environmental effects. Without a substantial understanding of the natural dynamics of a system, it will be difficult to reduce that uncertainty.KEYWORDS: Invertebrates, Wind energy, Receptors, Marine renewable energy, Wave energ

Interferometry of ϵ\epsilon Aurigae: Characterization of the asymmetric eclipsing disk

We report on a total of 106 nights of optical interferometric observations of the $\epsilon$ Aurigae system taken during the last 14 years by four beam combiners at three different interferometric facilities. This long sequence of data provides an ideal assessment of the system prior to, during, and after the recent 2009-2011 eclipse. We have reconstructed model-independent images from the 10 in-eclipse epochs which show that a disk-like object is indeed responsible for the eclipse. Using new 3D, time-dependent modeling software, we derive the properties of the F-star (diameter, limb darkening), determine previously unknown orbital elements ($\Omega$, $i$), and access the global structures of the optically thick portion of the eclipsing disk using both geometric models and approximations of astrophysically relevant density distributions. These models may be useful in future hydrodynamical modeling of the system. Lastly, we address several outstanding research questions including mid-eclipse brightening, possible shrinking of the F-type primary, and any warps or sub-features within the disk.Comment: 105 pages, 57 figures. This is an author-created, un-copyedited version of an article accepted for publication in Astrophysical Journal Supplement Series. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from i

A left-lateral strike-slip fault seaward of the Oregon convergent margin

We have mapped a recently active left-lateral strike-slip fault (the Wecoma fault) on the floor of Cascadia Basin west of the Oregon convergent margin, using SeaMARC I sidescan sonar, Seabeam bathymetry and multichannel seismic and magnetic data. The fault intersects the base of the continental slope at 45°10’N and extends northwest (293°) for at least 18.5 km. The fault’s western terminus was not identified and the eastern end of the fault splays apart and disrupts the lower continental slope. The fault extends to the base of the 3.5-km-thick sedimentary section and overlies a basement discontinuity that may be related to movement along the Wecoma fault. Prominent seafloor features crosscut by the fault individually display between 120 and 2500 m of left-lateral separation, allowing the general history of fault motion to be evaluated. The fault’s average slip rate since 10-24 ka is inferred to be 5-12 mm/yr, based on the age of an offset submarine channel. Surficial structural relationships, in conjunction with the maximum inferred slip rate, indicate that fault movement initiated at least 210 ka and that the fault has been active during the Holocene

A seismic reflection profile across the Cascadia subduction zone offshore central Oregon: New constraints on methane distribution and crystal structure

In 1989, we conducted an onshore/offshore seismic experiment to image the crustal structure of the Cascadia forearc. In this paper, we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort. This profile reveals several features of the forearc that were not apparent in an earlier, coincident relection profile. One of the most important of these features is a very strong bottom simulating reflection (BSR) beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m, where it appears to crop out on the seafloor. The pressure and temperature conditions at the BSR derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate/seawater system over a broad range of temperatures and pressures, assuming hydrostatic pressure and temperature gradiant measured near the base of the continental slope during Ocean Drilling Program (ODP) leg 146. Interval velocities and reflection coefficients derived from the data indicate that the BSR represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas, consistent with results obtained during leg 146. Although the regional distribution of the anomalously strong BSR beneath the midslope is poorly known, we speculate that it may be related to apparent slop instability. The data also provide constraints on the thickness and geometry of the Siletz terrane, which is the basement beneath the shelf and acts as the subduction zone backstop. A deep reflection, which might mistakenly be interpreted to be Moho if coincident large-aperture data were not available, is interpreted to be the base of the Siletz terrane. A "recently" active strike-slip (?) fault zone that overlies the seaward edge of the Siletz terrane suggests that the Siletz terrane controls the location of decoupling of the subduction complex from the rest of the forearc.Copyrighted by American Geophysical Union

Magnitude Limits of Subduction Zone Earthquakes

Maximum earthquake magnitude (m[subscript x]) is a critical parameter in seismic hazard and risk analysis. However, some recent large earthquakes have shown that most of the existing methods for estimating m[subscript x] are inadequate. Moreover, m[subscript x] itself is ill-defined because its meaning largely depends on the context, and it usually cannot be inferred using existing data without associating it with a time interval. In this study, we use probable maximum earthquake magnitude within a time period of interest, m[subscript p](T), to replace m[subscript x]. The term m[subscript p](T) contains not only the information of magnitude limit but also the occurrence rate of the extreme events. We estimate m[subscript p](T) for circum-Pacific subduction zones using tapered Gutenberg–Richter (TGR) distributions. The estimation of the two TGR parameters, β-value and corner magnitude (m[subscript c]), is performed using the maximum-likelihood method with the constraint from tectonic moment rate. To populate the TGR, the rates of smaller earthquakes are needed. We apply the Whole Earth model, a high-resolution global estimate of the rate of m ≥ 5 earthquakes, to estimate these rates. The uncertainties of m[subscript p](T) are calculated using Monte-Carlo simulation. Our results show that most of the circum-Pacific subduction zones can generate m ≥ 8.5 earthquakes over a 250-year interval, m ≥ 8.8 earthquakes over a 500-year interval, and m ≥ 9.0 earthquakes over a 10,000-year interval. For the Cascadia subduction zone, we include the 10,000-year paleoseismic record based on turbidite studies to supplement the limited instrumental earthquake data. Our results show that over a 500-year period, m ≥ 8.8 earthquakes are expected in this zone; over a 1000-year period, m ≥ 9.0 earthquakes are expected; and over a 10,000-year period, m ≥ 9.3 earthquakes are expected

Stellar Diameters and Temperatures. I. Main-Sequence A, F, and G Stars

We have executed a survey of nearby, main-sequence A-, F-, and G-type stars with the CHARA Array, successfully measuring the angular diameters of forty-four stars with an average precision of ~1.5%. We present new measures of the bolometric flux, which in turn leads to an empirical determination of the effective temperature for the stars observed. In addition, these CHARA-determined temperatures, radii, and luminosities are fit to Yonsei-Yale model isochrones to constrain the masses and ages of the stars. These results are compared to indirect estimates of these quantities obtained by collecting photometry of the stars and applying them to model atmospheres and evolutionary isochrones. We find that for most cases, the models overestimate the effective temperature by ~1.5%-4% when compared to our directly measured values. The overestimated temperatures and underestimated radii in these works appear to cause an additional offset in the star's surface gravity measurements, which consequently yield higher masses and younger ages, in particular for stars with masses greater than ~1.3 M_☉. Additionally, we compare our measurements to a large sample of eclipsing binary stars, and excellent agreement is seen within both data sets. Finally, we present temperature relations with respect to (B – V) and (V – K) colors as well as spectral type, showing that calibration of effective temperatures with errors ~1% is now possible from interferometric angular diameters of stars

Oblique strike-slip faulting of the central Cascadia submarine forearc

At least nine WNW trending left-lateral strike-slip faults have been mapped on the Oregon-Washington continental margin using sidescan sonar, seismic reflection, and bathymetric data, augmented by submersible observations. The faults range in length from 33 to 115 km and cross much of the continental slop. Five faults offset both the Juan de Fuca plate and North American plates and cross the plate boundary with little or no offset by the frontal thrust. Left-lateral separation of channels, folds, and Holocene sediments indicate active slip during the Holocene and late Pleistocene. Offset of surficial features ranges from 120 to 900 m, and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2.2 to 5.5 km of total slip. Near their western tips, fault ages range from 300 ka to 650 ka, yielding late Pleistocene-Holocene slip rates of 5.5 ± 2 to 8.5 ± 2 mm/yr. The geometry and slip direction of these faults implies clockwise rotation of fault-bounded blocks about vertical axes within the Cascadia forearc. Structural relationships indicate that some of the faults probably originate in the Juan de Fuca plate and propagate into the overlying forearc. The basement-involved faults may originate as shears antithetic to a dextral shear couple within the slab, as plate-coupling forces are probably insufficient to rupture the oceanic lithosphere. The set of sinistral faults is consistent with a model of regional deformation of the submarine forearc (defined to include the deforming slab) by right simple shear driven by oblique subduction of the Juan de Fuca plate.Copyrighted by American Geophysical Union