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USGS2020

Diurnal timing of nonmigratory movement by birds: The importance of foraging spatial scales

Timing of activity can reveal an organism's efforts to optimize foraging either by minimizing energy loss through passive movement or by maximizing energetic gain through foraging. Here, we assess whether signals of either of these strategies are detectable in the timing of activity of daily, local movements by birds. We compare the similarities of timing of movement activity among species using six temporal variables: start of activity relative to sunrise, end of activity relative to sunset, relative speed at midday, number of movement bouts, bout duration, and proportion of active daytime hours. We test for the influence of flight mode and foraging habitat on the timing of movement activity across avian guilds. We used 64570 days of GPS movement data collected between 2002 and 2019 for local (non‐migratory) movements of 991 birds from 49 species, representing 14 orders. Dissimilarity among daily activity patterns was best explained by flight mode. Terrestrial soaring birds began activity later and stopped activity earlier than pelagic soaring or flapping birds. Broad‐scale foraging habitat explained less of the clustering patterns because of divergent timing of active periods of pelagic surface and diving foragers. Among pelagic birds, surface foragers were active throughout the day while diving foragers matched their active hours more closely to daylight hours. Pelagic surface foragers also had the greatest daily foraging distances, which was consistent with their daytime activity patterns. This study demonstrates that flight mode and foraging habitat influence temporal patterns of daily movement activity of birds.

Journal of Avian Biology↗
USGS2020

Evaluation of a roughness length parametrization accounting for wind–wave alignment in a coupled atmosphere–wave model

The importance of wind energy as an alternative energy source has increased over the latest years with more focus on offshore winds. A good estimation of the offshore winds is thus of major importance for this industry. Up to now the effect of the wind–wave (mis)alignment has not yet been taken into account in coupled atmosphere–wave models to study the vertical wind profile and power production estimations of offshore wind farms. In this study the roughness length parametrization of Drennan et al . in 2003, and its extension addressing the wind–wave (mis)alignment proposed by Porchetta et al . in 2019, are investigated in the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) model. This study shows that the yearly mean wind estimation at hub height (100 m) is improved by the roughness length parametrization of Porchetta et al . compared to Drennan. This is mainly due to the increased roughness of the former parametrization compare to the latter, even in aligned wind–wave conditions. This difference in roughness is caused by the dataset used to obtain the constants, deep‐water conditions versus mixed offshore conditions. Moreover, the roughness length parametrization of Porchetta et al . performs better in two of three alignment categories. Furthermore, similar model performances are obtained if we exclude the wind directions from the wind shadow zone of the measurement mast or the wind directions from the recently built Alpha Ventus wind farm, which is in close vicinity of the measurement mast. Investigating different wind conditions shows that the new roughness length parametrization of Porchetta et al . performs best for both offshore and onshore winds. Additionally, we show that the coupled model estimations of the vertical wind are only slightly affected by significant wave height estimations. Similar model performances for different accuracies of significant wave height estimations are presented. One exception is the perpendicular alignment category where the new roughness length of Porchetta et al . outperforms the roughness length of Drennan when investigating the wind estimations related to significant wave heights with a higher accuracy. The roughness length parametrization of Porchetta et al . reduced the power production overestimation of the coupled model from 5.7 to 2.8%. We also show that the standalone atmospheric model including the roughness length of Charnock in 1955 has a degraded performance compared to the coupled model including the roughness length parametrization of Porchetta et al . for yearly average wind profiles.

Quarterly Journal of the Royal Meteorological Soci↗
USGS2012

Heat flow and energetics of the San Andreas fault zone

Approximately 100 heat flow measurements in the San Andreas fault zone indicate (1) there is no evidence for local factional heating of the main fault trace at any latitude over a 1000-km length from Cape Mendocino to San Bernardino, (2) average heat flow is high (∼2 HFU, ∼80 mW m −2 ) throughout the 550-km segment of the Coast Ranges that encloses the San Andreas fault zone in central California; this broad anomaly falls off rapidly toward the Great Valley to the east, and over a 200-km distance toward the Mendocino Triple Junction to the northwest. As others have pointed out, a local conductive heat flow anomaly would be detectable unless the frictional resistance allocated to heat production on the main trace were ≲100 bars. Frictional work allocated to surface energy of new fractures is probably unimportant, and hydrologic convection is not likely to invalidate the conduction assumption, since the heat discharge by thermal springs near the fault is negligible. Explanations for the low dynamic friction fall into two intergradational classes: those in which the fault is weak all of the time and those in which it is weak only during earthquakes (possibly just large ones). The first class includes faults containing anomalously weak gouge materials and faults containing materials with normal frictional properties under near-lithostatic steady state fluid pressures. In the second class, weakening is caused by the event (for example, a thermally induced increase in fluid pressure, dehydration of clay minerals, or acoustic fluidization). In this class, unlike the first, the average strength and ambient tectonic shear stress may be large, ∼1 kbar, but the stress allocated to elastic radiation (the apparent stress) must be of similar magnitude, an apparent contradiction with seismic estimates. Unless seismic radiation is underestimated for large earthquakes, it is difficult to justify average tectonic stresses on the main trace of the San Andreas fault in excess of ∼200 bars. The development of the broad Coast Range heat flow anomaly southward from Cape Mendocino suggests that heat flow increases by a factor of 2 within 4 m.y. after the passage of the Mendocino Triple Junction. This passage leaves the San Andreas transform fault zone in its wake; the depth of the anomalous sources cannot be much greater than the depth of the seismogenic layer. Some of the anomalous heat may be supplied by conduction from the warmer mantle that must occur south of the Mendocino transform (where there is no subducting slab), and some might be supplied by shear heating in the fault zone. With no contribution from shear heating, extreme mantle upwelling would be required, and asthenosphere conditions should exist today at depths of only ∼20 km in the northernmost Coast Ranges. If there is an appreciable contribution from shear heating, the heat flow constraint implies that the seismogenic layer is partially decoupled at its base and that the basal traction is in the sense that resists right lateral motion on the fault(s). As a result of these basal tractions, the average shearing stress in the seismogenic layer would increase with distance from the main fault, and the seismogenic layer would offer substantial resistance to plate motion even though resistance on the main fault might be negligible. These speculative models have testable consequences.

Journal of Geophysical Research Solid Earth↗
USGS1991

High-energy carbonate-sand accumulation, the Quicksands, southwest Florida Keys

High-resolution seismic-reflection profiles of the Quicksands, located along a broad ridge on the platform shelf west of Key West, Florida, indicate a significant deposit of non-oolitic carbonate sand occurs in a belt 47 km long by 28 km wide. The surface of the belt is ornamented by large (5 m), migrating tidal bars, oriented in a north-south direction, on which sand waves, oriented in an east-west direction, are superimposed. Some of the sand waves are awash at low tide. The sand waves are formed by strong reversing tidal currents flowing between the Gulf of Mexico and the Straits of Florida. The waves migrate directly over Pleistocene bedrock to the east, but the deposit thickens to the west and sand waves there overlie non-oolitic Holocene accumulations as thick as 12 m. Westward-dipping accretionary bedding indicates that net migration of the sands is to the west, despite north-south movement of tidal currents. The westward edge of the accumulation has accreted over deeper, muddier deposits. Although tidal currents and resultant bedforms appear identical to those of active ooid deposits in the Bahamas and elsewhere, no oolitically coated grains were found in this study. Thin-section analyses show the principal component (average 48%) of the sands is fragmented plates of species of the green alga Halimeda , followed by particulate coral (average 17%), which increases off the flanks of the main sand body. Short vibracores confirm the presence of cross-bedding.

Florida↗
USGS2011

Acetate availability and its influence on sustainable bioremediation of Uranium-contaminated groundwater

Field biostimulation experiments at the U.S. Department of Energy's Integrated Field Research Challenge (IFRC) site in Rifle, Colorado, have demonstrated that uranium concentrations in groundwater can be decreased to levels below the U.S. Environmental Protection Agency's (EPA) drinking water standard (0.126 μM). During successive summer experiments – referred to as “Winchester” (2007) and “Big Rusty” (2008) - acetate was added to the aquifer to stimulate the activity of indigenous dissimilatory metal-reducing bacteria capable of reductively immobilizing uranium. The two experiments differed in the length of injection (31 vs. 110 days), the maximum concentration of acetate (5 vs. 30 mM), and the extent to which iron reduction (“Winchester”) or sulfate reduction (“Big Rusty”) was the predominant metabolic process. In both cases, rapid removal of U(VI) from groundwater occurred at calcium concentrations (6 mM) and carbonate alkalinities (8 meq/L) where Ca-UO 2 -CO 3 ternary complexes constitute >90% of uranyl species in groundwater. Complete consumption of acetate and increased alkalinity (>30 meq/L) accompanying the onset of sulfate reduction corresponded to temporary increases in U(VI); however, by increasing acetate concentrations in excess of available sulfate (10 mM), low U(VI) concentrations (0.1–0.05 μM) were achieved for extended periods of time (>140 days). Uniform delivery of acetate during “Big Rusty” was impeded due to decreases in injection well permeability, likely resulting from biomass accumulation and carbonate and sulfide mineral precipitation. Such decreases were not observed during the short-duration “Winchester” experiment. Terminal restriction fragment length polymorphism (TRFLP) analysis of 16S rRNA genes demonstrated that Geobacter sp. and Geobacter -like strains dominated the groundwater community profile during iron reduction, with 13 C stable isotope probing (SIP) results confirming these strains were actively utilizing acetate to replicate their genome during the period of optimal U(VI) removal. Gene transcript levels during “Big Rusty” were quantified for Geobacter -specific citrate synthase ( gltA ), with ongoing transcription during sulfate reduction indicating that members of the Geobacteraceae were still active and likely contributing to U(VI) removal. The persistence of reducible Fe(III) in sediments recovered from an area of prolonged (110-day) sulfate reduction is consistent with this conclusion. These results indicate that acetate availability and its ability to sustain the activity of iron- and uranyl-respiring Geobacter strains during sulfate reduction exerts a primary control on optimized U(VI) removal from groundwater at the Rifle IFRC site over extended time scales (>50 days).

Geomicrobiology Journal↗
USGS2003

The saturated zone at Yucca Mountain: An overview of the characterization and assessment of the saturated zone as a barrier to potential radionuclide migration

The US Department of Energy is pursuing Yucca Mountain, Nevada, for the development of a geologic repository for the disposal of spent nuclear fuel and high-level radioactive waste, if the repository is able to meet applicable radiation protection standards established by the US Nuclear Regulatory Commission and the US Environmental Protection Agency (EPA). Effective performance of such a repository would rely on a number of natural and engineered barriers to isolate radioactive waste from the accessible environment. Groundwater beneath Yucca Mountain is the primary medium through which most radionuclides might move away from the potential repository. The saturated zone (SZ) system is expected to act as a natural barrier to this possible movement of radionuclides both by delaying their transport and by reducing their concentration before they reach the accessible environment. Information obtained from Yucca Mountain Site Characterization Project activities is used to estimate groundwater flow rates through the site-scale SZ flow and transport model area and to constrain general conceptual models of groundwater flow in the site-scale area. The site-scale conceptual model is a synthesis of what is known about flow and transport processes at the scale required for total system performance assessment of the site. This knowledge builds on and is consistent with knowledge that has accumulated at the regional scale but is more detailed because more data are available at the site-scale level. The mathematical basis of the site-scale model and the associated numerical approaches are designed to assist in quantifying the uncertainty in the permeability of rocks in the geologic framework model and to represent accurately the flow and transport processes included in the site-scale conceptual model. Confidence in the results of the mathematical model was obtained by comparing calculated to observed hydraulic heads, estimated to measured permeabilities, and lateral flow rates calculated by the site-scale model to those calculated by the regional-scale flow model. In addition, it was confirmed that the flow paths leaving the region of the potential repository are consistent with those inferred from gradients of measured head and those independently inferred from water-chemistry data. The general approach of the site-scale SZ flow and transport model analysis is to calculate unit breakthrough curves for radionuclides at the interface between the SZ and the biosphere using the three-dimensional site-scale SZ flow and transport model. Uncertainties are explicitly incorporated into the site-scale SZ flow and transport abstractions through key parameters and conceptual models. ?? 2002 Elsevier Science B.V. All rights reserved.

Journal of Contaminant Hydrology↗
USGS2026

Effects of a low-lipid diet on the gut microbiome and head kidney transcriptome of juvenile Chinook Salmon

Objective Pacific salmon Oncorhynchus spp. reared in production hatcheries are typically fed high-lipid, energy-dense diets to achieve large size and high body condition prior to release. In contrast, juveniles in natural environments tend to consume low-lipid, high-protein diets, and fish reared for research or conservation purposes are sometimes fed diets that are formulated to mimic natural diets and promote wild-like phenotypes. Understanding how these alternative diets affect fish health beyond growth and body condition could ultimately contribute to improving hatchery fish fitness. Methods In this work, we evaluated changes in the fecal microbiome and gene expression of juvenile Chinook Salmon O. tshawytscha on a standard high-lipid hatchery diet versus a low-lipid diet formulated to mimic the nutrition profile of natural-origin fish. To evaluate the time scale at which diet alters the fecal microbiome, we collected longitudinal samples over a 12-week period and switched the diets of a subset of fish twice during the experiment. We used 16S ribosomal RNA gene amplicon sequencing to characterize fecal microbiome differences between fish on the two diets as well as hatchery-reared fish at a production hatchery, hatchery fish that had been captured after release into a stream, and natural-origin, stream-reared fish of similar ages. Additionally, we conducted RNA sequencing on head kidney samples from laboratory-reared fish to evaluate changes in gene expression in this important immune organ. Results We found that the low-lipid diet and the hatchery diet resulted in microbiomes that differed from the microbiome of natural-origin fish and from each other and that diet-driven changes to the microbiome could occur in under 14 d. The low-lipid diet did not result in a microbiome that resembled the microbiome of naturally produced fish. Instead, the low-lipid diet resulted in a microbiome community that was distinct from those of fish reared on the hatchery diet and fish sampled from the wild. The RNA sequencing results indicated differential enrichment of pathways related to immunity, metabolism, and hormone synthesis between fish that were fed the two experimental diets. Conclusions The results suggest that additional environmental factors influence the microbiome more strongly than diet formulation or that the low-lipid diet has a smaller effect on the microbiome than a natural, ­invertebrate-based diet. Given that the gut microbiome and systemic immune function contribute significantly to disease resistance, our findings highlight the importance of understanding how diets fed to fish in captivity may affect fish health beyond growth and body condition metrics.

Journal of Aquatic Animal Health↗
USGS1987

Computer simulations of large asteroid impacts into oceanic and continental sites--Preliminary results on atmospheric, cratering and ejecta dynamics

Computer simulations have been completed that describe passage of a 10-km-diameter asteroid through the Earth's atmosphere and the subsequent cratering and ejecta dynamics caused by impact of the asteroid into both oceanic and continental sites. The asteroid was modeled as a spherical body moving vertically at 20 km/s with a kinetic energy of 2.6 × 10 30 ergs (6.2 × 10 7 Mt ). Detailed material modeling of the asteroid, ocean, crustal units, sedimentary unit, and mantle included effects of strength and fracturing, generic asteroid and rock properties, porosity, saturation, lithostatic stresses, and geothermal contributions, each selected to simulate impact and geologic conditions that were as realistic as possible. Calculation of the passage of the asteroid through a U.S. Standard Atmosphere showed development of a strong bow shock wave followed by a highly shock compressed and heated air mass. Rapid expansion of this shocked air created a large low-density region that also expanded away from the impact area. Shock temperatures in air reached ∼20, 000K near the surface of the uplifting crater rim and were as high as ∼2000K at more than 30 km range and 10 km altitude. Calculations to 30 s showed that the shock fronts in the air and in most of the expanding shocked air mass preceded the formation of the crater, ejecta, and rim uplift and did not interact with them. As cratering developed, uplifted rim and target material were ejected into the very low density, shock-heated air immediately above the forming crater, and complex interactions could be expected. Calculations of the impact events showed equally dramatic effects on the oceanic and continental targets through an interval of 120 s. Despite geologic differences in the targets, both cratering events developed comparable dynamic flow fields and by ∼29s had formed similar-sized transient craters ∼39km deep and ∼62km across. Transient-rim uplift of ocean and crust reached a maximum altitude of nearly 40 km at ∼30s and began to decay at velocities of 500 m/s to develop large-tsunami conditions. After ∼30s, strong gravitational rebound drove both craters toward broad flat-floored shapes. At 120 s, transient crater diameters were ∼80km (continental) and ∼105km (oceanic) and transient depths were ∼27km; crater floors consisting of melted and fragmented hot rock were rebounding rapidly upward. By 60 s, the continental crater had ejected ∼2 × 10 14 t, about twice the mass ejected from the oceanic crater. By 120 s, ∼70, 000km 3 (continental) and ∼90, 000km 3 (oceanic) target material were excavated (no mantle) and massive ejecta blankets were formed around the craters. We estimate that in excess of ∼70% of the ejecta would finally lie within ∼3 crater diameters of the impact, and the remaining ejecta (∼10 13 t), including the vaporized asteroid, would be ejected into the atmosphere to altitudes as high as the ionosphere. Effects of secondary volcanism and return of the ocean over hot oceanic crater floor could also be expected to contribute substantial material to the atmosphere.

International Journal of Impact Engineering↗
USGS2024

Diel temperature signals track seasonal shifts in localized groundwater contributions to headwater streamflow generation at network scale

Groundwater contributions to streamflow sustain aquatic ecosystem resilience; streams without significant groundwater inputs often have well-coupled air and water temperatures that degrade cold-water habitat during warm low flow periods. Widespread uncertainty in stream-groundwater connectivity across space and time has created disparate predictions of energy and nutrient fluxes across headwater networks, hindering predictions of cold-water habitat resilience under climate change scenarios. Recently, annual paired air and water temperature signals have been harnessed to indicate stream water thermal sensitivity and the dominance of deep versus shallow groundwater influence, although the utility of diel air–water temperature signal metrics for hydrologic inference has remained unexplored. Here we analyzed two consecutive years of locally paired, air–water temperature data from 47 headwater stream sites in the Catskill Mountains, New York, USA, and discovered characteristic seasonal patterns in diel temperature signal sinusoid metrics (amplitude ratio, phase lag, and mean ratio) driven by shifts in streamflow generation mechanisms and stream network position. Hydrologic interpretations of observed patterns were supported by stream heat budget model scenarios and additional analysis of paired air–water temperature data from two streams in Shenandoah National Park, Virginia, USA, with well characterized stream-groundwater connectivity. We found that within smaller tributaries, streamflow generation transitions from runoff to groundwater dominance were driven by hillslope drying during seasonal periods of lower precipitation. This was evidenced by significant correlations (p < 0.01) between daily water:air temperature signal amplitudes (non-linear decreases of ∼ 50 %) and derived base-flow index at 22 of the 28 sites, indicating enhanced local groundwater influence on streamflow promotes decoupling of diel air–water temperature signals. Additionally, ratios between daily water:air temperature signal means were lower in tributaries (∼0.68) when compared to main-stem (∼0.8) sites, increasing linearly throughout the observational period. In conceptual stream heat budget models, groundwater inflow had minimal effects on daily phase lags (∼0.2 hr), but increases in fractional groundwater discharge (0–50 %) depressed daily amplitude (∼20 % to 50 %) and mean ratios (∼15 %), supporting the sensitivity of daily metrics to interpreted changes in seasonal groundwater contributions to streamflow. During observational periods (i.e., April through October 2021 and 2022), significant differences (p < 0.01) between tributary and main-stem air–water metrics occurred when base-flow contributions were highest (∼0.93 vs. ∼ 0.68), as sites lower in the network had daily temperature metrics dominated by stream channel thermal inertia, rather than local groundwater connectivity, showing enhanced air–water diel signal coupling during warmer, drier periods. Divergent air temperature coupling across the network was interpreted as being driven by distance from local groundwater source zones, additional lateral groundwater inflows do not contribute a meaningful fraction to channel discharge lower in the network. Given the growing footprint of stream temperature observations, diel air–water temperature signals can provide distributed metrics sensitive to upstream groundwater discharge. Consequently, these metrics can support ongoing efforts by resource managers and researchers seeking to forecast the resilience of cold-water habitat to climate warming and changing precipitation regimes in mountain headwater streams.

New York, Virginia↗
USGS2017

Monitoring land surface albedo and vegetation dynamics using high spatial and temporal resolution synthetic time series from Landsat and the MODIS BRDF/NBAR/albedo product

Seasonal vegetation phenology can significantly alter surface albedo which in turn affects the global energy balance and the albedo warming/cooling feedbacks that impact climate change. To monitor and quantify the surface dynamics of heterogeneous landscapes, high temporal and spatial resolution synthetic time series of albedo and the enhanced vegetation index (EVI) were generated from the 500 m Moderate Resolution Imaging Spectroradiometer (MODIS) operational Collection V006 daily BRDF/NBAR/albedo products and 30 m Landsat 5 albedo and near-nadir reflectance data through the use of the Spatial and Temporal Adaptive Reflectance Fusion Model (STARFM). The traditional Landsat Albedo (Shuai et al., 2011) makes use of the MODIS BRDF/Albedo products (MCD43) by assigning appropriate BRDFs from coincident MODIS products to each Landsat image to generate a 30 m Landsat albedo product for that acquisition date. The available cloud free Landsat 5 albedos (due to clouds, generated every 16 days at best) were used in conjunction with the daily MODIS albedos to determine the appropriate 30 m albedos for the intervening daily time steps in this study. These enhanced daily 30 m spatial resolution synthetic time series were then used to track albedo and vegetation phenology dynamics over three Ameriflux tower sites (Harvard Forest in 2007, Santa Rita in 2011 and Walker Branch in 2005). These Ameriflux sites were chosen as they are all quite nearby new towers coming on line for the National Ecological Observatory Network (NEON), and thus represent locations which will be served by spatially paired albedo measures in the near future. The availability of data from the NEON towers will greatly expand the sources of tower albedometer data available for evaluation of satellite products. At these three Ameriflux tower sites the synthetic time series of broadband shortwave albedos were evaluated using the tower albedo measurements with a Root Mean Square Error (RMSE) less than 0.013 and a bias within the range of ±0.006. These synthetic time series provide much greater spatial detail than the 500 m gridded MODIS data, especially over more heterogeneous surfaces, which improves the efforts to characterize and monitor the spatial variation across species and communities. The mean of the difference between maximum and minimum synthetic time series of albedo within the MODIS pixels over a subset of satellite data of Harvard Forest (16 km by 14 km) was as high as 0.2 during the snow-covered period and reduced to around 0.1 during the snow-free period. Similarly, we have used STARFM to also couple MODIS Nadir BRDF Adjusted Reflectances (NBAR) values with Landsat 5 reflectances to generate daily synthetic times series of NBAR and thus Enhanced Vegetation Index (NBAR-EVI) at a 30 m resolution. While normally STARFM is used with directional reflectances, the use of the view angle corrected daily MODIS NBAR values will provide more consistent time series. These synthetic times series of EVI are shown to capture seasonal vegetation dynamics with finer spatial and temporal details, especially over heterogeneous land surfaces.

International Journal of Applied Earth Observation↗
USGS2002

Strike-slip earthquakes in the oceanic lithosphere: Observations of exceptionally high apparent stress

The radiated energies, E S , and seismic moments, M 0 , for 942 globally distributed earthquakes that occurred between 1987 to 1998 are examined to find the earthquakes with the highest apparent stresses (τ a =μ E S / M 0 , where μ is the modulus of rigidity). The globally averaged τ a for shallow earthquakes in all tectonic environments and seismic regions is 0.3 MPa. However, the subset of 49 earthquakes with the highest apparent stresses (τ a greater than about 5.0 MPa) is dominated almost exclusively by strike-slip earthquakes that occur in oceanic environments. These earthquakes are all located in the depth range 7–29 km in the upper mantle of the young oceanic lithosphere. Many of these events occur near plate-boundary triple junctions where there appear to be high rates of intraplate deformation. Indeed, the small rapidly deforming Gorda Plate accounts for 10 of the 49 high-τ a events. The depth distribution of τ a , which shows peak values somewhat greater than 25 MPa in the depth range 20–25 km, suggests that upper bounds on this parameter are a result of the strength of the oceanic lithosphere. A recently proposed envelope for apparent stress, derived by taking 6 per cent of the strength inferred from laboratory experiments for young (less than 30 Ma) deforming oceanic lithosphere, agrees well with the upper-bound envelope of apparent stresses over the depth range 5–30 km. The corresponding depth-dependent shear strength for young oceanic lithosphere attains a peak value of about 575 MPa at a depth of 21 km and then diminishes rapidly as the depth increases. In addition to their high apparent stresses, which suggest that the strength of the young oceanic lithosphere is highest in the depth range 10–30 km, our set of high-τ a earthquakes show other features that constrain the nature of the forces that cause interplate motion. First, our set of events is divided roughly equally between intraplate and transform faulting with similar depth distributions of τ a for the two types. Secondly, many of the intraplate events have focal mechanisms with the T -axes that are normal to the nearest ridge crest or subduction zone and P -axes that are normal to the proximate transform fault. These observations suggest that forces associated with the reorganization of plate boundaries play an important role in causing high-τ a earthquakes inside oceanic plates. Extant transform boundaries may be misaligned with current plate motion. To accommodate current plate motion, the pre-existing plate boundaries would have to be subjected to large horizontal transform push forces. A notable example of this is the triple junction near which the second large aftershock of the 1992 April Cape Mendocino, California, sequence occurred. Alternatively, subduction zone resistance may be enhanced by the collision of a buoyant lithosphere, a process that also markedly increases the horizontal stress. A notable example of this is the Aleutian Trench near which large events occurred in the Gulf of Alaska in late 1987 and the 1998 March Balleny Sea M = 8.2 earthquake within the Antarctic Plate.

Geophysical Journal International↗