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At least 559 records · Page 31Linked to original sources
USGS2008

Length variation in age-0 westslope cutthroat trout at multiple spatial scales

Phenotypic diversity provides ecological and evolutionary functions, stabilizing populations in variable environments. Although benefits of larger body size in juvenile fishes are well documented, size variation may have value as well. We explored the distribution of length and length variation in age-0 westslope cutthroat trout Oncorhynchus clarkii lewisi at three spatial scales: area (102 km2), stream (101 km2), and site (100 km2). In addition, we examined relationships between length variables (mean length and interquartile range of length) and instream (temperature and conductivity) and landscape (aspect, elevation, headwater distance, and valley width) variables that were expected to be associated with fish size. Conductivity was included as a surrogate for productivity. Most variation in mean length and interquartile range of fish length was found among areas (62.2% and 62.6%, respectively). Mean length also varied among streams and sites (21.9% and 15.8%, respectively). Similarly, interquartile range of fish length varied among streams and sites (19.1% and 18.3%, respectively). Both length variables were associated with temperature and elevation. Mean fish length was also associated with conductivity, but the association between interquartile length range and conductivity was weak. We conclude that the conservation of variation in phenotypic attributes, such as length, in westslope cutthroat trout may require conservation of viable populations across broad areas and across environmental gradients that are associated with growth. ?? Copyright by the American Fisheries Society 2008.

North American Journal of Fisheries Management↗
USGS2008

Meteorites on Mars observed with Mars Exploration Rovers

Reduced weathering rates due to the lack of liquid water and significantly greater typical surface ages should result in a higher density of meteorites on the surface of Mars compared to Earth. Several meteorites were identified among the rocks investigated during Opportunity's traverse across the sandy Meridiani plains. Heat Shield Rock is a IAB iron meteorite and has been officially recognized as 'Meridiani Planum.' Barberton is olivine-rich and contains metallic Fe in the form of kamacite, suggesting a meteoritic origin. It is chemically most consistent with a mesosiderite silicate clast. Santa Catarina is a brecciated rock with a chemical and mineralogical composition similar to Barberton. Barberton, Santa Catarina, and cobbles adjacent to Santa Catarina may be part of a strewn field. Spirit observed two probable iron meteorites from its Winter Haven location in the Columbia Hills in Gusev Crater. Chondrites have not been identified to date, which may be a result of their lower strengths and probability to survive impact at current atmospheric pressures. Impact craters directly associated with Heat Shield Rock, Barberton, or Santa Catarina have not been observed, but such craters could have been erased by eolian-driven erosion.

Journal of Geophysical Research E: Planets↗
USGS2017

Declines in low-elevation subalpine tree populations outpace growth in high-elevation populations with warming

Species distribution shifts in response to climate change require that recruitment increase beyond current range boundaries. For trees with long life spans, the importance of climate-sensitive seedling establishment to the pace of range shifts has not been demonstrated quantitatively. Using spatially explicit, stochastic population models combined with data from long-term forest surveys, we explored whether the climate-sensitivity of recruitment observed in climate manipulation experiments was sufficient to alter populations and elevation ranges of two widely distributed, high-elevation North American conifers. Empirically observed, warming-driven declines in recruitment led to rapid modelled population declines at the low-elevation, ‘warm edge’ of subalpine forest and slow emergence of populations beyond the high-elevation, ‘cool edge’. Because population declines in the forest occurred much faster than population emergence in the alpine, we observed range contraction for both species. For Engelmann spruce, this contraction was permanent over the modelled time horizon, even in the presence of increased moisture. For limber pine, lower sensitivity to warming may facilitate persistence at low elevations – especially in the presence of increased moisture – and rapid establishment above tree line, and, ultimately, expansion into the alpine. Synthesis . Assuming 21st century warming and no additional moisture, population dynamics in high-elevation forests led to transient range contractions for limber pine and potentially permanent range contractions for Engelmann spruce. Thus, limitations to seedling recruitment with warming can constrain the pace of subalpine tree range shifts.

Journal of Ecology↗
USGS2025

Tracking persistent declines in suspended sediment in the Lower Mississippi and Atchafalaya Rivers, 1992–2021: Harnessing WRTDSplus to characterize longitudinally varying trends and explore connections to streamflow

Suspended sediment (SS) continues a century-long decline in the Lower Mississippi and Atchafalaya Rivers, United States. In this study, we use the WRTDSplus model to estimate concentrations and loads for total, fine (<0.0625 millimeter (mm)), and coarse (≥0.0625 mm) SS for 11 sites. This extension of the Weighted Regressions on Time, Discharge, and Season (WRTDS) model allows a fourth explanatory variable in the model formulation. We incorporated hysteresis terms for most models based on a residual analysis, which allowed for the identification of decreased flushing over time at some sites. Total, fine, and coarse SS concentrations and loads decreased at all sites over two trend periods (water years (WY) 1992–2021 and WY 2012–2021). Declines were largely due to changes in fine SS (mud and silt) but decreases in coarse SS (sands) were also widespread. On average, recent declines are more severe in the Lower Mississippi River below the Old River Control Complex (ORCC, −3.7 mg per liter per year (mg/L/yr)) compared to the Atchafalaya River (−2.0 mg/L/yr), although there is longitudinal variability within each river. The reach below the ORCC is a net SS sink, leading to complex temporal changes for the sites in this area. Streamflows (low, moderate, and high) have increased over these periods, with the last decade being particularly wet. Increasing streamflow and decreasing SS, with little evidence of amelioration, may influence spillway operations during floods, sediment diversion construction and operation, coastal restoration efforts, and aquatic health.

Louisiana, Mississippi↗
USGS2017

Quantifying the effects of wildfire on changes in soil properties by surface burning of soils from the Boulder Creek Critical Zone Observatory

Study region This study used intact soil cores collected at the Boulder Creek Critical Zone Observatory near Boulder, Colorado, USA to explore fire impacts on soil properties. Study focus Three soil scenarios were considered: unburned control soils, and low- and high-temperature burned soils. We explored simulated fire impacts on field-saturated hydraulic conductivity, dry bulk density, total organic carbon, and infiltration processes during rainfall simulations. New hydrological insights for the region Soils burned to high temperatures became more homogeneous with depth with respect to total organic carbon and bulk density, suggesting reductions in near-surface porosity. Organic matter decreased significantly with increasing soil temperature. Tension infiltration experiments suggested a decrease in infiltration rates from unburned to low-temperature burned soils, and an increase in infiltration rates in high-temperature burned soils. Non-parametric statistical tests showed that field-saturated hydraulic conductivity similarly decreased from unburned to low-temperature burned soils, and then increased with high-temperature burned soils. We interpret these changes result from the combustion of surface and near-surface organic materials, enabling water to infiltrate directly into soil instead of being stored in the litter and duff layer at the surface. Together, these results indicate that fire-induced changes in soil properties from low temperatures were not as drastic as high temperatures, but that reductions in surface soil water repellency in high temperatures may increase infiltration relative to low temperatures.

Colorado↗
USGS2026

Deaf, deafblind, and hard of hearing university student experiences with earthquake early warning in the United States: Evaluating language planning and technology access

The growing literature on deaf and hard of hearing (DHH+) populations and disasters demonstrates that emergency communication (including alerts) is not reaching global DHH + individuals with dangerous impacts for morbidity and mortality. This is the first research study in the U.S. to qualitatively explore the experiences of DHH + persons with earthquake early warning (EEW) through group-based dialogue sessions. The study investigates eight DHH + university students'past earthquake experiences, access to EEW alerts, and perceptions of ShakeAlert Ⓡ , an EEW system for detecting earthquakes and alerting residents of California, Oregon, and Washington. Findings highlight key gaps in disaster alert usability within four thematic areas: lack of messaging in participants' language(s), unclear alert messaging, deficient message delivery mechanisms for deafblind persons, and insufficient access to earthquake information and training that leads to dependence on informal information networks. Weaknesses identified in these four themes reduce DHH + trust in EEW systems and compromise the capacity of alert recipients to take swift protective action or to mentally prepare before shaking starts. The study also underscores structural factors such as insufficient linguistic representation in disaster language planning and technology design, which ignores the linguistic and sensory access needs of DHH + individuals. Building on disaster language planning frameworks, we recommend involving DHH + populations to co-develop EEW alerts. By centering DHH + perspectives, this research contributes to ongoing efforts to ensure that EEW systems reach everyone.

California↗
USGS2023

Foraging behavior of Raramuri Criollo vs. Angus cattle grazing California Chaparral and Colorado Plateau shrublands

Selecting livestock genetics adapted to arid environments, such as Criollo cattle, is one of several strategies recommended for decreasing the vulnerability to climate change of ranching in the southwestern USA. Our objective was to determine whether desirable foraging traits of Criollo cattle previously documented in the Chihuahuan Desert, held true in two of the most climate-vulnerable ecosystems of the Southwest. We conducted a study at Rancho Corta Madera (RCM) in southern California and Dugout Ranch (DR) in southeast Utah. Twenty mature cows, 10 Raramuri Criollo and 10 Red or Black Angus, were monitored with GPS collars during multiple seasons between 2018 and 2021. Geolocation data were used to compute daily distance traveled (km*d −1 ), movement velocity (m*min −1 ), path sinuosity (SI), time spent grazing, resting, or traveling (h*d −1 ), and area of the pasture explored (ha*d −1 ) as well as to calculate selection of vegetation cover types ( E , Ivlev's Electivity Index) by cows of each breed. The effects of breed, season, year, and pasture on each of these metrics were modeled with repeated measures analyses of variance. At both ranches, statistically detectable differences (P ≤ 0.05) between breeds were observed for most behavior metrics during the dormant season. Conversely, few breed differences were observed during the growing season. Criollo cattle exhibited greater relative preference for a number of shrub dominated vegetation types at both ranches, and similar relative selection of grassland dominated sites compared to Angus counterparts. At both ranches, Criollo cattle exhibited similar or less relative preference for riparian areas vs. Angus counterparts. Breed divergence vs. convergence of foraging behaviors during the dormant vs. growing seasons, previously observed in the Chihuahuan Desert, was documented at both sites. Positive system outcomes associated with foraging traits of Criollo cattle could be expected to occur more broadly across the Southwest.

Journal of Arid Environments↗
USGS2003

Climate change and Arctic ecosystems: 2. Modeling, paleodata-model comparisons, and future projections

Large variations in the composition, structure, and function of Arctic ecosystems are determined by climatic gradients, especially of growing-season warmth, soil moisture, and snow cover. A unified circumpolar classification recognizing five types of tundra was developed. The geographic distributions of vegetation types north of 55°N, including the position of the forest limit and the distributions of the tundra types, could be predicted from climatology using a small set of plant functional types embedded in the biogeochemistry-biogeography model BIOME4. Several palaeoclimate simulations for the last glacial maximum (LGM) and mid-Holocene were used to explore the possibility of simulating past vegetation patterns, which are independently known based on pollen data. The broad outlines of observed changes in vegetation were captured. LGM simulations showed the major reduction of forest, the great extension of graminoid and forb tundra, and the restriction of low- and high-shrub tundra (although not all models produced sufficiently dry conditions to mimic the full observed change). Mid-Holocene simulations reproduced the contrast between northward forest extension in western and central Siberia and stability of the forest limit in Beringia. Projection of the effect of a continued exponential increase in atmospheric CO 2 concentration, based on a transient ocean-atmosphere simulation including sulfate aerosol effects, suggests a potential for larger changes in Arctic ecosystems during the 21st century than have occurred between mid-Holocene and present. Simulated physiological effects of the CO 2 increase (to >700 ppm) at high latitudes were slight compared with the effects of the change in climate.

Journal of Geophysical Research D: Atmospheres↗
USGS2016

Estimating 40 years of nitrogen deposition in global biomes using the SCIAMACHY NO 2 column

Owing to human activity, global nitrogen (N) cycles have been altered. In the past 100 years, global N deposition has increased. Currently, the monitoring and estimating of N deposition and the evaluation of its effects on global carbon budgets are the focus of many researchers. NO 2 columns retrieved by space-borne sensors provide us with a new way of exploring global N cycles and these have the ability to estimate N deposition. However, the time range limitation of NO 2 columns makes the estimation of long timescale N deposition difficult. In this study we used ground-based NO x emission data to expand the density of NO 2 columns, and 40 years of N deposition (1970–2009) was inverted using the multivariate linear model with expanded NO 2 columns. The dynamic of N deposition was examined in both global and biome scales. The results show that the average N deposition was 0.34 g N m –2 year –1 in the 2000s, which was an increase of 38.4% compared with the 1970s’. The total N deposition in different biomes is unbalanced. N deposition is only 38.0% of the global total in forest biomes; this is made up of 25.9%, 11.3, and 0.7% in tropical, temperate, and boreal forests, respectively. As N-limited biomes, there was little increase of N deposition in boreal forests. However, N deposition has increased by a total of 59.6% in tropical forests and croplands, which are N-rich biomes. Such characteristics may influence the effects on global carbon budgets.

International Journal of Remote Sensing↗
USGS2022

Cohort strength and body size in co-occurring salmonids in a small stream network: Variation in space and time

Trout and salmon commonly coexist in stream networks. Exploring similarities and differences among species can help explain coexistence and invasive ability. Here, we describe spatial distribution, cohort strengths and size-at-age of three co-occurring species in a small stream network. Spatial distributions varied dramatically among species; native brook trout occupied all stream reaches, naturalized brown trout were found in the mainstem and lower portions of tributaries and fry-stocked Atlantic salmon were limited to the mainstem. Size-at-age also differed among species, Atlantic salmon were consistently the smallest, brook trout were intermediate in size and brown trout were the largest. Despite size differences, mean lengths of brook trout and brown trout were highly correlated among years. Cohort strengths varied considerably across years but were also highly correlated for the two trout species, suggesting strong environmental control on cohort strength and a reduced role for species interactions. At low densities, we observed strong negative effects of density on body sizes and weaker effects otherwise. Overall, these results suggest differences in spatial distribution combined with similarities in response to environmental variation contribute to species coexistence in this small steam network.

Canadian Journal of Fisheries and Aquatic Sciences↗
USGS2020

Assessment of two techniques for remediation of lacustrine rocky reef spawning habitat

Rocky reef habitats in lacustrine systems constitute important areas for lithophilic‐spawning fishes. Interstitial spaces created by the structure of rocky reefs form microenvironments where incubating embryos and juvenile fish are potentially protected from predators and physical displacement. However, if interstitial spaces are filled or blocked by sediment or biofouling, the reef structure may lose these benefits. Common practices to restore reef habitat include augmentation of existing reef structures or construction of new reefs, though these practices can be costly. We explored an alternative approach for reef remediation. In 2018, we developed two benthic sled cleaning devices that used either propulsion or pressurized water jets and were towed behind a small vessel to clean reefs. We used the devices to clean two impaired natural rocky reefs in Saginaw Bay, Lake Huron. We indexed effectiveness of cleaning by measured changes in substrate relative hardness before and after cleaning. A biological response to reef cleaning was also measured by egg deposition of fall (Lake Whitefish Coregonus clupeaformis ) and spring (Walleye Sander vitreus ) lithophilic spawners. We found that our propulsion cleaning device was more effective in increasing substrate relative hardness than was the water jet device, although this was not consistent among all study locations. We also found that egg deposition on study plots was variable, but in general, egg deposition was highest on study plots that had the greatest increases in relative hardness post‐cleaning. The practicality of cleaning devices is likely related to the magnitude of site‐specific degradation. Our results indicate that the use of these or similar devices can potentially increase the quality of spawning habitat by displacing sediments that have deposited on reef structures.

North American Journal of Fisheries Management↗
USGS2006

Geomorphic control of landscape carbon accumulation

We use the CREEP process-response model to simulate soil organic carbon accumulation in an undisturbed prairie site in Iowa. Our primary objectives are to identify spatial patterns of carbon accumulation, and explore the effect of erosion on basin-scale C accumulation. Our results point to two general findings. First, redistribution of soil carbon by erosion results in a net increase in basin-wide carbon storage relative to a noneroding environment. Landscape-average mean residence times are increased in an eroding landscape owing to the burial/preservation of otherwise labile C. Second, field observations taken along a slope transect may overlook significant intraslope variations in carbon accumulation. Spatial patterns of modeled deep C accumulation are complex. While surface carbon with its relatively short equilibration time is predictable from surface properties, deep carbon is strongly influenced by the landscape's geomorphic and climatic history, resulting in wide spatial variability. Convergence and divergence associated with upland swales and interfluves result in bimodal carbon distributions in upper and mid slopes; variability in carbon storage within modeled mid slopes was as high as simulated differences between erosional shoulders and depositional valley bottoms. The bimodality of mid-slope C variability in the model suggests that a three-dimensional sampling strategy is preferable over the traditional two-dimensional analog or "catena" approach. Copyright 2006 by the American Geophysical Union.

Journal of Geophysical Research G: Biogeosciences↗
USGS2024

Likely ferromagnetic minerals identified by the Perseverance rover and implications for future paleomagnetic analyses of returned Martian samples

Although Mars today does not have a core dynamo, magnetizations in the Martian crust and in meteorites suggest a magnetic field was present prior to 3.7 billion years (Ga) ago. However, the lack of ancient, oriented Martian bedrock samples available on Earth has prevented accurate estimates of the dynamo's intensity, lifetime, and direction. Constraining the nature and lifetime of the dynamo are vital to understanding the evolution of the Martian interior and the potential habitability of the planet. The Perseverance rover, which is exploring Jezero crater, is providing an unprecedented opportunity to address this gap by acquiring absolutely oriented bedrock samples with estimated ages from ∼2.3 to >4.1 Ga. As a first step in establishing whether these samples could contain records of Martian paleomagnetism, it is important to determine their ferromagnetic mineralogy, the grain sizes of the phases, and the forms of any natural remanent magnetization. Here, we synthesize data from various Perseverance instruments to achieve those goals and discuss the implications for future laboratory paleomagnetic analyses. Using the rover's instrument payload, we find that cored samples likely contain iron oxides enriched in Cr and Ti. The relative proportions of Fe, Ti, and Cr indicate that the phases may be titanomagnetite or Fe-Ti-Cr spinels that are ferromagnetic at room temperature, but we cannot rule out the presence of non-ferromagnetic ulvöspinel, ilmenite, and chromite due to signal mixing. Importantly, the inferred abundance of iron oxides in the samples suggests that even <1 mm-sized samples will be easily measurable by present-day magnetometers.

Journal of Geophysical Research: Planets↗
USGS2015

A dynamic population model to investigate effects of climate and climate-independent factors on the lifecycle of the tick Amblyomma americanum (Acari: Ixodidae)

The lone star tick, Amblyomma americanum , is a disease vector of significance for human and animal health throughout much of the eastern United States. To model the potential effects of climate change on this tick, a better understanding is needed of the relative roles of temperature-dependent and temperature-independent (day-length-dependent behavioral or morphogenetic diapause) processes acting on the tick lifecycle. In this study, we explored the roles of these processes by simulating seasonal activity patterns using models with site-specific temperature and day-length-dependent processes. We first modeled the transitions from engorged larvae to feeding nymphs, engorged nymphs to feeding adults, and engorged adult females to feeding larvae. The simulated seasonal patterns were compared against field observations at three locations in United States. Simulations suggested that 1) during the larva-to-nymph transition, some larvae undergo no diapause while others undergo morphogenetic diapause of engorged larvae; 2) molted adults undergo behavioral diapause during the transition from nymph-to-adult; and 3) there is no diapause during the adult-to-larva transition. A model constructed to simulate the full lifecycle of A. americanum successfully predicted observed tick activity at the three U.S. study locations. Some differences between observed and simulated seasonality patterns were observed, however, identifying the need for research to refine some model parameters. In simulations run using temperature data for Montreal, deterministic die-out of A. americanum populations did not occur, suggesting the possibility that current climate in parts of southern Canada is suitable for survival and reproduction of this tick.

Journal of Medical Entomology↗
USGS2016

Prey size and availability limits maximum size of rainbow trout in a large tailwater: insights from a drift-foraging bioenergetics model

The cold and clear water conditions present below many large dams create ideal conditions for the development of economically important salmonid fisheries. Many of these tailwater fisheries have experienced declines in the abundance and condition of large trout species, yet the causes of these declines remain uncertain. Here, we develop, assess, and apply a drift-foraging bioenergetics model to identify the factors limiting rainbow trout (Oncorhynchus mykiss) growth in a large tailwater. We explored the relative importance of temperature, prey quantity, and prey size by constructing scenarios where these variables, both singly and in combination, were altered. Predicted growth matched empirical mass-at-age estimates, particularly for younger ages, demonstrating that the model accurately describes how current temperature and prey conditions interact to determine rainbow trout growth. Modeling scenarios that artificially inflated prey size and abundance demonstrate that rainbow trout growth is limited by the scarcity of large prey items and overall prey availability. For example, shifting 10% of the prey biomass to the 13 mm (large) length class, without increasing overall prey biomass, increased lifetime maximum mass of rainbow trout by 88%. Additionally, warmer temperatures resulted in lower predicted growth at current and lower levels of prey availability; however, growth was similar across all temperatures at higher levels of prey availability. Climate change will likely alter flow and temperature regimes in large rivers with corresponding changes to invertebrate prey resources used by fish. Broader application of drift-foraging bioenergetics models to build a mechanistic understanding of how changes to habitat conditions and prey resources affect growth of salmonids will benefit management of tailwater fisheries.

Arizona↗
USGS2008

Managing visitor impacts in parks: A multi-method study of the effectiveness of alternative management practices

How can recreation use be managed to control associated environmental impacts? What management practices are most effective and why? This study explored these and related questions through a series of experimental ?treatments? and associated ?controls? at the summit of Cadillac Mountain in Acadia National Park, a heavily used and environmentally fragile area. The treatments included five management practices designed to keep visitors on maintained trails, and these practices ranged from ?indirect? (information/education) to ?direct? (a fence bordering the trail). Research methods included unobtrusive observation of visitors to determine the percentage of visitors who walked off-trail and a follow-up visitor survey to explore why management practices did or didn?t work. All of the management practices reduced the percentage of visitors who walked off-trail. More aggressive applications of indirect practices were more effective than less aggressive applications, and the direct management practice of fencing was the most effective of all. None of the indirect management practices reduced walking off-trail to a degree that is likely to control damage to soil and vegetation at the study site. Study findings suggest that an integrated suite of direct and indirect management practices be implemented on Cadillac Mountain (and other, similar sites) that includes a) a regulation requiring visitors to stay on the maintained trail, b) enforcement of this regulation as needed, c) unobtrusive fencing along the margins of the trail, d) redesign of the trail to extend it, widen it in key places, and provide short spur trails to key ?photo points?, and e) an aggressive information/education program to inform visitors of the regulation to stay on the trail and the reasons for it. These recommendations are a manifestation of what may be an emerging principle of park and outdoor recreation management: intensive use requires intensive management.

Journal of Park and Recreation Administration↗
USGS2003

Exploration and discovery in Yellowstone Lake: Results from high-resolution sonar imaging, seismic reflection profiling, and submersible studies

Discoveries from multi-beam sonar mapping and seismic reflection surveys of the northern, central, and West Thumb basins of Yellowstone Lake provide new insight into the extent of post-collapse volcanism and active hydrothermal processes occurring in a large lake environment above a large magma chamber. Yellowstone Lake has an irregular bottom covered with dozens of features directly related to hydrothermal, tectonic, volcanic, and sedimentary processes. Detailed bathymetric, seismic reflection, and magnetic evidence reveals that rhyolitic lava flows underlie much of Yellowstone Lake and exert fundamental control on lake bathymetry and localization of hydrothermal activity. Many previously unknown features have been identified and include over 250 hydrothermal vents, several very large (>500 m diameter) hydrothermal explosion craters, many small hydrothermal vent craters (∼1–200 m diameter), domed lacustrine sediments related to hydrothermal activity, elongate fissures cutting post-glacial sediments, siliceous hydrothermal spire structures, sublacustrine landslide deposits, submerged former shorelines, and a recently active graben. Sampling and observations with a submersible remotely operated vehicle confirm and extend our understanding of the identified features. Faults, fissures, hydrothermally inflated domal structures, hydrothermal explosion craters, and sublacustrine landslides constitute potentially significant geologic hazards. Toxic elements derived from hydrothermal processes also may significantly affect the Yellowstone ecosystem.

Wyoming↗
USGS2009

Stress evolution following the 1999 Chi-Chi, Taiwan, earthquake: Consequences for afterslip, relaxation, aftershocks and departures from Omori decay

We explore how Coulomb stress transfer and viscoelastic relaxation control afterslip and aftershocks in a continental thrust fault system. The 1999 September 21 M w = 7.6 Chi-Chi shock is typical of continental ramp-d??collement systems throughout the world, and so inferences drawn from this uniquely well-recorded event may be widely applicable. First, we find that the spatial and depth distribution of aftershocks and their focal mechanisms are consistent with the calculated Coulomb stress changes imparted by the coseismic rupture. Some 61 per cent of the M ??? 2 aftershocks and 83 per cent of the M ??? 4 aftershocks lie in regions for which the Coulomb stress increased by ???0.1 bars, and there is a 11-12 per cent gain in the percentage of aftershocks nodal planes on which the shear stress increased over the pre-Chi Chi control period. Second, we find that afterslip occurred where the calculated coseismic stress increased on the fault ramp and d??collement, subject to the condition that friction is high on the ramp and low on the d??collement. Third, viscoelastic relaxation is evident from the fit of the post-seismic GPS data on the footwall. Fourth, we find that the rate of seismicity began to increase during the post-seismic period in an annulus extending east of the main rupture. The spatial extent of the seismicity annulus resembles the calculated ???0.05-bar Coulomb stress increase caused by viscoelastic relaxation and afterslip, and we find a 9-12 per cent gain in the percentage of focal mechanisms with >0.01-bar shear stress increases imparted by the post-seismic afterslip and relaxation in comparison to the control period. Thus, we argue that post-seismic stress changes can for the first time be shown to alter the production of aftershocks, as judged by their rate, spatial distribution, and focal mechanisms. ?? Journal compilation ?? 2009 RAS.

Geophysical Journal International↗