Physics of the Omori law: Inferences from interevent time distributions and pore pressure diffusion modeling

Empirical laws and statistics of earthquakes are valuable as a basis for a better understanding of the earthquake cycle. In this paper we focus on the postseismic phase and the physics of aftershock sequences. Using interevent time distributions for a catalogue of Icelandic seismicity, we infer that the parameter C₂ in the Omori law, often considered to represent incomplete detection of aftershocks, is at least in part related to the physics of the earthquake process. We investigate the role of postseismic pore pressure diffusion after two Icelandic earthquakes on the rate of aftershocks and what we can infer about the physical meaning of C₂ from the diffusion process. Using the Mohr–Coulomb failure criterion we obtain a rate of triggered points in our diffusion model that agrees with the modified Omori law, with a value of C₂ that is consistent with data. Our pore pressure diffusion model suggests that C₂ is related to the process of reducing high pore pressure gradients existing across a fault zone at short times after a main shock.     

Identifying biological effects of organic contaminants in surface- and groundwater

Thousands of chemicals are in daily use; many of these reach the aquatic environment, which serves as a medium of transport and deposition into sediment and organisms. It is often unknown which effects these contaminants, alone or in combination with other stressors, may have on organisms, populations and communities in the aquatic environment. Ecotoxicology strives to identify and mechanistically understand contaminant–biota interactions with the ultimate goal to predict potential adverse effects. There are two general routes to addressing this goal. The first is to understand how organisms and populations respond according to anticipated exposure routes, organisms’ genome and physiology, and structure- physico-chemical properties of chemicals. This knowledge should lead to improved criteria for chemical design and approval. The second general route is to identify the cause–effect relationships of effects detected in the environment without prior knowledge on the chemical’s identity. This approach serves to pinpoint critical chemicals in complex environmental samples to enable decisions or technologies for their reduction or removal. An example will be presented in which biological effect assessment contributed to a better understanding of groundwater contamination.     

Mid-depth Lagrangian pathways in the North Atlantic and their impact on the salinity of the eastern subpolar gyre

The role of Mediterranean Overflow Water (MOW) in creating subsurface salinity anomalies within the Rockall Trough, a gateway to high latitude areas of deep convection, has been examined closely in recent years. Eulerian investigations of high latitude property fields have suggested that these subsurface anomalies are likely the result of variability in the zonal extent of the eastern limb of the subpolar gyre: when expanded into the eastern North Atlantic, the gyre is presumed to limit the extent to which MOW is able to penetrate northward to subpolar latitudes. However, though the depth of the subsurface salinity anomalies in the Rockall Trough supports the hypothesis that the intermittent presence of MOW is involved in creating the anomalies, MOW pathways to the Rockall Trough have not yet been established. Here, Lagrangian trajectories from floats released in the eastern North Atlantic between 1996 and 1997 and synthetic trajectories launched within an eddy-resolving ocean general circulation model are used to demonstrate that two main density neutral transport pathways lead to the Rockall Trough. One pathway involves the transport of relatively fresh waters as part of the North Atlantic Current and the other involves the transport of relatively salty waters from the eastern reaches of the subtropical North Atlantic. The results from this study indicate that changes in these pathways over time can explain the subsurface salinity variability in the Rockall Trough.     

U-Th systematics and Th ages of carbonate chimneys at the Lost City Hydrothermal Field

The Lost City Hydrothermal Field (LCHF) is a serpentinite-hosted vent field located 15 km west of the spreading axis of the Mid-Atlantic Ridge. In this study, uranium-thorium (U-Th) geochronological techniques have been used to examine the U-Th systematics of hydrothermal fluids and the ²³⁰Th ages of hydrothermally-precipitated carbonate chimneys at the LCHF. Fluid sample analyses indicate that endmember fluids likely contain only 0.0073 ng/g U or less compared to 3.28 ± 0.03 ng/g of U in ambient seawater. For fluid samples containing only 2-21% ambient seawater (1.1-11 mmol/kg Mg), Th concentration is 0.11-0.13 pg/g and surrounding seawater concentrations average 0.133 ± 0.016 pg/g. The ²³⁰Th/²³²Th atomic ratios of the vent fluids range from 1 (±10) × 10⁻⁶ to 11 (±5) × 10⁻⁶, are less than those of seawater, and indicate that the vent fluids may contribute a minor amount of non-radiogenic ²³⁰Th to the LCHF carbonate chimney deposits. Chimney ²³⁸U concentrations range from 1 to 10 μg/g and the average chimney corrected initial δ²³⁴U is 147.2 ± 0.8, which is not significantly different from the ambient seawater value of 146.5 ± 0.6. Carbonate ²³²Th concentrations range broadly from 0.0038 ± 0.0003 to 125 ± 16 ng/g and ²³⁰Th/²³²Th atomic ratios vary from near seawater values of 43 (±8) × 10⁻⁶ up to 530 (±25) × 10⁻³. Chimney ages, corrected for initial ²³⁰Th, range from 17 ± 6 yrs to 120 ± 13 kyrs. The youngest chimneys are at the intersection of two active, steeply-dipping normal faults that cut the Atlantis Massif; the oldest chimneys are located in the southwest portion of the field. Vent deposits on a steep, fault-bounded wall on the east side of the field are all <4 kyrs old, indicating that mass wasting in this region is relatively recent. Comparison of results to prior age-dating investigations of submarine hydrothermal systems shows that the LCHF is the most long-lived hydrothermal system known to date. It is likely that seismic activity and active faulting within the Atlantis Massif and the Atlantis Fracture Zone, coupled with volumetric expansion of the underlying serpentinized host rocks play major roles in sustaining hydrothermal activity at this site. The longevity of venting at the LCHF may have implications for ecological succession of microorganisms within serpentinite-hosted vent environments.     

Barrier island stratigraphy and Holocene history of west-central Florida

Although the morphology of the barrier-inlet system along the west-central Florida coast is quite complicated, the stratigraphy of these barriers is rather simple. The basal Holocene unit in most cores is an organic-rich, muddy sand that represents a vegetated, paralic marine, coastal environment similar to that which is north and south of the present barrier system. Above that unit is a muddy, bioturbated sand that displays a marine fauna at most locations but also contains Crassostrea virginica in a few places. These sediments accumulated in a low-energy marine setting that may or may not have been protected by a barrier island. Much of this facies also represents sediment that was delivered as washover deposits in an intertidal or subtidal setting and was subsequently bioturbated. The facies that can be attributed to a barrier island with some certainty are no more than 3000 years old, and on most islands, are much younger. These are the shelly sand and sorted sand facies. The shelly strata represent deposition in nearshore, beach, supratidal washover or intertidal spillover environments, and tidal inlet and tidal delta channels, whereas the sorted sand is typical of eolian deposition in dunes or the backbeach and some tidal delta elements. The presence of Holocene oyster beds offshore of a present barrier suggests that some of these islands formed significantly offshore and moved to their present position through washover. It is likely that most of these barriers initially formed through upward shoaling by waves. Although there is significant morphologic difference between the wave-dominated and mixed-energy, drumstick barrier islands, their stratigraphy is quite similar. The only significant difference is the presence of extensive progradation on at least part of the drumstick islands and a relatively high amount of former washover deposits on the wave-dominated type.     

A Typology of Catalyst Events for Collaborative Watershed Management in the United States

While much has been written about the benefits of collaborative watershed management to address nonpoint source pollution and other water quality concerns in the United States, few scholars have addressed the catalytic nature of events that generate these collective action responses. Further, because equivalent catalyst events in different communities do not always lead to collective action, it is critical to understand the interaction between a community's baseline conditions and the catalyst events that lead to collective action. This article presents a conceptual framework that illustrates the relationship between baseline conditions and events that lead to collective action. In this article a theoretical typology of catalyst events is presented that includes both intentional and nonintentional types of events. Understanding these types of catalyst events can help water quality advocates create and/or seize opportunities to nurture a collective action. This article concludes with a call for future research into catalyst events.     

Variability of Sea Surface Heights in the Baltic Sea: An Intercomparison of Observations and Model Simulations

Sea level changes in the Baltic Sea are dominated by internal, short-term variations that are mostly caused by the ephemeral nature of atmospheric conditions over the Baltic area. Tides are small and their influence decreases from western parts of the Baltic Sea to the Baltic Proper. Superimposed to the large short-term sea level changes (up to few decimeters from day to day) are seasonal and interannual variations (centimeters to decimeters). This study focuses on the comparison of sea surface heights obtained from observations and from a high resolution oceanographic model of the Baltic Sea. From this comparison, the accuracy of the modeled sea surface variations is evaluated, which is a necessary precondition for the further use of the oceanographic model in geodetic applications. The model reproduces all observed Baltic sea level variations very reliably with an accuracy of 5 to 9 cm (rms) for short-term variations (up to 2 months) and 8 cm (rms) for long-term variations (>2 months). An additional improvement of the model can be attained by including long-period sea level variations of the North Sea. The model performs well also in the case of extreme sea level events, as is shown for a major storm surge that occurred at the southern coast of the Baltic Sea in November 1995.