Natural and Anthropogenic Sources of Seismic, Hydroacoustic, and Infrasonic Waves: Waveforms and Spectral Characteristics (and Their Applicability for Sensor Calibration)

The record of seismic, hydroacoustic, and infrasonic waves is essential to detect, identify, and localize sources of both natural and anthropogenic origin. To guarantee traceability and inter-station comparability, as well as an estimation of the measurement uncertainties leading to a better monitoring of natural disasters and environmental aspects, suitable measurement standards and reliable calibration procedures of sensors, especially in the low-frequency range down to 0.01 Hz, are required. Most of all with regard to the design goal of the Comprehensive Nuclear-Test-Ban Treaty Organisation's International Monitoring System, which requires the stations to be operational nearly 100% of the time, the on-site calibration during operation is of special importance. The purpose of this paper is to identify suitable excitation sources and elaborate necessary requirements for on-site calibrations. We give an extensive literature review of a large variety of anthropogenic and natural sources of seismic, hydroacoustic, and infrasonic waves, describe their most prominent features regarding signal and spectral characteristics, explicitly highlight some source examples, and evaluate the reviewed sources with respect to requirements for on-site calibrations such as frequency bandwidth, signal properties as well as the applicability in terms of cost-benefit. According to our assessment, earthquakes stand out across all three waveform technologies as a good natural excitation signal meeting the majority of the requirements. Furthermore, microseisms and microbaroms allow a calibration at very low frequencies. We also find that in each waveform technique man-made controlled sources such as drop weights or air guns are in good agreement with the required properties, although limitations may arise regarding the practicability. Using these sources, procedures will be established allowing calibration without record interrupting, thereby improving data quality and the identification of treaty-related events.

Back-Arc Spreading Centers and Superfast Subduction: The Case of the Northern Lau Basin (SW Pacific Ocean)

The Lau Basin is a back-arc region formed by the subduction of the Pacific plate below the Australian plate. We studied the regional morphology of the back-arc spreading centers of the Northern Lau basin, and we compared it to their relative spreading rates. We obtained a value of 60.2 mm/year along the Northwest Lau Spreading Centers based on magnetic data, improving on the spreading rate literature data. Furthermore, we carried out numerical models including visco-plastic rheologies and prescribed surface velocities, in an upper plate-fixed reference frame. Although our thermal model points to a high temperature only near the Tonga trench, the model of the second invariant of the strain rate shows active deformation in the mantle from the Tonga trench to ~800 km along the overriding plate. This explains the anomalous magmatic production along all the volcanic centers in the Northern Lau Back-Arc Basin.

Aspects of marine geoscience: a review and thoughts on potential for observing active processes and progress through collaboration between the ocean sciences

Much progress has been made in the UK in characterizing the internal structures of major physiographic features in the oceans and in developing understanding of the geological processes that have created or shaped them. UK researchers have authored articles of high impact in all areas described here. In contrast to terrestrial geoscience, however, there have been few instrumented observations made of active processes by UK scientists. This is an area that could be developed over the next decades in the UK. Research on active processes has the potential ability to engage the wider public: Some active processes present significant geo-hazards to populations and offshore infrastructure that require monitoring and there could be commercial applications of technological developments needed for science. Some of the suggestions could involve studies in shallow coastal waters where ship costs are much reduced, addressing tighter funding constraints over the near term. The possibilities of measuring aspects of volcanic eruptions, flowing lava, turbidity currents and mass movements (landslides) are discussed. A further area of potential development is in greater collaboration between the ocean sciences. For example, it is well known in terrestrial geomorphology that biological agents are important in modulating erosion and the transport of sediments, ultimately affecting the shape of the Earth's surface in various ways. The analogous effect of biology on large-scale geomorphology in the oceans is also known but remains poorly quantified. Physical oceanographic models are becoming increasingly accurate and could be used to study further the patterns of erosion, particle transport and deposition in the oceans. Marine geological and geophysical data could in turn be useful for further verification of such models. Adapting them to conditions of past oceans could address the shorter-period movements, such as due to internal waves and tides, which have been barely addressed in palaeoceanography.

Temporal change in deep-sea benthic ecosystems: a review of the evidence from recent time-series studies

Societal concerns over the potential impacts of recent global change have prompted renewed interest in the long-term ecological monitoring of large ecosystems. The deep sea is the largest ecosystem on the planet, the least accessible, and perhaps the least understood. Nevertheless, deep-sea data collected over the last few decades are now being synthesised with a view to both measuring global change and predicting the future impacts of further rises in atmospheric carbon dioxide concentrations. For many years, it was assumed by many that the deep sea is a stable habitat, buffered from short-term changes in the atmosphere or upper ocean. However, recent studies suggest that deep-seafloor ecosystems may respond relatively quickly to seasonal, inter-annual and decadal-scale shifts in upper-ocean variables. In this review, we assess the evidence for these long-term (i.e. inter-annual to decadal-scale) changes both in biologically driven, sedimented, deep-sea ecosystems (e.g. abyssal plains) and in chemosynthetic ecosystems that are partially geologically driven, such as hydrothermal vents and cold seeps. We have identified 11 deep-sea sedimented ecosystems for which published analyses of long-term biological data exist. At three of these, we have found evidence for a progressive trend that could be potentially linked to recent climate change, although the evidence is not conclusive. At the other sites, we have concluded that the changes were either not significant, or were stochastically variable without being clearly linked to climate change or climate variability indices. For chemosynthetic ecosystems, we have identified 14 sites for which there are some published long-term data. Data for temporal changes at chemosynthetic ecosystems are scarce, with few sites being subjected to repeated visits. However, the limited evidence from hydrothermal vents suggests that at fast-spreading centres such as the East Pacific Rise, vent communities are impacted on decadal scales by stochastic events such as volcanic eruptions, with associated fauna showing complex patterns of community succession. For the slow-spreading centres such as the Mid-Atlantic Ridge, vent sites appear to be stable over the time periods measured, with no discernable long-term trend. At cold seeps, inferences based on spatial studies in the Gulf of Mexico, and data on organism longevity, suggest that these sites are stable over many hundreds of years. However, at the Haakon Mosby mud volcano, a large, well-studied seep in the Barents Sea, periodic mud slides associated with gas and fluid venting may disrupt benthic communities, leading to successional sequences over time. For chemosynthetic ecosystems of biogenic origin (e.g. whale-falls), it is likely that the longevity of the habitat depends mainly on the size of the carcass and the ecological setting, with large remains persisting as a distinct seafloor habitat for up to 100 years. Studies of shallow-water analogs of deep-sea ecosystems such as marine caves may also yield insights into temporal processes. Although it is obvious from the geological record that past climate change has impacted deep-sea faunas, the evidence that recent climate change or climate variability has altered deep-sea benthic communities is extremely limited. This mainly reflects the lack of remote sensing of this vast seafloor habitat. Current and future advances in deep-ocean benthic science involve new remote observing technologies that combine a high temporal resolution (e.g. cabled observatories) with spatial capabilities (e.g. autonomous vehicles undertaking image surveys of the seabed).

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

The formation of oceanic detachment faults is well established from inactive, corrugated fault planes exposed on sea floor formed along ridges spreading at less than 80 km Myr(-1) (refs 1-4). These faults can accommodate extension for up to 1-3 Myr (ref. 5), and are associated with one of the two contrasting modes of accretion operating along the northern Mid-Atlantic Ridge. The first mode is asymmetrical accretion involving an active detachment fault along one ridge flank. The second mode is the well-known symmetrical accretion, dominated by magmatic processes with subsidiary high-angle faulting and the formation of abyssal hills on both flanks. Here we present an examination of approximately 2,500 km of the Mid-Atlantic Ridge between 12.5 and 35 degrees N, which reveals asymmetrical accretion along almost half of the ridge. Hydrothermal activity identified so far in the study region is closely associated with asymmetrical accretion, which also shows high levels of near-continuous hydroacoustically and teleseismically recorded seismicity. Increased seismicity is probably generated along detachment faults that accommodate a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which occurs primarily at segment ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. Both seismic evidence and geochemical evidence indicate that the axial lithosphere is thicker and colder at asymmetrical sections of the ridge, either because associated hydrothermal circulation efficiently penetrates to greater depths or because the rising mantle is cooler. We suggest that much of the variability in sea-floor morphology, seismicity and basalt chemistry found along slow-spreading ridges can be thus attributed to the frequent involvement of detachment faults in oceanic lithospheric accretion.

Widespread active detachment faulting and core complex formation near 13 degrees N on the Mid-Atlantic Ridge

Oceanic core complexes are massifs in which lower-crustal and upper-mantle rocks are exposed at the sea floor. They form at mid-ocean ridges through slip on detachment faults rooted below the spreading axis. To date, most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes. Here we present a survey of the Mid-Atlantic Ridge near 13 degrees N containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis. The detachment faults are apparently all currently active and at various stages of development. A field of extinct core complexes extends away from the axis for at least 100 km. Our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity. Within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity. Preliminary examination of seismicity and seafloor morphology farther north along the Mid-Atlantic Ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow-spreading ridge than previously suspected.