Ambient noise tomography reveals basalt and sub-basalt velocity structure beneath the Faroe Islands, North Atlantic

The Faroe Islands Passive Seismological Experiment (FIPSE) was funded by Sindri (contract C46-52-01) and formed a collaborative project between Dr. David Cornwell, Prof. Richard England (University of Leicester) and Prof. Graham Stuart (University of Leeds). Seismological equipment was loaned from the NERC geophysical equipment facility (GEF, loan 918), with field assistance from David Hawthorn and data processing assistance from Victoria Lane (SEIS-UK). We acknowledge the help, advice and support of Jarðfeingi, especially Thomas Varming, Uni Petersen, Bartal Højgaard, Romica Øster and Heri Ziska. Rannvá M. Arge and Magni Jøkladal are thanked for their assistance with fieldwork. Research undertaken in this article was supported by the Carnegie Trust for the Universities of Scotland, via a Collaborative Research Grant. Rosie Fletcher is thanked for her comments, which greatly improved the text.Peer reviewedPostprin

THE BIOMASS OF TROPICAL ANCHOVIES (Encrasicholina SPECIES) AT BACAN, EASTERN INDONESIA ESTIMATED BY THE DAILY EGG PRODUCTION METHOD

Tropical anchovies (Encrasicholdno species) are the target of the large baitfishery for the pole-and-line skipjack tuna fleet in eastern Indonesia. Recent studies have shown that the tuna catches are constrained by a lack of bait

Exploiting seismic signal and noise in an intracratonic environment to constrain crustal structure and source parameters of infrequent earthquakes

In many regions of the world characterized by a relatively low rate of seismicity, the determination of local and regional seismic source parameters is often restricted to an analysis of the first onsets of P waves (or first motion analysis) due to incomplete information about Earth structure and the small size of the events. When rare large earthquakes occur in these regions, their waveforms can be used to model Earth structure. This, however, makes the nature of the earthquake source determination problem circular, as source information is mapped as structure. Presented here is one possible remedy to this situation, where through a two-step approach we first constrain Earth structure using data independent of the earthquake of interest. In this study, we focus on a region in Western Australia with low seismicity and minimal instrument coverage and use the CAPRA/LP temporary deployment to demonstrate that reliable structural models of the upper lithosphere can be obtained from an independent collection of teleseismic and ambient noise datasets. Apart from teleseismic receiver functions (RFs), we obtain group velocities from the cross-correlation of ambient noise and phase velocities from the traditional two-station method using carefully selected teleseismic earthquakes and station pairs. Crustal models are then developed through the joint inversion of dispersion data and RFs, and structural Green's functions are computed from a layered composite model. In the second step of this comprehensive approach, we apply full waveform inversion (three-component body and surface waves) to the 2007 M L= 5.3 Shark Bay, Western Australia, earthquake to estimate its source parameters (seismic moment, focal mechanism, and depth). We conclude that the full waveform inversion analysis provides constraints on the orientation of fault planes superior to a first motion interpretation

High-frequency ambient noise tomography of southeast Australia: New constraints on Tasmania's tectonic past

The island of Tasmania, at the southeast tip of Australia, is an ideal natural laboratory for ambient noise tomography, as the surrounding oceans provide an energetic and relatively even distribution of noise sources. We extract Rayleigh wave dispersion curves from the continuous records of 104 stations with ∼15 km separation. Unlike most passive experiments of this type, which observe very little coherent noise below a 5 s period, we clearly detect energy at periods as short as 1 s, thanks largely to the close proximity of oceanic microseisms on all sides. The main structural elements of the eastern and northern Tasmanian crust are revealed by inverting the dispersion curves (between 1 and 12 s period) for both group and phase velocity maps. Of particular significance is a pronounced band of low velocity, observed across all periods, that underlies the Tamar River Valley and continues south until dissipating in southeast Tasmania. Together with evidence from combined active source and teleseismic tomography and heat flow data, we interpret this region as a diffuse zone of strong deformation associated with the mid-Paleozoic accretion of oceanic crust along the eastern margin of Proterozoic Tasmania, which has important implications for the evolution of the Tasman Orogen of eastern Australia. In the northwest, a narrower low-velocity anomaly is seen in the vicinity of the Arthur Lineament, which may be attributed to local sediments and strong deformation and folding associated with the final phases of the Tyennan Orogeny

A New Shear-Velocity Model of Continental Australia Based on Multi-Scale Surface-Wave Tomography

The Australian Seismological Reference Model (AuSREM) represents a state-of-the-art geophysical synthesis of the Australian continent. To date, its shear-wave component has limited resolution at lower-crustal to uppermost-mantle depths, where it is mainly constrained by sparse measurements collected at the local scale. In this study, we compile a large data set of surface-wave phase velocities based on seismic ambient noise and teleseismic earthquakes, to produce Rayleigh and Love phase-velocity maps of continental Australia in a broad period range (4–200 s). Via transdimensional Bayesian inversion, we translate the phase-velocity maps into a 3-D shear-wave velocity model extending to 300 km depth. Owing to the unprecedented seismic coverage and to the joint use of ambient-noise and teleseismic data, the retrieved model fills a tomographic gap in the known shear-wave velocity structure of the continent, comprising lower-crustal to uppermost mantle depths. Consistent with AuSREM, strong velocity heterogeneities in our model highlight the (faster) cratonic blocks and the (slower) sedimentary basins at upper-crustal depths. At mantle depths, the most prominent feature of the continent is a large-scale eastward decrease in shear-wave velocity. We interpret our observations in light of the relevant literature, and produce depth maps of the Moho and lithosphere-asthenosphere boundary (LAB). Notably, our LAB proxy features a stripe of thicker lithosphere extending to the east coast, which is not visible in the AuSREM LAB model. This observation supports the idea that lateral variations in lithospheric thickness control both the composition and volume of surface volcanism in eastern Australia

CHANGES IN FISHING EFFICIENCY OF THE POLE-AND-LINE SKIPJACK TUNA FLEET BASED AT SORONG IRIAN JAYA, INDONESIA

Data from the fishing operations of the pole-and-line fishing fleet based in Sorong, IrianJava, eastern Indonesia are examined to assess the status of the fishery in that region

The mechanisms underpinning Cenozoic intraplate volcanism in eastern Australia: Insights from seismic tomography and geodynamic modelling

Cenozoic intraplate volcanism is widespread throughout much of eastern Australia and manifests as both age-progressive volcanic tracks and non-age-progressive lava fields. Various mechanisms have been invoked to explain the origin and distribution of the volcanism, but a broad consensus remains elusive. We use results from seismic tomography to demonstrate a clear link between lithospheric thickness and the occurrence, composition, and volume of volcanic outcrop. Furthermore, we find that non-age-progressive lava fields overlie significant cavities in the base of the lithosphere. Based on numerical simulations of mantle flow, we show that these cavities generate vigorous mantle upwellings, which likely promote decompression melting. However, due to the intermittent nature of the lava field volcanics over the last 50 Ma, it is probable that transient mechanisms also operate to induce or enhance melting. In the case of the Newer Volcanics Province, the passage of a nearby plume appears to be a likely candidate. Our results demonstrate why detailed 3-D variations in lithospheric thickness, plate motion, and transient sources of mantle heterogeneity need to be considered when studying the origin of non age-progressive volcanism in continental interiors

Seismic tomography of the North Anatolian Fault: New insights into structural heterogeneity along a continental strike-slip fault

Knowledge of the structure of continental strike-slip faults within the lithosphere is essential to understand where the deformation occurs and how strain localizes with depth. With the aim to improve the constraints on the lower crust and upper mantle structure of a major continental strike-slip fault, we present a high-resolution teleseismic tomography of the North Anatolian Fault Zone (NAFZ) in Turkey. Our results highlight the presence of a relatively high velocity body between the two branches of the fault and significant along-strike variations in the NAFZ velocity structure over distances of ~20 km. We interpret these findings as evidence of laterally variable strain focussing caused by preexisting heterogeneity. Low velocities observed in the crust and upper mantle beneath the NAFZ support the presence of a narrow shear zone widening in the upper mantle, where we constrain its width to be ~50 km.Major funding was provided by the UK Natural Environment Research Council (NERC) under grant NE/I028017/1. Equipment was provided and supported by the NERC Geophysical Equipment Facility (SEIS-UK). This project is also supported by Bogaziçi University Scientific ˘ Research Projects (BAP) under grant 6922 and Turkish State Planning Organization (DPT) under the TAM project, number 2007K120610

Moving towards more responsible fishing practices in Australia’s northern prawn fishery

Bycatch in Australia s Northern Prawn Fishery (NPF) is extremely diverse in both species and size composition. This makes developing responsible fishing practices to reduce the amount of bycatch in the NPF, a complex process. During this study we assessed the performance of Bycatch Reduction Devices (BRDs) that could be used in the NPF. We tested 16 different BRDs which can be grouped into one of 3 categories: (1) inclined grids (2) fish exclusion devices and (3) square-mesh codends. Four inclined grids were tested during our study. All the grids were extremely effective at excluding large animals such as sharks, stingrays and sea turtles. Three different fish exclusion devices were tested. Fish exclusion ranged between 0 and 39 percent, depending on the device. Prawn retention rates for both inclined grids and fish exclusion devices varied between devices. Two sizes of square-mesh codend (38 mm and 45 mm) were also tested. More than 95% of market-sized prawns were retained. Fish exclusion varied greatly between species. Optimum exclusion of bycatch for the NPF could be provided by a combination of devices from each of the three categories

Transdimensional inversion of receiver functions and surface wave dispersion

We present a novel method for joint inversion of receiver functions and surface wave dispersion data, using a transdimensional Bayesian formulation. This class of algorithm treats the number of model parameters (e.g. number of layers) as an unknown in the problem. The dimension of the model space is variable and a Markov chain Monte Carlo (McMC) scheme is used to provide a parsimonious solution that fully quantifies the degree of knowledge one has about seismic structure (i.e constraints on the model, resolution, and trade-offs). The level of data noise (i.e. the covariance matrix of data errors) effectively controls the information recoverable from the data and here it naturally determines the complexity of the model (i.e. the number of model parameters). However, it is often difficult to quantify the data noise appropriately, particularly in the case of seismic waveform inversion where data errors are correlated. Here we address the issue of noise estimation using an extended Hierarchical Bayesian formulation, which allows both the variance and covariance of data noise to be treated as unknowns in the inversion. In this way it is possible to let the data infer the appropriate level of data fit. In the context of joint inversions, assessment of uncertainty for different data types becomes crucial in the evaluation of the misfit function. We show that the Hierarchical Bayes procedure is a powerful tool in this situation, because it is able to evaluate the level of information brought by different data types in the misfit, thus removing the arbitrary choice of weighting factors. After illustrating the method with synthetic tests, a real data application is shown where teleseismic receiver functions and ambient noise surface wave dispersion measurements from the WOMBAT array (South-East Australia) are jointly inverted to provide a probabilistic 1D model of shear-wave velocity beneath a given station