Description: Publication date: Available online 6 December 2014 Source: Polar Science Author(s): Alexey Shulgin , Hans Thybo Controlled source seismic investigation of crustal structure below ice covers is an emerging technique. We have recently conducted an explosive refraction/wide-angle reflection seismic experiment on the ice cap in east-central Greenland. The data-quality is high for all shot points and a full crustal model can be modelled. A crucial challenge for applying the technique is to control the sources. Here, we present data that describe the efficiency of explosive sources in the ice cover. Analysis of the data shows, that the ice cap traps a significant amount of energy, which is observed as a strong ice wave. The ice cap leads to low transmission of energy into the crust such that charges need be larger than in conventional onshore experiments to obtain reliable seismic signals. The strong reflection coefficient at the base of the ice generates strong multiples which may mask for secondary phases. This effect may be crucial for acquisition of reflection seismic profiles on ice caps. Our experience shows that it is essential to use optimum depth for the charges and to seal the boreholes carefully.

Description: Publication date: Available online 15 December 2014 Source: Polar Science Author(s): Genti Toyokuni , Hiroshi Takenaka , Masaki Kanao , Seiji Tsuboi , Yoko Tono We calculate regional synthetic seismograms for a realistic structure model beneath Greenland, including both surface topography and ice sheet thickness. In 2009, the multinational GreenLand Ice Sheet monitoring Network (GLISN) was initiated to monitor seismic events in and around the Greenland. Japan has been sending a field team to Greenland each year since 2011, to construct and maintain the GLISN stations especially on ice. However, the thick and heterogeneous Greenland ice sheet is considered to cause various kinds of distortion on seismic waveforms observed at these ice stations. We have been working for construction of a numerical technique, which can calculate accurate regional seismic wavefields with small computational requirements. In this paper, we calculate elastic wave propagation up to 2 Hz for four structural models of the Greenland ice sheet radiated from a seismic source with various depths and mechanisms. Our computations for a realistic ice sheet model, the near-surface seismic source produced a very characteristic wave train with the group velocity smaller than the S -wavespeed in the ice, which is considered as an ice-sheet guided S wave, developed by a superposition of post-critical reflections between the free surface and the ice bed. We named this wave “ Le ” on the analogy of the Lg wave, a crustally guided S wave (“ e ” comes from the German word “Eisdecke”, meaning the ice sheet). Furthermore, computation for a deeper seismic source resulted in reinforcement of the crustal Sg -coda wave with a group velocity range of ∼ 3.1-2.6 km/s, which well explains the characteristic waveform observed on the Greenland ice sheet .

Description: Publication date: Available online 11 November 2014 Source: Polar Science Author(s): Takaki Iwata , Masaki Kanao In this study, we evaluate the annual variation in teleseismic detection capability at Syowa Station (69.0°S, 39.6°E) located in East Antarctica, a variation that has been noted in previous studies. For the quantitative evaluation of the annual variation, we introduced a statistical model of a magnitude–frequency distribution of earthquakes covering the entire magnitude range. The annual variation in the model parameter that quantifies the detection capability was then estimated by using Bayesian analysis. In the estimation, we incorporated the annual variation in air temperature at the station and succeeded in clarifying the significant effect that the variation in temperature has on the teleseismic detection capability.

Description: Publication date: Available online 4 December 2014 Source: Polar Science Author(s): Marek Grad , Rolf Mjelde , Lech Krysiński , Wojciech Czuba , Audun Libak , Aleksander Guterch As a part of the large international panel “IPY Plate Tectonics and Polar Gateways” within the “4th International Polar Year” framework, extensive geophysical studies were performed in the area of southern Svalbard, between the Mid-Atlantic Ridge and the Barents Sea. Seismic investigations were performed along three refraction and wide-angle reflection seismic lines. Integrated with gravity data the seismic data were used to determine the structure of the oceanic crust, the transition between continent and ocean (COT), and the continental structures down to the lithosphere-asthenosphere system (LAB). We demonstrate how modeling of multiple water waves can be used to determine the sound velocity in oceanic water along a seismic refraction profile. Our 2D seismic and density models documents 4–9 km thick oceanic crust formed at the Knipovich Ridge, a distinct and narrow continent-ocean transition (COT), the Caledonian suture zone between Laurentia and Barentsia, and 30–35 km thick continental crust beneath the Barents Sea. The COT west of southern Spitsbergen expresses significant excess density (more than 0.1 g/cm 3 in average), which is characteristic for mafic/ultramafic and high-grade metamorphic rocks. The results of the gravity modeling show relatively weak correlation of the density with seismic velocity in the upper mantle, which suggests that the horizontal differences between oceanic and continental mantle are dominated by mineralogical changes, although a thermal effect is also present. The seismic velocity change with depth suggests lherzolite composition of the uppermost oceanic mantle, and dunite composition beneath the continental crust.

Description: Publication date: Available online 7 November 2014 Source: Polar Science Author(s): Olaf Eisen , Coen Hofstede , Anja Diez , Yngve Kristoffersen , Astrid Lambrecht , Christoph Mayer , Rick Blenkner , Sverrir Hilmarsson We present implementations of vibroseis system configurations with a snowstreamer for over-ice long-distance seismic traverses (>100 km). The configurations have been evaluated in Antarctica on ice sheet and ice shelf areas in the period 2010–2014. We discuss results of two different vibroseis sources: Failing Y-1100 on skis with a peak force of 120 kN in the frequency range 10–110 Hz; IVI EnviroVibe with a nominal peak force of 66 kN in the nominal frequency range 10–300 Hz. All measurements used a well-established 60 channel 1.5 km snowstreamer for the recording. Employed forces during sweeps were limited to less than 80% of the peak force. Maximum sweep frequencies, with a typical duration of 10 s, were 100 and 250 Hz for the Failing and EnviroVibe, respectively. Three different concepts for source movement were employed: the Failing vibrator was mounted with wheels on skis and pulled by a Pistenbully snow tractor. The EnviroVibe was operated self-propelled on Mattracks on the Antarctic plateau. This lead to difficulties in soft snow. For later implementations the EnviroVibe with tracks was put on a polyethylene (PE) sled. The sled had a hole in the center to lower the vibrator baseplate directly onto the snow surface. With the latter setup, data production varied between 20 km/day for 6-fold and 40 km/day for single fold for 9 h/day of measurements. The combination of tracks with the PE-sled was especially advantageous on hard and rough surfaces because of the flexibility of each component and the relatively lose mounting. The systems presented here are suitable to obtain data of subglacial and sub-seabed sediment layers and englacial layering in comparable quality as obtained from marine geophysics and land-based explosive surveys. The large offset aperture of the streamer overcomes limitations of radar systems for imaging of steep along-track subglacial topography. With joint international scientific and logistic efforts, large-scale mapping of Antarctica's and Greenland's subglacial geology, ice-shelf cavity geometries and sea-bed strata, as well as englacial structures can be achieved.

Description: Publication date: Available online 29 October 2014 Source: Polar Science Author(s): Vera Schlindwein , Andrea Demuth , Edith Korger , Christine Läderach , Florian Schmid The Arctic mid-ocean ridge system constitutes the most active source of earthquakes in the north polar region. However, the characteristics of its earthquake activity at teleseismic and local scales are not well studied because of the remote location of the ridge. We present here a comprehensive seismicity analysis that compares the teleseismic earthquake record of 35 years drawn from the catalogue of the International Seismological Centre with reconnaissance-style local earthquake records at six locations along the ridge that were instrumented either with ocean bottom seismometers or with seismometers on drifting ice floes. The teleseismic earthquake activity varies along the ridge and reflects ultraslow spreading processes with more and larger earthquakes produced in magma-rich regions than in magma-starved areas. Large magnitude earthquakes M  > 5.5 are common along this ultraslow spreading ridge. Locally recorded earthquakes are of small magnitude ( M  〈 2) and probably reflect the formation of the pronounced topographic relief. Their size and event rate is not as variable along the ridge as that of teleseismic events. Locally recorded earthquakes in the upper mantle are generated at several locations. Their focal depths do not depend on spreading rate but reflect the thermal state of the lithosphere with very deep earthquakes indicating an exceptionally cold lithosphere.

Description: Publication date: Available online 5 October 2014 Source: Polar Science Author(s): E.A. Melnik , V.D. Suvorov , E.V. Pavlov , Z.R. Mishenkina The estimate of seismic lithosphere thickness in Siberia remains controversial in spite of long-range controlled–source data available from peaceful nuclear explosions (PNE). Published models of layered upper mantle based on this evidence fail to unambiguously constrain the asthenospheric depth. The observed velocity changes may be due either to vertical layering or to lateral heterogeneity, which are difficult to discriminate because of large (1000 km) PNE spacing. Among the upper mantle models, obtained with reference to Moho velocities derived from higher-resolution chemical explosion data, we focus especially on lateral density heterogeneity. The model reveals three velocity layers, with velocities 8.0–8.5 km/s in Layer 1, 8.6–8.7 km/s in Layer 2, and ∼8.5 km/s in Layer 3. Layers 2, which varies strongly in thickness, may consist of dense eclogite, judging by the high velocities. Its base may correspond to the base of the lithosphere underlain by the lower-velocity asthenospheric material of Layer 3. The lateral variations in velocity within Layer 1 and in thickness of Layer 2 correlate with major tectonic units: the West Siberian basin, the Tunguska basin with the Permian–Triassic continental flood basalts (the large igneous province of Siberian Traps), as well as the Vilyui basin and the Yakutian kimberlite province. Isostasy in the West Siberian and Vilyui basins results in thick sediments and thin crust, while the large depths of the basement and the intra–crustal discontinuity in the Tunguska basin isostatically compensate the elevated surface topography due to voluminous lavas. The magmatism left its imprint in the mantle as an attenuated “eclogitic layer” beneath the Tunguska basin. However, the available data are still insufficient to understand the exact causes of this attenuation, because mantle conditions may have changed during the elapsed 250 m.y. since then.

Description: Publication date: Available online 7 October 2014 Source: Polar Science Author(s): Takeshi Inoue , Sakae Kudoh , Masaki Uchida , Yukiko Tanabe , Masakane Inoue , Hiroshi Kanda We used observational and experimental analyses to investigate the photosynthetic activity and water relationships of five lichen species attached to different substrates in a glacier foreland in the High Arctic, Ny-Ålesund, Svalbard (79°N) during the snow-free season in 2009 and 2010. After the rains ceased, lichens and their attached substrates quickly dried, whereas photosynthetic activity in the lichens decreased gradually. The in situ photosynthetic activity was estimated based on the relative electron transportation rate (rETR) in four fruticose lichens: Cetrariella delisei , Flavocetraria nivalis , Cladonia arbuscula ssp . mitis , and Cladonia pleurota . The rETR approached zero around noon, although the crustose lichen Ochrolechia frigida grown on biological soil crust (BSC) could acquire water from the BSC and retain its WC to perform positive photosynthesis. The light-rETR relationship curves of the five well-watered lichens were characterized into two types: shade-adapted with photoinhibition for the fruticose lichens, and light-adapted with no photoinhibition for O. frigida . The maximum rETR was expected to occur when they could acquire water from the surrounding air or from substrates during the desiccation period. Our results suggest that different species of Arctic lichens have different water availabilities due to their substrates and/or morphological characteristics, which affect their photosynthetic active periods during the summer.

Description: Publication date: Available online 30 September 2014 Source: Polar Science Author(s): M. Olivieri , G. Spada It has been suggested that the Greenland ice sheet is the cause of earthquake suppression in the region. With few exceptions, the observed seismicity extends only along the continental margins of Greenland, which almost coincide with the ice sheet margin. This pattern has been put forward as further validation of the earthquake suppression hypothesis. In this review, new evidence in terms of ice melting, post-glacial rebound and earthquake occurrence is gathered and discussed to re-evaluate the connection between ice mass unloading and earthquake suppression. In Greenland, the spatio-temporal distribution of earthquakes indicates that seismicity is mainly confined to regions where the thick layer of ice is absent and where significant ice melting is presently occurring. A clear correlation between seismic activity and ice melting in Greenland is not found. However, earthquake locations and corresponding depth distributions suggest two distinct governing mechanisms: post-glacial rebound promotes moderate-size crustal earthquakes at Greenland’s regional scale, while current ice melting promotes shallow low magnitude seismicity locally.

Description: Publication date: Available online 16 September 2014 Source: Polar Science Author(s): Galina Antonovskaya , Yana Konechnaya , Elena O. Kremenetskaya , Vladimir Asming , Tormod Kværna , Johannes Schweitzer , Frode Ringdal This paper presents preliminary results from a cooperative initiative between the Norwegian Seismic Array (NORSAR) institution in Norway and seismological institutions in NW Russia (Arkhangelsk and Apatity). We show that the joint processing of data from the combined seismic networks of all these institutions leads to a considerable increase in the number of located seismic events in the European Arctic compared to standard seismic bulletins such as the NORSAR reviewed regional seismic bulletin and the Reviewed Event Bulletin (REB) issued by the International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) organization. The increase is particularly pronounced along the Gakkel Ridge to the north of the Svalbard and Franz-Josef Land archipelagos. We also note that the vast majority of the events along the Gakkel Ridge have been located slightly to the south of the ridge. We interpret this as an effect of the lack of recording stations closer to and north of the Gakkel Ridge, and the use of a one-dimensional velocity model which is not fully representative for travel-times along observed propagation paths. We conclude that while the characteristics of earthquake activity in the European Arctic is currently poorly known, the knowledge can be expected to be significantly improved by establishing the appropriate cooperative seismic recording infrastructures.