Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere-asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.

Volcanic tremor has attracted considerable attention by seismologists because of its potential value as a tool for forecasting eruptions and better understanding the physical processes that occur inside active volcanoes. However, unlike tectonic earthquakes where the dominant source process is brittle failure of rock, the driving mechanism of tremor seems to involve complex interactions of magmatic fluids with the surrounding bedrock. These interactions are responsible for the following distinct characteristics found in volcanic tremor recorded at many volcanoes worldwide: (a) the onset of tremor may be emergent or impulsive, with its amplitude showing in many cases a direct relationship to the volcanic activity; (b) in the frequency domain the spectra consist of a series of sharp peaks in the band 0.1^7 Hz, representing either a fundamental frequency and its harmonics, or a random distribution, while quite often they exhibit temporal variations in their content; (c) the depth of the source can vary considerably from one volcano to another in the range of a few hundred metres to 40 km; (d) tremor may occur prior to and/or after eruptions with a duration that ranges from several minutes to several days or months. The methods used to study tremor include spectral analysis using both the Fast Fourier Transform and the Maximum Entropy Method, polarisation analysis of the wavefield and methods that make use of array data to deduce the backazimuth and type of the seismic waves as well as the location of the source. Visual and/or recorded acoustic observations of the ongoing volcanic activity have assisted in many cases to further constrain proposed physical mechanisms for the generation of tremor. The models suggested as possible sources of tremor can be grouped as follows: (a) fluid-flow-induced oscillations of conduits transporting magmatic fluids; (b) excitation and resonance of fluid-filled cracks; (c) bubble growth or collapse due to hydrothermal boiling of groundwater; (d) a variety of models involving the oscillations of magma bodies with different geometries. It has been proposed by many authors that the source of tremor is not unique and may differ from one volcano to another, a fact that adds more difficulty in the source modelling efforts. As data quality, computer power and speed are improving, it may be possible in the near future to decipher and accurately model tremor source processes at different volcanic environments. ß 2002 Elsevier Science B.V. All rights reserved.

East Greenland's continental crust is little explored compared to the geologically related Norway. As part of a comprehensive geophysical study of east Greenland, the Alfred Wegener Institute for Polar and Marine Research acquired eight seismic refraction profiles between 70°N and 76°N. We present the results of four profiles combined with magnetic and geological data. The data set revealed pronounced differences in the crustal architecture north and south of Kong Oscar Fjord (72°N). The crust north of Kong Oscar Fjord shows three structural units, a Caledonian, a Devonian, and a Mesozoic to Tertiary, whereas only a Caledonian and a Devonian to Tertiary unit can be distinguished in the southern area. A seismic high‐velocity layer was discovered in the lower crust north of Kong Oscar Fjord which is absent in the southern area. This high‐velocity layer coincides with a pronounced magnetic anomaly related to Tertiary magmatism and is interpreted as Tertiary magmatic underplate. By integrating geophysical and geological data, we showed that a different evolution of the areas north and south of Kong Oscar Fjord started after the Devonian extensional collapse of the Caledonides. Mesozoic extension shifted to the east and preserved Devonian structures in the northern area, whereas the Devonian crust was further thinned and weakened in the southern area. We propose that it allowed an easier ascent of Tertiary melts to the surface than the crust north of Kong Oscar Fjord, where most of the melts got trapped in the lower crust. The model explains the distribution of extrusives and intrusives in east Greenland's Tertiary igneous province.

Roughly 60% of the Earth's outer surface is comprised of oceanic crust formed by volcanic processes at mid-ocean ridges (MORs). Although only a small fraction of this vast volcanic terrain has been visually surveyed and/or sampled, the available evidence suggests that explosive eruptions are rare on MORs, particularly at depths below the critical point for

Mobility of glaciers such as rapid retreat or disintegration of large ice volumes produces a large variety of different seismic signals. Thus, evaluating cryospheric seismic events (e.g., changes of their occurrence in space and time) allows to monitor glacier dynamics. We analyze a 1 year data span recorded at the Neumayer seismic network in Antarctica. Events are automatically recognized using hidden Markov models. In this study we focused on a specific event type occurring close to the grounding line of the Ekström ice shelf. Observed waveform characteristics are consistent with an initial fracturing followed by the resonance of a water‐filled cavity resulting in a so‐called hybrid event. The number of events detected strongly correlates with dominant tide periods. We assume the cracking to be driven by existing glacier stresses trough bending. Voids are then filled by seawater, exciting the observed resonance. In agreement with this model, events occur almost exclusively during rising tides where cavities are opened at the bottom of the glacier, i.e., at the sea/ice interface.