G. C. Roult

Antarctica II: Upper-mantle structure from velocities and anisotropy

Abstract To improve the lateral resolution of three-dimensional seismic wave velocity models in Antarctica and the surrounding oceans, we have analysed direct earthquake-to-station Rayleigh-wave data observed on the vertical high-gain long-period and the very long period components of seven GEOSCOPE stations located in the southern hemisphere and three other stations at equatorial latitudes. The phase velocities of Rayleigh waves along 400 well-distributed paths are obtained in the period range 60–300 s, by fitting the data with synthetic seismograms computed with known source parameters in a reference earth model represented by the Preliminary Reference Earth Model (PREM). Corrections for shallow layers have been carefully applied to the observed phase velocities. The geographical distributions of phase velocities and azimuthal anisotropy are then computed with the tomographic method without any a priori regionalization developed by Montagner (Ann. Geophys., 4(B3): 283–294, 1986). The results show some new and important features of Antarctica and the southern hemisphere. The locations of velocity anomalies are well resolved. The eastern part of Antarctica corresponds to a craton-like structure down to depths of about 250 km, and the highest velocities are observed in Enderby Land, where some of the oldest rocks in the world have been sampled. The low velocities are located along the ridges encircling the Antarctic continent. The lowest velocities appear in some areas corresponding to hotspots (Crozet, Kerguelen, Macquarie and Balleny Islands). Also, an elongated low velocity is found on the western flank of the Transantarctic Mountains, which might be related to the existence of a rift zone similar to the African rift. The Australia-Antarctica Discordance (AAD) presents slow velocities near the surface but fast velocities below the lithosphere. These main features are discussed in the framework of the Gondwana hypothesis and the earlier supercontinent. The first azimuthal anisotropy results are also discussed. Anisotropy values are smaller within the Antarctic continent than in the surrounding oceans. They are also small in the AAD but particularly large in the areas around it, suggesting an active tectonic process characterized by a downward flow at depth, a good candidate for a cold spot or a new subduction zone.

Analysis of ‘background’ free oscillations and how to improve resolution by subtracting the atmospheric pressure signal

Abstract It is well established that the Earth oscillates continuously with its fundamental modes excited even on seismically quiet days but the source of this ‘background’ excitation is unclear. The source could be internal (for example, tectonic motions) or external (for example, atmospheric turbulence). The GEOSCOPE broad-band seismic network provides a long-term, high quality, seismic data set to better observe these oscillations. Nine years of data from the quietest GEOSCOPE station (WUS) show background oscillations as well as evidence of a small annual variation with the most energy in June and July. We also use 4 years of simultaneous pressure/seismometer data from the TAM station to show how to improve the resolution of these oscillations by subtracting the pressure effect on the acceleration signal. Removing the pressure effect allows us to isolate some very low frequency spheroidal fundamental modes as low as angular order l=2 after large earthquakes and down to l=8 during quiet periods.

Antarctica I: Deep structure investigations inferred from seismology; a review

Abstract The strategic position of Antarctica and its role have always been seen as clues to the tectonic and geodynamic history of the southern hemisphere. Direct investigations are difficult, and seismology was, and still is, a very efficient tool for answering some important questions. Major seismological results from the beginning of the century to the present are reviewed, taking into consideration their limitations owing to the lack of stations and data. The existing regional Earth models reviewed are very simple because of poor processing facilities. The availability of high-quality data, obtained with the GEOSCOPE network in this part of the southern hemisphere, has recently led to the construction of maps of lateral heterogeneities of Antarctica with an improved resolution. A brief report of our contribution is presented.

The 2007 eruptions and caldera collapse of the Piton de la Fournaise volcano (La Réunion Island) from tilt analysis at a single very broadband seismic station

Seismic records from La Reunion Island very broadband Geoscope station are investigated to constrain the link between the 2007 eruptive sequence and the related caldera collapse of the Piton de la Fournaise volcano. Tilt estimated from seismic records reveals that the three 2007 eruptions belong to a single inflation-deflation cycle. Tilt trend indicates that the small-volume summit eruption of 18 February occurred during a phase of continuous inflation that started in January 2007. Inflation decelerated 24 days before a second short-lived, small-volume eruption on 30 March, almost simultaneous with a sudden, large-scale deflation of the volcano. Deflation rate, which had stabilized at relatively low level, increased anew on 1 April while no magma was erupted, followed on 2 April by a major distal eruption and on 5 April by a summit caldera collapse. Long-term tilt variation suggests that the 2007 eruptive succession was triggered by a deep magma input.

Towards multiscalar and multiparameter networks for the next century: The French efforts

Abstract The design of the future networks cannot be separated from the scientific issues which scientists are faced to, nor from the new technology tools that are to be developed for the next century. The trilogy, science, scientist and instrument must be well balanced in order to make implementation of the network achievable. Since most of the scientific issues are multiscalar for spatial scales as well as for temporal scales, we present the general design of the network which should enable us to address these scientific issues. They must be able to explore different scales of heterogeneities from global scale to local scale. We review in this paper the different technological developments, which are presently explored, and which prefigure what will be the future geophysical networks. The technical developments carried out by various French scientific programs (GEOSCOPE, OPTIMISM, SOFM) will be presented as an illustration. The concept of multiparameter station is defined and it demonstrated the great scientific interest in installing different kinds of sensors at the same place in a seismic station. Every seismic station will have to be transformed into a multiparameter geophysical station. It is also necessary to cover the oceanic desert by a dense network of geophysical observatories which can fulfill the scientific requirements of the whole geoscientist community. The recent progresses made by Japanese, French and US groups show that the technical challenge of installing permanent geophysical ocean bottom observatories (coined GOBOs) is not out of reach. Therefore, the new generation network will have to be composed of multiparameter continental or oceanic stations including at least broadband seismometers, microbarometers, microthermometers, and eventually other sensors (electromagnetic sensors, strainmeters, GPS,…). The design of the complete chain of acquisition, from the sensor to the distribution of data, will imply the integration of all the technical progresses made in micromechanics, electronics, computers, space science, and telecommunication systems. Finally, the new generation of geophysical networks must be able to provide scientific data at all scales, from the global scale to the local scale. The networks at all scales must be coordinated in order to constitute a hierarchical or multiscalar network, which will constitute the basic tool for addressing scientific issues in geosciences. The efforts necessary to achieve such a hierarchical, multiparameter network represent a formidable technological challenge for the next ten years.

New refinements in attenuation measurements from free-oscillation and surface-wave observations

This paper deals with improvements in the accuracy and precision of attenuation measurements in the Earth, obtained from observations of surface waves and free oscillations. We focus our study on the observation of the resonant frequencies of the Earth and propose an improved method for deriving attenuation data from the free-oscillation amplitude decays that reduces uncertainties. We test our method on different theoretical seismograms computed for two different Earth models, a spherically symmetric one (the PREM model) and a laterally heterogeneous one (the M84A model). We compute seismograms for: the spheroidal fundamental mode only, the fundamental mode and overtones, without noise and with actual noise. Some biases related to the method are systematically found and corrected. Our conclusion is that if we fulfill some conditions in attenuation measurements by taking into account different parameters, such as the observed resonance period, the relative amplitude of the peak for the particular great-circle path considered, the noise level of the component in the station considered, it is possible to reduce the uncertainty in amplitude measurements. Doing this, we are more confident in free-oscillation measurements than in those of surface waves. It is evident that the presence of noise may increase the mean Q inferred from long-time series if no care is taken relative to the signal-to-noise level. The method is then systematically applied to a dataset of about one thousand vertical seismograms provided by the GEOSCOPE network after various large earthquakes, which occurred from 1982 to 1995. We determine reliable estimates of the mean frequency and attenuation of the fundamental spheroidal mode for the angular order range l=21–51 (period range 175–336 s). Some results are discarded for reasons explained. We compare the consistency of this new dataset with previously published Q observations and commonly used models (PREM, QM1, QL6). We point out the difficulties of obtaining reliable Q factors and show the discrepancies between surface waves and normal modes measurements. It is noted that surface wave attenuation measurements may be strongly affected by the time window used in order to extract each particular surface wave train, but fit normal modes results, in some cases.

THE GEOSCOPE PROGRAM : ITS DATA CENTER

Abstract The purpose of the GEOSCOPE program was the installation of about 20 stations well distributed worldwide (in particular in the southern hemisphere), in the standard configuration defined by the FDSN (VBB 24 bit, continuous recording at 20 samples/s (sps)). The installation is almost complete. The effort has been focused on the accessibility of data from our Data Center. Data can be obtained either on line through the WWW GEOSCOPE server (http://geoscope.ipgp.jussieu.fr), or through our anonymous ftp, or through CD-ROM production or through the IRIS/SPYDER system for large earthquakes. In the near future easier ways will be available, such as autoDRM (automatic Data Request Management) and NetDC requests (Networked Data Centers, protocol proposed by the IRIS DMC of Seattle).

Normal modes of the Earth

The free oscillations of the Earth were observed for the first time in the 1960s. They can be divided into spheroidal modes and toroidal modes, which are characterized by three quantum numbers n,l, and m. In a spherically symmetric Earth, the modes are degenerate in m, but the influence of rotation and lateral heterogeneities within the Earth splits the modes and lifts this degeneracy. The occurrence of the Great Sumatra-Andaman earthquake on 24 December 2004 provided unprecedented high-quality seismic data recorded by the broadband stations of the FDSN (Federation of Digital Seismograph Networks). For the first time, it has been possible to observe a very large collection of split modes, not only spheroidal modes but also toroidal modes.

Seismic Monitoring at Piton de la Fournaise

The Piton de la Fournaise Volcanological Observatory (“Observatoire Volcanologique du Piton de la Fournaise”, OVPF) started operating at the end of 1979. The first monitored eruption by the OVPF was that of 1981 (Bachelery et al. in Nature 297(5865):395–397, 1982; Chevallier et al. in L’eruption du mois de fevrier 1981 au Piton de la Fournaise (Ile de la Reunion, Ocean Indien). Phenomenologie et remarques structurales, 293(II):187–190, 1981). Since then, the surveillance networks, and particularly the seismic network, have been dramatically improved. In addition, various seismic experiments have been carried out in parallel with the monitoring of the volcano. This chapter is aimed at summarizing the main results obtained within both the framework of the monitoring and that of research experiments.