Deep Earth Dynamics Research Articles

Ocean bottom seismic noise (1–10 000 s)

There is now a good consensus in the geophysical scientific community of the necessity to install broadband seismic stations on the ocean bottom in order to improve our knowledge of the deep Earth dynamics. However, it is necessary to first investigate the level of seismic noise (0.01–10<th>000 s) and to compare it with the expected amplitude of seismic signal due to earthquakes. During the French pilot experiment SISMOBS/OFM (28 April–11 May 1992) in the Atlantic Ocean, two sets of Guralp CMG3 broadband seismometers were successfully installed and recovered. The first set, named OFP, was installed inside the ODP borehole 396B at 292 m below seafloor and the second one, named OFM, was installed on the sea bottom (semi-buried) close to the hole. A comparison of broadband seismic noise (1–3600 s) between OFP, OFM, and a typical continental GEOSCOPE seismic station SSB shows that the noise level of OFM is smaller (resp. larger) at periods longer than 20 s (resp. shorter than 2 s) than the one in the borehole and in SSB. The detection threshold is very low and makes it possible to get good records of teleseismic events as small as 5.4. These results were very encouraging since they demonstrate that in the future ocean bottom seismometers will provide very valuable information on earthquakes and Earth structure.

The core–mantle boundary layer and deep Earth dynamics

Recent seismological work has revealed new structures in the boundary layer between the Earth's core and mantle that are altering and expanding perspectives of the role this region plays in both core and mantle dynamics. Clear challenges for future research in seismological, experimental, theoretical and computational geophysics have emerged, holding the key to understanding both this dynamic system and geological phenomena observed at the Earth's surface.

Dipolar reversal states of the geomagnetic field and core–mantle dynamics

Palaeomagnetic records from lavas suggest that at least two specific inclined dipolar field configurations have dominated the reversal process for the past ten million years. These long-lived states provide a way to explain directional rebounds, aborted reversals and the recording in sediments of what appear to be preferred longitudinal paths of the virtual geomagnetic pole. The polar orientations correlate with near-radial flux concentrations recognizable when today's field is stripped of its axial dipole, and with lower-mantle seismic anomalies, suggesting a tie to deep-Earth dynamics.

150 years of magnetic observatories: Recent researches on world data

A joint Discussion Meeting of the Royal Astronomical Society and the Royal Irish Academy, held on January 11th, 1991, commemorated the establishment of some early magnetic observatories, discussed recent research using global geomagnetic data and described the present status of magnetic observatories in the United Kingdom. The observatory and instruments at the Dublin magnetic observatory; the origins of the Greenwich magnetic observatory, and why it eventually had to be resited; and the history of the Munich magnetic observatory formed the historical part of the proceedings. Current research topics discussed were the geomagnetic secular variation and deep Earth structure and dynamics; fluid flow patterns near the top of the core; the origin of the annual variation of the geomagnetic field; results of an analysis of monthly means from some British observatories; a new theory of the geomagnetic daily variation; and the interactions between ionospheric science and geomagnetism. The present-day observatory scene was described in terms of the information that can be derived from the almost 40 year series of data from Hartland magnetic observatory; of the methods used to process data from the three UK magnetic observatories, which nowadays are operated automatically and remotely; and (a look into the future) of a new project, INTERMAGNET, which aims to make available, in near real time, data from the world-wide network of magnetic observatories.