SUMMARY We report the detection, principally by the French Polynesian seismic network, of hydroacoustic signals generated inside large icebergs, either ‘parked’ along the Wilkes coast of Antarctica in the Indian Ocean, or drifting in the Southern Pacific Ocean between latitudes of 55° and 65°S, during the years 2002–2004. The signals can be classified into two very broad families, based on the nature of their spectra. A first group features prominently monochromatic signals, whose frequency can, however, fluctuate with time during a single sequence of emission (typically lasting a few to a few tens of minutes). Such signals are generally reminiscent of those detected in 2000 in the Ross Sea and are generated principally in the Indian Ocean ‘iceberg parking lot’, between longitudes 144°E and 156°E. A new family of signals features a much broader spectrum, superimposed on a number of preferential frequencies suggesting the background activation of a number of resonators; these signals occur both in the parking lot and in the Southern Pacific. Further variations in spectra are documented inside each family. On the basis of similar in situ observations on Ross Sea icebergs under project SOUTHBERG, the first family is generally interpreted as expressing a stick‐and‐slip process during collisions between large iceberg masses. The second family of signals are observed during exceptional episodes of the otherwise silent drift of the icebergs in the deep Pacific Basin, some of which correlate with their passage over the various fronts defining the oceanographic southern convergence zone. Finally, a most recent episode of activity, generally similar to the above first family, was detected on 2004 December 3–4, at the ocean entry of the Dibble Ice Tongue, 600 km west of the parking lot along the coast of Antarctica. It is interpreted as resulting from collisions between large drifting icebergs and fragments of the ice tongue calved off during its disintegration, as documented by satellite imagery.
This paper lays the theoretical groundwork for a variable period mantle magnitude, Mm, based on the measurement of the spectral amplitude X(ω) of very long period Rayleigh waves. We retain the concept of magnitude by restricting ourselves to single‐station measurements, ignoring the focal mechanism and the exact depth of the shallow earthquakes considered. Our measurements are made at a series of periods (in all cases greater than 40 s), and the largest value is retained. This procedure effectively avoids the well‐known interference effects leading to saturation of magnitude scales defined at a fixed period. Two corrections are used: a period‐dependent distance correction CD, and a source correction Cs, also period‐dependent, compensating for the variation of the excitation of Rayleigh waves with period. Both of these corrections are fully predictable on the basis of standard surface wave excitation and dispersion theory. The result is a formula of the type Mm = log10 X(ω) + CD + CS + C0 in which all coefficients, including the constant C0 are justifiable on sound theoretical grounds. The analysis of a data set of 256 records from the broadband seismograph at Papeete, Tahiti, the ultra‐long period system at Pasadena, and stations of the GEOSCOPE network, shows that the mean error in the estimation of the seismic moment is on the order of 0.1–0.2 units of magnitude, with the standard deviation at each station also on the order of 0.2 units of magnitude. No significant trend with either distance, period, or station can be identified. The method can also be transposed to the time domain, under some simple assumptions which are justifiable theoretically for typical teleseismic distances across the Pacific Basin. Both versions of the method lend themselves well to automation. Thus, by providing a real‐time estimate of the seismic moment of distant earthquakes, Mm has considerable potential for tsunami warning purposes. Its concept can easily be extended to Love waves and also to intermediate and deep earthquakes.
The 15‐station French Polynesian Seismic Network is used to study the intraplate seismicity of the South‐Central Pacific Ocean for the period January 1, 1965 to December 31, 1979. The overall pattern of seismicity shows a clustering of earthquakes at approximately 30 distinct localities, occurring both as discrete events and in swarms. Three localities are associated with known centers of active vulcanism (Moua Pihaa and Rocard seamounts in the Tahiti‐Méhétia area and Macdonald seamount in the Austral Islands). A set of eight localities is distributed along the tectonic axis of the Tuamotu Archipelago and its northwestward extention into the Line Islands, and a set of five is roughly aligned with the northeastern edge of the Tuamotu platform; both sets may be related to load inhomogeneities in the lithosphere. However, much of the seismicity is not correlated with major bathymetric features. With the exception of one isolated event, for example, none of the recorded activity occurs along the major fracture zones that cross the study area. Only three localities have been the sites of two or more earthquakes with body‐wave magnitudes greater than 5.0; these we designate Regions A, B and C. Excluding Hawaii, Regions A, B and C are the most intense centers of seismicity within the Pacific plate interior, and together they account for more than 90% of the seismic energy release in the South‐Central Pacific. An analysis of water multiples from events in Regions A, B and C indicates hypocentral depths within the oceanic crust. Eight focal mechanisms have been obtained; these all have nearly horizontal, NW‐trending compressional axes, oriented approximately parallel to the direction of Pacific plate motion. The uniformity of this orientation over the large distances separating the epicenters (>2000 km) suggests that the mechanisms are indicative of a regional tectonic stress field, rather than locally disturbed stress patterns. These data thus provide additional constraints on the force balance models of plate tectonics.
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