Intraplate seismicity of the Pacific Basin, 1913–1988

We establish here a comprehensive database of intraplate seismicity in the Pacific Basin. Relocation and analysis of 894 earthquakes yield 403 reliable intraplate earthquakes during 1913–1988. These numbers do not include earthquake swarms, which account for another 838 events. Most of the remainder (304 events) are actually plate boundary earthquakes that have been erroneously located in intraplate regions. A significant number occur in recent years when location capabilities should have guarded against this situation. Relocations involve a careful linear inversion ofP andS arrivals, accompanied by a Monte Carlo statistical analysis. We have also attentively removed the high number of clerical errors and nuclear tests that exist in epicenter bulletins.A geographical examination of the relocated epicenters reveals several striking features. There are three NW-SE lineaments north of the Fiji Plateau and in Micronesia; diffuse seismicity and incompatible focal mechanisms argue against the southernmost, discussed byOkal et al. (1986) andKroenke andWalker (1986), as the simple relocation of the Solomon trench to the North. Besides another striking lineament, along the 130°W meridian, there is also a strong correlation between seismicity and bathymetry in certain parts of the Basin. In the Eastcentral Pacific and Nazca plates there are many epicenters on fracture zones and fossil spreading ridges, and hot spot traces like the Louisville, Nazca and Cocos Ridges also display seismicity.     

Études bactériologiques des eaux du Léman,prélevées dans le Petit-Lac pendant les années 1963, 1964 et 1965

Summary The examination of the figures with regard to the qualitative point of view (Coliforms) as well as to the quantative one (total numbers of germs) for the period of 1957 to 1965 [1] reveals an aggravation of the bacterial situation in the last years and, especially, in 1963 and 1964.

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Zusammenfassung Bei der Prüfung der Zahlen sowohl in qualitativer (Kolibakterien) als auch in quantitativer Hinsicht (Total der vorhandenen Keime) wird für die Zeit von 1957 bis 1965 [1] eine merkliche Verschlechterung der bakteriologischen Verh?ltnisse festgestellt. Dies gilt besonders für die letzten Jahre, vor allem aber für die Jahre 1963 und 1964.

     

Qu'en est-il de la pollution microbiologique d'origine aérienne des eaux du Léman?

Résumé Les auteurs ont recherché, au moyen d'investigations dans le ≪Petit-Lac≫, le nombre de micro-organismes d'origine aérienne qui tombent sur le Léman en un an. Ce nombre s'élève, en moyenne, à 482×10¹⁵ germes; cela représente un poids de près de 2 tonnes. Ils mentionnent également les principales conditions climatiques et météorologiques qui conditionnent l'augmentation ou la diminution du nombre des microorganismes dans l'atmosphère.

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Zusammenfassung Die Autoren haben, aufgrund von Untersuchungen im sogennanten ≪Petit-Lac≫, die Zahl der Mikroorganismen, die w?hrend eines Jahres aus der Luft ins Wasser des Genfersees gelangen, gesch?tzt. Diese Zahl betr?gt im Mittel 482×10¹⁵ Keime, was einem Totalgewicht von nahezu zwei Tonnen entspricht. Die Autoren erw?hnen zudem die haupts?chlichen klimatischen und meteorologischen Bedingungen, welche die zahlenm?ssige Zunahme oder Abnahme dieser Mikroorganismen in der Atmosph?re beeinflussen.

     

On the observability of isotropic seismic sources: The July 31, 1970 Colombian earthquake

Theoretical calculations are made to study the observability of isotropic components of seismic sources. In particular we consider the 1970 deep Colombian earthquake, for which a precursory isotropic component was previously reported by Gilbert and Dziewonski.We compare an ultra-long period vertical record at Pasadena of the 1970 event to synthetic seismograms calculated both for Gilbert and Dziewonski's source model and for the pure double-couple source of Furumoto and Fukao, and obtain better overall agreement for the latter. The amplitude of the long-period synthetic for the isotropic source is about 5–15 times smaller than the synthetic for the deviatoric source, suggesting that the data may be relatively insensitive to the presence of a small isotropic source. When this possibility was tested, the overall agreement was found to be almost completely insensitive to the presence of even a reasonably large isotropic component.However, the isotropic source was derived from multi-station moment tensor inversion, rather than from single-station studies. A numerical experiment on the effect of lateral heterogeneity of eigenfrequencies and of Q on the inversion for the moment tensor shows that even relatively small amounts of heterogeneity can produce spurious isotropic sources from moment tensor inversion.     

Mapping the Miocene Farallon Ridge jump on the Pacific plate: a seismic line of weakness

Bathymetric and magnetic data are used to obtain estimates, on the Pacific and Nazca plates, of the boundaries separating lithosphere generated at the old Farallon Ridge from the more recent one created at the present-day East Pacific Rise. An excellent correlation is found with the sites of known teleseismically recorded intraplate seismicity, suggesting that these boundaries, which are lines of age discontinuity in the plate, must be zones of weakness of the lithospheric plate. In particular, the so-called Region C, identified by Okal et al. as a major site of seismic release, sits on a small piece of Farallon plate, in the immediate vicinity of the northern extension of the fossil Roggeveen Rise, cut across by the East Pacific Rise during the ridge jump.     

Investigation of the 600-km discontinuity under France through travel-time and amplitude anomalies

In this paper, we give a model of the 600-km discontinuity under France, which explains large travel-time and amplitude anomalies on teleseisms as recorded in the French network. It features a large descent from the Ardennes to the Pyrénées, of which tectonic implications are discussed.     

On the cessation of seismicity at the base of the transition zone

Through a detailed analysis of seismicity at the base of the transition zone, we obtain an updated value of the maximum reliable depth of confirmed seismicity, we investigate regional variation in the maximum depth of seismicity among those Wadati-Benioff zones which reach the bottom of the transition zone, and we attempt to quantify the maximum possible rate of seismic release in the lower mantle compatible with the failure to detect even a single event since the advent of modern seismological networks. We classify deep subduction zones into three groups: those whose seismicity does not reach beyond 620 km, those whose seismicity appears to terminate around 650–660 km, and Tonga-Kermadec (and the Vityaz cluster) whose seismicity extends to 685–690 km. We suggest that the depth extent of seismicity is controlled by the depth of the pv + mw transition responsible for the 660-km seismic discontinuity, which is deflected to greater depths in cold slabs than in warmer ones. We note that this transition marks the depth below which thermal perturbation of phase transitions no longer generates buoyancy anomalies and their large attendant down-dip compressive stresses and below which strain energy generated by other mechanisms may not accumulate to seismogenic levels due to superplastic weakness in fine-grained materials. We find that the maximum level of seismic activity in the lower mantle must be at least three orders of magnitude less than that observed in the transition zone.     

Normal Mode Energetics for Far-field Tsunamis Generated by Dislocations and Landslides

— We apply the normal mode representation of tsunami waves, as introduced by Ward (1980) to the systematic study of the excitation of far-field tsunamis by both dislocation sources (represented by double-couples of moment M ₀), and landslides (represented by single forces). Using asymptotic representations of the continuation of the tsunami eigenfunction into the solid Earth, we derive analytical expressions of the spectral amplitude generated by both systems. We show that the quadrupolar corrections defined by Dahlen (1993) in the case of landslides can result in an increase of 1 to 2 orders of magnitude of the effective force. Even so, the spectrum of tsunami waves generated by landslides is found to be offset significantly to relatively high frequencies (10 mHz), where dispersion becomes important and eventually diminishes time-domain amplitudes. We proceed to calculate the total energy delivered into the tsunami modes by integrating the energy of multiplets for an average source geometry. In the case of dislocation sources, and taking into account the corner frequency of the source, we reproduce the scaling with M ₀ ^4/3 which was derived from purely static arguments by Kajiura (1981). We compare the directivity patterns of far-field tsunami waves by dislocations and landslides, and conclude that the latter cannot give rise to pronounced lobes of directivity for physically acceptable values of the velocity of the slide. Directivity thus constitutes a robust discriminant of the nature of the source which, when applied to the 1946 Aleutian tsunami in the far-field, requires generation by a dislocative source.