Regional seismic discrimination

The discrimination of various phenomena, such as frost effects, sonic booms and explosions, from regional earthquakes is investigated on the basis of observations in Sweden. Explosions represent the greatest problem in this respect, for whose solution the short-period Rayleigh wave Rg provides the most reliable method. Its presence indicates an artificial origin, practically without exception.     

Seismicity of the Tanzania region

The seismicity of the East African Rift system within the region bounded by latitudes 2°N and 12°S and longitudes 28°E and 40°E has been studied as far as all available instrumentally based material permits. An earthquake catalogue is presented and the data contained therein are used for tectonophysical investigations, including frequency—magnitude relations and time and space distribution of the seismicity within the region. In addition, earthquake engineering aspects are discussed.     

T-phases from Atlantic earthquakes

Summary Underwater sound waves from earthquakes or so-calledT-waves are investigated for the Atlantic-Arctic area for the years 1953–1968, mainly from Swedish seismograph records, and for comparison also from an earthquake in the equatorial Atlantic withT-waves recorded on both sides of the ocean. The waves travel as sound waves through water, and asPg, Sg andRg over the land path. The North Atlantic source area of theT-waves, recorded at Scandinavian stations, is very well limited in extent with a strong concentration northeast of Jan Mayen, probably due to favourable bottom topography. The calculated sound velocities in water are 1.43 km/sec for the Arctic case and 1.52 km/sec for the equatorial one. TheT-waves exhibit a clear inverse dispersion. The dispersion explains their long duration. The duration of theT-phase increases logarithmically with the maximum amplitudes within theT-wave group and decreases linearly with distance over the land path, corresponding to a quality factor of about 700. Propagation across the ocean by multiple reflections between surface and bottom appears as the most probable mechanism. The particle motion ofT Sg is dominantly transverse horizontal, which is explained by refraction when the waves are transmitted to land. TheT-wave spectra at two different localities show clear relations, depending upon the ocean depths.     

Ultra-long-period motions from the Alaska earthquake of July 10, 1958

Summary Ultra-long-period motions from the Alaska eerthquake of July 10, 1958, have been studied on records at Uppsala. Mantle Rayleigh waves with up to six passages around the earth were recorded. The dispersion curve for the period range 100–400 sec was deduced, showing a minimum group velocity of 3.56 km/sec at a period of 225 sec, thus confirming earlier results byEwing & Press. A value of the internal friction in the mantle of 480×10^–5 for the period range 120–260 sec was obtained. The particle motion of the mantle Rayleigh waves is retrograde elliptical in the plane of propagation, the vertical axis being 0.68 of the horizontal axis. A transverse horizontal wave motion is observed on theE-component. Both its period (12 min 10 sec) and the delay of its onset (8 hours after the earthquake) confirm the hypothesis that it represents a free torsional vibration of the whole earth, apparently observed for the first time.

---

Zusammenfassung Bewegungen mit ultra-langen Perioden von dem Alaska-Beben am 10. Juli 1958 wurden mittels Registrierungen in Uppsala studiert. Mantel-Rayleigh-Wellen, die die Erde sechsmal umkreist haben, wurden registriert. Die Dispersionskurve für den Periodenbereich 100–400 Sek wird bestimmt; sie zeigt ein Minimum der Gruppengeschwindigkeit von 3.56 Km/Sek bei einer Periode von 225 Sek, womit frühere Resultate vonEwing & Press bestätigt werden. Ein Wert der inneren Reibung im Mantel von 480×10^–5 für den Periodenbereich 120–260 Sek wird abgeleitet. Die Partikelbewegung der Mantel-Rayleigh-Wellen ist retrograd elliptisch in der Ausbreitungsebene und die Länge der vertikalen Achse ist 0.68 der horizontalen Achse. Eine transversale, horizontale Wellenbewegung ist auf derE-Komponente registriert. Sowohl ihre Periode (12 Min 10 Sek) wie die Verzögerung ihres Auftretens (8 Stunden nach dem Beben) bestätigen die Hypothese, dass es sich um eine freie Torsionsschwingung der ganzen Erde handelt, die wahrscheinlich zum ersten Mal beobachtet ist.

     

Pa andSa waves and the upper mantle

Summary This investigation is based on records of 96 earthquakes withPa andSa written by the Press-Ewing instruments at Uppsala in the interval June 1961–December 1962.Pa andSa waves are observed for all earthquake regions, irrespective of distance, focal depth or path properties. They have significantly higher velocities under continents than under oceans, which demonstrates corresponding differences in the upper mantle. Continental velocities are 8.35 km/sec (Pa) and 4.56 km/sec (Sa), oceanic velocities 8.01 km/sec (Pa) and 4.45 km/sec (Sa). The most frequent periods are 10 sec (Pa) and 20 sec (Sa). They are independent of distance forPa but increase with distance forSa. The best developedPa andSa are obtained for earthquakes at focal depths less than about 60 km. The particle motion ofSa may be anything from pureSV to pureSH motion and has high correlation to the particle motion ofS. The apparent angles of emergence (in average 51° forPa and 54° forSa) vary with distance. On the basis of our observations it is suggested thatPa andSa propagate by multiple reflections under grazing incidence under the Moho discontinuity.

---

Zusammenfassung Die vorliegende Untersuchung gründet sich auf die Registrierungen von 96 Erdbeben mitPa- undSa-Wellen, die mit Hilfe der Press-Ewing Instrumente zu Uppsala im Zeitraum Juli 1961–Dezember 1962 aufgenommen wurden. DiePa- undSa-Wellen sind für alle seismischen Regionen beobachtet worden, unabhängig von Entfernung, Herdtiefe oder Wellenweg. Sie haben bedeutend höhere Geschwindigkeiten unter den Kontinenten als unter den Ozeanen, was einen entsprechenden Unterschied im oberen Erdmantel beweist. Die kontinentalen Geschwindigkeiten betragen 8.35 km/sec (Pa) und 4.56 km/sec (Sa), die ozeanischen Geschwindigkeiten 8.01 km/sec (Pa) und 4.45 km/sec (Sa). Die am häufigsten vorkommenden Perioden betragen 10 sec (Pa) und 20 sec (Sa). Sie sind unabhängig von der Entfernung fürPa aber wachsen mit der Entfernung fürSa. Die am besten entwickeltenPa- undSa-Wellen werden für Erdbeben mit kleinerer Herdtiefe als rund 60 km beobachtet. FürSa wird jede beliebige Orbitalbewegung zwischenSV undSH beobachtet. Sie hat eine hohe Korrelation mit der Orbitalbewegung vonS. Die scheinbaren Emergenzwinkel (durchschnittlich 51° fürPa und 54° fürSa) variieren mit der Entfernung. Auf Grund unserer Beobachtungen wird die Hypothese aufgestellt, dass sich diePa-undSa-Wellen durch Mehrfachreflexionen, bei tangentialem Einfall, unter der Moho-Diskontinuität ausbreiten.

---

Resumen La presente investigación está basada en los sismogramas de 96 terremotos, que muestranPa ySa, registrados por los sismógrafos de Uppsala (Press-Ewing), durante el periodo Junio 1961 a Diciembre 1962. Las fasesPa ySa se observan en todas las regiones sismicas, independientemente de la distancia, de la profundidad focal ó de las propiedades de la trayectoria. Tienen velocidades significativamente mayores bajo continentes que bajo los océanos, lo que demuestra la existencia de diferencias en el manto superior. Las velocidades continentales son 8.35 km/seg (Pa) y 4.56 km/seg (Sa) y las oceánicas 8.01 km/seg (Pa) y 4.45 km/seg (Sa). Los periodos mas frecuentes son 10 segundos paraPa y 20 segundos paraSa. Tales periodos son independientes de la distancia en el caso dePa pero crecen con ella paraSa. Las fasesPa ySa mejor desarrolladas se obtienen para terremotos cuya profundidad focal es inferior a los 60 kilómetros. El movimiento de la particula del suelo debido aSa puede ser de cualquier tipo, desde puroSV a puroSH, y muestra gran correlación con el movimiento de la particula deS. Los ángulos de emergencia aparentes (de promedio 51° paraPa y 54° paraSa) varian con la distancia. Basándonos en nuestras observaciones sugerimos quePa ySa se propagan por reflexión múltiple, bajo incidencia rozante bajo la discontinuidad de Mohorovii.

     

Early Cambrian oceanic plagiogranite in the Silvretta Nappe,eastern Alps: geochemical,zircon U-Pb and Rb-Sr data from garnet-hornblende-plagioclase gneisses

 Garnet–hornblende–plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the Austro-Alpine Silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates. It is proposed here that the gneisses were formerly oceanic plagiogranites. An ε_{Nd( 530 )} value of +5.6 for the gneisses as well as initial ⁸⁷Sr/⁸⁶Sr values of 0.7036–0.7037 for the gabbroic rocks and 0.7026–0.7027 for the ultramafic rocks suggest a mantle source for this rock association. The geochemical characteristics of the garnet–hornblende–plagioclase gneisses indicate that their precursors were derived by fractional crystallization from a basaltic parent magma, by the same process which produced the adjacent clinopyroxene norites and ultramafic cumulates as well. The combined U–Pb upper intercept ages of zircons from two gneiss samples yield an igneous crystallization age of 532±30 Ma, similar to previously dated (mostly calc-alkaline) orthogneisses in the same area. High-quality transparent zircons showed the least degree of discordance, but contain extremely low U and Pb levels. The rock suite, including this plagiogranite, was emplaced within oceanic crust which formed in the latest Precambrian–early Palaeozoic off the northern continental margin of Gondwana. Received: 26 April 1996 / Accepted: 2 August 1996     

Particle accumulation in a cylindrical sediment trap under laminar and turbulent steady flow: An experimental approach

The hydrodynamical, fluid and particle parameters which control flushing rates, flow cells, and accumulation rates of particulate matter in cylindrical (MultiPIT) sediment traps were quantified in a flume simulation using a seeding technique for 25–45 µm particles. Particle collection was found to be a trap- and particle-specific filtering process encompassing advective and gravitational entry of particles over a reduced trap aperture area, and gravitational-turbulent removal of particles at the bottom of the internal flow cell. Trapping efficiency increased up to 10-fold with increasing horizontal flow velocity (1–30 cm · s^–1). For given flow velocity, the trap over-and undercollected particles relative to their weight, i.e. (theoretical) Stokes settling velocity. The trapping efficiency increased with increasing trap Reynolds number Re_T, changed by the approaching velocity in our experiments. Opposite findings from earlier experiments using the flume seeding technique and changing Re_T by altering the trap diameter (Butman, 1986) are discussed. Semi-empirical equations are derived for the accumulation process of light, heavy and intermediate particles. From these, measured trap fluxes can be converted into in-situ verticle particle flux except for light particles.