A pollen-based record of late glacial–Holocene climatic variability in the southern lake district, Chile     

Lourdes Vargas-Ramirez  Emile Roche  Philippe Gerrienne  Henry Hooghiemstra 《Journal of Paleolimnology》2008,39(2):197-217

A pollen record from Puyehue area (40°S; 72°W) in the southern Lake District, Chile, indicates that prior to 13,410 ¹⁴C yr BP (ca. 16,500–15,200 cal yr BP), cold resistant and hygrophilous vegetation, particularly Nothofagus forest and myricaceous vegetation, covered the area. From ca. 15,000 cal yr BP onward, the forest became increasingly dense. Between 10,010 and 7450 ¹⁴C yr BP (ca. 11,000–8000 cal yr BP), the expansion of Nothofagus obliqua and the spread of grasses suggests the climate became warmer and semi-arid. Lowland deciduous forest (Nothofagus obliqua, Aextoxicon punctatum, Laurelia sempervirens) and Valdivian rainforest (Nothofagus dombeyi, Eucryphia cordifolia, Caldcluvia paniculata, Aextoxicon punctatum, Laureliopsis philippiana) were abundant. During the next two thousand years, stable warm climatic conditions prevailed, and the diversity of the vegetation increased. From 5760 to 1040 ¹⁴C yr BP (ca. 6500–900 cal yr BP), the North Patagonian rainforest expanded. The presence of Pilgerodendron/Fitzroya, together with Nothofagus forest, suggests that humid conditions prevailed. During the last millennium, human impact intensified and regional vegetation was disturbed, particularly the lowland deciduous forest and Valdivian rainforest. North-Patagonian and subantartic taxa, such as Podocarpus nubigena, Pilgerodendron/Fitzroya, Nothofagus dombeyi type, Austrocedrus chilensis and Drimys winteri, occupied the low and high-altitude parts of the Cordillera. Five hundred years ago, shrub and grasses expanded in the Nothofagus forest, suggesting that forest became more open under cool–cold, and humid climatic conditions. These conditions prevail to the present day. This is the fourth in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M.-F. Loutre and E. Chapron.  相似文献   

Crustal structure in the Society and Tuamotu Islands, French Polynesia     

Jacques Talandier  Emile A. Okal 《Geophysical Journal International》1987,88(3):499-528

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Rayleigh-wave phase velocities in French Polynesia     

Emile A. Okal  Jacques Talandier 《Geophysical Journal International》1980,63(3):719-733

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Rayleigh-wave phase velocities are investigated in the period range 17–100 s by the two-station method over several paths covering most of French Polynesia. Our results confirm the validity of theoretical models obtained through regionalization of data pertaining to longer paths. They also exhibit a 2–3.5 per cent anisotropy, with the axis of maximum velocity oriented in the direction of spreading of the plate. Part of this anisotropy is, however, due to the presence of the Tuamotu archipelago; when this is removed, the remaining anisotropy (about 1.5 per cent) correlates with the present direction of spreading, indicating that a relaxation of the anisotropy has taken place since the East Pacific ridge jump. Finally, the presence of the Tuamotu Islands explains anomalous waveshapes for surface waves travelling in their vicinity, due to multipathing through their faster structure.  相似文献   

From 3-Hz P Waves to 0 S 2: No Evidence of A Slow Component to the Source of the 2011 Tohoku Earthquake     

Emile A. Okal 《Pure and Applied Geophysics》2013,170(6-8):963-973

In the hours following the 2011 Honshu event, and as part of tsunami warning procedures at the Laboratoire de Géophysique in Papeete, Tahiti, the seismic source of the event was analyzed using a number of real-time procedures. The ultra-long period mantle magnitude algorithm suggests a static moment of 4.1 × 10²⁹ dyn cm, not significantly different from the National Earthquake Information Center (NEIC) value obtained by W-phase inversion. The slowness parameter, $\Uptheta = -5.65, $ is slightly deficient, but characteristic of other large subduction events such as Nias (2005) or Peru (2001); it remains significantly larger than for slow earthquakes such as Sumatra (2004) or Mentawai (2010). Similarly, the duration of high-frequency (2–4 Hz) P waves in relation to seismic moment or estimated energy, fails to document any slowness in the seismic source. These results were confirmed in the ensuing weeks by the analysis of the lowest-frequency spheroidal modes of the Earth. A dataset of 117 fits for eight modes (including the gravest one, ₀ S ₂, and the breathing mode, ₀ S ₀) yields a remarkably flat spectrum, with an average moment of 3.5 × 10²⁹ dyn cm (*/1.07). This behavior of the Tohoku earthquake explains the generally successful real-time modeling of its teleseismic tsunami, based on available seismic source scaling laws. On the other hand, it confirms the dichotomy, among mega-quakes (M ₀ > 10²⁹ dyn cm) between regular events (Nias, 2005; Chile, 2010; Sendai, 2011) and slow ones (Chile, 1960; Alaska, 1964; Sumatra, 2004; and probably Rat Island, 1965), whose origin remains unexplained.  相似文献   

The Dwarskersbos,South Africa local tsunami of August 27, 1969: field survey and simulation as a meteorological event   总被引：1，自引：1，他引：0  

Emile A. Okal  Johan N. J. Visser  Coenraad H. de Beer 《Natural Hazards》2014,74(1):251-268

We investigate the hitherto unexplained wave which inundated the village of Dwarskersbos, South Africa, in the early hours of August 27, 1969, in the absence of any seismic disturbance or major meteorological storm. A field survey, based on the interview of nine elderly witnesses still residing in the community, documented maximum run-up of 2.9 m, concentrated on an extremely short segment of coastline, less than 2 km in length. These characteristics are incompatible with generation by a seismic source (which, at any rate, should have been felt by the population). A landslide source, located at the only canyon featuring a steep enough ocean floor, is also ruled out since a numerical simulation fails to reproduce the concentration of the wave at Dwarskersbos. By contrast, the wave can be explained as a “meteo-tsunami” resulting from resonance between a meteorological squall propagating at 18 m/s in the azimuth \(\hbox {N101}^{\circ }\hbox {E}\) and a gravity wave propagating in the shallow waters off the eastern shore of St. Helena Bay. This is confirmed by numerical simulation under the formalism of Proudman (Dynamical oceanography. Methuen, London, 1953), which provides a satisfactory model of the distribution of run-up along the beach.  相似文献