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991.
Geophysical data have identified four submarine segments of the Kerepehi Fault, roughly bisecting a back-arc rift (Hauraki Rift). These segments have been traced through the shallow waters of the Firth of Thames, which lies at the southern end of the Hauraki Gulf, New Zealand. No historical or paleotsunami data are available to assess the tsunami hazard of these fault segments.Analysis of the fault geometry, combined with paleoseismic data for three further terrestrial segments of the Fault, suggest Most Credible Earthquake (MCE) moment magnitudes of 6.5–7.1. Due to the presence of thick deposits of soft sediment, and thesemi-confined nature of the Firth, the MCE events are considered capable of generating tsunami or tsunami-like waves. Two numerical models (finite element and finite difference), and an empirical method proposed by Abe (1995), were used to predict maximum tsunami wave heights. The numerical models also modelled the tsunami propagation.The MCE events were found not to represent a major threat to the large metropolitan centre of Auckland City (New Zealand's largest population centre). However, the waves were a threat to small coastal communities around the Firth, including the township of Thames, and 35,000 ha of low-lying land along the southern shores of the Firth of Thames.The Abe method was found to provide a quick and useful method of assessing the regional tsunami height. However, for sources in water depths < 25 m the Abe method predicted heights 2–4 times larger than the numerical models. Since the numerical models were not intended for simulating tsunami generation in such shallow water, the Abe results are probably a good guide to the maximum wave heights. 相似文献
992.
New constraints on the pyroclastic eruptive history of the Campanian volcanic Plain (Italy) 总被引:2,自引:0,他引:2
B. De Vivo G. Rolandi P. B. Gans A. Calvert W. A. Bohrson F. J. Spera H. E. Belkin 《Mineralogy and Petrology》2001,73(1-3):47-65
Summary The ∼ 150 km3 (DRE) trachytic Campanian Ignimbrite, which is situated north-west of Naples, Italy, is one of the largest eruptions in the
Mediterranean region in the last 200 ky. Despite centuries of investigation, the age and eruptive history of the Campanian
Ignimbrite is still debated, as is the chronology of other significant volcanic events of the Campanian Plain within the last
200–300 ky. New 40Ar/39Ar geochronology defines the age of the Campanian Ignimbrite at 39.28 ± 0.11 ka, about 2 ky older than the previous best estimate.
Based on the distribution of the Campanian Ignimbrite and associated uppermost proximal lithic and polyclastic breccias, we
suggest that the Campanian Ignimbrite magma was emitted from fissures activated along neotectonic Apennine faults rather than
from ring fractures defining a Campi Flegrei caldera. Significantly, new volcanological, geochronological, and geochemical
data distinguish previously unrecognized ignimbrite deposits in the Campanian Plain, accurately dated between 157 and 205 ka.
These ages, coupled with a xenocrystic sanidine component > 315 ka, extend the volcanic history of this region by over 200 ky.
Recent work also identifies a pyroclastic deposit, dated at 18.0 ka, outside of the topographic Campi Flegrei basin, expanding
the spatial distribution of post-Campanian Ignimbrite deposits. These new discoveries emphasize the importance of continued
investigation of the ages, distribution, volumes, and eruption dynamics of volcanic events associated with the Campanian Plain.
Such information is critical for accurate assessment of the volcanic hazards associated with potentially large-volume explosive
eruptions in close proximity to the densely populated Neapolitan region.
Received August 1, 2000; accepted November 2, 2000 相似文献
993.
D. Melas T. Persson H. De Bruin S.-E. Gryning E. Batchvarova C. Zerefos 《Theoretical and Applied Climatology》2001,70(1-4):105-116
Summary A mesoscale numerical model, incorporating a land-surface scheme based on Deardorffs’ approach, is used to study the diurnal
variation of the boundary layer structure and surface fluxes during four consecutive days with air temperatures well below
zero, snow covered ground and changing synoptic forcing. Model results are evaluated against in-situ measurements performed
during the WINTEX field campaign held in Sodankyl?, Northern Finland in March 1997. The results show that the land-surface
parameterization employed in the mesoscale model is not able to reproduce the magnitude of the daytime sensible heat fluxes
and especially the pronounced maximum observed in the afternoon. Additional model simulations indicate that this drawback
is to a large extent removed by the implementation of a shading factor in the original Deardorff scheme. The shading factor,
as discussed in Gryning et al. (2001), accounts for the fact that in areas with sparse vegetation and low solar angles, both
typical for the northern boreal forests in wintertime, absorption of direct solar radiation is due to an apparent vegetation
cover which is much greater than the actual one (defined as the portion of the ground covered by vegetation projected vertically).
Moreover, the observed asymmetry in the diurnal variation of the sensible heat flux indicates that there might be a significant
heat storage in the vegetation. The implementation of an objective heat storage scheme in the mesoscale model explains part
of the observed diurnal variation of the sensible heat flux.
Received November 12, 1999 Revised October 4, 2000 相似文献
994.
995.
200 0年 4月 5日~ 7日袭击我国西北地区北部、华北、东北南部、黄淮地区的扬沙或沙尘暴波及到长春市 ,带来了大量的降尘。据长春市西部 4个采样点测定 ,4月 7日伴随降雪的降尘量最高可达 13.0 g/m2 ,市区总降尘量超过 30 0 0t。泥雪样品在室温条件下自然融化后 ,经实验室 48h细菌培养 ,其细菌含量为每毫升 8.9~ 10 .5万个 ,每平方米泥雪的细菌侵入量达 6 .4亿个左右。扬沙或沙尘暴对城市造成了巨大的环境危害 ,而伴随着大量微生物的侵入 ,势必引起城市微生态环境的改变 ,威胁到城市的生态安全。 相似文献
996.
997.
Caracas, Venezuela, Site Effect Determination with Microtremors 总被引:1,自引:0,他引:1
A.-M. Duval S. Vidal J.-P. Méneroud A. Singer F. De Santis C. Ramos G. Romero R. Rodriguez A. Pernia N. Reyes C. Griman 《Pure and Applied Geophysics》2001,158(12):2513-2523
—?Caracas 1967 earthquake caused heavy damage to multi-story buildings. In 1995, 184 microtremor measurement points were performed over the city. The measurement grid was more or less dense and covered the main part of the alluvial basin as well as surrounding rock basement. For each point, the horizontal record spectrum was divided by the vertical one (H/V ratio). Subsequently, the strongest value (Ao) of this ratio in a given frequency band was kept, as well as the frequency (Fo) where it occurred. Spatial interpolations of Ao and Fo were performed among all points of Palos Grandes district. A map was plotted representing a single surface where Ao is relief and Fo is represented by grey gradation. An alluvion thickness (H) map can be compared with this result. Damaged buildings are located on the same map. Fo decreases until 0.6?Hz when alluvion thickness (H) increases. Fo values fit with frequencies previously predicted from computation and with 1967 earthquake observations. Moreover, interpolation surfaces show that amplification (Ao) of H/V on microtremor is quite low above rock but is high on the south part of the basin. The maximum occurs over the non-urbanized zone. However the other area, where Ao is higher than 5, roughly corresponds to the location of the four collapsed buildings. Furthermore, the estimated natural frequency of these buildings was around Fo. Graphics showing H, Fo and Ao evolution through the basin were composed. Links between all these values are noticeable. Fo is claimed to be very similar to the resonance frequency of soil. As for Ao, it could be a fairly relevant sign of damage seriousness. Microtremor technique is an economic tool and it allows measurement grid as dense as desired. H/V ratio processing followed by interpolation of maximum values provides precise and useful information about expected site effect. 相似文献
998.
999.
The nature and restricted range of Dupal-type Sr, Nd and Pb isotopic compositions of Cretaceous kimberlites, tuffaceous diatremes of kamafugitic affinity and carbonatite complexes which intrude the southwestern São Francisco craton margin in Brazil, indicate that these magmas either interacted extensively with, or were derived from, a light rare earth element (LREE) enriched homogeneous lithospheric mantle source isotopically similar to the “enriched mantle I” (EMI) component. The shallow-derived alkalic rocks contain a greater proportion of this EMI-like component, whereas the lower time-averaged Rb/Sr, Nd/Sm and Pb/U ratios of the kimberlites compared to the other rock types suggest mixing of the EMI-like mantle material with variable amounts of mantle with a high 238U/204Pb (HIMU-like) component. Systematic variations in rock types and geochemistry on a regional scale are believed to be indicative of vertical geochemical heterogeneities which are translated into lateral heterogeneities by different depths of melting. It is proposed that HIMU- and EMI-like signatures in particular, are concentrated in laterally extensive but vertically distinctive portions of the mantle beneath the São Francisco craton. The EMI-type signatures appear to be restricted to shallow-derived volcanism, whereas the HIMU-type signatures may originate from a source that started melting deeper in the mantle. The Nd signatures of the EMI-type volcanics follow the evolution path defined by the NeoProterozoic crustal sequences which overlie and flank the craton margin. This suggests that the source of the EMI-type mantle signatures might be related to the tectono-thermal processes which led to the formation and evolution of such crustal sequences. The isotopic similarity of the sources of the studied rocks and of the high-Ti basalts of the northern Paraná basin to those of some Ocean Island Basalts with Dupal signatures in the South Atlantic (viz. in Walvis Ridge) is ascribed to processes by which continental lithosphere became firstly delaminated, and then contaminated a zone of South Atlantic asthenosphere from which hotspot islands have been erupting. 相似文献
1000.