The tsunami caused by the Great East Japan Earthquake on 11 March 2011 greatly influenced the coastal benthic environment on the northern Pacific coast of Japan. We used the direct count method to investigate changes in the abundance and distribution of Alexandrium (Alexandrium tamarense and Alexandrium catenella) cysts before and after the tsunami in Sendai Bay. Densities of Alexandrium cysts in sediments collected in summer 2011 ranged from 0 to 8,190 cysts cm?3. In the western part of the bay, the density increased greatly after the tsunami, the highest density being approximately 10 times the density recorded in 2005. Molecular identification of single cysts with multiplex polymerase chain reaction (PCR) showed that Alexandriumtamarense dominated the cyst population in the southwestern part of the bay in 2011. Furthermore, accumulation of cysts on the surface sediment after disturbance of the sediment was confirmed by a laboratory experiment. The main factor causing the drastic changes in abundance and distribution of Alexandrium cysts after the earthquake was considered to be vertical and horizontal redistribution of the cysts in sediments after the tsunami. 相似文献
Mean net annual balance and the related spatio-temporal variations have been determined on the basis of well-dated artificial layers in shallow ice cores (Chernobyl, 1986, and atmospheric thermonuclear tests, mainly in 1961-62 in Novaya Zemlya). Seventy ice cores from 13 Svalbard glaciers have been analysed. On each glacier, in its accumulation area and at the highest elevation, one ice core was recovered down to about 40 m and sampled for radioactivity measurements to determine the 1986 and 1962-63 layer (1954 was the initial date of the nuclear tests). For each glacier, at least five complementary ice cores from the accumulation area were analysed to determine the Chernobyl reference layer. Six ice cores exhibit both the Chernobyl and nuclear tests layers and are of special interest in this study. This work provides new data on the deposition rates of natural and artificial radioisotopes. Using ice cores samples from the Arctic glaciers, even with superimposed ice accumulation, it is possible to distinguish between the Chernobyl and the nuclear tests fallouts. This work also shows that the mean annual net balance did not significantly change for at least five ice core locations in the Svalbard glaciers for the two periods extending from 1963 to 1986 to the recent date of drilling. 相似文献
The duration, total power, rms amplitude, and peak ground acceleration (PGA) are examined using the data from the KiK-net digital borehole accelerograph arrays in Japan during three large earthquakes. We investigate correlations between the duration, PGA on the surface and hypocentral distance, instrumental seismic intensity, and the local site conditions. Also considered in this paper, are correlation between the duration and PGA. This present study suggests that PGA is inversely proportional to the duration.Comparing the duration, total power, rms amplitude, and PGA on the surface with those at the bottom of borehole, the duration and maximum amplitude characteristics of earthquake ground motions are investigated with emphasis on site amplification due to the local site conditions. The difference of amplification factors is not only site-dependent but also event-dependent. The comparison of the site amplification factors and the average shear-wave velocity in the upper 30-m showed a good agreement.This paper indicated that the duration and amplitude characteristics suffer a reciprocal effect due to the local site conditions and the site dependency of maximum amplitude PGA is greater than that of the duration. In view of the results, the duration is not less important than the maximum amplitude and frequency content in earthquake engineering. Moreover, this study provided meaningful information of input motion available to earthquake-resistant design. 相似文献
Transfer functions represent the ratio in the frequency domain of one ground motion to another. Transfer functions are a convenient way by which the variation of earthquake ground motions from the free-field to the foundation level of a structure can be quantified for studies of kinematic soil–structure interaction. Aside from ordinary filtering and baseline correction, substantial signal processing occurs in the computation of transfer functions, including windowing (to extract the S-wave portion of the record) and smoothing (to reduce scatter that can obscure physically significant trends). Utilizing several signal processing techniques on a sample data set, we find that detailed features of the transfer function ordinates (i.e., frequency-to-frequency variations) can be affected by the degree of smoothing and by the window length (e.g., whole record versus S-window). However, the overall shape and magnitude of the transfer functions are relatively consistent. More important than signal processing details is the frequency bandwidth over which the results are considered valid, because significant portions of the spectrum can be dominated by stochastic processes with little physical meaning. We argue that transfer functions should be interpreted over those portions of the spectrum having minimal noise impact, as indicated by high coherence. 相似文献
The effects of irradiance on the excystment of tintinids from marine sediments were examined by incubating sediment samples in filtered seawater under various light levels. The total incidence of tintinnid excystment over five days was significantly less at low irradiance at and less than 2.5 E m–2sec–1 comparing to that at higher irradiance, at and more than 6.9 E m–2sec–1 (p<0.01), indicating that excystment of tintinnids is inhibited under low irradiance conditions. These results imply that water depth, disturbance of surface sediments and vertical mixing of water influence the excystment of tintinnids in coastal areas because these factors determine the light levels to which the tintinnid cysts are exposed. 相似文献
The objective of this paper is to make it clear how strain behaviors are affected by the lateral inhomogeneity of ground based on both theoretical simulation and array observation of earthquake motions. We use a two-dimensional pseudo-spectral method to numerically obtain strain motions as well as acceleration motions. We applied the method to some model grounds, which have typical inhomogeneity in the horizontal direction, to estimate general features of strains and accelerations. In addition, the technique was also applied to a site where subsurface irregularity is typically found and an array observation system of earthquake motions was densely deployed. In the latter application of the method, we placed emphasis on comparisons between the observed and simulated motions. 相似文献
A simple-extended array of observation for strong motions, named ‘Small-Titan’, was installed in Sendai City, Japan in 1997. This array system consists of 20 observation sites, which have an average of about 4 km for the station-to-station distance. Each observation station was located at various kinds of soils to effectively obtain local site effects. Since its completion, the system has obtained about 160 motion records. As is well known, ground motions are influenced by the three main factors such as source, path and local site effects. An accurate prediction of ground motions should thus be performed based on the three factors. In this context, theoretical methods used commonly lack enough information related with source and path effects. A generalized inversion scheme to separate the effects of source, propagation path and local site from S-wave spectra was applied to the records of Small-Titan obtained in the Sendai region, Japan. The objective of this paper is to estimate local site effects in Sendai City and to obtain meaningful information of input motions available to earthquake-resistant design, aimed at setting in terms of local soil conditions. This paper also describes the applicability of the generalized inversion scheme to seismic zoning.
Site amplification factors estimated from the generalized inversion scheme vary significantly, reflecting the effects of soil characteristics at each site. Strong site-dependency was found in the site amplification factors. In particular, the local site amplification factors in the western side are smaller than those in the eastern side, across the Nagamachi–Rifu Fault that exists in the central part of the Sendai area. Results from the generalized inversion scheme are compared with those obtained by the direct spectral ratio scheme, the H/V method and the bootstrap inversion. There is a good agreement in the amplification factors between the generalized inversion scheme, the direct spectral ratio scheme and the bootstrap inversion scheme. The H/V method for estimating site amplification is found to extract the same dominant periods as the other methods. This paper especially emphasizes a good agreement in not only dominant period, but also amplification factor.
The generalized inversion scheme is a useful method in separating the effects of source, propagation path and local site and in revealing the site amplification. In addition, the generalized inversion scheme provides meaningful information of input motions available to earthquake-resistant design, aimed at setting in terms of local soil conditions. Moreover, this paper indicates that the generalized inversion scheme is applicable to the seismic zoning toward disaster mitigation. 相似文献