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11.
We extend to the regional field of distances the procedure of one-station estimation of seismic moments using the mantle magnitudeM
m, as introduced earlier in the case of teleseismic events. A theoretical analysis of the validity of the asymptotic expansion of normal modes in terms of surface waves, which was used in the development ofM
m, upholds the validity of the algorithm for distances as short as 1.5°. This is confirmed by the analysis of a dataset of 149 GEOSCOPE records obtained at distances ranging from 1.5 to 15°, from earthquakes with moments between 1024 and 2.5×1027 dyn-cm. The performance ofM
m as measured in terms of average residual with respect to published values ofM
0, and standard deviation of the residuals, is not degraded in this distance range, with respect to the teleseismic case. This indicates that the mantle magnitudeM
mcan be reliably used at regional distances, notably for tsunami warning applications. 相似文献
12.
Emile A. Okal 《Pure and Applied Geophysics》1992,139(1):59-85
The mantle magnitudeM
m
is used on a dataset of more than 180 wavetrains from 44 large shallow historical earthquakes to reassess their moments, which in many cases had been previously estimated only on the basis of the earthquake's rupture area. We provide 27 new or revised values ofM
o, based on the spectral amplitudes of surface waves recorded at a number of stations, principally Uppsala and Pasadena. Among them, and most significantly, we document a large low-frequency component to the source of the 1923 Kanto earthquake: the low-frequency seismic moment is 2.9×1028 dyn-cm, in accord with geodetic observations. On the other hand, we revise downwards the seismic moment of the 1906 Ecuador event, which did not exceed 6×1028 dyn-cm.Finally, the study of the 1960 Chilean and 1964 Alaskan earthquakes whose exceptionally large moments are properly retrieved throughM
m
measurements, serves proof that this approach performs flawlessly even for the very greatest earthquakes, and is therefore successful in its goal to avoid the saturation effects plaguing any magnitude scale measured at a fixed period. 相似文献
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16.
The study of transitory signals and local variability structures in both/either time and space and their role as sources of climatic memory, is an important but often neglected topic in climate research despite its obvious importance and extensive coverage in the literature. Transitory signals arise either from non-linearities, in the climate system, transitory atmosphere–ocean couplings, and other processes in the climate system evolving after a critical threshold is crossed. These temporary interactions that, though intense, may not last long, can be responsible for a large amount of unexplained variability but are normally considered of limited relevance and often, discarded. With most of the current techniques at hand these typology of signatures are difficult to isolate because the low signal-to-noise ratio in midlatitudes, the limited recurrence of the transitory signals during a customary interval of data considered. Also, there is often a serious problem arising from the smoothing of local or transitory processes if statistical techniques are applied, that consider all the length of data available, rather than taking into account the size of the specific variability structure under investigation. Scale-dependent correlation (SDC) analysis is a new statistical method capable of highlighting the presence of transitory processes, these former being understood as temporary significant lag-dependent autocovariance in a single series, or covariance structures between two series. This approach, therefore, complements other approaches such as those resulting from the families of wavelet analysis, singular-spectrum analysis and recurrence plots. A main feature of SDC is its high-performance for short time series, its ability to characterize phase-relationships and thresholds in the bivariate domain. Ultimately, SDC helps tracking short-lagged relationships among processes that locally or temporarily couple and uncouple. The use of SDC is illustrated in the present paper by means of some synthetic time-series examples of increasing complexity, and it is compared with wavelet analysis in order to provide a well-known reference of its capabilities. A comparison between SDC and companion techniques is also addressed and results are exemplified for the specific case of some relevant El Niño-Southern Oscillation teleconnections. 相似文献
17.
Michael C. Murrell James D. Hagy Emile M. Lores Richard M. Greene 《Estuaries and Coasts》2007,30(3):390-402
The relationships between phytoplankton productivity, nutrient distributions, and freshwater flow were examined in a seasonal
study conducted in Escambia Bay, Florida, USA, located in the northeastern Gulf of Mexico. Five sites oriented along the salinity
gradient were sampled 24 times over the 28-mo period from 1999 to 2001. Water column profiles of temperature and salinity
were measured along with surface chlorophyll and surface inorganic nutrient concentrations. Primary productivity was measured
at 2 sites on 11 dates, and estimated for the remaining dates and sites using an empirical regression model relating phytoplankton
net production to the product of chlorophyll, euphotic zone depth, and daily solar insolation. Freshwater flow into the system
varied markedly over the study period with record low flow during 2000, a flood event in March 2001, and subsequent resumption
of normal flow. Flushing times ranged from 1 d during the flood to 20 d during the drought. Freshwater input strongly affected
surface salinity distributions, nutrient flux, chlorophyll, and primary productivity. The flood caused high turbidity and
rapid flushing, severely reducing phytoplankton production and biomass accumulation. Following the flood, phytoplankton biomass
and productivity sharply increased. Analysis of nutrient distributions suggested Escambia Bay phytoplankton alternated between
phosphorus limitation during normal flow and nitrogen limitation during low flow periods. This study found that Escambia Bay
is a moderately productive estuary, with an average annual integrated phytoplankton production rate of 290 g C m−2 yr−1. 相似文献
18.
M
TSU
: Recovering Seismic Moments from Tsunameter Records 总被引:1,自引:0,他引:1
We define a new magnitude scale, MTSU, allowing the quantification of the seismic moment M0 of an earthquake based on recordings of its tsunami in the far field by ocean-bottom pressure sensors (``tsunameters') deployed
in ocean basins, far from continental or island shores which are known to affect profoundly and in a nonlinear fashion the
amplitude of the tsunami wave. The formula for MTSU, MTSU = log10 M0 − 20 = log10 X (ω) + CDTSU + CSTSU + C0, where X (ω) is the spectral amplitude of the tsunami, CDTSU a distance correction and CSTSU a source correction, is directly adapted from the mantle magnitude Mm introduced for seismic surface waves by Okal and Talandier. Like Mm, its corrections are fully justified theoretically based on the representation of a tsunami wave as a branch of the Earth's
normal modes. Even the locking constant C0, which may depend on the nature of the recording (surface amplitude of the tsunami or overpressure at the ocean floor) and
its units, is predicted theoretically. MTSU combines the power of a theoretically developed algorithm, with the robustness of a magnitude measurement that does not take
into account such parameters as focal geometry and exact depth, which may not be available under operational conditions in
the framework of tsunami warning. We verify the performance of the concept on simulations of the great 1946 Aleutian tsunami
at two virtual gauges, and then apply the algorithm to 24 records of 7 tsunamis at DART tsunameters during the years 1994–2003.
We find that MTSU generally recovers the seismic moment M0 within 0.2 logarithmic units, even under unfavorable conditions such as excessive focal depth and refraction of the tsunami
wave around continental masses. Finally, we apply the algorithm to the JASON satellite trace obtained over the Bay of Bengal
during the 2004 Sumatra tsunami, after transforming the trace into a time series through a simple ad hoc procedure. Results are surprisingly good, with most estimates of the moment being over 1029 dyn-cm, and thus identifying the source as an exceptionally large earthquake. 相似文献
19.
Experimental determination of magnesium isotope fractionation during higher plant growth 总被引:1,自引:0,他引:1
Two higher plant species (rye grass and clover) were cultivated under laboratory conditions on two substrates (solution, phlogopite) in order to constrain the corresponding Mg isotope fractionations during plant growth and Mg uptake. We show that bulk plants are systematically enriched in heavy isotopes relative to their nutrient source. The Δ26Mgplant-source range from 0.72‰ to 0.26‰ for rye grass and from 1.05‰ to 0.41‰ for clover. Plants grown on phlogopite display Mg isotope signatures (relative to the Mg source) ∼0.3‰ lower than hydroponic plants. For a given substrate, rye grass display lower δ26Mg (by ∼0.3‰) relative to clover. Magnesium desorbed from rye grass roots display a δ26Mg greater than the nutrient solution. Adsorption experiments on dead and living rye grass roots also indicate a significant enrichment in heavy isotopes of the Mg adsorbed on the root surface. Our results indicate that the key processes responsible for heavy isotope enrichment in plants are located at the root level. Both species also exhibit an enrichment in light isotopes from roots to shoots (Δ26Mgleaf-root = −0.65‰ and −0.34‰ for rye grass and clover grown on phlogopite respectively, and Δ26Mgleaf-root of −0.06‰ and −0.22‰ for the same species grown hydroponically). This heavy isotope depletion in leaves can be explained by biological processes that affect leaves and roots differently: (1) organo-Mg complex (including chlorophyll) formation, and (2) Mg transport within plant. For both species, a positive correlation between δ26Mg and K/Mg was observed among the various organs. This correlation is consistent with the link between K and Mg internal cycles, as well as with formation of organo-magnesium compounds associated with enrichment in heavy isotopes. Considering our results together with the published range for δ26Mg of natural plants and rivers, we estimate that a significant change in continental vegetation would induce a change of the mean river δ26Mg that is comparable to analytical uncertainties. 相似文献
20.
Geoffrey Blewitt William C. Hammond Corné Kreemer Hans-Peter Plag Seth Stein Emile Okal 《Journal of Geodesy》2009,83(3-4):335-343
We identify the key design aspects of a GPS-based system (and in the future, GNSS-based systems) that could contribute to real-time earthquake source determination and tsunami warning systems. Our approach is based on models of both transient and permanent displacement of GPS stations caused by large earthquakes, while considering the effect of GPS errors on inverted earthquake source parameters. Our main conclusions are that (1) the spatial pattern, magnitude, and timing of permanent displacement of GPS stations can be inverted for the earthquake source and so predict the 3D displacement field of the ocean bottom, thus providing the initial conditions for tsunami models, and (2) there are no inherently limiting factors arising from real-time orbit and positioning errors, provided sufficient near-field GPS stations are deployed. This signal could be readily exploited by GPS networks currently in place, and will be facilitated by the IGS Real-Time Project as it comes to fruition. 相似文献