Sensitivity of goodness-of-fit statistics to rainfall data rounding off

An analysis based on the L-moments theory suggests of adopting the generalized Pareto distribution to interpret daily rainfall depths recorded by the rain-gauge network of the Hydrological Survey of the Sardinia Region. Nevertheless, a big problem, not yet completely resolved, arises in the estimation of a left-censoring threshold able to assure a good fitting of rainfall data with the generalized Pareto distribution. In order to detect an optimal threshold, keeping the largest possible number of data, we chose to apply a “failure-to-reject” method based on goodness-of-fit tests, as it was proposed by Choulakian and Stephens [Choulakian, V., Stephens, M.A., 2001. Goodness-of-fit tests for the generalized Pareto distribution. Technometrics 43, 478-484]. Unfortunately, the application of the test, using percentage points provided by Choulakian and Stephens (2001), did not succeed in detecting a useful threshold value in most analyzed time series. A deeper analysis revealed that these failures are mainly due to the presence of large quantities of rounding off values among sample data, affecting the distribution of goodness-of-fit statistics and leading to significant departures from percentage points expected for continuous random variables. A procedure based on Monte Carlo simulations is thus proposed to overcome these problems.     

Impacts of permafrost changes on alpine ecosystem in Qinghai-Tibet Plateau

Alpine cold ecosystem with permafrost environment is quite sensitive to climatic changes and the changes in permafrost can significantly affect the alpine ecosystem. The vegetation coverage, grassland biomass and soil nutrient and texture are selected to indicate the regime of alpine cold ecosystems in the Qinghai-Tibet Plateau. The interactions between alpine ecosystem and permafrost were investigated with the depth of active layer, permafrost thickness and mean annual ground temperature (MAGTs). Based on the statistics model of GPTR for MAGTs and annual air temperatures, an analysis method was developed to analyze the impacts of permafrost changes on the alpine ecosystems. Under the climate change and human engineering activities, the permafrost change and its impacts on alpine ecosystems in the permafrost region between the Kunlun Mountains and the Tanggula Range of Qinghai-Tibet Plateau are studied in this paper. The results showed that the per- mafrost changes have a different influence on different alpine ecosystems. With the increase in the thickness of active layer, the vegetation cover and biomass of the alpine cold meadow exhibit a significant conic reduction, the soil organic matter content of the alpine cold meadow ecosystem shows an exponential decrease, and the surface soil materials become coarse and gravelly. The alpine cold steppe ecosystem, however, seems to have a relatively weak relation to the permafrost environment. Those relationships resulted in the fact that the distribution area of alpine cold meadow decreased by 7.98% and alpine cold swamp decreased by 28.11% under the permafrost environment degradation during recent 15 years. In the future 50 years the alpine cold meadow ecosystems in different geomorphologic units may have different responses to the changes of the permafrost under different climate warming conditions, among them the alpine cold meadow and swamp ecosystem located in the low mountain and plateau area will have a relatively serious degradation. Furthermore, from the angles of grassland coverage and biological production the variation characteristics of high-cold eco- systems in different representative regions and different geomorphologic units under different climatic conditions were quantitatively assessed. In the future, adopting effective measures to protect permafrost is of vital importance to maintaining the stability of permafrost engineering and alpine cold eco- systems in the plateau.     

Recurrence Interval Statistics of Cellular Automaton Seismicity Models

The recurrence interval statistics for regional seismicity follows a universal distribution function, independent of the tectonic setting or average rate of activity (Corral, 2004). The universal function is a modified gamma distribution with power-law scaling of recurrence intervals shorter than the average rate of activity and exponential decay for larger intervals. We employ the method of Corral (2004) to examine the recurrence statistics of a range of cellular automaton earthquake models. The majority of models has an exponential distribution of recurrence intervals, the same as that of a Poisson process. One model, the Olami-Feder-Christensen automaton, has recurrence statistics consistent with regional seismicity for a certain range of the conservation parameter of that model. For conservation parameters in this range, the event size statistics are also consistent with regional seismicity. Models whose dynamics are dominated by characteristic earthquakes do not appear to display universality of recurrence statistics.     

Virtual California: Fault Model, Frictional Parameters, Applications

Virtual California is a topologically realistic simulation of the interacting earthquake faults in California. Inputs to the model arise from field data, and typically include realistic fault system topologies, realistic long-term slip rates, and realistic frictional parameters. Outputs from the simulations include synthetic earthquake sequences and space-time patterns together with associated surface deformation and strain patterns that are similar to those seen in nature. Here we describe details of the data assimilation procedure we use to construct the fault model and to assign frictional properties. In addition, by analyzing the statistical physics of the simulations, we can show that that the frictional failure physics, which includes a simple representation of a dynamic stress intensity factor, leads to self-organization of the statistical dynamics, and produces empirical statistical distributions (probability density functions: PDFs) that characterize the activity. One type of distribution that can be constructed from empirical measurements of simulation data are PDFs for recurrence intervals on selected faults. Inputs to simulation dynamics are based on the use of time-averaged event-frequency data, and outputs include PDFs representing measurements of dynamical variability arising from fault interactions and space-time correlations. As a first step for productively using model-based methods for earthquake forecasting, we propose that simulations be used to generate the PDFs for recurrence intervals instead of the usual practice of basing the PDFs on standard forms (Gaussian, Log-Normal, Pareto, Brownian Passage Time, and so forth). Subsequent development of simulation-based methods should include model enhancement, data assimilation and data mining methods, and analysis techniques based on statistical physics.     

前兆异常与地震预测的一点思考

对1991—2000年中国震例提供的前兆观测异常和首都圈地区前兆观测项目进行了统计分析,对2005年11月黑龙江前兆异常进行了介绍,也对开展前兆异常研究提出了一些想法。根据对地震前兆观测项目预测效能初步研究,作者建议“十一五”期间应该建设两种地震预测试验场。地震频发地区的试验场,用于验证观测项目效果;无震地区试验场,用于试验仪器的抗干扰性能和确定背景场。     

Co-Phased Median Filters, Some Peculiarities of Sweep Signal Processing

Median filters are nonlinear and a theoretical analysis of their behavior is very difficult and so are rarely used for the processing of seismic data. However, they are able to preserve steps, sharp discontinuities and edges that are lost using most other standard filters. As seismic data are mostly harmonic or frequency modulated signals in the frequency range 1 to 10 Hz, the median filter must be adapted so as to suppress noise but not unduly distort the signal. This can be accomplished by using a median filter whose length is n times that of signal period where n is even. By use of weighted-order statistics, samples of the signals closest in phase can be obtained. To illustrate the method, a signal from a frequency band of 6.3–7.5 Hz was processed and the quality of the signal enhanced two fold over the quality of the signal without processing.     

How to cut a sediment core for 210Pb geochronology

²¹⁰Pb geochronology is described mathematically, but how to cut a sediment core is not explicit. Thick sectioning may reduce dating reliability; but on the contrary, thin sectioning is time-consuming. Considering the counting error of excess ²¹⁰Pb, a new method was proposed for the determination of meaningful thickness for sectioning. The authors applied this method to a core from Tokyo Bay. To increase the thickness with depth, the treatment is helpful in reducing the sample number for measurement and improving the dating accuracy. In addition, the averaging effect involved in sectioning was discussed, and it was confirmed that the averaging effect in the new method on ²¹⁰Pb geochronology may generally be neglected.     

Using Field-Portable XRF to Assess Geochemical Variations Within and Between Dolerite Outcrops of Preseli, South Wales

Eight dolerite outcrops in Preseli, south Wales were measured in situ using field-portable XRF analysis, in order to compare two different analysis ("sampling") strategies, and to investigate geochemical variability within and between outcrops. A sampling strategy of two (neighbouring but independent) measurements at each of a maximal number of locations dispersed over an outcrop was the more effective in indicating the overall chemical variance of that outcrop. Analysis of variance indicated that much of the observed variance within individual outcrops stems from real geochemical variability rather than from sampling and analytical factors. Standard ANOVA F tests showed that several of the studied outcrops are heterogeneous at the 5% significance level for one or more elements. Geochemical distinctions between some outcrops were demonstrated using discriminant analysis. PXRF analysis offers an alternative approach to conventional characterisation of outcrops, which is often based on laboratory analysis of small numbers of samples. However, PXRF data are affected by rock weathering and may require correction for this if they are to be compared with analyses of fresh rock.     

Multivariate analysis of landscape wildfire dynamics in a Mediterranean ecosystem of Greece

This paper focuses on spatial distribution of long-term fire patterns versus physical and anthropogenic elements of the environment that determine wildfire dynamics in Greece. Logistic regression and correspondence analysis were applied in a spatial database that had been developed and managed within a Geographic Information System. Cartographic fire data were statistically correlated with basic physical and human geography factors (geomorphology, climate, land use and human activities) to estimate the degree of their influence at landscape scale. Land cover types of natural and agricultural vegetation were the most influential factors for explaining landscape wildfire dynamics in conjunction with topography and grazing.