Updating default depths in the ISC bulletin

The International Seismological Centre (ISC) publishes the definitive global bulletin of earthquake locations. In the ISC bulletin, we aim to obtain a free depth, but often this is not possible. Subsequently, the first option is to obtain a depth derived from depth phases. If depth phases are not available, we then use the reported depth from a reputable local agency. Finally, as a last resort, we set a default depth.In the past, common depths of 10, 33, or multiples of 50 km have been assigned. Assigning a more meaningful default depth, specific to a seismic region will increase the consistency of earthquake locations within the ISC bulletin and allow the ISC to publish better positions and magnitude estimates. It will also improve the association of reported secondary arrivals to corresponding seismic events.We aim to produce a global set of default depths, based on a typical depth for each area, from well-constrained events in the ISC bulletin or where depth could be constrained using a consistent set of depth phase arrivals provided by a number of different reporters.In certain areas, we must resort to using other assumptions. For these cases, we use a global crustal model (Crust2.0) to set default depths to half the thickness of the crust.     

地震震级空间分布的非参数分析

描述了应用非参数局部线性回归对震级平均趋势空间结构的研究。如果推测特定区域的地震数据集有空间相关,则需要使用修正版本的广义交叉验证方法计算光滑参数,以获得平均震级估计量。这种方法允许我们将空间依赖考虑进去从而获得更好的光滑参数。而且,应用参数自助法(bootstrap)可以量化由非参数估计方法得出的空间图的变异,也可以计算在特定震中发生的地震观测的震级大于等于给定阈值的概率。这些方法应用于两个不同的地震数据集:伊比利亚半岛西北部历史地震目录和加利福尼亚1998年1月到2008年4月的地震。     

A semi-empirical method for predicting hydrological drought magnitudes in the Canadian prairies

Abstract

A hydrological drought magnitude (M _T ) expressed in standardized terms is predicted on annual, monthly and weekly time scales for a sampling period of T years in streamflow data from the Canadian prairies. The drought episodes are considered to follow the Poisson law of probability and, when coupled with the gamma probability distribution function (pdf) of drought magnitude (M) in the extreme number theorem, culminate in a relationship capable of evaluating the expected value, E(M _T ). The parameters of the underlying pdf of M are determined based on the assumption that the drought intensity follows a truncated normal pdf. The E(M _T ) can be evaluated using only standard deviation (σ), lag-1 autocorrelation (ρ) of the standardized hydrological index (SHI) sequence, and a weighting parameter Φ (ranging from 0 to 1) to account for the extreme drought duration (L _T ), as well as the mean drought duration (L_m ), in a characteristic drought length (L_c ). The SHI is treated as standard normal variate, equivalent to the commonly-used standardized precipitation index. A closed-form relationship can be used for the estimation of first-order conditional probabilities, which can also be estimated from historical streamflow records. For all rivers, at the annual time scale, the value of Φ was found equal to 0.5, but it tends to vary (in the range 0 to 1) from river to river at monthly and weekly time scales. However, for a particular river, the Φ value was nearly constant at monthly and weekly time scales. The proposed method estimates E(M _T ) satisfactorily comparable to the observed counterpart. At the annual time scale, the assumption of a normal pdf for drought magnitude tends to yield results in close proximity to that of a gamma pdf. The M _T , when transformed into deficit-volume, can form a basis for designing water storage facilities and for planning water management strategies during drought periods.

Editor D. Koutsoyiannis; Associate editor C. Onof

Citation Sharma, T.C. and Panu, U.S., 2013. A semi-empirical method for predicting hydrological drought magnitudes in the Canadian prairies. Hydrological Sciences Journal, 58 (3), 549–569.     

Variations with time in the detection thresholds of P for seismological stations reporting to the ISC

To determine maximum-likelihood magnitudes for a given seismological network requires estimates of the detection thresholds of short-period P for the individual stations. Ideally the detection threshold of a station with fixed instrumentation and operating procedures would be constant. However, some stations reporting to the International Seismological Centre (ISC) show systematic variations with time. Other stations are unreliable or report amplitudes (A) and periods (T) only intermittently. Ways are therefore needed for choosing stations that have a consistent performance over many years. Two rough measures are described here that can be used to look at variation in the apparent detection thresholds with time: one is the number of A/T observations (at distances greater than 20^∘) and the other , for the P detections associated each month by the ISC; being the average . Some of the factors that control detection thresholds and clearly revealed by these rough measures are: array stations have lower thresholds than conventional observatories; installation of automatic detectors can lower the threshold; and annual variations in noise level are reflected in annual variations in detection thresholds. For some stations an apparent lowering of the threshold appears not to be due to changes at the station, but is a consequence of the lowering of the ISC reporting thresholds in the region around the station. For stations of the Swiss seismological network there is a slow rise in the threshold from the mid-1970s to 1992 which is difficult to explain but seems to be due to a gradual changes in the detail with which the seismograms are analysed.     

A procedure for estimation of the upper bound for earthquake magnitudes

A procedure is described for estimation of the maximum feasible magnitude for earthquakes in a given region. This procedure provides a confidence interval for the upper bound without the need for any numerical calculation.     

Uncertainties in the estimation of earthquake magnitudes in Greece

Instrumental magnitudes in Greece have been reported as: a) Mmagnitudes based on the records of the Wiechert or Mainka seismographs,b) M_LGR magnitudes based on the records of the Wood-Anderson(WA) seismographs (T_o = 0.8 sec, V_effective 1000) or othershort period seismographs calibrated against WA records and,c) M_LSM magnitudes based on strong motion records(accelerograms). Comparison of such magnitudes with momentmagnitudes, M_w, for 329 earthquakes, with epicenters in thebroader Aegean area, performed in this study, showedthat M, M_LGR+0.5 and M_LSM are practically equalto M_w, with a small overall standard error ( = 0.23).Therefore, equivalent moment magnitudes, M_w ^*,estimated from these magnitudes and reported in the catalogues of theGeophysical Laboratory of the University of Thessaloniki are equal tomoment magnitudes for all practical purposes with reasonable uncertainties.It has been further shown that surface wave magnitudes, M_s,for M_s <6.0, can be also transferred into momentmagnitudes, M_w ^*, but the larger uncertaintiesencountered make its use rather problematic.     

A note on the differences in magnitudes estimated from strong motion data and from Wood-Anderson seismometer

Magnitude dependent differences between estimates of local magnitude, M_L (Richter^12.13) based on strong motion data and on the response of Wood-Anderson Seismometer are analyzed. It is suggested that the principal cause for differences for intermediate and small magnitudes (M < 5.5) could be associated with anelastic attenuation; indicating Q values between 100 and 200 in Southern California. For large magnitudes the observed differences are interpreted to result from saturation of strong-motion amplitudes relative to other long period estimates of magnitude.     

The third asymptotic distribution of largest magnitudes in the Balkan earthquake provinces

Summary The third asymptotic distribution of the theory of largest values is applied to sixteen earthquake provinces in the Balkan area. The model which assumes an upper magnitude limit fits the observations better than the first distribution. Graphs and tables provide estimates of return periods of largest magnitudes.Communication presented at the XV General Assembly of the European Seismological Commission in Krakow, 22–28 September, 1976.     

A linearization scheme for hysteretic systems subjected to random excitation

The problem of predicting the response of hysteretic yielding systems under random excitation is considered. The paper presents a linearization scheme which is motivated by extensive simulation studies of the bilinear hysteretic system. The response process is decomposed into a low frequency drift component and a component with a frequency near the linearized natural frequency of the system. Effective stiffness and damping parameters are presented. The accuracy of the various steps of the scheme is examined. The approach gives results which are in close agreement with simulation estimates even for large-system non-linearity.     

A new integration scheme for application to seismic hybrid simulation

Hybrid simulation is a powerful test method for evaluating the seismic performance of structural systems. This method makes it feasible that only critical components of a structure be experimentally tested. This paper presents a newly proposed integration algorithm for seismic hybrid simulation which is aimed to extend its capabilities to a wide range of systems where existing methods encounter some limitations. In the proposed method, which is termed the variable time step (VTS) integration method, an implicit scheme is employed for hybrid simulation by eliminating the iterative phase on experimental element, the phase which is necessary in regular implicit applications. In order to study the effectiveness of the VTS method, a series of numerical investigations are conducted which show the successfulness of the VTS method in obtaining accurate, stable and converged responses. Then, in a comparative approach, the improved accuracy of the VTS method over commonly used integration methods is demonstrated. The stability of the VTS method is also studied and the results show that it provides conditional stability; however, its stability limit is well beyond the accuracy limit. The effect of time delay on the VTS method results is also investigated and it is shown that the VTS method is quite successful in handling this experimental error.     

Remarks about magnitudes and maximum intensities of the earthquakes in Friuli, 1976

Summary The magnitudesM and maximum intensitiesI ₀ of 66 earthquakes which occurred in the period from 1901 to 1973 in the eastern alpine area are compared with those of 62 earthquakes occurred in Friuli, northern Italy between 6 May and 15 September 1976. The average focal depth of the Friuli events is about 10 km. The existing empirical relation betweenM andI ₀ for this area is improved by many new data.     

A semi-implicit scheme for the shallow water equations for application to shelf sea modelling

A semi-implicit scheme for the numerical solution of the shallow water equations is proposed. The scheme is suitable for the simulation of shelf sea dynamics as is demonstrated by some examples of successful application covering a range of grid sizes typical for shelf sea models. The basic outlines of the method are presented. Some practical aspects of computation are discussed which illustrate that an explicit model can be modified easily to the semi-implicit version proposed here. Compared to explicit schemes the semi-implicit approach has two major advantages: (1) its economy (a saving of at least 50% in computing time can be achieved); (2) a closer match is obtained between the time-stepping procedure and the time scales of processes, the spatial scales of which are close to the lower limit of the resolution of the model grid.     

A scheme to treat the singularity in global seismic wavefield simulation using pseudospectral method with staggered grids

The pseudospectral method has been applied to the simulation of seismic wave propagation in 2-D global Earth model.When a whole Earth model is considered,the center of the Earth is included in the model and then singularity arises at the center of the Earth where r=0 since the 1/r term appears in the wave equations.In this paper,we extended the global seismic wavefield simulation algorithm for regular grid mesh to staggered grid configuration and developed a scheme to solve the numerical problems associated with the above singularity for a 2-D global Earth model defined on staggered grid using pseudospectral method.This scheme uses a coordinate transformation at the center of the model,in which the field variables at the center are calculated in Cartesian coordinates from the values on the grids around the center.It allows wave propagation through the center and hence the wavefield at the center can be stably calculated.Validity and accuracy of the scheme was tested by compared with the discrete wavenumber method.This scheme could also be suitable for other numerical methods or models parameterized in cylindrical or spherical coordinates when singularity arises at the center of the model.     

A scheme to treat the singularity in global seismic wavefield simulation using pseudospectral method with staggered grids

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A numerical test of two approximate solutions to the radiative transfer equation using the Elsasser scheme

In a recent paperCharlock, Herman andZdunkowski (1976) disagree on how best to apply the Radiative Transfer Equation (RTE) in the construction of Elsasser type radiation tables. The two proposed approximation solutions are analyzed and compared against a quasiexact solution. Infrared fluxes and cooling rates are calculated for part of the 6.3 m water vapor band for two model atmospheres. It is found that the Zdunkowski (Z) approximation yields more accurate downward fluxes, while the Charlock-Herman (CH) approximation, in general, results in more accurate upward fluxes. For the two model atmospheres studied the cooling rates for theZ approximation are usually of better quality than those due to the CH solution, unless the divergence of the net flux is extremely small.     

模拟退火法建立数字岩心的一种补充优化方案

针对传统的模拟退火建立数字岩心方法建模效率低、计算结果存在较多的孤立点等缺陷,设计了后续的补充搜索方案,定义了孔隙与骨架的边界点选取准则,提出了基于择多算子算法选取的对象点与孔隙骨架边界点交换作为新系统的生成方案.使用传统模拟退火算法对随机的初始模型优化作为第一阶段,用新系统生成方案继续优化作为第二阶段,最后减少模型中孤立孔隙点或骨架点,输出结果.试验表明,新的补充优化方案不仅可行而且效果良好.最终优化模型与传统模拟退火建模结果相比,无论是在孔隙形态还是在孔隙连通性上都与原始岩心较为接近,由反映岩石孔隙空间特征的变差函数可以看出补充搜索方案优化结果优于传统模拟退火法建模结果.