Retrospective validation of the time association of precursors

This paper examines the problems raised by the retrospective association of events, a process which underlies any search for precursors to geophysical phenomena. Association is analysed here with respect to time only, disregarding the spatial component, so the results are more readily applicable to volcanic eruptions than to earthquakes. Two sources of bias are examined, viz. the time lag between the precursor and event series, and the tolerance or time window accepted on that lag, which is also linked to the number of associated events. These parameters are typically chosen a posteriori to minimize the apparent probability of random association in the given series. In general, given different input data, optimal association of the two series will be achieved for different values of these parameters. Thus, any valid system of significance testing must include a step which estimates the effect of retrospectively choosing optimal parameters rather than fixing them in advance. This paper shows that, as long as at least one of the two time-series—either the precursors or the events—is Poissonian, the correction factors for the significance level can be written in a simple closed form. The correction scheme is applied to a proposed association between seismic clusters and flank eruptions of Mount Etna. Although the association appears to be highly significant when assessed using simple standard tests, this significance is not confirmed when the effect of retrospective parameter optimization is taken into account. The key point is that retrospective studies can be investigated using standard techniques, but where such techniques are employed, the threshold for significance of association must be greatly lowered in order to account for the bias due to the optimal choice of parameters.     

Scale dependence in habitat selection: the case of the endangered brown bear (Ursus arctos) in the Cantabrian Range (NW Spain)

Animals select habitat resources at multiple spatial scales. Thus, explicit attention to scale dependency in species–habitat relationships is critical to understand the habitat suitability patterns as perceived by organisms in complex landscapes. Identification of the scales at which particular environmental variables influence habitat selection may be as important as the selection of variables themselves. In this study, we combined bivariate scaling and Maximum entropy (Maxent) modeling to investigate multiscale habitat selection of endangered brown bear (Ursus arctos) populations in northwest Spain. Bivariate scaling showed that the strength of apparent habitat relationships was highly sensitive to the scale at which predictor variables are evaluated. Maxent models on the optimal scale for each variable suggested that landscape composition together with human disturbances was dominant drivers of bear habitat selection, while habitat configuration and edge effects were substantially less influential. We found that explicitly optimizing the scale of habitat suitability models considerably improved single-scale modeling in terms of model performance and spatial prediction. We found that patterns of brown bear habitat suitability represent the cumulative influence of habitat selection across a broad range of scales, from local resources within habitat patches to the landscape composition at broader spatial scales.     

盐池湾自然保护区喜马拉雅雪鸡的巢址选择

2005年的4～7月,采用样方法和直接观察法,对甘肃省盐池湾自然保护区喜马拉雅雪鸡(Tetraogallus himalayensis)的巢址选择进行了研究.共测量16个巢址.研究表明,喜马拉雅雪鸡的巢主要分布于3 301～3 500 m之间、多在灌丛草地和高山岩石草地、灌木下或草丛中及突出大石下、中坡以上的坡位、坡度...     

创业者区位选择的家乡偏好及其驱动机制

作为家乡认同感浓厚且出现大规模流动人口的转型中国家,中国的创业者在区位选择上究竟偏好家乡还是异地亟待回答,这关乎地方创业政策的方向性布局。基于2014年中国劳动力动态调查数据,从微观个体层面实证检验了创业者在区位选择上是否存在家乡偏好,并探究了其驱动机制。研究发现：创业者在区位选择上的确存在家乡偏好,且这种偏好在生存型和机会型的创业者中均普遍存在;在剔除可能造成家乡偏好高估的样本后,上述发现依然成立。进一步的机制分析发现,创业者偏好在家乡创业是为了借助熟悉的社会网络拓展营销渠道并照顾子女和父母。上述发现为制定创业人才政策提供了科学依据。     

国家基本/基准站地面气温资料城市化偏差订正

城市化偏差是中国地面气温观测记录中最大的系统性偏差,订正该偏差可为大尺度气候变化监测和研究提供准确的基础资料。论文介绍了用于单站地面月平均气温序列城市化偏差订正的一个方法,并利用该方法订正了685个国家基本/基准站1961—2015年地面年及月平均气温序列中的城市化偏差。采取自东往西迭代订正的方法,即从东往西逐经度订正,订正完的目标站也可作为参考站。首先,规定目标站的参考站在300 km范围内,并利用2站的去线性趋势年均气温的相关系数作为标准,规定相关系数最大且通过信度水平为0.005显著性检验的4个候选参考站作为该目标站的参考站;然后,对各个参考站年均气温与其对应目标站年均气温求相关,并以其平方为权重计算各参考站月和年均气温的平均值序列,即为各目标站年和月平均地面气温参考序列;其次,利用目标站气温序列趋势及其参考序列趋势之差作为总的订正值,订正目标站气温序列中包含的城市化偏差。较大的城市化偏差出现在华北地区、华中部分地区、东北北部、西南及西部部分地区,介于0.1~0.3 ℃/10 a;在中国西北部分地区、西藏西部及南部、东北南部、华南沿海、华东及华中个别站存在负偏差;对整个中国而言,相对城市化偏差为19.6%。以北京、武汉、银川、深圳作为华北、华中、西北和华南地区的大城市代表站,发现其在过去55 a的相对城市化偏差分别为67.0%、75.4%、32.7%和50.3%,与前人针对单站评估城市化影响的结果基本一致,说明论文的订正方法较为合理。论文介绍的城市化偏差订正方法,可用于订正中国等快速城市化地区地面气温观测资料的系统偏差,订正后的气温数据在很大程度上消除了城市化因素引起的不确定性。     

Smaller source earthquakes and improved measuring techniques allow the largest earthquakes in Iceland to be stress forecast (with hindsight)

A group of three earthquakes in 2000 June in SW Iceland included the two largest earthquakes in Iceland in the past 30 yr. Previously, temporal changes in shear-wave splitting had not been recognized before these earthquakes as there were too few small earthquakes to provide adequate shear-wave data, and they were not stress forecast, even with hindsight. These large earthquakes were subject to a special investigation by the European Community funded PREPARED Project during which the seismic catalogue was extended to include smaller magnitude earthquakes. This more detailed data set, together with a semi-automatic programme for measuring the parameters of shear-wave splitting greatly increased the number of time-delay measurements.
The new measurements displayed the typical temporal variations before larger earthquakes as seen elsewhere: a long-term increase in time delays, interpreted as stress accumulation before the earthquake; and a decrease, interpreted as crack coalescence, immediately prior to the earthquake. The logarithms of the durations of both the implied accumulation of stress and the crack coalescence have the same self-similar relationships to earthquake magnitude as found elsewhere in Iceland. This means that, in principle, the time and magnitude of the larger earthquakes could have been stress forecast in real time had the smaller source earthquakes of the extended catalogue and the improved measuring procedures been available at the time.     

A representativeness-directed approach to mitigate spatial bias in VGI for the predictive mapping of geographic phenomena

Volunteered geographic information (VGI) contains valuable field observations that represent the spatial distribution of geographic phenomena. As such, it has the potential to provide regularly updated low-cost field samples for predictively mapping the spatial variations of geographic phenomena. The predictive mapping of geographic phenomena often requires representative samples for high mapping accuracy, but samples consisting of VGI observations are often not representative as they concentrate on specific geographic areas (i.e. spatial bias) due to the opportunistic nature of voluntary observation efforts. In this article, we propose a representativeness-directed approach to mitigate spatial bias in VGI for predictive mapping. The proposed approach defines and quantifies sample representativeness by comparing the probability distributions of sample locations and the mapping area in the environmental covariate space. Spatial bias is mitigated by weighting the sample locations to maximize their representativeness. The approach is evaluated using species habit suitability mapping as a case study. The results show that the accuracy of predictive mapping using weighted sample locations is higher than using unweighted sample locations. A positive relationship between sample representativeness and mapping accuracy is also observed, suggesting that sample representativeness is a valid indicator of predictive mapping accuracy. This approach mitigates spatial bias in VGI to improve predictive mapping accuracy.     

中国城市群科学选择与分级发展的争鸣及探索

城市群作为国家参与全球竞争与国际分工的全新地域单元,是中国新型工业化和新型城镇化发展到较高阶段的产物,是"一带一路"建设的主战场,因而在推进国家新型城镇化和经济社会发展中具有举足轻重的战略地位。城市群的发展不仅主宰着国家经济发展的命脉,也主导着中国新型城镇化的未来。但中国城市群在选择与培育过程中出现了一系列亟待解决的现实问题,需要从科学角度提出推进城市群健康发展的技术路径和科学方案。基于这一意图,中国地理学会、中国城市百人论坛和中国科学院地理科学与资源研究所于2014年12月20日联合举办了首届《中国城市群发展高层论坛》。通过近100位专家激烈的辩论、争鸣与思考,一致认为:城市群在国家新型城镇化中占据主体地位并发挥重要作用,城市群的研究与发展是一个复杂科学问题和漫长过程,是一个自然自需过程,不能违背客观规律;城市群基本内涵和空间识别范围标准存在着激烈争论与思考,政策空间层面的城市群和学术空间层面的城市群有着不同的利益导向和价值取向;城市群选择和培育中存在着"滥圈滥划、扩容贪大、拔苗助长、无中生有、拼凑成群"等一系列亟待解决的"城市群病";未来中国城市群空间格局正在形成不同的组织方案;不同发育程度的城市群存在着不同的问题和不同的发展模式,如京津冀城市群的协同发展优化模式、长江三角洲城市群的扩围模式、珠江三角洲城市群的"两条腿"并行模式、辽中南城市群的空间整合模式、哈长城市群的"井"字型空间组织模式、中原城市群的战略整合模式、关中城市群的均衡组织模式等。     

Display-sites selection by houbara bustard (Chlamydotis [undulata] macqueenii) in Mori, Xinjiang, People''s Republic of China

Display-site selection by houbara bustards was studied in Mori, Xinjiang, China during the breeding seasons from April to June 2000. Most of the habitats chosen for displaying were short sub-shrubby and open areas close to high shrub patches. The displaying males clearly prefer low covered areas and avoid densely covered and high vegetation sites. The vegetation density and number of plant species at display sites were significantly lower from that at randomly selected sites. The average distance to the closest shrub patch was significantly shorter at display sites than at random sites. Plant species richness, vegetative density, vegetative cover and distance to the shrub patches are possibly the most important factors that determined the display-sites selection of houbara bustard.     

Quantitative mapping of a precursory seismic quiescence to the Izu–Oshima 1990 (M6.5) earthquake, Japan

A highly significant seismic quiescence with a standard deviate Z = 10.1. corresponding to a 99 per cent confidence level, lasted from 1987.7 up to the 1990 February 20 Izu-Oshima M 6.5 earthquake. The quiescent volume had dimensions of 30 km N-S and 10 km E-W and was centred below 14 km depth. Within the recently upgraded seismograph network of the Earthquake Research Institute (ERI), this main shock was the only one with a magnitude M > 5.8 in the upper 30 km of the crust for which the precursory quiescence hypothesis could be tested. Within a radius of 50 km, and during the observation period (1983.5–1995.9), there were no other 1.5 yr or longer periods of quiescence that were rated Z > 6.5 in the declustered earthquake catalogue, except one that was associated with volcanic activity. The total space-time covered by alarms, including the volcanic one, was less than 1 per cent at the Z = 6.5 level. The rarity of highly significant episodes of quiescence, and the correlation in space and time suggest that a precursory seismic quiescence started 2.5 yr before the Izu-Oshima 1990 earthquake in its source volume and to the north of it, and that it can be recognized with an alarm level of Z = 6.0, generating no false alarms. During the 1.5 yr quiescence window, only 10 earthquakes occurred in the quiet volume, whereas 50 events were expected based on the rate seen at other times. In randomly selected volumes containing 50, 100 and 200 events, the anomaly scored Z = 6.1 to 10.1. On the basis of the data from May 1983 to 1995, there is no highly significant quiescence currently present in the Izu-Oshima area.     

北京湿地越冬灰鹤食性的初步分析

2005年12月~2006年3月,采用粪便分析法对北京野鸭湖湿地自然保护区和密云水库北部地区越冬灰鹤(Grus grus)的食性进行了研究。结果表明,灰鹤冬季为植食性,主要的食物有5科13种。谷物为其冬季食物的主要来源,占食物总量的76.40%。除农作物外,灰鹤也取食湿地植物的种子或植株,但仅占食物总量的13.27%。将野鸭湖灰鹤和密云水库灰鹤的食物组成进行比较,发现两越冬种群虽然都主要以农作物为食,但野鸭湖灰鹤以取食玉米(Zea mays)为主,密云水库越冬灰鹤取食最多的食物则为高粱(Sorghun bicolar)。