Seismic anisotropy — the state of the art: II

Summary. The theory, causes, observations, and possible applications of seismic anisotropy in the Earth have developed considerably since the previous state of the art paper was published in 1977. The behaviour of waves in layered anisotropic media is now much better understood and the evidence for seismic anisotropy indicates that anisotropy is likely to be present throughout much of the crust and upper mantle. The top few hundred kilometres of the mantle appears to be anisotropic with the orientations aligned by the present or palaeo stress-field. The upper part of the crust is frequently anisotropic, probably due to cracks differentially aligned by the non-lithostatic stresses. The possibility of being able to monitor crack geometry by seismic techniques opens a wide range of applications in currently important activities.     

大数据对人文—经济地理学研究的促进与局限

大数据技术的诞生不仅快速推动着社会的进步,而且也将科学研究不断引向新的高度。以人类社会经济活动为主要研究对象的人文—经济地理学与当前大数据建设和发展趋势具有高度一致性,大数据的发展对丰富和完善人文—经济地理学势必起到积极的推动作用,同时也对人文—经济地理学的学科思维和研究方法提出了新的挑战。梳理和分析了目前大数据在人文—经济地理学主要研究领域,包括城市内部空间研究、交通与消费行为、社会空间与社会网络研究中的最近进展,以及大数据对参与式研究和决策平台的作用。着重剖析了大数据对人文—经济地理学数据获取,研究思维与范式,研究内容、研究时空尺度与研究目标等方面的促进作用与存在问题,特别是由于大数据自身发展的不完善,在数据收集特别是数据属性方面还存在很大的局限,缺乏理论基础将会使得大数据与实际应用受到很大限制,同时,数据本身也不能替代研究者思维和决策过程。因此,人文—经济地理学者应该科学对待大数据所带来的机遇,弥补和丰富以往发展中的短板,即完善学科数据建设、建立大数据应用较为完善的研究方法体系,促进跨域数据整合和跨域研究,以及推进研究对象和研究目的的转变。     

老龄化社会中的地方和非正式照护①——地理老年学最新研究进展

发达国家政策制定者和执行者正面临着一个重要问题,即如何照护身体衰弱的老年群体,以应对人均寿命延长给社会经济与福利带来的影响。当然寿命延长本身是人类取得的一个巨大成功。然而,预期寿命的增加也引发了老年人口尤其是高龄老人数量的增长,这些高龄老人面临疾病高发和工具性日常生活活动能力(Instrumental Activities of Daily Life, IADLs)下降的情况,而这种能力对保持他们晚年生活的独立和尊严非常重要。与此同时,政策和实践的重心已经从机构养老转为“就地养老”,即支持老年人尽可能长时间地住在自己家里。从概念上讲,这意味着之前由机构提供的服务和照护(care)将改为由家庭来提供,这样脆弱的老年人也可以得到来自家人、朋友和邻居的非正式照护。一方面,这意味着许多老年人能够享受到由家庭照护所带来的亲密感、安全感和情感支持;另一方面,家庭结构的变化,社区的衰落,以及经济紧缩时期保障和福利的大幅削减,造成越来越多的老年人要面对孤独、孤立及风险。于是,老年人的照护者、照护地点和照护方式,及老年人对这些因素的差异化体验,将成为对健康和老龄化研究感兴趣的地理学家越来越关心的问题。本文回顾了地理老年学对非正式照护和家庭研究的最新进展,指出该领域的工作对老年人口照护的多学科探讨具有重要贡献。     

Seismic anisotropy in the core–mantle transition zone

Split S waves observed at Hockley, Texas from events in the Tonga–Fiji region of the southwest Pacific show predominantly vertically polarized shear-wave ( SV  ) energy arriving earlier than horizontally polarized ( SH ) energy for rays propagating horizontally through D" . After corrections are made for the effects of upper-mantle anisotropy beneath Hockley, a time lag of 1.5 to 2.0  s remains for the furthest events (93.9°–100.6° ), while the time lags of the nearer observations (90.5°–92.9° ) nearly disappear. At closer distances, the S waves from these same events do not penetrate as deeply into the lower mantle, and are not split. These observations suggest that a patch of D" beneath the central Pacific is anisotropic, while the mantle immediately above the patch is isotropic. The thickness of the anisotropic zone appears to be of the order of 100–200  km.
  Observations of shear-wave splitting have previously been made for paths that traverse D" under the Caribbean and under Alaska. SH leads SV , the reverse of the Hockley observations, but in these areas the fact that SV  leads SH in the HKT data shown here suggests a different sort of anisotropy under the central Pacific from that under Alaska and the Caribbean. The case of SH travelling faster than SV  is consistent with transverse isotropy with a vertical axis of symmetry (VTI) and does not require variations with azimuth. The case of SV  leading SH is consistent with transverse isotropy with a horizontal axis of symmetry (HTI), an azimuthally anisotropic medium, and with a VTI medium formed by a hexagonal crystal. Given that (Mg,Fe)SiO₃ perovskite appears unlikely to form anisotropic fabrics on a large scale, the presence of anisotropy may point to chemical heterogeneity in the lowermost mantle, possibly due to mantle–core interactions.     

The generalization of the Mie—Grüneisen equation of state

Summary. A generalized Mie—Grüneisen equation of state is found without thermodynamical limits of validity. The range of applicability of the classical Mie—Grüneisen equation is discussed and results are presented of a particular case of the present equation. The various definitions of the Grüneisen's parameter are analysed in comparison with their generalized theoretical expression, given here.     

Locating art worlds: London and the making of Young British art

The international prominence of Young British art (YBa) in the 1990s gave London a contemporary art movement to match its role as one of the world's key centres of art exchange. Examining the rise of YBa in retrospect, this paper is concerned with the difference place makes in helping to shape the (hi)story of art. It is argued that London's established role as an international art centre was crucial in providing the density of networks, associations and facilities necessary to sustain an international art movement. At the same time, YBa's success can be linked to London's changing status as a cultural capital, as well as profound changes in the business of contemporary art. Attention is drawn to the ways in which the international art world is dominated by networks formed within and across a limited number of world art cities.     

Preliminary Results on the 3-dimensional Seismic Structure of the Lithosphere under the USGS Central California Seismic Array

About 1500 readings of teleseismic P -time residuals obtained from the US Geological Survey seismograph network in central California have been used to obtain a three-dimensional image of seismic velocity anomalies for this area by the method of Aki, Christoffersson & Husebye We found that the California network is less suitable than the LASA and NORSAR arrays for this kind of studies because of its greater proportion of peripheral blocks in which the resolution is very poor for the stochastic inverse solution and the random error effect is severe for the generalized inverse solution. Nevertheless, the resultant velocity anomalies show a remarkable correlation with the San Andreas fault zone to a depth of 75 km. The anomaly pattern changes drastically as the depth exceeds 75 km, suggesting that the asthenosphere has been reached.     

Seismic excitation of the Chandler wobble revisited

Summary. An overview is taken of the last decade of studies of the effect of earthquakes on the polar motion. The treatment of the liquid outer core in static deformation is reviewed and some misconceptions in a number of papers are pointed out. Volterra's formula is generalized to the case of a liquid core which does not obey the highly idealized Adams—Williamson density law. The focal mechanism representation of Smylie & Mansinha (1971) is corrected for neglected terms arising from coordinate curvature, bringing the computed polar shifts into near numerical agreement with those of other workers. On the basis of the comparison of the observed and computed polar shifts for the Chile 1960 and Alaska 1964 events, it is suggested that the observed polar shifts for large earthquakes may be useful as discriminators in selecting focal mechanism parameters. The observed level of Chandler wobble excitation provides a constraint on some of the more extreme values of seismic moment recently proposed, unless these are supposed to depend only weakly on magnitude. The cumulative effect of the 30 largest earthquakes in the period 1901–64, recently examined by O'Connell & Dziewonski, is found to yield a rms Chandler wobble excitation of 0".10, using the random walk theory of Mansinha & Smylie (1967). This is close to the observed level (∼ 0".15). In addition to yielding the solution to a very long-standing geophysical puzzle, the study of the effect of earthquakes on the polar motion over the last decade may have produced a useful tool for the elucidation of seismic mechanism.     

Seismic Delays in the Eastern Caribbean

i
The intervals between the arrivals of the same body wave phases from distant earthquakes at a close network of stations are compared with those expected from the known surface speeds of the phases. The arrival at the station on the island of Barbados is shown to be
2.94 = 0.34 s
later than expected relative to the other stations.
This is believed to be due to differences in crustal structure associated with the belt of negative gravity anomalies east of the West Indian arc.
A crustal section consistent with the seismic delays, gravity anomalies and seismic refraction profiles is presented.