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地球科学

地球科学

开拓新天地

05 Jan 2009

预测地震的能力每年可以挽救数千人的生命。但是对于大多数科学家而言,提前知道此类事件的发生时间和地点仅是白日梦。 乔恩·卡特赖特 讲述了一位物理学家的故事,他相信这样的警告很快就会出现。

开拓新天地

从他在NASA的实验室出发’s Ames Research Center just south of San Francisco Bay, California, Friedemann 弗氏 is a regular visitor at the local memorial mason. Among the rows of ready-chiselled gravestones, he likes to browse the sundry stacks of raw, unfinished rock imported from as far afield as Norway and China. “看着一块岩石,我可以说,是的,这对我们来说看起来不错,” 弗氏 explains. “黑色的最好。他们往往会做好指控。”

电导率不是通常与岩石相关的属性,岩石在正常条件下是绝缘体。但是,弗洛因德对异常条件下的岩石感兴趣。他的实验室实验涉及研究高机械应力施加于火成岩时会发生什么—从岩浆中凝固并在地球深处常见的材料’地壳,地震形成的地方。程序很简单:他将一块大岩石放在两个活塞下面,并压碎它们,同时探测相关的电磁效应。“一开始我总是打破它们,” 弗氏 says. “现在,我们变得更加温柔了,一块岩石可以持续数周或数月。我们可以在一块岩石上进行数十次实验而不会破坏它。”他希望有一天他的工作可以帮助挽救数千人的生命。 Friedemann 弗氏 想了解如何预测地震。

动摇的理解

地震是科学家无法可靠预测的唯一自然灾害。美国地质调查局(USGS)等机构监视着地球上的应变’通过地面上的运动传感器的表面,以及地震活动的历史记录,它们通常可以预测活跃地区发生地震的长期前景,通常在30天内 年份。事实证明,需要明确说明即将发生的震颤的确切时间,地点和大小的预测。问题的一部分是地震学家对地面如何破裂没有清晰的认识。尽管对板块构造的研究使研究人员能够隔离出最容易发生地震的地区,但目前的想法是地球上每个微小的裂缝’地壳以一种混乱的方式传播。这意味着很难说出哪些裂纹会很快终止,哪些会破裂成令人震惊的事件。大多数地震学家认为,即将发生的地震不会发出可靠的警告信号。

弗氏 has a different opinion. Deep down in the crushing boundaries between the Earth’s tectonic plates —地震在哪里形成— the conditions are far from normal. As the plates struggle to grind past each other, the stresses grow until the plates finally slip with a devastating release of energy. 弗氏 thinks that this huge stress build-up prior to an earthquake can flood the surrounding rock with electric charge. Indeed, he believes that in the hours or days before an earthquake the ground could brim with so much charge that it generates a host of visible effects above the surface, such as infra-red emissions and vivid corona discharges. These electromagnetic phenomena could be earthquake precursors.

But despite living within walking distance of the infamous San Andreas Fault, 弗氏 was not always involved in earthquake science. The interest spawned from his research in the early 1980s, when he was studying the properties of simple crystals like magnesium oxide (MgO). He found that MgO always absorbs infra-red light at the characteristic wavelengths of hydrogen molecules. The only way for hydrogen to be present, he thought, would be if water crept into the structure during crystallization as defect OH 镁之间的组2+ 和O2– 离子。Then, pairs of the OH 组可以合并形成H2,剩下的O 离子to bond into more stable O22– 基团,称为过氧连接。

Peroxy links, according to 弗氏 , are key in turning MgO and other insulators into conductors. With a little heat, the wavefunctions of a peroxy link’s constituent O 离子“loosen up”并散布在数百个相邻离子中;进一步提高温度和O 离子completely dissociate from each other. In this state, each of the O 离子缺少稳定电子所需的电子。但缺少电子或正“hole” — which 弗氏 prefers to call a “phole” —能够跳到附近的无缺陷O 2– ion. Indeed, the heated crystal acts like a pure semiconductor in which the 羽毛s repel one another through the sea of O2– 离子to form a blanket of positive charge on the surface.

By 1994 弗氏 had collected strong evidence for 羽毛s through measurements of electrical conductivity and other properties, and was beginning to think of other ways to recreate the phenomenon. As he recalls, “逻辑是,破坏过氧键需要什么?当您加热水晶时,您’实际上只是增加离子的振动幅度。从那时起,我开始怀疑由机械变形产生的位错是否也能起到作用。”

弗氏 realized that MgO crystals are too brittle to sustain much deformation, so he turned his attention to stronger crystalline materials that could allow peroxy-link defects —岩石。在最初的实验中,他采取了一种非常规的方法:使用改良的玩具cross,将豌豆大小的钢丸烧成100 m s–1 放入小的岩石圆柱体中。他的预感得到了回报。他不仅测量了约400的正表面电位 mV从撞击区域扩展到整个岩石,但他还记录了并发的红外辐射爆发(2002年 J.地球动力学 33 543). 弗氏 attributed the latter event to the 羽毛s liberating their stored energy as they recombined into peroxy links at the surface. “这就是你所说的偶然发现,” he says.

坏兆头

弗氏 believes that his discovery can explain some of the bizarre events that are said to signal an imminent earthquake, such as eerie lights and strange animal behaviour. In 1966 in Matsushiro, Japan, ghostly lights were photographed during a string of tremors. Last year in the UK, following a moderately strong earthquake that rippled through the small Lincolnshire town of Market Rasen, The Times reported one frightened woman’s account of a “葡萄柚大小的发光球”那东西在她的卧室里变成了现实,飘向她,还有其他人声称即使没有暴风雨也看到了闪电。

1975年冬天,在中国海城市,有大量关于奇特动物行为的报道:狗变得非常烦躁;牛奔腾甚至蛇因冬眠而突然从冬眠中醒来而死。在也开始记录低振幅地震活动增加的地震学家的鼓励下,当局决定撤离该地区。几天后,发生了7.3级地震  袭击该地区,造成2000多人丧生。如果不撤离人口,这个数字可能会高出100倍。

但是,地震学家怀疑这些先兆的重要性。一百多年来,他们徒劳地试图将此类事件与地震活动联系起来,并发现它们是不可靠的警告信号,特别是因为大多数事件是在事件发生后报告的。像海城这样的成功预测,被认为是骗人的,因为没有一致的准确预测模式。

弗洛伊德同意我们还没有成熟的预测技术,但是他认为研究人员没有把握大局。他有信心,他有一个潜在的机制可以表明在哪里寻找前体—如此有信心,事实上,他已经用自己的超过一百万美元的现金来支持他的工作。

弗氏 ’我们的想法是,在地下数公里内,地震成核的应力可能会产生一团波状的云团,涌到地表,产生电磁干扰,例如地震光。他已经在实验室中看到了相关现象。通过将活塞放置在一块岩石板的上方和下方以施加集中的载荷,他发现在几吨重的物体上方,附近的一个负偏置的金属板可以拉出10–25 跨5个正离子的nA电流 气隙。另一方面,如果薄板正偏,则可能导致电子以短暂的100倾泻到岩石上 nA电流。这种电击穿还会产生可见光的闪光或所谓的电晕放电。

弗洛因德(Freund)认为,大多数其他假定的地震前兆也起源于管柱的传播。他指出过去的临床测试表明,正离子会困扰动物—其中包括引起呼吸系统问题和对疼痛的敏感性增强—这可能就是为什么有时会看到它们表现异常的原因。他说,正离子也可能吸引或排斥电离层区域,研究人员显然在1999年至2002年台湾周围发生的大部分地震中都记录到了这种现象。然后是红外辐射。几颗卫星记录了被认为是什么“thermal anomalies”高于重大震荡的震中,包括6.2之前的震荡 1998年,发生在中国张北县的中级地震。弗伦德认为这些异常与他的实验中的红外线发射具有相同的来源,即将磷脂重组为过氧连接(地球与行星科学快报 提交)。

All this may be a lot to take in, but that is the point. In the past almost all those trying to search for signs of earthquakes have only had the facilities to monitor a single type of precursor, whereas 弗氏 claims that his mechanism could show them how the precursors are all related, and thus where to look. “例如,有人在分析电离层,如果他们看到数据出现颠簸,则声称这表明即将发生地震,” he explains. “然后人们正确地说这太多了— you can’•仅用一个参数绘制一对一的相关性。我的工作可以使人们查看几个参数,以寻找预警,每个参数都可以作为指标。”

工作压力大

One of the more common questions directed at 弗氏 is whether there could be any other explanation for the rocks’电导率和相关的电磁现象。最明显的是压电性,其中某些材料— notably quartz — build up a charge imbalance when they are stressed. But while it is true that many of the rocks chosen by 弗氏 , such as the “Sierra White”来自加利福尼亚州的花岗岩含有三分之一或更多的石英,而来自更远地区的花岗岩(如来自中国北方的黑辉长岩)则不含石英。

Another possibility is that the conductivity is caused by a phenomenon known as a streaming potential. This type of voltage is sometimes generated in machines when weakly conducting fluids such as fuel or transformer oil are pumped through pipes, though in rocks it can also occur if there is salt water present. The water seeps through pores in the rock, picking up 离子of one charge while leaving aside 离子of the opposite charge. 弗氏 points out that the charge in his impact experiments flows at between 100 to 300 m s–1, which is too fast to be a streaming potential in rocks. Furthermore, when 弗氏 later upgraded his crossbow to a canon at NASA’的Ames研究中心,非正式地称为“Big Gun” —通常用于研究陨石坑形成的研究工具—在1.5的冲击下产生的冲击波 km s–1 似乎可以瞬间激活整个岩石的电荷。

但是,据加州斯坦福大学地球物理学家托尼·弗雷泽·史密斯(Tony Fraser-Smith)称,水的另一面是,它实际上可能通过“shorting out” any charges present. 弗氏 admits that the 羽毛s could react with water, although he thinks that the process would actually complete the circuit to keep the charges moving (地球与行星科学快报 时)。“弗雷泽·史密斯(Fraser-Smith)说的对,水可能对水流有所帮助,” he says. “但这并没有他想像的那么具有破坏性。”

In any event, 弗氏 is not the only researcher to have noticed the effect. Al Duba, a retired geophysicist who used to work at Lawrence Livermore National Laboratory, California, spent the better part of his career investigating anomalous conductivity in rocks. However, he concluded that the conductivity it is due to contamination, and he managed to destroy it by heating samples above 700 °C in a mixture of carbon dioxide and carbon monoxide. 弗氏 argues that this process only serves to react away the crucial O 离子。“我们曾经对此进行过友好的讨论,” he says. “杜巴(Duba)认为它一定是垃圾,并且您应该摆脱垃圾。但是我说‘No, you’重新摆脱了金蛋!’.”

唐’t mention the “p” word

地震预测是一个贬义词— at least among most seismologists, who make up the bulk of earthquake researchers. Although 弗氏 prefers not to use the word prediction, saying it is “too strong a word”,因此他的研究不可避免地落在了这个旗帜之下,因为他声称这可能会导致预警系统。

预测的麻烦在于它具有悠久的失败历史。在上个世纪的大部分时间里,地震学家一直致力于寻找统计上可靠的前兆,但未能发现任何在大地震之前持续发生的事件。现在,最接近共识的是,预测是不可能实现的目标,至少在短期内是这样,研究人员首先需要更好地了解裂缝如何在地球上成核并扩散’s crust.

但是对于某些人而言,应该完全禁止进行预测研究。东京大学地震学家罗伯特·盖勒(Robert Geller)不喜欢这样的事实,即某些国家(如日本)向试图改进预测方法的国家提供了不成比例的资金,他说这是行不通的。“我的立场如下” he says. “任何想要进行地震预测研究的人都应将其建议书发送到正常的资助系统,在该系统中应与其他所有地球物理学研究竞争对它进行审查—就是说,既不优待也不优待。通过此类正常审核的所有工作均应获得资助。”

弗氏 sees nothing fair about the US funding system. He says that he has sent grant proposals to the USGS annually for the past five years only to have each rejected on what he insists are “unscientific”理由。尽管在研究初期,NASA给予了他少量的支持,但此后他不得不自筹资金。“我基本上已经被地震学界列入黑名单,” he says.

加州理工学院的地球物理学家汤姆·希顿(Tom Heaton)认为,主要问题是地震学界“betrayed”过去,那些认为自己在实验室中的观察会扩展到现实世界的人经常这样做。这发生在1970年代,当时许多地震学家感到兴奋的是,由于许多微裂缝,实验室中受压的岩石似乎膨胀了。但是,随后利用这种效应来预测地震的尝试失败了。“现在,当我们监测大地震开始和小地震开始时的压力时,’t see a difference,” he explains. “因此,即使人们可以预测地震,他们也可能对数百种小地震和大地震做出预测。我们对约一百个预测会怎么做?”

But 弗氏 insists that this does not rule out the possibility of useful precursors. “当地震学家谈论应力和应变时,他们的意思是将仪表放入200至1000之间的钻孔中 m underground,” he says. “但是大多数地震在10到30发生核形 公里范围。地震学家必须依靠外推法和线性模型,他们知道’t much good.”

Recently, though, the USGS appears to have had a change of heart: it has invited 弗氏 to give a talk next month. However, for the man who has made such a huge personal and financial commitment to his work, entering what he calls “the lion’s den”这不是一个骄傲的问题,而是唯一的机会来修补将他的工作与既有的科学区分开来的感知断层,并可能为生活在不确定地面上的数百万人提供稳定。

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