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[编贴](方舟子版)USGS回答地震预测问题;“直言了”大地震:预报PK预测

已有 4074 次阅读 2008-6-12 22:23 |个人分类:科技与知识|关键词:地震,预测,预报| 地震, 预测, 预报

(方舟子版)美国地质调查局回答地震预测问题

【方舟子按:《南方周末》近期登了一篇《美国地质调查局:人类不能预报地震》,是根据美国地质调查局网站上的常见问答翻译的。里面有很多错译、漏译,我本来想在其译文的基础上根据原文做些修改,发现错误实在太多,干脆重新翻译。原文见:http://earthquake.usgs.gov/learning/faq.php

  问:你能预测地震吗?

  答:不能。美国地质调查局(USGS)、加州理工学院或其他任何地方的科学家都不曾预测过一次大地震。他们不知道如何预测地震,预计在可预见的未来也不知道。不过,根据科学数据,我们能够计算出未来可能发生的地震的概率。例如,科学家估计在未来30年内旧金山湾区发生一次大地震的概率是67%,南加州则是60%。

  USGS致力于地震灾害的长期减轻,是通过帮助提高建筑的安全性,而不是通过试图实现短期预测。

  问:动物能预测地震吗?

  答:我们能找到的有关大地震发生前出现动物异常行为的最早记载见于公元前373年的希腊。据称,在一次破坏性地震发生的前几天,老鼠、黄鼠狼、蛇和蜈蚣离开他们的窝巢,逃往安全地带。在某次地震发生前数周到数秒的时间范围内,哺乳动物、鱼、鸟、爬行动物和昆虫表现出异常行为,这类传闻极多,随处可见。然而,我们仍未发现地震发生前有一致和可靠的动物异常行为,也未发现有何机理能够对此做出解释。探索这一神秘现象的科学家多数但并非全部在中国或日本。

  问:是否有些人能够感觉到地震即将发生?

  答:有些人声称在地震发生前他们出现了症状,但是这并无科学解释,而且大部分时候,在这些症状出现之后并无地震。

  问:存在地震天气吗?

  答:在公元前4世纪,亚里斯多德曾提出地震是由关在地下洞穴中的风引起的。他认为小地震是源自空气压迫洞顶,而大地震是源自空气冲破了地表。这个理论导致人们相信存在地震天气,因为有大量的空气被关在地下,那么地震前的天气就会是炎热而平静的。后来有一种理论声称地震发生于平静、多云天气,通常在发生前有强风、火球和流星。

  然而,天气和地震无关。地震是地球内部地质活动的结果,能够发生于任何天气和一年的任何时间。地震源于数英里的地下。刮风、降雨、气温和气压的变化只能影响地表和地下浅层。震源所在的位置远超出了天气能影响到的深度,而导致地震的力量也远大于天气力量。有时候,我们被问到:“地震是否会以某种方式改变天气?”地震本身并不导致天气的改变。不过,地震是地球构造运动的一部分,这个过程通常会改变地高和地貌。构造运动能够导致内陆变成海岸,或者相反。但是,能够改变气候的重大变化要历经数百万年才能发生,而且是在许多次地震之后。

  问:月球或行星的位置能否影响地震的活动?

  答:月球、太阳和其他行星通过引力场的扰动对地球产生影响。这一影响的相对大小与物体质量成正比,与它和地球的距离的三次方成反比。用大型地震目录并未发现地震发生率与半日潮存在显著的相关性。不过,在某些火山地带,例如猛犸湖,半日潮与余震的发生率据称存在微弱但显著的相关性。

  问:为什么我们现在有这么多地震?是否地震的活动程度增加了?这是否意味这就会有一次大地震?或者,我们很长时间没有地震了,这是否意味着压力正在累积?

  答:虽然看上去似乎我们现在有更多的地震,但是一个世纪以来7级或以上的地震的次数相当恒定,而且根据我们的记录,实际上近年来似乎减少了。

  (中略)

  地震活动的暂时增加并不意味着大地震即将发生。类似的,缺乏地震活动也不意味着大地震即将发生。地震活动率的暂时增加或降低通常只是地震活动的自然变异的一部分。我们没有办法知道这次它是否会导致更大的地震。大量的小震,特别是在地热带,是常见的,而中-大地震通常伴随着一系列的余震。所有这一切都是正常和预料中的地震活动。

  问:一次地震是大地震的前震的概率有多大?

  答:在世界范围内,一次地震之后,3天内在其附近发生一次大地震的概率略多于6%。在加利福尼亚,这个概率是大约6%。这意味着任何一次地震都有大约94%的概率不是前震。在加利福尼亚,大约一半的较大地震有前震,另一半则没有。在现在,我们无法分辨某次地震是否是前震,直到在它之后发生了更大的地震……因此只能是事后回顾。

  问:两次地震在同一天发生。它们是否有关?

  答:人们常常纳闷在阿拉斯加发生的地震是否会引发加州的地震,或智利发生的地震是否与一周后在墨西哥发生的地震有关。就这样的距离而言,答案是否定的。即便是地壳的岩石也没有坚硬到足以把应力场有效地传播数千英里。

  问:在早晨、晚上或一个月的某个时候是否有更多的地震?

  答:地震在一年、一个月、一天的任何时候发生的机会相等。一年、一个月、一天的不同时间的因素差异并不会影响到地球内部导致地震的力量。

出处:新语丝 (XYS20080611)


***编后语:本人于2008年6月10日中午12点(美国东部夏令时为2008年6月9日晚12点)曾致信方舟子先生,不知他是否收到

 

 

 

根据游客“无名小辈”的建议和提供的线索,于2008年6月15日14:35转载“直言了”的博文“大地震:预报 PK 预测”,供大家参考(http://blog.gmw.cn/u/36000/archives/2008/17460.html)。

大地震:预报 pk 预测  

   

     

(作者;直言了,2008-05-28 21:17)。


    四川5/12大地震爆发后,中国和世界上的许多人提出了一个大问题:能否预测地震?

   

    阅读听取政府新闻发布会、媒体评论、互联网讨论,您可以发现,一个中文词汇“预报”代表了地震监测服务中的预测、预报、预警和警报系统。这种翻译造成了混乱和误解,官员和媒体的自相矛盾使情况变得更糟糕。结果,基于目前的科技能力,若有人说地震预报是不可能的(或有条件地部分可能),就可能被理解为地震预测和早期预警系统也不可能,而事实上,多年来,该两者发挥了相当好的作用。

    在讨论5/12地震的时候,美国地质观测局(USGS)的科学家M·伯兰彼德博士,向中国人民表示了深深的同情,同时,提供了有关地震预测和预报的翔实材料。正如他所说,此时此刻,对许多中国民众来说,理解地震预报是一个非常重要的课题。

    通过各种讨论,本人发现,USGS的科学家显得很保守和很小心,尤其是讨论预报的时候、他们是极端地小心谨慎。对比之下,美国航天局(NASA)的科学家显得更积极进取。事实上,USGS和NASA的地震预测工作,都做得很漂亮。本人相信,总有一天能实现更准确地预报大地震。不过,就目前而言,美国USGS的科学家有他们的充分理由选择很保守的工作方式。

    M·伯兰彼德博士明确说明,“预测”和“预报”使用不同的方法来服务于不同的目的。这就是说,把“预测”和“预报”两个意义不同的英文术语用一个中文词汇“预报”表达,是制造混乱。5/12大地震说明,术语的准确翻译是非常必要的,须知,只有对“预测”和“预报”的正确深入的了解、才能充分地受惠于地震监测服务。

    为此,这里与有兴趣的读者分享讨论,后面是伯兰彼德博士解释说明的原文。若您需要音频,您可以访问如下USGS的网页:《我们能预报地震吗?》,地址是:

     http://www.usgs.gov/corecast/details.asp?ID=76 
     

    附带说明:在英文里,“forecast”和“prediction”是近义词,都有对未来发生事件的估测的含义,但有明显区别。譬如,就地震监测服务来说,后面文字明白指明:

    

    ·“forecast”说明的是某段时间、某个区域和某个范围震级的一组地震事件的趋势,因而,它的概率取值倾向是“小概率”。地震监测服务的“forecast”是完全可以做到的;实际上,美国等国家不但天天做、且每小时更新一次数据。 

    “prediction”说明的是特定时刻点、特定地点和特定震级的一个地震事件,带有断定性“预言”的色彩,因而,它的概率取值倾向是“大概率”。 做“prediction”的地震监测是很困难的。

   

    到目前,地震监测中服务“forecast”和“prediction”的英文术语,在中文里都用“预报”表达,结果造成概念混淆和误解误导。       

          

    

         

伯兰彼德博士解释说明的原文:
   

     

From: Michael Blanpied
          USGS Earthquake Hazards Program, Reston, Virginia, USA
Date:  Sat, 24 May 2008 11:42:58 -0400. 

       

       

      Paul Spudich and I have discussed your correspondence about earthquake prediction, which is a very interesting subject and an important one to many Chinese at this moment.

   

     

      Responding to your first question: "If the above statement is true, then please explain the following rumors (successful earthquake forecasts with certain time and place) made by The Chinese State Seismological Bureau (including its local offices):"  [*]

          

         Most American seismologists agree that the 1975 Haicheng earthquake was successfully predicted. A team of Canadian and Chinese scientists examined the events preceding the Haicheng earthquake and agreed with this assessment. Their study is published in an American seismological journal at:

       

      < http://bssa.geoscienceworld.org/cgi/content/abstract/96/3/757 >,

      

        and summarized in plain-English form in the magazine GeoTimes at:

   

      < http://www.geotimes.org/june06/WebExtra062606.html >.

   

      Most American seismologists also agree that the Haicheng earthquake is the -only- successful prediction of a large earthquake. Despite decades of work and many attempts, using many methods, no other large earthquakes have been successfully predicted in the short term.

       

        Unlike most earthquakes, the Haicheng earthquake was preceded by hundreds of small foreshocks that made the prediction easier. Only a few big earthquakes have foreshocks, and it is extremely rare to have a strong sequence of foreshocks. Usually a foreshock cannot be distinguished from any of the other thousands of small earthquakes that happen every day.

      

        Unfortunately we have no information on the predictions of the other earthquakes that you listed, so cannot comment on them.

        

       

       You also asked:  "If the statement is true, then the fact becomes: The USGS does not make earthquake predictions, but conducts forecasting, such as 50/30/10-years/probability-10% forecast and 24-hours forecasting nationwide. Correct or false?"

          

      You make a good point. In the U.S. the term 'prediction' usually means a statement that an earthquake having a specified magnitude will occur in a short time interval (a day to a year), and in a well defined region (a city, county, or part of a U.S. state), and with a high probability much higher than the usual, day-to-day probability. The only U.S. prediction that has ever been endorsed by the USGS was that of the Parkfield earthquake; in that case a magnitude-6 earthquake was predicted to occur on a specific part of the San Andreas fault between 1985 and 1993. The earthquake occurred, but not until 2004.

       

        The U.S. public really wants us to predict earthquakes, and unfortunately most American seismologists (and the USGS) agree that we seismologists do not know how to make predictions with accuracies like the above definition. We are good at predicting the locations, magnitudes and shaking of future earthquakes, but we have no way to predict the time.

        

        The USGS uses the term 'forecast' to mean a statement that an earthquake (or ground shaking) will occur in a specific place sometime during a longer time interval (5 - 50 years), usually at a much lower probability level. We cannot evacuate cites based on these forecasts because of the long time interval, but the public can use these forecasts to prepare their homes, offices, and factories for the future shaking. Such forecasts form the basis for improved building codes, insurance rates, city planning, and response planning.

         

        Note that seismologists usually use the word 'earthquake' to mean slip on a fault (also called fault rupture); the public uses 'earthquake' to mean ground shaking. The USGS forecasts both of these. Here are examples:

      

       So, for an example of a shaking forecast, see:
       <
http://www.earthquake.usgs.gov/research/hazmaps/products_data/1996/hazmaps/CAmap1r.gif>

   

        This is a map of California and Nevada that forecasts the strength of shaking (specifically the peak ground acceleration in % of g, the acceleration of gravity) that has a 90% chance of not being exceeded in 50 years. So, if you look at the intersection of the California, Nevada, and Oregon borders (at 42.0 degrees north, 120.0 degrees west), you will see that the map is yellow-green, which indicates that at that location there is a 90% probability that the ground motion will not exceed about 14% g (or a 10% probability that the shaking will exceed 14% g) sometime in the next 50 years.

        

       For an example of a forecast of slip (rupture) on faults, see:
       <
http://www.scec.org/core/public/sceccontext.php/3935/13661 >

   

        The colors on this California map represent the probabilities of having nearby (within about 6 km) fault slip during an earthquake of magnitude 6.7 or larger in the next 30 years. So the blue of Sacramento means that there is a 0.01% chance in the next 30 years that an earthquake fault within 6 km of Sacramento will slip in a magnitude 6.7 (or larger) earthquake.

           

         There is one time when we can say more specific things about the timing of earthquakes, and that is during aftershock sequences. We know very well that a large earthquake will generate many aftershocks, that the aftershock rate will generally decrease with time, and that there will generally be more smaller aftershocks and fewer large ones. We cannot predict the time or place of individual aftershocks, and we cannot predict how large the largest ones will be. For the state of California we publish a web map showing the probability of shaking from aftershocks and regular earthquakes in the next 24 hours:

   

           < http://pasadena.wr.usgs.gov/step/ >

      

        The documentation is excellent, but I can briefly summarize here: The blue background shows the probability of a large earthquake in any 24 hours; this probability comes from observations of faults and past earthquakes and does not change. The yellow spots are areas of higher probability due to aftershocks of previous earthquakes; those spots typically fade away over a few days.

        

        Note that the 24-hour map cannot predict the time or place of a large earthquake, and the map does not use any observations other than small earthquakes. At this time we do not know of any method for predicting the time and place of an earthquake. #

        

        

 [*] The list of successful earthquake predictions (declared by CEA offices):   

          

- 02-04-1975, Haicheng, Liaoning; M=7.3.

- 05-29-1976, 20:23, Longling county, Yunnan, M=7.3.        

- 07-28-1976, 3:00 AM, Qinglong County, Hebei Province.

- 08-16-1976, Songpan-Pingjiang county, M=7.2.       

- 05-21-1984, Huanghai, Jiangsu, M=6.2. 

- 07-12-1995, morning, Menglian county, M=7.3.        

 

 



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