20110319-日本地震預報

20110319-日本地震預報 

此篇新聞是經濟學人（The Economist）於20110317所發表，題為"The myth and reality of the Japanese earthquake"的文章，建議直接看原文。

其中，最令人感到震驚的是，日本地質學家在一月份就提出即將在未來三十天內，將發生規模8級以上的大地震，其中在宮城港及茨城縣所發生的機率，分別高達99%及90%，說的白一點，就是這兩個地方，將會發生地震，其距離本次2011年日本東北地方太平洋近海地震之震央及引起輻射外洩的福島第一核電廠的相關位置，如圖一所示。也因為有如此精確的預測，在大地震發生時，地質學家並沒有太多的意外，頂多，就是時間上延後了將近半個月，以及沒料到規模會達到9級那麼大！！

但是，既然都能做出這麼精確的預測，往後再有相同的情況發生，人們對於地質學家的言論，相信可信度會更高，畢竟這種人命關天的預測，不像總統大選一樣，預測得不準確，大家笑笑就過去了。如同這次災害，既然能預測，那就有時間做預防性工作，只是，規模那麼大，能擋得了多少，真的很難說，至少，有拜有包庇，盡力降低傷亡損失才是重點。

對於地質學家，真的，佩服。

圖一、日本東北地方太平洋近海地震示意圖

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【譯文】

時間回到一月，日本地質學家警告日本東北海岸下的太平洋板塊碰撞，將會造成災難性的滑動。經過其計算，在未來的30天內，有99%的機會，將在宮城港 (Miyagi coast) 發生8級以上的地震，而有90%的機會在茨城縣 (Ibaraki prefecture) 發生。令地質學家感到驚訝的，只有在三月11日發生的地震時，其規模竟高達9級。

而地質學上傳統的斷層分隔模型，並不能同意：逆斷層（上斷層穿過下斷層的底部的斷層類型）發生破裂時，將使裂縫缺口形成更大的破裂此一論點。但在20110311禮拜五，距日本北太平洋海岸約130公里處，卻發生北美板塊沿著一隱沒帶（subduction zone）越過太平洋板塊，在震源深度24公里處引起震盪，造成海床猛烈的向上推，並觸發巨大海嘯。

很多人會問，會不會因此引起一連串的地震？而大部分的東京居民也擔心地震正在漸漸靠近中，且媒體也注意到此一現象，在該主震發生後，分別在長野縣（Nagano）和新瀉縣（Niigata）間出現規模6.6級地震、在靜岡縣（Shizuoka ）也有6.1級地震。但是這些正常的能量釋放，都是由不同的板塊運動所造成。

很難想像，東京將來有一天，會形成此災害，最有可能的原因來自於，被數百公里外的大陸板塊所衝擊，其最可能的根據是西南方100公里外的菲律賓板塊沉入歐亞板塊所造成的一連串震動，終將引發如同此次之大地震，但是此一論點，尚存在許多爭議。

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【原文】Plate tectonics，Faulty thinking，The myth and reality of the Japanese earthquake。

BACK in January, Japanese seismologists warned that the tectonic plates colliding beneath the Pacific Ocean off the northeast coast of Japan were poised to slip catastrophically. By their reckoning, there was a 99% chance of an earthquake of magnitude 8.0 occurring off the Miyagi coast, and a 90% chance of one off Ibaraki prefecture, within the next 30 years. They were surprised only by the sheer size of the magnitude 9.0 monster that was unleashed when the plates at last let go on March 11th.

It seems that, on occasions, the rupture along this particular type of 「reverse fault」 (where the upper part of one side of the fault is thrust over the foot of the other) can jump across gaps and other boundaries along the fissure, linking up with other parts of the fault to extend the breach alarmingly. The traditional fault-segmentation model used in seismology does not allow for this. But on that fateful Friday, when the North American plate slid over the Pacific plate along a subduction zone running 130km (80 miles) off the Pacific coast of northern Japan, the shock leapt from the first segment to a second and on to a third, extending the fault zone some 400km and increasing its intensity more than 30-fold. With all the action taking place only 24km down, the seabed was thrust violently upwards, triggering huge waves.

Could such a chain of seismic events happen elsewhere in Japan? Many in Tokyo fear that earthquakes may be creeping closer. The Japanese media have drawn attention to a quake of magnitude 6.6 on the far side of the country, between Nagano and Niigata prefectures, and to a quake of magnitude 6.1 in Shizuoka prefecture, both within days of the main quake. But these fairly common events occurred on entirely different tectonic plates. It is hard to imagine how faults on one continental plate might communicate with those on another that is hundreds of kilometres away.

No doubt Tokyo will be sideswiped one day. The most likely spawning ground for that earthquake will be 100km or more to the south-west, where the Philippine plate dives under the Eurasian plate, creating a continuous sequence of shudders. This could feasibly cause a megaquake of the kind the north-east has just suffered. But the evidence remains largely against it.

【Reference】
http://www.economist.com/node/18398762?story_id=18398762