地核內部的攪動或許刺激了大地震發生
世界從未停止轉動，不過它的腳步經常會慢下來。科學家繪製長達數十年的圖表顯示地球每一天的長度會有些微變動：這天多了一毫秒，那天則少了一毫秒。上周於西雅圖舉行的美國地質年會上，兩名地球物理學家主張這些細微的變化可能足以影響大地震發生的時間點――因此或許能用來幫助預測大地震。
據科羅拉多大學(CU)波德分校的Roger Bilham和蒙大拿大學的Rebecca Bendick所言，過去100年內地球變慢的時期，和全球規模7以上的大地震數目增加的時期，兩者之間有驚人的對應關係。這類高峰期的地震比平常多上二到五次，其有用之處在於它是地球開始變慢很久一段時間之後才發生。發表此研究成果的Bilham表示：「令人驚訝的是地球可以在地震發生5年之前就向我們發出預警。」
多數地震學家同意「地震預測」是個相當棘手的議題。對於他們發現的模式可能成因為何，Bilham和Bendick到目前為止也只有模糊且難以證實的想法，但其他研究人員表示此發現確實新穎到令人無法忽視。「他們發現的相關性令人印象深刻，值得深入研究。」同為科羅拉多大學的地質學家Peter Molnar說。
這項研究一開始是在尋找地震發生時間的同步性(synchrony)。許多個別的震盪單元，可以是螢火蟲、心臟肌肉或是節拍器，最後可以因為某種類型的串擾(cross-talk)――也就是共同受到某些影響，使它們的震盪步調變得一致。產生地震的斷層會反覆累積應力並猛烈釋放出來，此特質讓Bendick可以輕易把斷層想成她所描述的「雜音十分眾多，品質相當低劣的震盪器」。她和Bilham深入探討有關數據，利用的資料為過去100年來唯一完整的地震目錄：其中僅包含規模7以上的地震。
在八月刊登於《地球物理通訊》( Geophysical
Research Letters )的研究成果中，他們發表了兩種模式：第一種是大地震的發生時間似乎會成群聚集，但空間上並沒有此關係。第二種是大地震的數目似乎每隔32年就會達到高峰。地震也許能透過某種方式互通有無，或者地球因為受到某種外力輕輕擠壓而產生破裂。
研究人員探索這類可以影響全球的力量，最後他們發現此模式跟日長度變化一致。雖然聖嬰現象之類的天氣模式，可以在一年或數年之中讓日長度反覆增減一毫秒；然而，以好幾十年為週期的數毫秒變化，尤其是每三十年左右因地球轉速減緩而形成的低鋒，卻可以跟地震群聚的趨勢完美地對齊。「當然，這看起來有些異想天開。」Bendick表示。但她或許是對的。當日長度以數十年發生變化時，地球磁場也會出現短暫的波動。研究人員認為外核液態鐵的流動方式發生變化或許是這兩種現象的成因，但他們無法確定到底發生了什麼作用，或許是熔融的外核有部分被上方地函給卡住了。這可能會改變液態金屬的流動方式使得磁場發生變化，同時在地函和地核之間傳遞足夠動量而影響到日長度。
地震學家以往不認為深埋在地殼下方2900公里的地核會是地震發生之處，但Bilham在西雅圖的演講中認為它應當如此。他說地核「其實跟我們非常接近。它和我們的距離比這裡(西雅圖)距紐約還近。」
Molnar表示呈現固態的地殼和地函，與液態地核轉速之間的些微差異，或許會轉換成一股作用力，能用某種方式微調地震使它們同步發生。由於地球在赤道的自轉速度為每秒460公尺，考慮到速度如此之快，這樣的想法確實有合理之處。他接著說：「當然，也許這根本就是一派胡言」。不過，加州大學柏克萊分校的地球物理學家Michael Manga表示，呈現出他們兩者之間有某種關聯的證據相當有力。他說：「我一直在研究由季節變化，像是融雪而造成的地震。他(Bilham)呈現的關聯性比我之前看過的好上許多。」
洛杉磯南加州大學的地質學家James Dolan說：「無論如何，我們在5年之內就會知道其對錯，因為地球自轉從4年多前已開始進入另一個減速周期。地球平均一年會有17到20場大型地震，相較於今年目前為止的地震數目比平均還少了四場，從明年開始地球應該就會比平均多發生5場。如果此模式維持不變，那它將會對地震預警重下新的定義。」
Sloshing of Earth’s core may spike major earthquakes
The world doesn’t stop spinning. But
every so often, it slows down. For decades, scientists have charted tiny fluctuations
in the length of Earth’s day: Gain a millisecond here, lose a millisecond
there. Last week at the annual meeting of the Geological Society of America
here, two geophysicists argued that these minute changes could be enough to
influence the timing of major earthquakes—and potentially help forecast them.
During the past 100 years,
Earth’s slowdowns have correlated surprisingly well with periods with a global
increase in magnitude-7 and larger earthquakes, according to Roger Bilham of
the University of Colorado (CU) in Boulder and Rebecca Bendick at the University
of Montana in Missoula. Usefully, the spike, which adds two to five more quakes
than typical, happens well after the slow-down begins. “The Earth offers us a
5-years heads up on future earthquakes, which is remarkable,” says
Bilham, who presented
the work.
Most seismologists agree
that earthquake prediction is a minefield. And so far, Bilham and Bendick have
only fuzzy, hard-to-test ideas about what might cause the pattern they found.
But the finding is too provocative to ignore, other researchers say. “The
correlation they’ve found is remarkable, and deserves investigation,” says
Peter Molnar, a geologist also at CU.
The research started as a
search for synchrony in earthquake timing. Individual oscillators, be they
fireflies, heart muscles, or metronomes, can end up vibrating in synchrony as a
result of some kind of cross-talk—or some common influence. To Bendick, it
didn’t seem a far jump to consider the faults that cause earthquakes, with
their cyclical buildup of strain and violent discharge, as “really noisy,
really crummy oscillators,” she says. She and Bilham dove into the data, using
the only complete earthquake catalog for the past 100 years: magnitude-7 and
larger earthquakes.
In work
published in August in Geophysical Research Letters they
reported two patterns: First, major quakes appeared to cluster in time—although
not in space. And second, the number of large earthquakes seemed to peak at
32-year intervals. The earthquakes could be somehow talking to each other, or
an external force could be nudging the earth into rupture.
Exploring such global
forces, the researchers eventually discovered the match with the length of day.
Although weather patterns such as El Nino can drive day length to vary back and
forth by a millisecond over a year or more, a periodic, decades-long
fluctuation of several milliseconds—in particular, its point of peak slow down
about every three decades or so—lined up with the quake trend perfectly.
"Of course that seems sort of crazy," Bendick says. But maybe it
isn’t. When day length changes over decades, Earth’s magnetic field also
develops a temporary ripple. Researchers think slight changes in the flow of
the molten iron of the outer core may be responsible for both effects. Just
what happens is uncertain—perhaps a bit of the molten outer core sticks to the
mantle above. That might change the flow of the liquid metal, altering the
magnetic field, and transfer enough momentum between the mantle and the core to
affect day length.
Seismologists aren’t used
to thinking about the planet’s core, buried 2900 kilometers beneath the crust
where quakes happen. But they should, Bilham said during his talk here. The
core is “quite close to us. It’s closer than New York from here,” he said.
At the equator, Earth spins
460 meters per second. Given this high velocity, it’s not absurd to think that
a slight mismatch in speed between the solid crust and mantle and the liquid
core could translate into a force somehow nudging quakes into synchrony, Molnar
says. Of course, he adds, “It might be nonsense.” But the evidence for some
kind of link is compelling, says geophysicist Michael Manga of the University
of California, Berkeley. “I’ve worked on earthquakes triggered by seasonal
variation, melting snow. His correlation is much better than what I’m used to
seeing.”
One way or another, says
James Dolan, a geologist at the University of Southern California in Los
Angeles, “we’re going to know in 5 years.” That’s because Earth’s rotation
began a periodic slow-down 4-plus years ago. Beginning next year, Earth should
expect five more major earthquakes a year than average—between 17 to 20 quakes,
compared with the anomalously low four so far this year. If the pattern holds,
it will put a new spin on earthquake forecasting.
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