順暢無阻的板塊運動：發生在超慢速中洋脊的地震詳細記錄

原文網址：www.sciencedaily.com/releases/2016/06/160629135238.htm

Plate tectonics without jerking: Detailed recordings of earthquakes on ultraslow mid-ocean ridges

順暢無阻的板塊運動：發生在超慢速中洋脊的地震詳細記錄

The earthquake distribution on ultraslow mid-ocean ridges differs fundamentally from other spreading zones. Water circulating at a depth of up to 15 kilometres leads to the formation of rock that resembles soft soap. This is how the continental plates on ultraslow mid-ocean ridges may move without jerking, while the same process in other regions leads to many minor earthquakes, according to geophysicists of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). Their stultraslow mid-ocean ridgeudy is going to be published advanced online in the journal Nature on Wednesday, June 29, 2016.

在超慢速中洋脊(ultraslow mid-ocean ridge)發生的地震其分佈模式相較於其他擴張帶有著根本上的差異。此處的水可以循環至地下15公里深處，而形成跟軟皂十分相似的岩石。根據阿爾弗雷德•韋格納暨亥姆霍茲極地與海洋研究所的地球物理學家表示，這便是超慢速中洋脊上的大陸板塊可以順暢無阻地移動，然而同樣過程在其他中洋脊卻會產生許多小地震的原因。他們的研究將會於2016/6/29預先刊登在期刊《自然》(Nature)的線上版。

Mountain ranges like the Himalayas rise up where continental plates collide. Mid-ocean ridges, where the continents drift apart, are just as spectacular mountain ranges, but they are hidden in the depths of the oceans. On the seabed, like on a conveyor belt, new ocean floor (oceanic lithosphere) is formed as magma rises from greater depths to the top, thus filling the resulting gap between the lithospheric plates. This spreading process creates jerks, and small earthquakes continuously occur along the conveyor belt. The earthquakes reveal a great deal about the origin and structure of the new oceanic lithosphere. On the so-called ultraslow ridges, the lithospheric plates drift apart so slowly that the conveyor belt jerks and stutters and, because of the low temperature, there is insufficient melt to fill the gap between the plates. This way, the earth's mantle is conveyed to the seabed in many places without earth crust developing. In other locations along this ridge, on the other hand, you find giant volcanoes.

當大陸板塊碰撞時會隆起像喜馬拉雅山這類的宏偉山脈。中洋脊身為陸地彼此漂離之處雖然也跟山脈同樣壯觀，但它們卻隱藏在海洋深處。海底就像輸送帶一樣，當岩漿從地球深處往上湧至地表，因而填補板塊岩石圈之間的間隙時，就會形成新的海床(海洋岩石圈)。此種擴張過程會產生顫動，使輸送帶周邊不斷發生小地震，這些地震會透露跟新的海洋岩石圈來源和構造有關的大量訊息。在所謂的「超慢速中洋脊」(ultraslow ridge)板塊岩石圈漂離的速度相當慢，使得輸送帶僅能斷斷續續地運作。且因為此處溫度相對較低造成融熔物質的量不足以填滿板塊之間的空隙，使得此區域中有許多地方的地函到達海床時並未伴隨新地殼形成。另一方面，在此種洋脊的其他地方則可以發現許多巨型火山。

Ultraslow ridges can be found under the sea ice in the Arctic and south of Africa along the Southwest Indian Ridge in the notorious sea areas of the Roaring Forties and Furious Fifties. Because these areas are so difficult to access, earthquakes have not been measured there. And so until now, little was known about the structure and development of around 20 percent of the global seabed.

在南極海的海冰之下以及沿著西南印度洋海脊可以發現超慢速中洋脊。西南印度洋海域因為「咆哮四十度及瘋狂五十度」(Roaring Forties and Furious Fifties)而惡名昭彰。由於這些區域相當難以抵達，使得此處的地震活動尚未有人觀測，也造成全球海床約20%的區域其構造和發育到目前為止仍所知甚少。

With the research vessel Polarstern, a reliable workhorse even in heavy seas, the researchers around Dr Vera Schlindwein of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), have now for the first time risked deploying a network of ocean bottom seismometers (OBS) at the Southwest Indian Ridge in the Furious Fifties and recovered them a year later. At the same time, a second network was placed on a volcano in the more temperate latitudes of the Southwest Indian Ridge. "Our effort and our risk were rewarded with a unique set of earthquake data, which for the first time provides deep insights into the formation of the ocean floor when spreading rates are very slow," explains AWI geophysicist Vera Schlindwein.

今日，藉著即使在波濤洶湧的海域依然能良好運作的研究船「 Polarstern」，以阿爾弗雷德•韋格納暨亥姆霍茲極地與海洋研究所(AWI)的 Vera Schlindwein博士為首的研究人員，首度能冒險去西南印度洋海脊於咆哮四十度及瘋狂五十度的海域架設海底地震儀(OBS)觀測網，並在一年之後回收這些儀器。於此同時，第二個觀測網則設立在西南印度洋海脊較接近中緯度的一座火山之上。「這些獨一無二的地震數據讓我們首度有機會能仔細探究板塊擴張速率十分緩慢的情況下海床會如何形成，花費於此的心力和冒的風險可說是相當值得。」AWI的地球物理學家 Vera Schlindwein如此解釋。

Her results turn current scientific findings on the functioning of ultra-slow mid-ocean ridges upside down: Schlindwein and her PhD student Florian Schmid found that water may circulate up to 15 kilometres deep in the young oceanic lithosphere, i.e. the earth crust and the outer part of the earth mantle. If this water comes into contact with rock from the earth mantle, a greenish rock called serpentinite forms. Even small quantities of ten percent serpentinite are enough for the rock to move without any earthquakes as if on a soapy track. The researchers discovered such aseismic areas, clearly confined by many small earthquakes, in their data.

她的結果徹底顛覆了目前對超慢速中洋脊運作方式的科學認知：Schlindwein和她的博士生Florian Schmid發現這裡的海水可以循環滲透至年輕海洋岩石圈15公里深之處，也就是整個幼年地殼及地函外圍。如果海水跟地函岩石接觸，就會形成一種稱作蛇紋岩(serpentinite)的綠色岩石。即使蛇紋岩含量僅有少少的10%，也足以讓岩石移動時不發生地震，如同在滑溜的道路上前進一般。研究人員從他們的數據中發現這種無震帶清楚地被包夾在許多小型地震之間。

Until now, scientists thought that serpentinite only forms near fault zones and near the surface. "Our data now suggest that water circulates through extensive areas of the young oceanic lithosphere and is bound in the rock. This releases heat and methane, for example, to a degree not previously foreseen," says Vera Schlindwein.

科學家迄今認為蛇紋岩只會形成於斷層帶周遭接近地表處。「從數據看來，我們現在認為水可以循環至年輕海洋岩石圈中許多區域並且跟岩石互相結合。這個過程會釋放出像是熱能及甲烷的物質，其量之大可能遠超出我們的預期。」Vera Schlindwein說。

The AWI geophysicists were now able to directly observe the active spreading processes using the ocean floor seismometers, comparing volcanic and non-volcanic ridge sections. "Based on the distribution of earthquakes, we are for the first time able to watch, so to speak, as new lithosphere forms with very slow spreading rates. We have not had such a data set from ultra-slow ridges before," says Vera Schlindwein.

如今，這位AWI的地球物理學家利用海底地震儀可以直接觀測正在發生的擴張過程，並且可以比較洋脊中有火山活動跟沒有的區段之間的差異。「我們根據地震的分布情況首度能『監看』在極度緩慢的擴張速率下新生岩石圈的形成過程。在這之前我們從未自超慢速洋脊得到這樣的數據。」

"Initially, we were very surprised that areas without earth crust show no earthquakes at all down to 15 kilometres depth, even though OBS were positioned directly above. At greater depths and in the adjacent volcanic areas, on the other hand, where you can find basalt on the sea floor and a thin earth crust is present, there were flurries of quakes in all depth ranges," says Vera Schlindwein about her first glance at the data after retrieving the OBS with RV Polarstern in 2014.

「起初我們發現即使直接於地殼不存在的區域上方架設海底地震儀，直到地下15公里深都沒有偵測到地震發生，這讓我們十分訝異。另一方面，在鄰近海床上有玄武岩及薄層地殼覆蓋的火山帶以及更深的區域，在各個深度地震都毫不停歇地發生。」Vera Schlindwein如此描述她於2014年藉由Polarstern研究船取回海底地震儀後，對其中所含數據的最初觀察結果。

The results also have an influence on other marine research disciplines: geologists think about other deformation mechanisms of the young oceanic lithosphere. Because rock that behaves like soft soap permits a completely different deformation, which could be the basis of the so-called "smooth seafloor" that is only known from ultra-slow ridges. Oceanographers are interested in heat influx and trace gases in the water column in such areas, which were previously thought to be non-volcanic and "cold." Biologists are interested in the increased outflow of methane and sulphide on the sea floor that is to be expected in many areas and that represents an important basis of life for deep-sea organisms.

研究結果對於其他海洋科學研究領域也有相當影響。地質學家認為這是幼年海洋岩石圈的另一種變形機制。行為像軟皂的岩石可以讓一種截然不同的變形機制發生，這可能是形成僅在超慢速洋脊觀察到的「平滑海床」的基礎原理。過往海洋學家認為這個區域沒有火山活動發生且十分「寒冷」，現在他們則對當地水層的熱流量和稀有氣體相當感興趣。生物學家關注此研究預期許多區域海床上的甲烷和硫化物有較高的排放量，因為這些物質對深海生物來說是維持生命相當重要的基石。

引用自：Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research. "Plate tectonics without jerking: Detailed recordings of earthquakes on ultraslow mid-ocean ridges." ScienceDaily. ScienceDaily, 29 June 2016.