Why was the Turkey-Syria earthquake so bad? 土耳其—敘利亞震災為何如此嚴重？

The magnitude 7.8 earthquake that struck Turkey and Syria on Monday last week is likely to be one of the deadliest this decade, seismologists said, with a more than 100km rupture between the Anatolian and Arabian plates.

Here is what scientists said happened beneath the Earth’s surface and what to expect in the aftermath:

The epicenter was about 26km east of the Turkish city of Nurdagi at a depth of about 18km on the East Anatolian Fault. The quake radiated towards the northeast, bringing devastation to central Turkey and Syria.

During the 20th Century, the East Anatolian Fault yielded little major seismic activity. “If we were going simply by (major) earthquakes that were recorded by seismometers, it would look more or less blank,” said Roger Musson, an honorary research associate at the British Geological Survey.

Only three earthquakes have registered above 6.0 on the Richter Scale since 1970 in the area, according to the US Geological Survey. But in 1822, a 7.0 quake hit the region, killing an estimated 20,000 people.

HOW BAD WAS THIS EARTHQUAKE?

On average, there are fewer than 20 quakes over 7.0 magnitude in any year, making last Monday’s event severe.

Compared with the 6.2 earthquake that hit central Italy in 2016 and killed some 300 people, the Turkey-Syria earthquake released 250 times as much energy, according to Joanna Faure Walker, head of the University College London Institute for Risk and Disaster Reduction.

Only two of the deadliest earthquakes from 2013 to 2022 were of the same magnitude as last Monday’s quake.

WHY WAS IT SO SEVERE?

The East Anatolian Fault is a strike-slip fault.

In those, solid rock plates are pushing up against each other across a vertical fault line, building stress until one finally slips in a horizontal motion, releasing a tremendous amount of strain that can trigger an earthquake.

The San Andreas Fault in California is perhaps the world’s most famous strike-slip fault, with scientists warning that a catastrophic quake is long overdue.

The initial rupture for the Turkey-Syria earthquake kicked off at a relatively shallow depth. “The shaking at the ground surface will have been more severe than for a deeper earthquake of the same magnitude at source,” David Rothery, a planetary geoscientist at the Open University in Britain, said.

WHAT KIND OF AFTERSHOCKS CAN BE EXPECTED?

Eleven minutes after the initial quake, the region was hit by a 6.7-magnitude aftershock. A 7.5-magnitude quake came hours later, followed by another 6.0 spasm in the afternoon.

“What we are seeing now is the activity is spreading to neighboring faults,” said Musson. “We expect seismicity to continue for a while.”

After the deadly 1822 event, aftershocks carried on into the following year.

WHAT MIGHT THE FINAL DEATH TOLL BE?

Earthquakes of similar magnitudes in populated areas have killed thousands of people. Nepal’s 7.8-magnitude earthquake in 2015 claimed nearly 9,000 lives.

“It’s not going to be good,” said Musson. “It will be in the thousands, and could be in the tens of thousands.”

Cold winter weather, he added, means that people trapped under rubble have less chance at survival.

(Reuters)

地震學家表示，上週一襲擊土耳其和敘利亞的7.8級地震，可能是十年來傷亡最慘重的地震之一，安納托利亞和阿拉伯板塊之間斷裂了超過100公里。

以下是科學家所描述地表下所發生的事，以及預期後果：

震源位於何處？

震央位於土耳其城市努爾達伊以東約26公里處，深度約18公里，處於東安納托利亞斷層上。地震朝東北方向發散出去，蹂躪土耳其中部及敘利亞。

東安納托利亞斷層在二十世紀幾乎沒有重大地震活動。「如果我們只根據地震儀記錄的（大）地震來判斷，它看來多少是空白的」，英國地質調查局名譽研究員羅傑．穆森說。

根據美國地質調查局的資料，自1970年以來，該地區僅發生過三次芮氏6.0級以上的地震。但在1822年，一場7.0級地震襲擊了該地區，估計有兩萬人喪生。

這次地震有多嚴重？

平均來說，每年超過7.0級的地震只有不到20次，這讓上週一的地震變得非常嚴重。

倫敦大學學院風險與減災研究所所長喬安娜•佛爾•沃克表示，與2016年發生在義大利中部、造成約300人死亡的6.2級地震相比，土耳其—敘利亞地震所釋放的能量，是義大利地震的250倍。

2013年至2022年傷亡最嚴重的地震中，只有兩次與上週一的地震震度相同。

為何如此嚴重？

東安納托利亞斷層屬於橫移斷層【又譯平移斷層、走滑斷層】。

在橫移斷層中，堅硬的岩石板塊在垂直斷層線上相互推擠，不斷增加壓力，最後造成一塊岩石板塊水平滑動，釋放出可引發地震的巨大壓力。

加州的聖安德列斯斷層或許是最著名的橫移斷層，科學家警告說，一場災難性的地震早該發生了。

土耳其—敘利亞地震的斷裂是從深度較淺的地方開始。英國空中大學的行星地質學家大衛．羅瑟里表示：「相同的震級，若發生在地表，其震度將比震源更深的地震更加嚴重」。

預計會發生什麼樣的餘震？

地震第一次發生的11分鐘後，該地區發生了6.7級餘震。數小時後是7.5級地震，下午又發生了6.0級地震。

「現在我們可以見到，地震活動正蔓延到鄰近的斷層」，穆森說。「我們預計地震活動將會持續一段時間」。

在1822年造成嚴重傷亡的地震，其餘震一直持續到次年。

死亡人數會是多少？

在人口稠密地區發生的類似震級的地震，已造成數千人死亡。2015年尼泊爾發生的7.8級地震，奪走了近九千人的生命。

「情況並不樂觀」，穆森表示，「死亡人數將會有數千，而且可能會以數萬計」。

他補充說，在寒冷的冬天，表示被困在瓦礫下的人生存機會更小。

(台北時報林俐凱編譯)