Reports of “earthquake lights,” like the ones seen in videos captured before last Friday’s 6.8-magnitude earthquake in Morocco, go back centuries to ancient Greece.

These outbursts of bright, dancing light in different colors have long puzzled scientists, and there’s still no consensus on what causes them, but they are “definitely real,” said John Derr, a retired geophysicist who used to work at the U.S. Geological Survey. He has coauthored several scientific papers on earthquake lights, or EQL.

“Seeing EQL depends on darkness and other favorability factors,” he explained in an email.

He said the recent video from Morocco shared online looked like the earthquake lights caught on security cameras during a 2007 quake in Pisco, Peru.

Juan Antonio Lira Cacho, a physics professor at Universidad Nacional Mayor de San Marcos in Peru and the Pontifical Catholic University of Peru, who has studied the phenomenon, said cell phone video and the widespread use of security cameras has made studying earthquake lights easier.

Earthquake lights can take several different forms.Sometimes, the lights may appear similar to ordinary lightning, or they may be like a luminous band in the atmosphere akin to polar aurora. Other times they resemble glowing spheres floating midair. They may also look like small flames flickering or creeping along or near the ground or larger flames emerging from the ground.

To better understand earthquake lights, Derr and his colleagues gathered information on 65 American and European earthquakes associated with trustworthy reports of earthquake lights dating back to 1600. They shared their work in a 2014 paper published in the journal Seismological Research Letters.

The researchers found that some 80% of the EQL occurrences studied were observed for earthquakes with magnitudes greater than 5.0. In most cases, the phenomenon was observed shortly before or during the seismic event, and it was visible up 600 kilometers from the quake epicenter.

The 2014 study also found that the vast majority of earthquakes linked to luminous phenomena occurred within tectonic plates, rather than at their boundaries.

One theory explains that when certain defects or impurities in crystals in rocks are put under mechanical stress — such as during the build-up of tectonic stresses before or during a major earthquake — they instantly break apart and generate electricity. Rock is an insulator that, when mechanically stressed, becomes a semiconductor.

Other theories about what causes earthquake lights include static electricity produced by the fracturing of rock and radon emanation, among many others.

(SD-Agencies)

Words to Learn 相关词汇

【地震的】dìzhèn de seismic relating to or caused by an earthquake

【绝缘体】juéyuántǐ insulator a material that is a poor conductor (as of electricity or heat)

有人在上周五摩洛哥6.8级地震前拍摄到的视频中看到了“地震光”，事实上，关于这一现象的报道可以追溯到几个世纪前的古希腊。曾在美国地质调查局工作的退休地球物理学家约翰•德尔说地震光“绝对真实存在”。他曾与他人合作撰写了多篇关于地震光的科学论文。

他在一封电子邮件中解释说：“能否看到地震光取决于周围环境的黑暗程度和其他因素。他说最近网上流传的摩洛哥视频看起来很像2007年秘鲁皮斯科地震时监控摄像头捕捉到的地震光。

秘鲁国立圣马科斯大学和秘鲁天主教宗座大学的物理学教授胡安•安东尼奥•里拉•卡乔曾研究过这一现象。他说手机视频和遍布的监控摄像头使得研究地震光更容易。

地震光有几种不同的形式。它有时看起来像普通闪电或者大气中的发光带，类似极光。其他时候，它们就像漂浮在半空中的发光球体，也可能像沿着地面或靠近地面闪烁蠕动的小火焰，或者从地下冒出的大火焰。

为了更好地了解这个现象，德尔和同事收集了65次此前发生在美国和欧洲的地震信息，都有关于地震光的可信报道，最早可追溯到 1600 年。2014 年他们在《地震学研究快报》杂志上发表了一篇论文来分享研究成果。

研究发现，在有记录的地震光事件中，约 80% 是在震级大于 5.0 的地震中观察到的。在大多数情况下，地震光是在地震发生前不久或地震期间观测到的，在距离震中600公里的范围内都能看见。

2014 年的研究还发现，绝大多数有地震光的地震都发生在板块构造内部，而非板块边缘。

有一种理论解释说，当岩石中晶体的某些缺陷或杂质受到机械应力时--例如在大地震前或大地震时积累的构造应力--它们会瞬间破裂并产生电能。岩石是绝缘体，但受到机械应力时会变成半导体。

关于地震发光原因，其他理论包括岩石断裂产生的静电和氡散发等。

(Translated by Debra)