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24 小时内发生 40 多次地震正在美国西北部引起轰动

  发表于 Dec 9, 2021 02:48:19 | 显示全部楼层 |阅读模式
周二,在俄勒冈州海岸发生了 40 多次地震(震级从 3.5 5.8 级不等)后,北美最活跃的断层线之一活跃起来,引起了该地区数百万人的关注和关注。

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这一系列地震从周二早上开始一直持续到周三,都聚集在俄勒冈州沿海城镇纽波特以西 200-250 英里的地方,足以在陆地上大部分未被发现,但考虑到该地区的地震历史,正在引起轰动。

“如果你昨天问我地球上的哪个地方最有可能在一天内产生 5.0 级以上的地震,这在我的名单上会很高,”太平洋西北地震台网主任哈罗德托宾说。华盛顿大学告诉 CNN。

造成地震的断层线是布兰科断裂带。根据俄勒冈州立大学的一项分析,它比加利福尼亚臭名昭著的圣安德烈亚斯断层更活跃,自 1970 年代以来已经产生了 1,500 多次 4.0 级或更大的地震。

并非所有地震都是生而平等的

本周地震最令人印象深刻的是,地震群至少发生了 9 次震级达到 5.0 5.8 级的地震,其中大部分发生在仅 10 公里的浅层深度。

根据美国地质调查局的数据库,自 1980 年以来,该地区 5.0 级或更大地震的绝对数量是年平均水平的三倍(每年发生 3 5.0+ 地震)。

幸运的是,据美国国家海啸预警中心称,本周的地震都没有触发海啸警报。

然而,由于该地区是北美最容易发生地震的地区之一,并且在 1700 1 26 日已经发生了美国大陆最大的地震之一,因此该活动引起了一些人的关注。地震发生在邻近的卡斯卡迪亚俯冲带,一个离陆地更近的巨型逆冲断层,胡安德富卡板块在北美板块下方潜入。这个断层不仅会引发毁灭性的海啸,还会引发破坏性的震动。

这不一定是所谓“大”的前身

尽管本周地震群的震中布兰科断裂带是北美地震最活跃的地区之一,但它很少会导致破坏性地震。

断层位于俄勒冈州海岸线以西约 275 英里和卡斯卡迪亚俯冲带以西约 200 英里处,历史上发生过最大和最具破坏性的西北地震。

“布兰科断裂带地震是走滑型(两侧地壳块的横向运动,而不是上下位移),因此它们不太可能构成海啸威胁,即使发生了更大的地震,例如例如 7.0 级,”托宾告诉 CNN。

根据地震学家露西·琼斯博士的说法,自 1980 年以来,布兰科断裂带上发生了超过 133 5 级或更大的地震,而且从未发生过陆地上的地震。

“今天的地震可以被认为是一次主震和一组余震,唯一的区别是在这种情况下,它们之间没有太大的差异,”托宾说。

尽管本周的地震引发了所谓的“大地震”可能即将到来的担忧,但托宾保证情况并非如此。

“从这些地震到卡斯卡迪亚俯冲带有很长的距离,”托宾解释说。 “我们目前对压力如何通过地壳(和地幔)传递的最佳理解表明,这些事件不会明显改变俯冲带的压力。”

他指出,虽然我们在短时间内发生了数量惊人的 5.5 级以上地震,但这并不是本质上令人震惊,而是在地震学上很有趣。

今年早些时候,南加州的索尔顿海附近发生了类似的地震群,虽然没有本周的震级。

A swarm of more than 40 earthquakes in 24 hours is causing a buzz in the northwest US

One of North America's most active fault lines sprung to life on Tuesday after a swarm of more than 40 earthquakes -- ranging from a magnitude 3.5 to 5.8 -- rattled off the coast of Oregon, catching the attention and concern of millions in the region.

The series of quakes, which began early Tuesday morning and continued into Wednesday, were all clustered between 200-250 miles west of the coastal town of Newport, OR, far enough to be mostly undetected on land, but given the area's seismic history, it is creating quite a buzz.

"If you had asked me yesterday where on Earth would be most likely to produce a bunch of magnitude 5.0+ quakes in a single day, this would have been high on my list," Harold Tobin, Director of the Pacific Northwest Seismic Network at the University of Washington, told CNN.

The fault line responsible for the quakes is the Blanco Fracture Zone. According to an analysis by Oregon State University, it is more active than the infamous San Andreas Fault in California, having produced more than 1,500 quakes of magnitude 4.0 or greater since the 1970s.

Not all quakes are created equal

What has been most impressive about this week's quakes is the swarm has included at least 9 tremors reaching a magnitude 5.0 to 5.8, with the majority occurring at a shallow depth of only 10 km.

The sheer number of magnitude 5.0 or greater quakes in the region triples the annual average (three 5.0+ quakes per year) since 1980, according to the USGS database.

Fortunately, according to the US National Tsunami Warning Center, none of this week's quakes have triggered a tsunami alert.

However, the activity has heightened the concern level for some, as the region is among the most earthquake-prone areas in North America and has already produced one of the largest quakes in the continental United States on January 26, 1700. The quake occurred on the neighboring Cascadia Subduction Zone, a megathrust fault much closer to land, where the Juan de Fuca plate dives underneath the North American Plate. This fault can not only trigger devastating tsunamis but also destructive shaking.

This is not necessarily a precursor for the so-called 'big one'

Though the epicenter of this week's swarm of quakes, the Blanco Fracture Zone, is among the most seismically active in North America, it rarely leads to destructive quakes.

The fault is some 275 miles west of the Oregon coastline and about 200 miles west of the Cascadia Subduction Zone, where the largest and most destructive Northwest quakes have historically taken place.

"Blanco Fracture zone quakes are strike-slip (lateral motions of the crustal blocks on either side, rather than up-down displacement), so it is very unlikely for them to pose a tsunami threat, even if a bigger quake happened, like a magnitude 7.0 for example," Tobin told CNN.

According to seismologist Dr. Lucy Jones, there have been more than 133 quakes of magnitude 5 or greater on the Blanco Fracture Zone since 1980 and have never been followed by something on land.

"Today's quakes can be thought of as something like a main-shock and a swarm of aftershocks, the one distinction being that in this case, there's not a lot of magnitude difference among them," Tobin said.

Though this week's quakes have raised the concerns the so-called "big one" could be near, Tobin assured it is not necessarily the case.

"There's quite a lot of distance from these quakes to the Cascadia Subduction Zone," Tobin explained. "Our best current understanding of how stress transfers through the crust (and mantle) would suggest that these events don't change stress on the subduction zone appreciably."

He pointed out, although we have had a remarkable number of magnitude 5.5+ quakes in a short period of time, it is not inherently alarming but rather seismologically interesting.

A similar swarm of quakes, though not to this week's magnitude, occurred earlier this year near the Salton Sea in Southern California.

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