Эрдэмтэд газрын царцдасын доорх мантийн давхаргад тохиолддог ховор төрлийн газар хөдлөлтийн шалтгааныг олж тогтоов.
1979 онд АНУ-ын Юта мужийн хойд хэсэгт бүртгэгдсэн 3.8 магнитудын хүчтэй газар хөдлөлт нь эрдэмтдийн анхаарлыг олон арван жилийн турш татаж ирсэн юм. Уг чичирхийлэл нь дэлхийн царцдасаас 55 милиэс илүү гүн буюу газрын гадаргаас маш алсад, мантийн давхаргад тохиолдсон нь тогтоогджээ. Тухайн үед судлаачид ийм гүнд газар хөдлөлт болох боломжгүй гэж үзэж байсан ч сүүлийн үеийн судалгаагаар энэ нь хэмжилтийн алдаа биш, харин бодит үзэгдэл болохыг баталлаа.
Ютагийн их сургуулийн судлаач Жорж Зандт болон профессор Кит Копер нар анхны тохиолдлоос гадна тус бүс нутагт бүртгэгдсэн дахин найман гүний чичирхийллийг нарийвчлан судалжээ. Эдгээр есөн газар хөдлөлт бүгд дэлхийн царцдас-мантийн зааг давхаргын доор болсон нь тогтоогдсон бөгөөд тэдгээр нь гадаргын газар хөдлөлтөөс ялгаатай нь урьдчилсан болон дараачилсан чичирхийлэл үүсгэдэггүй байна.
Судалгааны багийнхны таамаглаж буйгаар, эдгээр чичирхийлэл нь Вайомингийн кратоны баруун ирмэг дэх геологийн тогтоцтой холбоотой ажээ. Эртний литосферийн энэхүү хатуу хэсэг нь мантийн урсгалыг чиглүүлэхэд саад болж, улмаар их хэмжээний даралт, хэв гажилтыг үүсгэдэг байна. Мөсөн уул урсгалын дундуур явахад эргэн тойрныхоо орчинд нөлөөлдөг шиг, энэхүү кратон нь мантийн бодисын хөдөлгөөнийг хазайлгаж, гүний газар хөдлөлтийг өдөөдөг гэж эрдэмтэд тайлбарлаж байна.
Энэхүү нээлт нь мантийн давхаргад өрнөдөг үл мэдэгдэх үйл явцыг ойлгоход чухал ач холбогдолтой юм. Гэсэн хэдий ч ийм төрлийн газар хөдлөлт хамгийн ихдээ хэр хүчтэй болохыг одоогоор таамаглах боломжгүй байгаа нь шинжлэх ухааны хувьд томоохон сорилт хэвээр байна.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
A modest earthquake recorded beneath northern Utah in 1979 has turned out to be far more unusual than anyone first realized. New research suggests the tremor originated deep in the Earth’s upper mantle, in a place where scientists did not expect seismic events to occur.
The original event measured magnitude 3.8, hardly the kind of quake that grabs headlines. Yet its estimated depth made it stand out. Seismic data indicated that the earthquake occurred more than 55 miles below sea level, well beneath the Earth’s crust.
For years, researchers debated whether that depth could be correct. Now, a fresh analysis of the 1979 quake and eight similar events has strengthened the case that these earthquakes were real mantle quakes rather than measurement errors.
A Seismic Mystery That Began In 1979
When the earthquake was recorded in northern Utah, scientists quickly noticed something odd. The signals pointed to a source much deeper than typical earthquakes in the region.
The depth estimate placed the quake inside the upper mantle, a layer below the crust where continental earthquakes are rarely observed. That conclusion was difficult for many researchers to accept at the time. According to the recent study published in The Seismic Record, George Zandt, then a seismology researcher at the University of Utah, remained convinced the data were accurate despite the skepticism.
“I did some other analysis that convinced me of the reality of the deep depth but it was hard to convince others of the highly anomalous mantle earthquake occurring in a region where none should exist,” Zandt said.
The debate lingered for decades. The earthquake itself was relatively small, but its location raised questions that researchers could not easily answer.
Eight More Earthquakes Point To The Same Conclusion
The mystery gained new momentum when University of Utah geology professor Keith Koper and Zandt revisited the case. Their investigation looked beyond the original earthquake and examined eight additional deep seismic events recorded in the same area.
As reported in The Seismic Record, all nine earthquakes appear to have occurred within the Earth’s upper mantle, dozens of miles below the crust-mantle boundary. The researchers describe them as an “archetypal continental mantle event,” linking them to movements taking place within the mantle over extremely long periods of time.
The discovery highlights how different these earthquakes are from the ones people usually associate with fault lines near the surface. One notable difference is that the deep events do not seem to produce foreshocks or aftershocks. Scientists are still trying to understand exactly how they happen.
“It’s sort of a mystery in terms of fundamental physics,” Koper said. “How in the world can these things happen?”
There is another unanswered question as well. Researchers do not know how large these deep earthquakes could become.
“Another reason why it’s a big deal is that we have no idea how big they can be,” he added. “With crustal earthquakes, we can measure what we think their maximum size is going to be.”
An Ancient Block Of Earth May Be Involved
The study found that the powerful quakes cluster near the western edge of the Wyoming Craton, an ancient section of lithosphere stretching across northern Utah and southwestern Wyoming. Researchers believe this geological feature may play a role in generating the unusual seismic activity. Temperatures in the surrounding mantle can exceed 1,300 degrees Fahrenheit, creating conditions very different from those found closer to the surface.
As noted by the researchers, mantle material slowly flows around the craton over millions of years. As that movement is redirected around the rigid structure, stresses can build up.
“On the scale of millions of years, the mantle is hitting the craton and then flowing around it,” Koper explained. “It’s that interaction where that mantle flow is being diverted around this hard cratonic root that’s causing the increased strain rate, the increased deformation and it’s also creating extra stresses.”
To illustrate the idea, Koper compared the process to an iceberg moving through the surrounding medium. The craton acts as an obstacle, while mantle material slowly moves around it.
“We think it’s that interaction between the keel of the iceberg and the medium around it that’s leading to these earthquakes,” he said.
The new research does not solve every aspect of the mystery. What it does show is that northern Utah hosts a rare form of deep seismic activity.
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