Traces of three strong ancient earthquakes are found in the Altai

Scientists have found traces of three ancient earthquakes in the Altai. Their power was no less than the 2003 Chui earthquake.

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Results from this study are published in the International Geology Review scientific journal. The first author of the article, paleoseismologist Yevgeny Deev, Associate Professor at the Novosibirsk State University Geological and Geophysical Faculty and a senior researcher at the Institute of Oil and Gas Geology and Geophysics SB RAS talked about this work.

“Paleoseismologists study traces of ancient and historical earthquakes that have occurred. With strong earthquakes, the surface cracks and breaks and faulty ledges are formed. They indicate to scientists the location of the focal zone and allow them to eventually determine the magnitude and intensity of the earthquake.”

Evgeny Deev and his colleagues conduct paleoseismological studies on the territory of the Altai Mountains. During two field seasons 2015–2016, scientists discovered traces of three strong ancient earthquakes.

“We found a faulted ledge in Southeast Altai, in the articulation zone of the Kuraiski Range and in the Northwestern part of the Chui Basin. Outwardly it looks like an extended elevation, more than 6 km in length with a height from 0.5 to 8 meters. The large height of the ledge tells us that it was formed as a result of several seismic movements”, explained Evgeny Deev.

“While examining the ledge, we were able to identify a seismogenic rupture, during which a shift occurred during the earthquake. We estimated the amplitude of the discontinuity in the rupture and calculated the magnitude, which was in the range of 7.2 to 7.6. This indicates that the intensity of the ancient earthquake was 9–10”, continued Deev.

Scientists also managed to estimate the age of the seismic event. According to radiocarbon analysis, the earthquake occurred 3,100–3,400 years ago.

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On one of the sections of the faulted ledge, the paleo-seismologists discovered a previously unknown archaeological site – a group of 17 ancient burial mounds. It is interesting that some of them had obvious traces of destruction that occurred during an earthquake. Archaeologist  Professor Andrei Borodovsky from the Institute of Archeology and Ethnography SB RAS and Polish researcher Lukash Oleshchak from the Institute of Archeology at Jagiellonian University joined the work. The researchers assessed the age of the burial ground based on the location of the mounds and the appearance of the stone embankments. Necropolis of Turalu-Djurt III is the name they gave the monument. It was probably formed in the second half of the first millennium BC, the first millennium AD. The age interval is very big, because we are discussing the results of preliminary analysis.

“It turned out that one more earthquake happened on this territory and it was quite strong. It is known that the mounds were built before the earthquake, which means the second seismic event occurred less than two millennia ago”, according to Deev.

Paleoseismologists began to study the fault directly under deformed mounds to learn about the history of a younger earthquake. It is interesting that scientists discovered traces of one more, a third earthquake in this territory! It was the oldest. Using radiocarbon analysis, specialists were able to determine the age of these two earthquakes.

“Thus, in two field seasons we found traces of three earthquakes. They occurred about 5,500, 3,400–3,100 and 1,300 years ago. In the last two, the youngest events, we were able to estimate the magnitude and intensity, which was 9–10 and possibly even higher”, summed up the paleoseismologist.

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“ This is the first paleoseismological work in this investigated area, completely done on the study of faulty ledges. Previously, for paleosysmological analysis, landslides and avalanches were used and these are not always associated with earthquakes. We found traces of primary surface ruptures on the Northern outskirts of the Chui basin that told us there were focal zones of strong ancient earthquakes.

A couple of decades ago the Altai Mountains were considered a region with a very moderate level of seismic activity. The situation radically changed on September 27, 2003 when a powerful earthquake with an intensity of 9–10 occurred on the Southern outskirts of the Chui basin. Subsequent studies by Russian scientists have shown that in the past strong earthquakes took place on this territory.

“Now we can talk about a fairly sustained repetition of strong earthquakes on the opposite Northern border of the Chui basin. Modern seismological observations present this territory as weak seismic, just as they used to consider the Southern territory. However, at least once every two or two and a half thousand years there are strong earthquakes with a magnitude of 7 and above”, according to Deev.

The scientific article in the International Geology Review journal ends with a reminder that, “the seismogenic faulty ledge is located less than one km from the planned pipeline route from Russia to the PRC and this fact undoubtedly requires consideration when laying the gas pipeline”.

Author: Dina Golubeva
Photos: Yevgeny Deev