An earthquake occurred in southern Mongolia on December 4, 1957, measuring 7.8–8.1 and assigned XII (''Extreme'') on the

Modified Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

Surface faulting In seismology, surface rupture (or ground rupture, or ground displacement) is the visible offset of the ground surface when an earthquake rupture along a fault affects the Earth's surface. Surface rupture is opposed by buried rupture, where the ...

was observed in the aftermath with peak vertical and horizontal scarp reaching . Because of the extremely sparse population in the area, this event, despite its magnitude, was not catastrophic. However, 30 people died and the towns of Dzun Bogd, Bayan-leg and Baruin Bogd were completely destroyed.

Tectonic setting

As the

Indian Plate The Indian Plate (or India Plate) is a minor tectonic plate straddling the equator in the Eastern Hemisphere. Originally a part of the ancient continent of Gondwana, the Indian Plate broke away from the other fragments of Gondwana , began ...

collides with the

Eurasian Plate The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent and ...

, the force of the collision causes deformation. A primary effect of this collision is the uplift of the

Himalayas The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 10 ...

; however, deformation extends further. In Mongolia, escape tectonics created a network of active faults to support the strike-slip stresses. During this earthquake, two main faults sustained a rupture: the

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that sys ...

Gurvan Bulag fault, and the related strike-slip Bogd fault. The Gurvan Bulag has a slip rate of /yr for the vertical component, with slip rate increasing at the end of the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in ...

epoch. Paleoseismological investigation revealed that the average recurrence interval of earthquakes like the 1957 event on the fault had decreased from 50 kyr to 3-14 kyr in the late Pleistocene. The Bogd fault is a large left-lateral strike-slip fault. It is split into five distinct segments. Slip rates vary between segments, but it is between /yr overall. Recurrence intervals of 1957 type events on the Bogd fault have been calculated at around 1,000 years. Other large () earthquakes had struck Mongolia in the previous half century, including the 1905 Tsetserleg, 1905 Bolnai, and 1931 Fuyun earthquakes. Some studies indicate that these earthquakes triggered each other, with the earlier ones triggering the later events.

Earthquake

The earthquake struck southern Mongolia at on December 4, 1957. Rupture was complex, with multiple scenarios proposed. The original hypothesis was that the earthquake occurred along the strike-slip Bogd fault and ruptured for , however, the more recently adopted conclusion is that there was a long strike-slip rupture at a width of with of simultaneous reverse faulting in a roughly east-west direction. Offsets from surface rupture reached up to of strike-slip rupture, with vertical offsets, and an average slip of . According to a study on the earthquake's surface rupture, the authors consider the rupture the "world's best preserved surface rupture of a great earthquake". The average slip decreased from west to east. Ruptured fault splays were observed up to away from the main fault trace. Large aftershocks struck the epicentral region after the mainshock. Shortly after the mainshock, a 6.5 event struck. On April 7, 1958, another large earthquake measuring M 6.8 struck, located in the epicentral region of the mainshock.

Impact

Despite attaining the maximum value on the

of XII (''Extreme''), the earthquake did not cause significant disruption to population centers owing to its remote and unpopulated location. Dzun Bogd, Bayan-leg and Baruin Bogd, however, were destroyed. Due to good weather, shepherds of these herding communities were outside, which limited the death toll to 30. Major geological effects were also observed. Surface offsets reaching of both strike-slip and vertical motion were observed after the event. Subsidence was also observed. At the , a long and wide portion of the earth subsided due to the earthquake. In the Bitüüt valley, a large landslide was triggered.

Notes

References

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