As per multiple sources, a seismic event happened on 2019-12-20 11:39:52 (UTC) which jolt Afghanistan, Pakistan and even parts of India. The epicenter of the event was 51 km of Jarm, Afghanistan(36.534°N 70.437°E210.2) with an intermediate focal depth of 210.2 km and a magnitude of 6.4..
I was wondering that at a depth of 200 km in the upper mantle, the temperature ranges between 900 to 1600 F. At this temperature it is assumed to be ductile natured lithosphere. But for generation events like this, we would need solid and brittle. the lithosphere. How the lithosphere maintains its solidity at such depth and temperatures and a brittle mood fault like feature can accumulate such huge stresses which can produce such devastating earth quacks??
Hi Mukhtiar,
As you imply in your question, the consensus among seismologists is that temperature (including temperature contrasts across plate interfaces) is a principal determinant controlling the seismicity-depth limit, so variations in the maximum depth of earthquakes in different geodynamic settings therefore carry information on their thermal structure. A figure of around 600 Deg. C is taken as an upper isentropic temperature limit to generate seismic energy in mantle materials (see review by Emmerson & McKenzie, 2007).
In the case of the Tian Shan range, beneath which your specified quake is located, intermediate depth earthquakes collectively define geometries (clustering), focal mechanisms and strain rates that are not easy to reconcile with the simple model geometries of active lithospheric subduction (see, for example Billington et. al., (1977) or Negredo et. al., (2007)) This has prompted some interesting alternative hypotheses for this type of intermediate depth seismicity, eg Lorinczi & Houseman (2009) for the Vrancea region of the Carpathians, and most recently, by Peter Molnar (2019) for the Tien Shan itself.
In both the latter cases, however, the underlying assumption is that these earthquakes are triggered within the conventional thermal envelope for seismic behaviour, and seek rather to rationalize the presence of "cold" (ie
Thanks G. N. Howes , indeed it was very helpful to understand the phenomena. I found another related paper too.
( Seno, Tetsuzo, and Hafiz Ur Rehman. "When and why the continental crust is subducted: examples of Hindu Kush and Burma." Gondwana Research 19, no. 1 (2011): 327-333. )
Dear colleagues, thank you very much for your interesting question and answer.
I trying to give another explanation to the Mukhtiar´s question, may I share this work of Hagen and Azevedo entitled “Deep and Ultra-Deep Earthquakes Worldwide, Possible Anomalies in South America” in which the authors explain the possible origin of the ultra-deep earthquakes that occur in South America.
I suggest you pay attention to the fact that, although there are more than a few places in the world where there are phenomena of ultra-deep earthquakes, statistically only in three of them the authors can it be alleged that they are such as a systematic phenomenon: the border of the Pacific Ocean that include the North Pacific, South Pacific, and South America.
Also, please, pay attention that in the South America study-case, the ultra-deep earthquakes happened in the interior of the continent. Hagen and Azevedo explain that the reason for those existences was an anomaly created in the asthenosphere as part of the process of the South America collision with the Nazca plate. They proposed that South America has a lithospheric-slab buried in the region with an approximate depth between 500 - 670 km.
Although beyond the scope of this answer, it would be interesting to observe that the southernmost limit in South America of ultra-deep earthquakes is interpreted as the boundary of the “flat-slab subduction”, a phenomenon that ultimately favors Andean growth and uplift, e.g., Espurt, N., F. Funiciello, J. Martinod, B. Guillaume,V. Regard, C. Faccenna, and S. Brusset (2008), Flat subduction dynamics and deformation of the South American plate: Insights from analog modeling, Tectonics, 27, TC3011, doi:10.1029/2007TC002175.
Best regards,
Mario E. Sigismondi
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