'Quake risk higher than estimated'

'Quake risk higher than estimated'

There are 2 comments on the Times of Oman story from May 20, 2013, titled 'Quake risk higher than estimated'. In it, Times of Oman reports that:

Muscat: Recent research has revealed that the Makran Subduction Zone is more prone to earthquakes and tsunami hazards than previously thought.

Join the discussion below, or Read more at Times of Oman.

“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#1 May 22, 2013
Interesting article.

The 1945 quake was mentioned in passing; this quake had an Mw 8.3 and caused landsliding of oversteepened forearc folds above the thrust and splay faults leading to the surface. The landslide produced a 15m tsunami near the epicentre zone. in Balochistan.

The mentioned Mw 9.2 is an average for a Rebound/long duration mercalli X (r/lx) quake which relaxes the overlying crust and lurches the descending plate. This Mw is an estimate based on the length of the Makran subduction zone (Hormuz-karachi) multiplied by the runout of the 2004 rupture (~30m) characterised in the case of 2004 by a very small descending plate lurch due to the Indo-Australasian plate being backed up against the Himalayas and in the latest stage of recompression.

In rality as the Makran is a very compressed backarc basin, and the Arabian plate is not as heavily backed up against the Zagros, the descending plate lurch will be larger and the runout of the Makran backarc basin is likely to be larger (up to 60m and possibly greater) as a result of compression of the backarc basin sides from Zagros and Quetta arc convergence which squeezes the Makran south in a wedge.

The good news for the inhabitants of Oman (recently bounced about by 2 Makran quake sequences), is that the initial quake series was the result of the sideways transfer of magma from the Quetta arc backed up against the Pashtun cordillera backrange,(fought over by Afghanistan, Talibanistan, and Pakistan with little mercy). This as a result of E. compression of the Quetta arc by an r/lx of the N.part of the Hindu Kush in 1505, with an ability of the Quetta arc to compress to far more than the Makran backarc basin, with the deep magma filled dike like fracture exiting the Quetta arc stage left (west) sometime between 1940 and 2000 AD. The continued compression of the Quetta arc since the exit of the magma produced a small volcanic eruption at Tor Zawar which erupted the residual magma in the fracture and the squeeze out of the partially molten walls of the now empty fracture with a 2 phase mix erupting classed as trachyandesite.
The magma at depth (~90-100Km) entered the Makran backarc basin and has been jacking apart the deep crust in the arc graben while travelling sideways to the least compressed part of the arc graben in the Makran backarc basin (the volcanoes N. of the 1945 quake segment). The result of the dilation of the deep crust is that the top of the magma filled fracture falls in, generating a series of rift quakes with the mainshock deepest and aftershocks climbing to the surface. The wider the fracture, the larger the rift quake, the last being the largest so far.

The fact that the magma is still moving with the forearc compressible means that r/lx is not due in the near future. What will instead happen is that the magma will eventually contact the other side of the makran where the backarc basin is being overcompressed by the Oman shelf and the Zagros range. The magma, left with nowhere else to go will respond to future compression via convergence in the simple way, and go up. The position of epicentres indicates that Bazman is the most likely volcano to go "pop" in a spectacular way as a result.

R/lx and a bloody great big tsunami is likely in the future when the volcanic activity has died down due to the magma filled fracture being squeezed flat by continued convergence, The volcanism is due in decades, the r/lx is due in a century or two.

There is also a possibility before eruption of one of the 3 Makran volcanoes, that a major forearc pop out will be caused by magma pooling @ depth in a dike under the volcanoes. In that case any of the 3 volcanoes may erupt after the event as the forearc pop out & tsunami will be accompanied by a large rift quake @ the arc graben; Bazman is at the front, an unnamed cinder cone field is in the centre, and Taftan sits over the graben backfault.

Have a nice day: Ag

Since: May 13

Location hidden

#2 May 22, 2013
Interesting indeed thx

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