This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Tuesday, September 24, 2013

Pakistan earthquake and its new island

A view of the new island off the coast of Pakistan
from NBC 
A magnitude 7.7-7.8 earthquake hit Pakistan today with an epicenter at about 15 km depth. It was well inside the borders of Pakistan and far from the ocean. Although only a few dozen (46) casualties have been reported as I write this (10:00 p.m. Central Time), the location of the quake is remote and it is likely that casualties will rise, with some estimates being in the thousands  (WAPMERR, The World Agency of Planetary Monitoring and Earthquake Risk Reduction). As this unfolds, an intriguing observation with speculations has hit the news--a mysterious island has risen from the ocean. It is reported to be 20-40 feet high and about 100 feet wide and lies about 350 feet out in the sea from the Gwadar coast (these numbers are very tentative--one report says that it's a mile out to sea, which seems more consistent with the picture that I've included above). It appears to be several hundred miles from the epicenter.
     Residents of Gwadar have reported that an earthquake in 1968 produced an island that remained for about a year before vanishing (but it's reported also to have occurred in the 1940's so I don't know if this is an inconsistency in historical reporting or if it has happened more than once). What could have caused this mysterious phenomenon? Speculation at the moment is our old culprit, liquefaction--the sudden transformation of wet, but solid, sediments to a weak mush upon shaking, just like jiggling quicksand turns it to a mush (for more on liquefaction and the general phenomena discussed here, see my book "The Dynamics of Disaster" to be released by Norton Press on October 21!!). When this happens, structures such as buildings can sink into the ground because it can no longer support their weight.  The liquified material can also squirt up through cracks in the ground forming sand- or mud-volcanoes, and the speculation here is that the island is a big mud volcano.
     This area of the Arabian Sea is known for its mud volcanoes.* Here the oceanic crust of the Arabian Sea is being subducted under Eurasia at a rate of about 4 cm/year.  A sediment pile has been building up on top of this subduction region to a thickness exceeding 6 km, a pile of wet gooey muck. The coastal region is known for mud volcanoes on land, and for the episodic formation of islands of them in the shallow waters off the coast. These are typically destroyed within months. Here's a seismic reflection image of a buried mud volcano from the reference * listed below:

Sometimes these mud islands appear in places that others have appeared before. One, called Malan island, appeared in 1999, unaccompanied by any noticeable earthquake, reoccupying a site of one that had been formed in 1945. It appears to have been driven by methane of bacterial origin.

One of the largest reported mud volcanoes is a complex 100-m high. It has been hanging around since at least 1840 and appears not to have changed much in the intervening decades. Methane of bacterial origin is persistently discharged into its crater mud lake. Eruptions of these gases often show periodicities of several hours that may be related to ocean tides. I'm not sure how the dating is done, but the article * below says that the presence of mud volcano activity in this region can be demonstrated for the last 460,000 years.

*G. Delisle, "The mud volcanoes of Pakistan," Environmental Geology, 46: 1024-1029, 2004.

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