October 2, 2013
The creative forces of earthquakes
Posted by Austin Elliott
For all the destruction wrought when human settlements and infrastructure are shaken by tectonic forces, earthquakes are the result of processes that create and rejuvenate the landscapes we live in.
In late September, 2013, a mighty earthquake ripped through the Pakistani desert, causing a surprisingly small number of casualties, but nonetheless rendering homeless over 100,000 people, a major swath of nearby mountain villages’ populations. In the vicinity of the epicenter you can see the topographic results of the same forces that caused the earthquake. The fault that ripped apart in this case accommodates the north- and west-ward crumpling of central Pakistan as the Indian plate collides violently into Asia. You can see the regional mountain ranges “draped” around the rigid Indian sub-continent like an errant theater prop holding up a curtain.
The fault slipped mainly horizontally (i.e., strike-slip like the San Andreas), but along a non-vertical plane. Seismological estimates show up to 13 meters of slip–a huge amount for a single earthquake. With a hypocentral depth of only 15 km this magnitude 7.7 earthquake is probably a major surface rupturing event–a rare occurrence on the continents, and one that can produce a huge wealth of information about how fault ruptures progress… if scientists can access the area to study it.
The effect of this earthquake that has garnered the most attention so far, however, is the emergence of a fairly large island just offshore in the Indian Ocean. Beautiful pictures of the feature come from NASA’s satellites, and thankfully circulated the internet a bit before Congress deprived us of the original source. [The original link is currently out of commission, but should be available once the US Legislative Branch resides deep in outer space]. The following images were nabbed from an excellent post on the phenomenon by EarthSky’s founder. Clicking on the images links to the original NASA source.
Pakistanis and international journalists have swarmed the new island, earning us some super cool scenes of this myth-grade phenomenon.
The island is an uplifted welt in the seafloor, an emergence of corals, mud, and lofted sea creatures that were stranded and died. Have a close look at the photo opening this post. There’s been a lot of rumination, speculation, and investigation about the source of this island (see nice articles at LiveScience and Paleoseismicity). Initial reports called the feature a “mud volcano,” but the images now available suggest a slightly different origin, at least of the surface material: a wider pocket of buoyant mud and gases that rose and carried the seafloor with it. Rather than having the characteristic vent and smooth depositional slopes of a mud volcano,
this island is an elliptical dome of seafloor sediments criss-crossed by extensional fractures, a clear signature of upward bulging of the seafloor itself. Such features–domes of rising sediments and rocks–are called “diapirs,” and are prevalent in regions with salt, gaseous mud, and other less-dense deposits underlying deformable, denser rocks. When these sediments get shaken or otherwise squeezed they gain the extra forces they need to push up buoyantly through the rocks trapping them from above, and emerge as we’ve seen here. Salt diapirs are found throughout the Zagros Mountains just one country to the west, although these huge features are formed gradually over time as opposed to this remarkably quickly formed bulge.
The new island is a remarkable signature of an active planet, and its cracked, muddy volume will probably be eaten away by ocean waves within a few years. In the meantime, what a sight! You can see where many an ancient legend may have come from, right before our modern eyes!
Other articles about this phenomenon:
EarthSky – http://earthsky.org/earth/view-from-space-earthquake-island-before-and-after
NASA Earth Observatory – http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=82146
LiveScience – http://www.livescience.com/39958-what-caused-pakistan-earthquake-island.html
Photo Gallery from LiveScience – http://www.livescience.com/40085-pakistan-earthquake-island-gallery.html
Paleoseimicity.org – http://www.paleoseismicity.org/blog/2013/09/26/a-mud-volcano-as-an-earthquake-environmental-effect/
Mud Volcanoes are very common in southern Pakistan, The island formed is the result of methane gas spewing out the seafloor,the phenomenon related to basin forming processes dependent upon tectonics and eustasy.
The island is rather a result of collision of the Eurasian and the Arabian Plate. The Arabian Plate is moving north along a zone of subduction below the Eurasian Plate