Saturday, June 20, 2015
Nepal Quake 2015
Why Nepal?
5-16-15
On April 25 this year Nepal suffered a magnitude 7.8
earthquake that killed over 8000 people leaving the country devastated. Many
villages and towns in the rugged Himalayan terrain were isolated by landslides
blocking transportation routes. Food and
water continue to be in short supply, even in Katmandu, the capital located
about 50 miles southeast of the quake’s epicenter.
By April 27, 44 aftershocks ranging in magnitudes
from 4.0 to 6.7 rattled the region, terrifying residents and increasing
damage. Then on May 12, a magnitude 7.3 aftershock
struck about 50 miles east of Katmandu, killing 135 and injuring over 2000. Each whole number increase in earthquake
magnitude represents ten times as much energy.
The M7.8 quake was equivalent to the amount of energy released by the
1980 eruption of Mt. St. Helens.
Why is this poor nation plagued with so many strong
earthquakes? Over
the previous century alone, Nepal had experienced four earthquakes stronger
than magnitude 6. This is equal to the
largest historic quake in New England that occurred in 1755 offshore from Cape
Ann. This is not a new phenomenon since
the region has been tectonically active for tens of millions of years, long
before the Himalayas even existed.
Geologists have long known that earthquakes are not
randomly distributed around the globe but occur along the boundaries between
the huge tectonic plates that make up the earth’s outer skin called the
lithosphere. Lithosphere consists of the
crust and the underlying upper portion of the mantle. Continental crust averages about 20 miles in
thickness compared to the 4-7 mile average thickness of oceanic crust. Continental crust consists of all kinds of
rocks (igneous, metamorphic, and sedimentary) which, if averaged, would
approximate granite in composition. Oceanic crust consists of more dense basaltic
rock overlain by a thin skin of sedimentary deposits. As a result of these differences in densities,
when a denser oceanic plate encounters a continental plate it plunges beneath
it forming what is called a subduction zone.
There are three general types of lithospheric plates:
those consisting only of oceanic crust and the underlying upper mantle rock, those
consisting of only continental crust and underlying mantle rock, and those
consisting of both oceanic and continental crust and the underlying mantle rock.
The North American plate, an example of the latter, extends from the middle of
the Atlantic Ocean to the west coast of the United States and Canada then north
through Alaska and the Aleutians to eastern Russia. Lithospheric plates are constantly moving a
few millimeters a year, bumping and grinding against each other along their
margins. Where two plates move directly
toward each other and collide, the resulting margin is called convergent. If one plate is oceanic and the other is
continental, the more dense oceanic plate slowly plunges back into the mantle
below where it breaks up and sinks, eventually melting. This process is called subduction. The deep
ocean trenches are located over subduction zones. The great global belts of earthquakes and
active volcanoes occur along subduction zones.
This is what is occurring along the west coast of South America for
example.
If two plates are moving directly away from each
other, their mutual boundary is divergent.
This is what is happening along mid-ocean ridges. This pulling apart allows for much basaltic magma
to form in the mantle and intrude up into the lithosphere to fill in the
gap. This is how new ocean floor is
produced. Earthquakes occur at
relatively shallow depths along these divergent zones that stretch for
thousands of miles along the floors of all the great oceans. Sometimes divergent zones form beneath
continents, tearing them apart to form great rift zones such as in eastern
Africa. Many ancient inactive rifts lie
buried beneath younger sedimentary rock across much of the interior of the
United States. Numerous rift basins
associated with the opening of the Atlantic Ocean, beginning about 200 million
years ago, are present throughout New England.
Reactivation of faults along these basins may account for some of the
modern day earthquake activity in New England.
In a few places, two plates are moving past one
another to form what are called transform plate boundaries. The famous San Andreas fault in California is
such a boundary. Much earthquake
activity occurs along these boundaries but generally little or no volcanic
activity.
The last major type of plate boundary is a
convergent one in which two continental plates collide, resulting in tremendous
deformation including faulting, folding, and uplift extending hundreds of miles
into each plate. In some cases what is
called an island arc, rather than a second continent, collides with a
continental plate. Japan, Indonesia, and the Aleutian Islands are island
arcs. Southeastern Massachusetts is
believed to be a portion of an ancient island arc that collided with the North
American plate several hundred million years ago. The stresses along convergent
margins are enormous. During collisions
huge slabs of rock extending laterally for hundreds of miles and thousands of
feet thick are slowly forced up and over rock farther inland along what are
called overthrust faults. There are
numerous such faults in New England resulting from repeated collisions of island
arcs with the North American continent over hundreds of millions of years and
ending about 300 million years ago. The
Appalachian Mountains were the result.
Nepal has the misfortune of being located in region where
two enormous tectonic plates are colliding. About 300 million years ago, India
was a part of a great supercontinent called Gondwana located in the southern
hemisphere. Eurasia was far to the north and part of the supercontinent of
Laurasia in the northern hemisphere.
For unknown reasons, sometime before 100 million years ago, India broke
loose from Gondwana and began its long, slow journey north toward Eurasia. There is
much debate about the timing of the rest of the story and whether or not
some other land mass, possibly an island arc, first collided with Eurasia about
50 million years ago. A subduction zone formed all along the
northern margin of the plate were it was in contact with the Eurasian plate and
the Pacific plate farther east. The
ocean between the two was consumed by subduction as the India slowly moved
north-northeast at about the rate your fingernails grow, bringing India ever
closer to Eurasia.
Perhaps around 30 million years ago India finally began
colliding with the Eurasian plate and the subduction zone became a continental
collision zone. India is currently
moving toward and against Eurasia at an average rate just under 2 inches per
year. The result is tremendous
compression with faulting and uplift of the Himalayas and Tibetan plateau that
is continuing to this day.
Labels: Nepal Quake
