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| Map of the San Andreas Fault, showing relative motion. |
By Matthew Blackett, Coventry
University | Updated May 11, 2016 at 3:05 PM
The director of the Southern California Earthquake Center,
Thomas Jordan, made an announcement recently that would have sent a chill down
the spine of every Californian: that the San Andreas fault appears to be in a
critical state and as such, could generate a large earthquake imminently.
Of course, the
reiteration of the seismic hazard to Californians will be nothing surprising,
but what is new is the warning that the southern portion of the fault "looks like it's locked, loaded and ready to go."
Why is this eminent
seismologist making these alarming statements?
Well, the fact is that there has not
been a major release of stresses in the southern portion of the San Andreas fault system since 1857.
In simple terms, the San Andreas is
one of many fault systems roughly marking the border between the Pacific and
North American tectonic plates. Both plates are moving in an approximately
northerly direction, but the Pacific plate is moving faster than its North
American counterpart, meaning that stresses between the plates are constantly
building up.
In 1906, some of
these stresses were catastrophically released in the San Francisco Bay
area in a 7.8
magnitude event and again, in northern California, during the 6.9 magnitude 1989
Loma Prieta earthquake.
Events of these magnitudes, however,
have not occurred along the San Andreas fault in the south of the state – the 1994 Northridge event was associated with a nearby, but
separate, fault system – leading to the suggestion that one is imminent and,
given the amount of stress that might actually have accumulated, when it
arrives it will be the "Big One."
How big is 'Big'?
So just how big
could this potential earthquake be and is it possible that the destruction
demonstrated in the film San Andreas could actually come to fruition?
Fiction ... or fact?
In short, Californians will be
(reasonably) pleased with the answers to these questions.
In the film, the San Andreas fault
produces an earthquake with a magnitude of
9.0. While not unheard of globally, earthquakes of this size are generally
confined to regions of the earth where subduction – where one tectonic plate is being
forced below another – is happening, for example in Chile and Japan. The
tectonic situation in California is different. Here, two plates are sliding
past each other.
As such, recent predictions limit the possible maximum earthquake
magnitude along the San Andreas fault system to 8.0, although with a 7 percent
probability estimate that such an event could occur in Southern California in
the next 30 years; over the same period, there is a 75 percent chance of a
magnitude 7.0 event.
While magnitudes of 7.0, 8.0 and 9.0
might sound negligibly different, the energy that such events would unleash
varies significantly, with a magnitude 9.0 event releasing 32 times more energy
than a magnitude 8.0 and 1,000 times more energy than a magnitude 7.0.
Obviously, however,
be it a 7.0 or an 8.0, damage is inevitable, but the whole sequence of events,
as depicted in the film, is unlikely.
For example, the San Andreas fault
is not beneath the ocean and as such, any slippage along it could not displace
water to the extent that a tsunami would be generated. The opening up of a
massive chasm is also from the land of fantasy, as the plates are sliding
relative to each other, not away from each other.
What is realistic, however, is that
a great amount of destruction is likely. While the building codes in California are stringent, recommending
retrofitting of seismic protection measures to older buildings and preventing
the construction of new buildings near to known fault lines, there is no way to
make a building 100 percent safe.
Predicting
devastation
In an attempt to
understand the effects of a large, southern San Andreas earthquake, the United
States Geological survey modelled
a 7.8 magnitude event, with slippage of 2-7 metres, to represent the stresses
that have built up in the area since the last large event.
From this model, it
was found that damage would be most severe to constructions straddling the
fault. Fortunately, constructions of this sort are few and far between
following the 1972 Alquist-Priolo Earthquake Fault Zoning Act.
What would be affected by this slippage, however, are the 966 roads, 90 fibre
optic cables, 39 gas pipes and 141 power line that cross the fault zone.
The total cost of
damage to buildings was estimated at $33 billion, with modern buildings faring
well but older buildings being particularly susceptible. Fires would rage – as
they did following the Northridge earthquake – as gas mains, and water pipes,
become severed; in fact, the damage from resulting fires is estimated as more
costly than that resulting from the initial shaking.
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| San Andreas Fault in the Carrizo Plain, aerial view from 8,500 feet altitude. |
The overall death
toll is estimated at 1,800. And just when things don't look like they can get
any worse, the main event will have destabilized the tectonics of the region to
such an extent that a series of potentially powerful aftershocks will begin.
For example, in 2011,
Christchurch, New Zealand was struck by a 6.2 magnitude event and since then the
city and surrounding region have experienced more than 10,000 aftershocks.
Fortunately, the film San
Andreas is pure fiction, featuring the levels of exaggeration we are
all used to from film makers who are, ironically, also based in southern
California.
Even so, in all
probability, the San Andreas is likely to generate a significant earthquake in
the not too distant future. When it arrives, the damage will be significant and
southern California will be massively affected.
But Californians are
no strangers to these events and the infrastructure of the state, in recent
times, has been designed with earthquake protection in mind.
Forget tsunamis and
deep chasms opening up, but do expect violent shaking, building damage, fires
and widespread economic impacts as the region is out of action for potentially
a long period of time.
Related Posts:
Matthew
Blackett is a senior lecturer in physical geography and natural
hazards at Coventry
University in England.
http://www.upi.com/Top_News/Opinion/2016/05/11/San-Andreas-fault-is-about-to-crack-heres-what-will-happen-when-it-does/6741462974519/
hey, there is a broken link in this article, under the anchor text - recent predictions
ReplyDeleteHere is the working link so you can replace it - https://selectra.co.uk/sites/default/files/pdf/fs2015-3009.pdf