Introduction
Turkey's tragedy
Long time shaking
Plate tectonics
Finding faults
Proving the theory
Perilous predictions
Safe buildings
1999-2004 Activity
Transform plate boundary (North Anatolian and San Andreas Faults): The plates move laterally (as shown).
Divergent plate boundary (usually found at mid-oceanic ridges): The plates move away from each other, and new rock solidifies from hot magma rising from the upper mantle.
Convergent plate boundary (found in the U.S. Pacific Northwest): The denser oceanic plate "subducts" beneath the less dense continental plate. The result is a deep trench in the ocean and an arc of volcanoes on land.
A bird's-eye view of the San Andreas fault: Where plates collide.
Courtesy USGS.
Three kinds of boundaries
Continental drift -- or plate tectonics -- involves a lot of complicated motion at the plate boundaries. The tamest version generally occurs beneath the oceans, when plates move away from each other. At these divergent plate boundaries, molten magma rises and solidifies into solid rock, filling the gap formed as the two plates move apart. These spreading centers (see figure below or the graphic at the top of this page) form ridges on the seafloor. As spreading continues and the rocks move away from the ridge, they cool and contract as they age. The movement usually occurs at a relatively constant rate of a few centimeters per year and tends not to produce large earthquakes.
Either of the other two types of boundaries may be associated with large
earthquakes. At transform
plate boundaries one plate moves laterally against and past
another. Some transform boundaries are also described as strike
slip faults. At a convergent
boundary, one plate must either slip beneath the other (a subduction
zone) or the two plates must collide (a collision zone). A
classic subduction zone has a denser oceanic plate diving, or "subducting",
beneath a less dense continental plate.
A
simple meeting
The recent quake off the coast of El Salvador was caused by subduction
of the Pacific plate beneath Central America. This
figure shows that earthquakes are very common in this region while
this figure shows that big
quakes happen regularly as well. Earthquakes, some killers, are a
fact of life for those people living around the border of the Pacific
Ocean.
A complicated
intersection The result is that the Anatolian plate is moving about 30 millimeters
per year, relative to the Eurasian plate. The movement occurs along the
North Anatolian Fault that ruptured this past August, and pushes or rotates
the Anatolian plate counterclockwise (to the west). Having trouble juggling
this geometry in your head? Don't
miss this link for a figure.
Slippin' and
slidin' But when a giant rock hits an immovable object -- when one tectonic
plate moves suddenly against another -- the havoc of a major earthquake
can result. Lacking the stress relief
of regular, minor earthquakes, strong rock gets stuck at the fault zone,
allowing strain to build up.
When the strain gets too great, it is relieved by the sudden movement
causing a major earthquake. The greater the strain, the larger the earthquake.
Thus in a sense, earthquakes should be predictable if we know the strain
and the strength of the rocks.
Unfortunately, despite that simple equation, precise predictions of
quakes are not possible now. Beyond problems measuring the strength of
rocks, we have only a foggy picture of the triggering mechanism. "How
the slip on a fault starts is a fundamental problem in seismology," says
Clifford Thurber, a geophysicist at the University of Wisconsin-Madison.
"We don't really know the conditions and state that a fault is in when
it starts moving."
The problem, simply, is inaccessibility. Earthquakes start underground
-- sometimes dozens or hundreds of kilometers deep, and "there is no direct
way to detect conditions," as Thurber says. Indirect measurements may
offer a guideline, but direct observations would be preferable. The project would be a step toward the eventual -- and we stress eventual
prediction of earthquakes. We'll get to that prospect shortly.
But first, let's look at how plate tectonics became the law
of the land, geologically speaking.
Turkey's tectonic situation is complex. Long a crossroads of people and
cultures, it's also a crossroads of tectonic plates. While California's
quakes result from the intersection of two plates (the North American
and the Pacific), Turkey and the Anatolian Plate are being squeezed by
three pretty good sized ones: the Eurasian, Arabian and African plates.
Faults can move in several ways. Where the rock is weak, as it is along
portions of the San Andreas and Anatolian faults, one side may "creep"
gradually against the other, forming weak earthquakes that do little or
no damage.
That's one reason for a recent proposal to drill 3.5 kilometers into the
San Andreas Fault to obtain direct measurements from that active region.
Thurber notes that cores removed from the hole would be examined for strength
and fluid content. Instruments in the hole itself would look at fluid
pressure, which may be implicated in initiating quakes. "We want to get
direct measurements of the physical properties for the first time," Thurber
says.
