The Geology Behind the Deadly Earthquakes in Turkey
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The Geology Behind the Deadly Earthquakes in Turkey and Syria |
Why Have So Many Earthquakes Hit Turkey and Syria?
Earthquakes
in Syria and Turkey are common, but the magnitude 7.8 that shook the
region on 6 February at 4:17am local time is clearly impressive. To find
earthquakes this strong on this particular fault, we would have to go
back to the year 1114.
Ten minutes after the strongest
earthquake, an aftershock of magnitude 6.7 struck near the epicentre.
“Aftershocks” are earthquakes that occur after every major earthquake,
and their statistical behaviour is well known. At the time of writing,
others continue to affect an area stretching over 350 kilometres from
eastern Turkey to the Syrian border.
More surprisingly and
dramatically, a second earthquake of magnitude 7.5 struck at 1:24pm
local time, further north. This earthquake was not an aftershock:
according to the first data processed live by the major international
seismological agencies, it would have occurred on an east-west fault
crossing-cutting the main rupture trace.
Seismologists consider
Turkey a tectonically active area, where three tectonic plates—the
Anatolia, Arabia, and Africa plates—touch and interact with each other.
The two major fault lines surrounding it, the North Anatolian Fault and
the East Anatolian Fault—which has a slip rate of between 6 and 10
millimeters per year—are gradually squeezing the country westward toward
the Mediterranean Sea. Yet, many buildings in the region are not built
to withstand large earthquakes, according to the US Geological Survey
(USGS), making the destruction worse.
Plate tectonics
The
Anatolian Plate is a continental tectonic plate that is separated from
the Eurasian plate and the Arabian plate by the North Anatolian Fault
and the East Anatolian Fault respectively. Most of the country of Turkey
is located on the Anatolian plate. Most significant earthquakes in the
region have historically occurred along the northern fault, such as the
1939 Erzincan earthquake. The devastating 2023 Turkey–Syria earthquake
occurred along the active East Anatolian fault at a strike slip fault
where the Arabian plate is sliding past the Anatolian plate
horizontally.
The Anatolian transform fault system is "probably the most active in the world." The East Anatolian Fault, a left lateral transform fault, forms a boundary with the Arabian Plate. To the south and southwest is a convergent boundary with the African Plate. This convergence manifests in compressive features within the oceanic crust beneath the Mediterranean as well as within the continental crust of Anatolia itself, and also by what are generally considered to be subduction zones along the Hellenic and Cyprus arcs. The northern edge is a transform boundary with the Eurasian Plate, forming the North Anatolian Fault Zone (NAFZ).
Research indicates that the Anatolian Plate is rotating counterclockwise as it is being pushed west by the Arabian Plate, impeded from any northerly movement by the Eurasian Plate. In some references, the Anatolian Plate is referred to as a "block" of continental crust still coupled to the Eurasian Plate. But studies of the North Anatolian Fault indicate that Anatolia is de-coupled from the Eurasian Plate. It is now being squeezed by the Arabian Plate from the east and forced toward the west as the Eurasian Plate to its north is blocking motion in that direction. The African Plate is subducting beneath the Anatolian Plate along the Cyprus and Hellenic Arcs offshore in the Mediterranean Sea.
Earthquakes may trigger other earthquakes
An
earthquake happens when a fault – a fracture in the first kilometres of
the Earth’s crust – slips rapidly, within seconds, abruptly releasing
the energy that has been growing over tens to hundreds of years by the
slow motion of tectonic plates. When this happens, the released energy
leads to shaking of the ground: the earthquake.
Some earthquakes
are linked to each other: when the fault breaks, earthquakes release
part of the energy and reorganise part of it in the Earths’ crust, which
can trigger new earthquakes. For instance, one can consider the series
of earthquakes of magnitude greater than 7 that have cascaded from east
to west for about 800 kilometres over the course of the 20th century
along the North Anatolian fault. Each earthquake brought the nearby
fault segment of the North Anatolian fault closer to a rupture.
The
notable point is that the entire length of the North Anatolian Fault
ruptured between 1939 and 1999. The last unbroken segment runs across
the Sea of Marmara, very close to Istanbul, between the epicenters of
the 1999 Izmit and 1912 Ganos earthquakes.
If a fault section is
already well loaded (close to rupture), when a big earthquake hits
nearby, then a second earthquake might happen. Otherwise, it will be
necessary to wait for the motion of tectonic plates to bring the
remaining energy necessary to trigger an earthquake. This is called
“static triggering”.
When giant earthquakes trigger other earthquakes… at a distance
There
is also a type of triggering known as “dynamic”. In some cases, the
energy excess resulting from a large earthquake is not large enough to
explain the occurrence of certain earthquakes, especially if they are
located far away from the epicentre of the main shock.
For
example, following the 1992 Landers and 1999 Hector Mine earthquakes in
California, earthquake swarms were observed several hundreds of
kilometres from the epicentre. It has been shown that these earthquakes
occurred exactly during the passage of the strongest seismic waves
emitted by these two earthquakes. At such distances, this shaking, the
seismic waves, cannot be felt by humans, but apparently, seismic faults
do.
Similar observations have been made in the laboratory to show
that during the passage of these seismic waves, the material that makes
up the core of the fault weakens, causing a sudden sliding, i.e., an
earthquake.
This kind of behaviour comes from the physics of
granular media, which behave like fluids while being shaken. Shaking a
pile of sand quickly will cause it to flatten under its own weight,
whereas without the shaking, it would stand still.
So shaking a
fault quickly can cause it to slip, producing earthquakes. It has also
been observed that these seismic waves can trigger slow slippage at
colossal distances. The seismic waves emitted by the Maule earthquake, a
magnitude 8.9 earthquake that struck Chile in 2010, caused a slow
earthquake along the Mexico subduction, about 7,000 kilometres from the
epicentre.
See also: How Turkey's Deadly Earthquake Moved the Country 10 Feet
Leylines?
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