What triggered the Turkey quakes? Why was the second so big? Key questions, answered.

The Washington Post / Matthew Cappucci

16:12 JST, February 8, 2023

At least 7,000 people have been killed and countless others remain missing after a pair of exceptionally destructive earthquakes shook Turkey and a broad swath of the Middle East. Particularly hard hit was Aleppo in northern Syria, a nation embroiled in a brutal civil war that is now facing compounding disasters.

Amid the lengthy rescue, recovery and rebuilding process that lies ahead, many have raised questions regarding the impetus of the earthquakes, how long aftershocks can be expected and just how bad the sequence was.

We’ve broken down seven things to know when it comes to the earthquake sequences.

How long did the 7.8-magnitude earthquakelast?

Earthquake surface waves are like ocean waves – even long after you drop a stone in a body of water to create a disturbance, the waves propagate outward. The same is true with an earthquake.

The actual slip that triggered the 7.8-magnitude quake occurred over a period of about 75 seconds, according to the U.S. Geological Survey. For most people in Turkey, that quake probably resulted in one to two minutes of shaking.

Earthquake waves rippled around the world

The shaking from Turkey’s earthquakes were felt by seismometers across the world.

But that’s perfectly normal. Seismometers within the Global Seismic Network are sensitive enough that they can detect the equivalent of a dime placed under one corner of North America.

Scientists can visualize the progress of earthquake waves by producing what’s called a record section. If they take plots from seismometers around the world, as shown below, and stack them atop each other (with distance on the vertical axis and time going from left to right), they can see how the earthquake waves travel.

So, yes – even in Canada, Alaska, Mexico and Australia, waves from the Turkish earthquakes were registered by ultrasensitive seismometers.

Was the second quake an aftershock? Why was it so big?

It’s typical for aftershocks to occur for weeks or months after the main earthquake in a sequence. They tend to diminish in intensity and frequency with time.

There’s a 1 in 15 chance that a given earthquake is a “foreshock” preceding a bigger quake, according to USGS. That’s unlikely in this case, but it does occasionally happen. (Consider the July 4-6, 2019, Ridgecrest, Calif., earthquake sequence, when a 6.4-magnitude main shock was followed by a 7.1 two days later.)

About nine hours after Turkey’s initial 7.8-magnitude earthquake, which struck near Nurdagi, at 4:15 a.m. local time Monday, a second potent quake ― a shallow 7.5-magnitude temblor – occurred 60 miles to the north. Ordinarily this could easily be considered an aftershock (although one of unusual intensity), but it’s more complicated in this case. Why?

If we look at a map of seismicity in the region and annotate both our 7.8-magnitude and subsequent 7.5 quakes, we notice two distinct clusters of activity:

That means the 7.5-magnitude quake was not an aftershock on the same fault, but rather a secondary main shock on a different fault triggered by the initial 7.8-magnitude quake to the south.

“There is room for debate; I might consider the second earthquake a second mainshock, given its size,” Judith Hubbard, a seismologist and researcher, wrote in a message to The Post. “Aftershocks don’t have to be on the original fault, but as the second earthquake was close to the first in size, it stands out and is not a typical aftershock.”

Turkey is highly susceptible to earthquakes – a result of sitting on a junction of three tectonic plates, making it a region of high seismic unrest. Still, both quakes stand out for their intensity.

“Since 1970, only three earthquakes of magnitude 6 or larger have been registered in this region,” the USGS wrote. “The largest was a magnitude 6.7 that occurred Jan. 24, 2020.”

What triggered the first earthquake?

After each earthquake, the USGS publishes information on the mechanism that triggered an earthquake. It concluded that the 7.8 quake was triggered by two faults moving laterally against each other, or a strike slip. In this case, it was a “left lateral” strike slip, since both masses of Earth’s crust moved left in relation to the other.

The earthquake “beach ball” diagram below represents a view of the ground motions and the forces involved when looking down from above.

It probably occurred on the East Anatolian Fault

In a news article, the USGS wrote that all of the related earthquake activity was “occurring within the East Anatolian fault system,” which is shown on the map below:

Along the East Anatolian Fault, the Anatolian and Arabian plates slide and chafe against each other. That results in ever-building stress, which is occasionally released in catastrophic slips. (Farther to the northeast, the Arabian Plate collides with the Eurasian Plate at the Bitlis-Zagros Fold and Thrust Belt, forming the Zagreb, Alborz and Caucasus mountains.)

In the case of the East Anatolian Fault, the Arabian Plate is moving north-northwest at about 15 millimeters (0.59 inches) per year, while the Anatolian Plate slides west-southwest at 22 millimeters (0.87 inches) per year:

The two plates are moving at about 15 millimeters per year relative to each other.

The two masses of land slid approximately 10 feet past each other

According to the USGS, the two masses of land slid up to 10 to 12 feet (roughly 3 to 3.5 meters) past each other during the “slip” of the 7.8-magnitude quake.

The greatest slip occurred about 50 kilometers, or about 30 miles, from the epicenter.

How much of the fault ruptured? The USGS indicates a stretch a little over 250 kilometers long, so roughly around 150 miles, slipped.

However, different modeling from the USGS indicates that the quake may have ruptured a shorter segment of the fault but featured a greater rupture along the “dip,” or the vertical interface of the two masses of crust.

The area of the slip, as modeled by USGS, was about 60 miles long and 45 miles wide.

The ground also moved up to about 40 centimeters per second, or 15 to 16 inches per second. That doesn’t sound like much, but consider nudging a home, office building or any other structure by that much – all in a series of disjointed rolls and back-and-forth jerks.

At peak, the ground accelerated at 40 percent of one “g,” or the acceleration due to gravity. If you jump into the air, you are pulled back to Earth by the acceleration due to gravity. Imagine 40 percent of that acceleration enacting a force on the entire landscape. That illustrates the power of the earthquake.

Hundreds of years of pent-up stress was released

Figuring out just how long this earthquake has been in the works can be estimated using simple math.

Let’s assume the quake did occur along the East Anatolian Fault (which can’t be 100 percent confirmed but is the likely location). One can divide the total magnitude of the slip (10 feet) by the yearly movement of the plates relative to each other (15 millimeters, or 0.6 inches). That would mean 200 years of stress-buildup.

That could be a conservative estimate. Hubbard said on Twitter that the stress could have built over as much as 300 years.