Tens of thousands of people were killed and more than a hundred thousand injured after the first quakes, and the United States Geological Survey estimated the latest quake led to hundreds of fatalities and likely caused between $1 billion and $10 billion in additional damages.
What are aftershocks?
Aftershocks are secondary earthquakes that follow in the days, weeks and months after the main event of an earthquake sequence. Once in a while, what initially appears to be the main event is actually a foreshock — a precursor to a larger earthquake — though that only happens about 6 percent of the time.
Every earthquake gives rise to aftershocks, their strength proportional to the magnitude of the initial shaking. In the case of the back-to-back quakes in Turkey, many of the ensuing aftershocks have been large enough to cause additional damage and casualties.
“The likelihood of aftershocks becomes smaller with time, as does the likelihood of large, and potentially damaging events,” wrote Jackie Caplan-Auerbach, a seismologist at Western Washington University, in a message to The Post. “That said, these sequences can go on for months or even years.”
What have the significant aftershocks been so far? What could still come?
The United States Geological Survey outlined what it deemed to be “the most likely scenario” with regard to aftershocks — a situation with a 90 percent chance of occurring.
It wrote that the aftershocks will “continue to decrease in frequency, with no aftershocks larger than [magnitude 7] within the next month.”
“Moderate to large aftershocks (magnitude 5 and 6) are likely and could cause localized damage, particularly in weak structures,” the agency added. “Smaller magnitude earthquakes (magnitude 3 and magnitude 4) will be felt by people close to the epicenters.”
The agency took the opportunity to remind residents and rescuers — many of whom have lost everything — to avoid entering damaged or collapsed structures.
Since the initial slip, or fault rupture, on Feb. 6, there have been three earthquakes of magnitude 6 or greater, and 32 others in the magnitude 5 or greater range. That magnitude is a rough threshold for when structural damage becomes a big problem. The majority of them — 23, to be exact — occurred within the 48 hours that followed the dual main shock events, according to USGS.
The main shock itself was a magnitude 7.8 quake that struck 20 miles west-northwest of Gaziantep in south-central Turkey near the Syrian border. That jostled the ground enough to trigger a magnitude 7.7 earthquake nine hours later on a neighboring fault roughly 60 miles to the north-northeast in Kahramanmaras province.
What causes aftershocks?
After every earthquake, the fault lines remain active as pieces of the earth’s crust continue to settle following a “slip.” Decades’ or centuries’ worth of pent-up stress and tension is released in a matter of seconds — but that in itself can introduce new stress.
“The compounding challenge is that it’s also possible for a quake to trigger other quakes in a manner that is not a traditional aftershock,” Caplan-Auerbach wrote. “When a fault slips it changes the stress field in the area, relieving stress in some areas but potentially adding it in others. This can increase the chances of a nearby fault faulting.”
If a slip occurs on another fault line nearby, that could technically be a separate earthquake event.
That manifested already with the Turkey earthquake sequence. Seismologists are still divided on whether the magnitude 7.7 earthquake should be considered an aftershock of the magnitude 7.8 or a separate event, since it occurred on a different fault. Regardless, ground motions from the first shuffled the landscape, which played a role in instigating the second.
“Every day that passes reduces the likelihood of another triggered event,” Judith Hubbard, a visiting professor at Cornell, wrote in a message to The Post. “However, that [risk] does not decay to zero — instead, it decays to the background rate. So while the hazard will be less elevated as time passes, it will never be nonzero — and in fact, in many areas, it will remain quite high.”
What should residents plan for? When will the shocks end?
For the next couple of weeks, a few more damaging aftershocks may be a realistic expectation — and, truth be told, a biggie can’t be ruled out for months to come.
The risk will wane, but, in a region as seismically active as Turkey and northern Syria, earthquakes are an ever-present concern.
“The hazard will still be elevated for years, but at some point the incremental additional hazard will be minimal compared to the underlying hazard,” Hubbard wrote.
Seismologists have two basic tools to broadly model aftershock intensity and frequency — a pair of mathematical “rules” that can be used to model earthquake sequence behavior. Omori’s Law dictates that, after a strong earthquake, the frequency of aftershocks decays at a predictable hyperbolic rate (a “hyperbola” is just the shape of a type of graph). The Gutenberg-Richter relationship, meanwhile, characterizes the percentage of aftershocks likely to reach a certain threshold in magnitude.
Putting them together offers crude — but holistically reliable — aftershock forecasts.
Once the dust fully settles, Hubbard believes the conversation should shift away from hoping against aftershocks to instead strengthening infrastructure to handle seismic vulnerability inveterate to the region. She feels that, following the Golcuk earthquake of Aug. 17, 1999, which struck Marmara, Turkey, many have grown complacent. That one was a magnitude 7.6 that claimed about 18,000 lives.
“My hope for people in seismic hazard zones both in Turkey and elsewhere is that they don’t live in fear, but instead, take realistic and functional steps to protect themselves, and then live their lives as normal,” Hubbard wrote. “This means living and working in buildings built to seismic code, organizing their living spaces, having emergency supplies on hand, and participating periodically in earthquake drills.”