By Miguel Neves, Ph.D. Candidate, Georgia Institute of Technology (@Waves2Miguel)
A new study based on earthquakes in the Marmara Sea, Turkey, showed how a moderate earthquake initiated and suggests a two-stage preparation that reconciles two sides of an old debate.
Citation: Neves, M., 2020, Earthquakes in Turkey support two disparate models of earthquake initiation, Temblor, http://doi.org/10.32858/temblor.133
The Main Marmara Fault is an approximately 150-kilometer-long section of the North Anatolian Fault that runs along the Marmara Sea in Turkey, just south of Istanbul. Though the western section of the fault is partially creeping, the eastern part of the Marmara Fault is locked and hasn’t ruptured in more than 100 years and poses a risk to large cities such as Istanbul. In September 2019, two moderate earthquakes, a magnitude-4.7 and a magnitude-5.8, occurred at the transition to the locked part of the fault. Seismologist Virginie Durand and a team of researchers at GFZ Potsdam have since conducted a new study investigating the initiation of the two earthquakes and whether the earthquakes affected the locked section. Using template matching, a technique that finds earthquakes similar to earthquakes previously detected in the area, the team compiled a high-resolution catalog of the seismic activity in the area and a very detailed “movie” of what happened in the days before and after the earthquakes. Their observations, reported in Seismological Research Letters, suggest that the earthquakes were initiated in two phases.
The beginning of an earthquake
An earthquake occurs when the two sides of a locked fault suddenly slip. But seismologists are still trying to understand what makes the fault decide to slip at a specific time and some have turned to studying foreshocks, earthquakes that sometimes precede larger earthquakes. The scientific community currently entertains two possible explanations. One model, the cascade model, suggests that a foreshock is not physically different than any other earthquake and there is nothing in how it ruptures that could fortell a subsequent larger quake, says Fatih Bulut, a professor of geodesy at Bogazici University in Turkey. In this model, an earthquake can trigger another earthquake nearby, and “a series of smaller cascading earthquakes could smoothly disappear or randomly trigger a major earthquake,” Bulut says.
The other model, called the nucleation or preparation model, suggests that foreshock occurrence is not random. It suggests some process happens before the earthquake event that alters the contact between the two sides of the fault, preparing the fault to rupture — what is known as a nucleation or preparation phase. Per this model, slow slip, or aseismic slip, events may initiate earthquakes; in this case, foreshocks would be byproducts of this preparation phase and could be used as warning signs of larger earthquakes to come.
Observations have yet to settle the debate. Observations suggest a preparation phase before many large earthquakes, but, Bulut notes, “we sometimes observe foreshocks for a few days preceding the mainshock, sometimes for only few minutes and sometimes nothing.” In some cases, such as the 1999 Izmit earthquake, both a cascading model and a slow slip have been used to explain the same foreshock sequence. That seems to be the case for the 2019 Marmara Fault quakes.
What did researchers observe in the Marmara Sea?
Using a high-resolution network of instruments in the Marmara Sea, Durand and her colleagues observed the preparation or nucleation phase of the 2019 moderate-sized earthquakes. According to their observations, bursts of smaller earthquakes were detected across the entire area ruptured during the magnitude-4.7 and magnitude-5.8 earthquakes in the days before the larger quakes occurred. This suggests that a large-scale process activated the entire area in the days before through aseismic slip, Durand says. However, the observations also suggest a second preparation phase, she says. In the hours before each of the magnitude-4.7 and magnitude-5.8 earthquakes, small foreshocks migrated toward the epicenter of the soon-to-be earthquakes. This observation can be interpreted as cascade triggering, Durand explains: This second phase might have acted as “a final impulse to trigger the rupture, but the rupture may not have been that large if we didn’t have the aseismic preparation.”
Can we predict earthquakes now?
No. “For the moment, all these observations are not useful for forecasting,” Durand says. Science still can only say a foreshock is a foreshock after a larger earthquake occurs. Scientists must obtain a better understanding of the physics behind earthquakes if we are to try to make forecasts, Durand says. Durand and Bulut agree that the challenge right now is to observe in more detail the deformation occurring before an earthquake. To this end, they suggest installing instruments capable of detecting slow slip closer to seismic faults to understand if it is characteristic.
It is helpful to have high-resolution seismic networks near faults, Bulut says. It was Turkey’s high-resolution seismic monitoring network — a network that now regularly detects earthquakes as low as magnitude-1.0 — that captured the foreshock sequence of the magnitude-5.8 earthquake and others in Turkey (such as the Kutahya 2011 and Ayvacik 2017). Having geodetic networks as close to the fault as possible that can identify slow slip and allow a correlation with seismic observations, Durand notes, is the future focus.
Was there an impact on the locked section?
Regarding possible changes in the locked section that could endanger Istanbul, the study shows that seismic activity decreased to normal levels after a few weeks. There is “no clear increase of the seismicity east of the rupture,” Durand says. Therefore, there is no indication that this sequence will lead to a large earthquake or activation of the locked section. However, Bulut adds, this study may help in the future to identify whether there is a “particular pattern that makes Marmara Sea foreshocks different than ordinary micro-earthquakes.”
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