Mw5 aftershock occurs near epicenter of 2023 M7.8 Turkey-Syria earthquake
Largest aftershock since January 25, 2024.
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Citation: Hubbard, J. and Bradley, K., 2024. Mw5 aftershock occurs near epicenter of 2023 M7.8 Turkey-Syria earthquake. Earthquake Insights, https://doi.org/10.62481/fad55236
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On September 7, 2024 at 09:31 (06:31 UTC), a magnitude 5 earthquake struck about 30 km northwest of the city of Gaziantep, Turkey. The earthquake was reported as a Mw5.0 by the Disaster and Emergency Management Authority (AFAD), a Mw4.9 by the Kandilli Observatory (KOERI), a Mww5 by the United States Geological Survey (USGS), and a mb4.8 by the Euro-Mediterranean Seismological Center (EMSC). This range of magnitudes is normal, and reflects a combination of different seismic networks and different approaches to measuring magnitude.
The earthquake occurred close to the epicentral location of the first large earthquake that devastated parts of Turkey and Syria on February 6, 2023. The aftershock was felt by many people within ~200 km, although shaking intensities remained relatively light.
We have written many posts about the Turkey-Syria earthquakes and their aftershocks, which caused strong shaking within a region ~500 km wide. This latest event is the largest aftershock to have occurred since January 25, 2024, when a M5.2 struck near Malatya. While aftershocks do become less frequent with time, elevated seismicity rates can be expected to continue for years. On average, aftershocks decay as 1/time, but the actual pattern can be pretty irregular. For instance, in August 2023 (six months after the mainshocks), there were nearly 90 events above M3, with about 10 of those above M4; far fewer than in the months just after the mainshocks. However, since November 2023, there have continued to be around 30-40 M3+ events per month, with a few reaching above M4 — it has become quite difficult to detect a decay since this time.
How does this latest aftershock fit in?
While the 2023 earthquake sequence started close to Gaziantep and Kahramanmaraş, most of the aftershocks have been concentrated elsewhere. On the map below, more recent earthquakes are shown in shades of orange and red. These recent events are concentrated along and around the Çardak–Sürgü Fault to the north, and around Malatya in the northeast and Hatay in the southwest. The northern faults in particular continue to be quite active (sustaining the aftershock rate), while aftershocks to the south have diminished considerably. This spatial difference seems to match with a geometric difference: aftershocks around the Çardak–Sürgü Fault display significant complexity, with activation of a number of different fault structures off of the main fault.
On the seismicity timeline above, you might notice that the number of very small (M1-2) earthquakes suddenly increases in mid-2023. This is not because more small earthquakes started happening — it is because more small earthquakes were detected. The jump in M1-2 earthquakes could arise from several reasons: quieting down of larger earthquakes that cause a lot of noise, obscuring small events; improvement of the seismic network by installation of new or better seismometers; or simply an administrative decision to locate and publish smaller earthquakes using the same data stream.
Moderate aftershocks near the epicentral region are not unexpected. For the first anniversary of the earthquake last February, we wrote a summary post about early scientific studies of the earthquakes and their aftershocks. In that post, we highlighted a paper published by Shinji Toda and Ross Stein, who had generated an aftershock forecast. This type of statistical forecast is created by modeling the pattern of expected stress changes caused by the earlier ruptures. Their forecast, which was valid for the 12-month period from December 1, 2023 to November 30, 2024, indicated that 1-3 Mw≥5 aftershocks were expected across the area, with higher likelihoods in some specific areas. While most of those areas are along the areas that have experienced the most seismicity, the area near Gaziantep and Kahramanmaraş is also shown as higher probability. This latest aftershock occurred near, but not exactly in, this zone.
Until now, the forecast window has only seen one earthquake of at least Mw5 — a M5.2 on January 25 near Malatya. This recent aftershock is right around the cutoff, depending on which agency you ask. Thus, the record so far is consistent with the forecast.
While the M7.8 earthquake on February 6, 2023 mostly ruptured the East Anatolian Fault, it did not start on that fault. Instead, the rupture started on a smaller splay fault to the south, near Gaziantep, as shown in this movie below by Diego Melgar and colleagues.
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This recent aftershock appears to have occurred on the same splay — the largest earthquake to have occurred on this fault since February 2023. The map and timeline below are centered on the nucleation area of the first mainshock. The new M5 event is not evidence of new behavior of the system, but rather is part of the expected sequence of aftershocks. As expected, the earthquake focal mechanism shows that it occurred as left-lateral slip on a fault striking northeast-southwest.
All earthquakes can trigger their own aftershock sequences. It is likely that this M5 will be followed by a few more felt events. However, that sequence will decay relatively quickly, and the region will probably revert to the overall aftershock patterns caused by the mainshocks.
So, this earthquake may be unsettling but it is not unexpected. It is still likely that several more similar magnitude aftershocks will occur within the region over the ensuing years.
References:
Hubbard, J., 2024. M5.2 earthquake strikes near Malatya, Türkiye. Earthquake Insights, https://doi.org/10.62481/7b340ca3
Hubbard, J. and Bradley, K., 2024. One year after the February 6 Türkiye-Syria earthquakes. Earthquake Insights, https://doi.org/10.62481/266f3dbf
Melgar, D., Taymaz, T., Ganas, A., Crowell, B.W., Öcalan, T., Kahraman, M., Tsironi, V., Yolsal-Çevikbil, S., Valkaniotis, S., Irmak, T.S. and Eken, T., 2023. Sub-and super-shear ruptures during the 2023 Mw 7.8 and Mw 7.6 earthquake doublet in SE Türkiye. Seismica, 2 (3). https://doi.org/10.26443/seismica.v2i3.387
Toda, S. and Stein, R.S., The Role of Stress Transfer in Rupture Nucleation and Inhibition in the 2023 Kahramanmaraş, Türkiye, Sequence, and a One-Year Earthquake Forecast. Seismological Research Letters, 95 (2A), https://doi.org/10.1785/0220230252
Zelenin, E., Bachmanov, D., Garipova, S., Trifonov, V. and Kozhurin, A., 2022. The Active Faults of Eurasia Database (AFEAD): the ontology and design behind the continental-scale dataset. Earth System Science Data, 14(10), pp.4489-4503. https://doi.org/10.5194/essd-14-4489-2022