M 6.7 earthquake broke a seismic gap in eastern Turkey

The January 2020 M 6.7 Elaziğ earthquake

On January 24, 2020, a magnitude 6.7 earthquake occurred at 20:55 (local time), in Elaziğ province of eastern Turkey (Fig. 1 and 2). The epicenter was located at 38.390°N 39.081°E, close to the towns of Sivrice and Doğanyol, with a focal depth estimated at 11.9 km (source USGS). The shock was strong in the neighboring provinces of Diyarbakır, Malatya and Adıyaman, and felt as far as Armenia, Syria and Iran.

Fig. 1: Location of the epicenter, focal mechanism, and source-time function of the 2020 Elaziğ earthquake. On the right are shown the two possible fault plane solutions compatible with the focal mechanism. The geological setting of the WSW-striking East Anatolian fault points toward the first solution (source Geoscope).
Fig. 2: Map of the aftershocks detected until January 25, 21:12. The orange star locates the epicenter of the mainshock. A few focal mechanisms, all consistent with left-lateral slip, are shown. Modified from AFAD report.

The main rupture extended over 40 km along strike southwest of the epicenter. The focal mechanism suggests left-lateral motion on a near-vertical fault plane striking WSW, which is consistent with a rupture of the East Anatolian strike-slip fault. The maximum slip was estimated around 1.6 m (Fig. 3). Models, observations of the deformation field from satellite imagery (Fig. 4), and field investigations suggest that most of the slip occurred at depth and did not reach the surface. Many aftershocks were detected during the day following the earthquake, extending over 75 km between Lake Hazar and Pütürge (Fig. 2).

Fig. 3: Finite fault slip model of the 2020 Elaziğ earthquake. This model is derived from seismological data and shows the distribution of coseismic slip on a SW-NE striking fault plane. The star locates the hypocenter of the earthquake (source USGS).
Fig. 4: Coseismic surface displacement field derived from the comparison of pre- and post-earthquake satellite radar data (InSAR technique). The displacement is displayed relative to the line of sight (LOS) of the satellite (red toward SW, blue toward NE). Courtesy of Geohazards Exploitation Platform.

The earthquake caused serious damage within 40 km of the epicenter, impacting 19 towns and over 200 villages (Fig. 5). Almost a hundred buildings collapsed and more than a thousand were heavily damaged. 41 people were killed, mostly in the cities of Elaziğ (400,000 inhabitants) and Malatya (760,000 inhabitants), and more than 1,600 were injured.

Fig. 5: Collapsed building in Elaziğ city center (source Daily Sabah).

Regional tectonic setting

The tectonics of the eastern Mediterranean are dominated by the collision between the Arabian and Eurasian plates and the associated westward escape of the Anatolian plate (Fig. 6). This escape is accommodated by two large strike-slip fault zones: the North Anatolian fault, a 1000-km long right-lateral fault zone crossing the north of Anatolia and the Marmara Sea region, and the East Anatolian fault, a 600-km long left-lateral fault zone oriented SW-NE between Karlıova (Bingöl province) and northern Syria. These two faults are marked by high horizontal slip-rates, of about 2 cm/yr and 1 cm/yr respectively and are among the most important seismic sources of the Mediterranean region.

Fig. 6: Tectonic setting of Turkey. Anatolia-Aegean block escapes westward from Eurasia-Arabia collision zone. The black arrows show current motion relative to Eurasia. NAF=North Anatolian Fault, EAF=East Anatolian Fault, DSF=Dead Sea Fault, NAT=North Anatolian Trough, CR=Corinth Rift. Modified from Armijo et al. (1999).

A seismic gap

Thanks to historical archives and descriptions of past earthquakes, it is known that the East Anatolian fault zone has been significantly active in the 19th and early 20th centuries AD (Fig. 7). The compilation of these historical data shows that the Palu-Sivrice fault section was ruptured by two important events: a M ~7.1 earthquake in 1874, followed by a smaller event in 1875. Geological investigations seem to confirm these historical ruptures (Duman and Emre, 2013). Two decades later, a resembling sequence occurred south of Malatya, with a M ~7.1 earthquake in 1893 followed by a smaller event in 1905 (Ambraseys, 1989).

Fig. 7: Surface ruptures produced by large historical earthquakes along the East Anatolian fault. Ruptured fault segments are highlighted in colors. The fault section between Lake Hazar and Pütürge, where the 2020 earthquake occurred, has not known any big rupture for several centuries. Modified from Duman and Emre (2013).

Therefore, the 2020 Elaziğ earthquake and its aftershocks highlight a fault section where no big earthquake has been reported in the last centuries. This is generally called a “seismic gap”. As the fault zone keeps accumulating tectonic stress over time, it is important to know the date and size of the last earthquakes in these regions in order to assess the type of earthquakes which might be generated in the future. For example, if the fault long-term slip-rate is known, it is possible to estimate the magnitude associated with a seismic release of all the stress accumulated since the last big rupture.

For earlier periods, historical documentation is lacking along the East Anatolian fault. It is known that the towns of Sivrice, Palu and Bingöl were destroyed in an apparently major earthquake in 995 AD (Ambraseys, 2009). Another way to document the past seismicity consists in studying seismic features in the sediments deposited along the fault zone. The sediment stratigraphy seen in trenches dug across the faults east of Lake Hazar suggests previous earthquakes in the 15-16th, 5th and 2nd centuries AD (Cetin et al., 2003). However, the precise extent of these older ruptures and their magnitudes are yet to be constrained more accurately. This would enable researchers to discuss the level of stress potentially available for a future earthquake along the Sivrice-Pütürge section.

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Yacine Benjelloun
About Yacine Benjelloun 1 Article
Yacine Benjelloun currently works at the Institut de Physique du Globe de Paris. Yacine's research interests encompass Geoarcheology, Tectonic Geomorphology and Holocene Paleoenvironment. His last project aimed to document the tectonic activity of the middle strand of the North Anatolian fault through a coupled geomorphological and archeoseismological approach.