Active seismotectonics of the East Anatolian Fault

Abstract

Summary The East Anatolian Fault (EAF) is a 700 km long left-lateral transform fault located between the Anatolian and Arabian plates. The proximity of the Euler Pole to the Arabia-Anatolia plate boundary leads to rapid changes in plate velocity along the boundary, which is manifested by the decreasing slip rates from east (10 mm/yr) to west (∼1-4 mm/yr). The EAF displays heterogeneous seismicity patterns with seismic gaps, localized clusters and broad diffuse zones. In this study, in order to understand the origin of these complexities and quantify the seismic hazard along the EAF, we present an improved seismicity catalog with more than 26.000 earthquakes and 160 focal mechanisms from regional moment tensor inversion between 2007 and 2020. The focal mechanisms and seismicity show that the EAF dips toward north and forms a well-defined plate boundary in the east between Palu and Çelikhan with almost pure left-lateral motion. Further west, the boundary becomes broader with activity along subparallel faults. Focal mechanisms show heterogeneous stress orientations in consistence with geodetically determined strain rate field. The stress orientations show a transition from strike-slip to extension toward the west of Çelikhan. Amongst all segments of EAF, the Pütürge segment, which holds the near-repeating earthquakes in the vicinity of the nucleation of the 2020 Mw6.8 earthquake, is distinguished with its steady and high rate of seismicity. Further east, the neighboring Palu segment is characterized by several distinct moderate earthquakes. We do not observe any change in the seismicity rate on these segments of the EAF following large earthquakes. In order to quantify the seismic hazard along the EAF, we calculate the recurrence time and maximum magnitude for each segment by using an extended seismicity catalog of 150 years including the large historical earthquakes and the geodetic strain rate. The results show ∼150 year recurrence time with Mmax∼6.7-7.0 along the seismically active Palu and Pütürge segments on the east, while relatively silent western segments yield longer recurrence times; 237- 772 for Pazarcık and 414-917 for Amanos segments with slightly larger magnitudes (Mmax∼7-7.4). We infer that the seismicity patterns and strain-rate field along the EAF are shaped by several factors such as strong geometrical irregularities, heterogeneous coupling and complex plate motion leading to rapid change of fault slip rate.

Months-long preparation of the 2023 MW 7.8 Kahramanmaraş earthquake, Türkiye

“…S5). Clusters C3 and C4 present b-values (both b=0.8±0.2) close to that reported for the Pazarcık and Amanos fault segments during the interseismic period (b=0.9, see [21]). The features of these clusters are in striking contrast to clusters C1 and C2 directly preceding the mainshock.…”

“…The junction between the Pazarcık and Erkenek segments hosted very few seismic events during the five weeks before the mainshock rupture, in agreement with the low seismicity rates observed during the interseismic period (Fig. 4, see also [21]). Interestingly, co-seismic slip during the mainshock was the largest around this segment (Petersen et al, 2023, in prep.…”

“…Episodic interaction between main and secondary fault branches was previously documented from space-time evolution of seismic clusters 21,36 .…”

“…Inset shows the major tectonic plates and boundaries in this region. Pink dots represent the seismicity during 2007-2019 21 . Fault surface traces are from [22].…”

“…This results in a catalogue of 1,097 possible seismic events with a total of 5,868 P-and 4,960 S-picks. We calculated new event locations by employing the probabilistic location software NLLoc 56 while using the regional, 1-D velocity model from [21]. The search area encompassed a 340 km x 400 km x 82 km volume with its lower left corner positioned at 36.0 N and 35.4 E. Based on the event origin times obtained from NLLoc we visually investigated all events with an origin time difference of less than 5 s for possible duplicate events.…”