# Slow and progressive deformation of the Narlı pull-apart basin prior to the 2023 Mw7.8 Kahramanmaraş, Turkey earthquake

H. Karabulut, M. Bouchon, Marie-Paule Bouin, A. Ö. Konca, J. Schmittbuhl, S. E. Güvercin

► To cite this version:

H. Karabulut, M. Bouchon, Marie-Paule Bouin, A. Ö. Konca, J. Schmittbuhl, et al. Slow and progressive deformation of the Narlı pull-apart basin prior to the 2023 Mw7.8 Kahramanmaraş, Turkey earthquake. Geophysical Journal International, 2025, 241, pp.1679-1696. 10.1093/gji/ggaf108. insu-05080888

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Submitted on 23 May 2025

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# Slow and progressive deformation of the Narlı pull-apart basin prior to the 2023 M<sub>w</sub> 7.8 Kahramanmaraş, Turkey earthquake

H. Karabulut,<fnref n="1" /> M. Bouchon,<fnref n="2" /> M.-P. Bouin,<fnref n="3" /> A.Ö. Konca,<fnref n="1" /> J. Schmittbuhl<fnref n="4" /> and S.E. Güvercin<fnref n="5" />

Accepted 2025 March 12. Received 2025 February 10; in original form 2024 October 21

## SUMMARY

The ~300 km-long rupture of the 2023 February 6 Kahramanmaraş earthquake began in the Narlı section of the Karasu trough, a pull-apart basin sandwiched and sheared between the two major strike-slip faults of the region, the East Anatolian Fault (EAF) on the west and the Dead Sea Fault (DSF) on the east. Rupture started where the northern segment of the DSF enters the Narlı Basin with M<sub>w</sub> 7.0 sub-event and propagated across the basin before making its junction with the EAF. In the seven months preceding the earthquake this basin was the seat of anomalous seismic activity. This activity occurred in bursts interweaved with periods of quiescence. It started near-concomitantly in two clusters located on the opposite edges of the pull-apart basin ~20 km apart. The organization of this seismicity into families of numerous repeating earthquakes suggests an aseismic process linked to fault healing and rapid reloading in a critically stressed zone. By December 2022, two months before the earthquake, activity had migrated to a cluster located along the path that rupture was to follow during the initial stage of the earthquake. These observations show that the pull-apart basin where rupture started was progressively deforming in a succession of bursts before the earthquake. This regional-scale deformation is closely linked with the transitional nature of geodynamics and kinematics influenced by large-scale fault interactions in the surrounding area. The location of the epicentre near the northern termination of the rupture of the 1822 M7.4 earthquake suggests that the ~45 km long Narlı sub-rupture which constituted the first stage of this giant earthquake was closing a long-present seismic gap between the DSF and the EAF.

**Key words:** Earthquake interaction, forecasting, and prediction; Seismicity and tectonics; Fractures, faults, and high strain deformation zones; Transform faults.

## INTRODUCTION

After a long silence during the 20th century, several earthquakes with magnitudes larger than 6 occurred along the left-lateral East Anatolian Fault (EAF), including the 1971 M<sub>w</sub> 6.7 Bingöl earthquake, the 2010 M<sub>w</sub> 6.1 Kovancılar Earthquake and the 2020 M<sub>w</sub> 6.8 Sivrice Earthquake (Fig. 1). On 2023 February 6, two major earthquakes with magnitudes of M<sub>w</sub> 7.8 and M<sub>w</sub> 7.6 ruptured a significant portion of the EAF within a complex plate boundary zone where the Arabian, Nubian plates converge towards the Anatolian block (Fig. 1). The M<sub>w</sub> 7.8 earthquake originated at the northern end of the 1822 M<sub>w</sub> 7.4 rupture (Ambraseys 1989) in the Narlı Basin, located to the north of the Karasu Valley where the Dead Sea Fault (DSF) intersects with the EAF system. The M<sub>w</sub> 7.8 event, initiating with a

magnitude of M<sub>w</sub> 7.0 sub-event, propagated toward the EAF, resulting in a bilateral rupture extending over 300 km (Karabulut et al. 2023).

The seismic activity prior to the 2023 doublet indicates that the ruptured sections of the EAF and nearby faults were relatively quiet (Güvercin et al. 2022). However, following a gradual increase in seismic activity after the 2020 Sivrice earthquake (Konca et al., 2021), anomalous seismicity was observed in the Narlı Basin, where the rupture initiated (Karabulut et al. 2023; Kwiatek et al. 2023). Despite the region being closely monitored by seismic and GNSS stations (Ergintav et al. 2024), no slow slip events or seismic tremors were detected related to the incremental seismic activity in the Narlı Basin. The increase in seismicity has been interpreted as evidence of a large-scale preparatory phase for the M<sub>w</sub> 7.8 earthquake (Kwiatek

<footnote n="1">¹Boğaziçi University, Kandilli Observatory and Earthquake Research Institute, Department of Geophysics, 81684 Çengelkoy, Istanbul, Turkey</footnote>

<footnote n="2">²Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, ISTERRE, 38058 Grenoble, France</footnote>

<footnote n="3">³Institut de Physique du Globe de Paris, 75238, France</footnote>

<footnote n="4">⁴EOST, CNRS/University of Strasbourg, 67000 Strasbourg, France</footnote>

<footnote n="5">⁵Department of Earth Sciences, University of Southern California, 90089, USA E-mail: sezim.guvercin@gmail.com</footnote>
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**Figure 1.** Tectonic setting of the east Anatolia and northeastern Mediterranean region Faults are represented by grey lines (Emre *et al.* 2013), while the suture zones are marked by black saw-toothed lines. Green lines indicate the surface ruptures from the 1822 M 7.4 earthquake (Karabacak *et al.* 2023). The surface ruptures of the *M*<sub>w</sub>7.8 and *M*w7.6 earthquakes are shown as thick red and orange lines, respectively. The epicentres of two events are marked by green stars. GNSS annual velocities are sourced from Özbey et al. (2021). Blue arrowheads indicate sites integrated from previous studies, while red arrowheads denote data from Özbey et al. (2021). GNSS velocities are referenced to station MARA, marked by a red filled circle. The scale of the GNSS velocities is shown on the lower right corner (5 cm yr<sup>-1</sup>). The rupture extents of earthquakes along the EAF over the past 50 yr are also displayed (thick magenta and black lines). The Nubian plate is highlighted in dark blue. Abbreviations: EGB (El-Gharb Basin), KB (Karasu Basin), SF (Sürgü Fault), CF (Çardak Fault), EFZ (Engizek Fault Zone).

*et al.* 2023). Although the spatial and temporal evolution of these clusters has been known since the *M*<sub>w</sub>7.8 rupture, a more detailed analysis is necessary to gain deeper insight into the relationship between the fault stress state and regional deformation patterns, as well as the temporal and spatial progression of seismic clusters followed by the rupture initiation.

In this study, we analyse the long-term activity and the prepara-
tory processes of the 2023 M<sub>w</sub>7.8 earthquake in the Narlı Basin. We
first present a geodynamic and tectonic framework for the Karasu
Basin, encompassing the Narlı Basin. We demonstrate that, prior
to the earthquake, the stress accumulation in the Narlı Basin, sand-
wiched between the EAF and DSF, was evident through the small-
scale seismic clusters. Finally, we explore possible factors that con-
tributed to the critically stressed state of the Narlı Basin before the
rupture began.

Geodynamic and tectonic setting

The Aegean-Anatolian Domain is located in the convergence zone of the African, Arabian and Eurasian plates (McKenzie 1972). Since the late Cretaceous period, the subduction of the Neo-Tethys Ocean beneath the Pontide arc and the accretion of terranes derived from Gondwana have contributed to the formation of enclosed Mediterranean basins, influencing the development of Mediterranean subduction zones (Şengör & Yılmaz 1981). The Anatolian crust, containing these inherited accreted terranes, possesses weakness zones that may have developed into faults of varying scales, activated by regionally favourable deformation (Şengör et al. 1985). Transition from subduction to continental collision in east Anatolia and segmentation of the subduction in the west of Anatolia have notably impacted the thermal and mechanical state of the Anatolian lithosphere (Biryol et., al. 2011). The thinning of the mantle lithosphere beneath the Anatolian crust has led to a mechanical behaviour (Şengör et al. 2003; Kind et al. 2015), characterized by a brittle upper crust and a more ductile lower crust (Karabulut et al. 2019). The regional
deformation is overall driven by the convergence of Arabian plate and the rollback of the Aegean subduction.

The strain localization in the Anatolian domain is controlled by interactions of the plates along the boundaries involving Arabia, Africa and Eurasia (McKenzie 1970; McKenzie 1972; Şengör 1979; Şengör et al. 1985). These plate boundaries are typically recognized by sharp lithospheric-scale velocity contrasts in regional tomographic images (Cambaz & Karabulut 2010; Delph et al. 2015) and crustal thickness variations (Karabulut et al. 2019). However, such contrasts are often less visible across faults within the seismo-genic zones of the crust due to the complexity of suture zones where active faults form (Güvercin 2023). On the other hand, Pn velocity perturbations, as shown in Fig. 2, reveal a distinct contrast between Arabia and Anatolia in the uppermost mantle at the convergence of the Arabia, Africa and Anatolia plates, effectively characterizing the convergent plate boundary (Mutlu & Karabulut 2011). The GPS velocities relative to the station at Kahramanmaraş (Özbey et al., 2021), (represented by the red filled circle in Fig. 1), illustrate the relative plate motions of the Anatolia, Arabia and Cyprus regions around the proposed triple junction (Karig & Kozlu 1990; Duman & Emre 2013). Within this convergence zone, the EAF exhibits a slightly different orientation (Fig. 1), manifesting as a broad surface feature extending from the northeast to the southwest. This zone widens, in the southwest, where surface morphology is characterized by major fault traces running nearly parallel to the EAF, these are identified as the Sürgü, Engizek and Çardak zones (Karig & Kozlu 1990; Duman & Emre 2013). Moving further southwest, the zone sharply turns southward, connecting this poorly localized zone to the Cyprus subduction zone. The GPS velocities between the ruptured faults (solid red lines in Fig. 1) and the southwestern extension of the convergence boundary exhibit a transitional character compared to those of Anatolia and Arabia.

The transition to the Cyprus subduction zone exhibits complex geometry and kinematics near the Kahramanmaraş Triple Junction (KMTJ), where the Arabian, African (Nubian) and Anatolian plates converge. The relative motion between the African and Arabian plates is accommodated by the left-lateral DSF, a ~1000 km long fault zone extending from the Red Sea spreading centre to the KMTJ, characterized by reduced Pn velocities (Fig. 2). These low upper mantle velocities suggest elevated temperatures associated with the rifting processes that led to the development of the DSF. The northern extension of the rifting into the Karasu basin, which links the EAF and the El-Gharb Rift (Mart et al., 2005), is evident from the low Pn velocities shown in the Fig. 2.

The interactions between the El-Gharb Rift (Fig. 1) and the Karasu Basin with the Cyprus subduction zone are complex and cannot be fully understood through surface deformation patterns and kinematics constrained by GNSS observations alone. This complexity arises from the present state of the subducting Nubian plate. The inset in Fig. 2 illustrates the *P* wave velocity perturbations at a depth of 80 km relative to the AK135 velocity model (Karabulut et al. 2019). The eastern end of Cyprus marks a transition from the segmented Aegean slab to the Arabian lithosphere. The velocity perturbations in the inset of Fig. 2 as well as Pn velocity perturbations indicate that the eastern edge of the Cyprus slab has created a low-velocity zone originating from the asthenosphere of the Nubian plate. This low-velocity zone now occupies the boundary between the Nubian plate and the Arabian lithosphere, separated by the DSF, extending from the Dead Sea Rift to the northern termination of the Karasu basin, where it intersects the EAF. Consequently, a ~100 km long northern section of the plate boundaries between Nubia and Arabia, which hosts the DSF, exhibits different deformation patterns

from its ~1000 km continuation. The widespread extensional strain determined from GPS data in this zone (Özbey et al., 2021) also suggest a thin lithosphere, and high heat flow and low lithospheric strength in this portion of the Nubian plate east of Cyprus.

The thermal weakening and fragmented nature of the Anatolian crust is essential for understanding its seismic response during large ruptures and active seismic clusters throughout Anatolia. The most striking insight into how the crust responds during a large earthquake came from the recent 2023 $M_w$ 7.8 rupture. GNSS data from this earthquake reveal far-reaching deformations across Anatolia, extending over 800 km from the rupture zone, which are not accounted for by elastic rebound models (Ergintav et al. 2024). These observations indicate that nearly the entire Anatolian plate shifted westward by approximately 10 mm during the co-seismic phase, equivalent to half of its annual displacement from relative plate motion. This westward movement continued during the post-seismic period (Ergintav et al. 2024).

The fragmented and thermally weakened nature of the crust is also reflected in seismicity patterns across Anatolia during the instrumental seismicity period (Karabulut et al. 2022). Persistent seismic clusters indicate localized stress concentrations within the crust's weaker zones, with their timing and distribution occasionally synchronized. These clusters, which can last over a decade, are followed by periods of quiescence, suggesting that local seismicity is influenced by large-scale regional deformation (Karabulut et al. 2022).

## Karasu Rift–Narlı Basin

Despite the differences in geologic processes, the Karasu basin (Fig. 3) shares key characteristics with the Dead Sea Rift, including the same plate boundary fault (DSF) and extensive Quaternary volcanism (Rojay et al. 2001). The western boundary of this approximately 160 km long and narrow (~15–20 km wide) depression is formed by the Amanos Fault. The Karasu Basin surrounded by elevated mountain ranges broadens into a ~30 km wide rhomboidal shaped structure in the north, where it connects to the EAF, containing the Saglık basin to the west and the Narlı basin to the east, starting at the terminus of the surface trace of the Yesemek Fault (Fig. 3). These two left-lateral strike-slip faults overlap with each other with a left-stepover along the Karasu Valley (Duman & Emre 2013). Although the tectonic role of the Yesemek Fault has been debated, recent geological field studies (Duman & Emre 2013; Karabacak et al. 2023) and past seismological evidence (Ambraseys 1989; Ambraseys & Jackson 1998) strongly suggest that it represents the northern segment of the DSF. The Yesemek segment, along with its northward extension through the Narlı Fault Zone (Fig. 3, inset a), defines the eastern boundary of the Karasu Basin, where the Amanos, Pazarcık segments of EAF and DSF converge in a region bordered by the Arabian, Anatolian and Nubian plates (Fig. 3).

Along the Karasu Basin, slip partitioning between the DSF and Amanos Fault (AF) is approximately even, indicating that both faults almost equally accommodate the lateral motions of the African and Arabian plates (Duman & Emre 2013). The combined slip rate in this region, as inferred from GPS data, is around $6.8 \text{ mm yr}^{-1}$ (Reilinger et al. 2006), a value consistent with radiometric dating and offset measurements along the Amanos and Yesemek faults (Westaway 2003; Seyrek et al. 2007; Duman & Emre 2013).

The region under consideration, and indeed all southeastern Turkey, has been seismically quiet throughout the 20th century. As documented by Ambraseys (1989) in his study of historical
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Regional tectonic features and tomographic images of the Cyprus subduction and Suture zones. Pn velocity perturbations from 8 km s⁻¹ is overlain on the topography. The thick black solid lines with black triangles show the suture zones. The grey solid lines are the fault traces from Emre *et al.* (2013). The inset figure shows the teleseismic tomographic velocity perturbations from AK135 velocity model at 80 km depth (Karabulut *et al.* 2019). The yellow triangles show neogene-quaternary volcanics.</figcaption></figure>

earthquakes in the area, this extended period of quiescence dates back to 1822, when the last major earthquake occurred. With an estimated magnitude of at least 7.4, the 1822 earthquake stands as the largest earthquake to hit the region in the last five centuries (Ambraseys 1989). According to Ambraseys: ‘*The shock almost entirely destroyed the area between Gaziantep and Antakya in Turkey, and Aleppo and Han Sheikhun in northwest Syria, killing a large number of people.*’ While this earthquake has often been attributed to the Amanos segment of the EAF, Ambraseys’ estimated location (centroid/epicentre near 36.7°N 36.9°E), based on the geographical extent of the observed damage, suggests that it likely occurred on the Yesemek Fault. This view is further supported by Duman & Emre (2013), who noted: ‘*Fresh fault-related morphology is more apparent along the Yesemek Fault than the Amanos Fault*’ and concluded: ‘*We suggest that the 1822 earthquake might have occurred on the Yesemek Fault. If so, the last earthquake on the Amanos Fault would have been the 521 event.*’

Fig. 4 presents a map of seismic activity in the region since 2000, a period marked by improvements in the seismic network following the devastating 1999 Marmara earthquakes. The map reveals that most seismic activity occurs within the diffuse Narlı Fault Zone (NFZ), which stretches from the future epicentre of the 2023 M<sub>w</sub> 7.8 event to the junction where the DSF segment meets the EAF. Notably, the seismic activity shown in Fig. 4 does not align

with the segments of the 2023 M<sub>w</sub> 7.8 rupture, except for the 2022 cluster, which stands out as the most significant activity along this fault zone during the 23-yr period analysed.

The relocation of the hypocentre reveals that the rupture of the M7.8 Kahramanmaraş earthquake began not on what was previously considered a secondary splay fault of the NFZ (Fig. 3, Inset a), but on the northernmost segment of the 1000 km-long DSF. These findings, along with the epicentre of the M<sub>w</sub> 7.0 sub-event, suggest that the rupture initiated near the northern termination of the 1822 earthquake that ruptured the Yesemek Fault (Fig. 3). Despite the two-century gap between the events, this interval seems consistent with the low slip rate on this segment (~2–3 mm yr<sup>-1</sup>), which would require a long period to accumulate sufficient stress for a significant slip. This also explains why the 2023 earthquake rupture did not propagate southward along the Yesemek Fault, where approximately four centuries of strain accumulation since the last rupture in 1822 would have been needed to produce a metre of slip. Additionally, it clarifies the complete rupture of the Amanos segment, which, according to Ambraseys (1989), had not broken since 521. The average slip observed on this segment in 2023, around 4 m (Karabulut et al. 2023), matches the expected value for a ~3 mm yr<sup>-1</sup> slip rate over the past 1500 yr.

The rupture geometry of the M<sub>w</sub> 7.0 sub-event, which filled a ~45 km seismic gap between the Yesemek segment of the DSF
<figure><img src="image_4.png" /><figcaption>**Figure 3.** The tectonic setting of the region under study is depicted with shaded topography (http://dds.cr.usgs.gov/srtm/version21/SRTM30/). Thick black lines represent the surface ruptures from the 2023 February 6, $M_w$ 7.8 Kahramanmaraş earthquake (MTA 2023, http://yerbilimleri.mta.gov.tr/anasayfa.aspx). The red circles show the aftershocks of the $M_w$ 7.8 Kahramanmaraş earthquake and thick dark red circle marks the epicentre of the main shock (Güvercin 2024). Green lines indicate the surface ruptures from the 1822 M 7.4 earthquake (Karabacak et al. 2023). Blue triangles and circles indicate the locations of broad-band seismic stations used for event detection, with the blue circle showing the KHMR seismic station used for template matching and spectral analysis. The inset (a) provides a zoomed view of the topography of the northern Karasu valley. The inset (b) provides a zoomed view (region with the large gray rectangle on the topographic map) of the northern Karasu valley with the aftershock seismicity. The thick black dashed line show the surface projection of the $M_w$ 7.0 sub-rupture. The inset (c) shows a zoomed view (region with the small grey rectangle on the topographic map) of the epicentral region of the $M_w$ 7.0 sub-event. The blue arrow indicates step over observed from the aftershock seismicity. Thin black contour in the inset figures show 650 m elevation. Dark grey areas show the elevations greater than 650 m and white regions less than 650 m. Green line indicates the northern segment of the surface ruptures from the 1822 M 7.4 earthquake. Abbreviations: HA: Hellenic Arc, CA: Cyprus Arc, DSFZ: Dead Sea Fault Zone, MTJ: Kahramanmaraş Triple Junction, MS: Mediterranean Sea.</figcaption></figure>

and the EAF, follows a straight path from the epicentre to the junction with the EAF (Fig. 3). The aftershock zone of this sub-event, extending from the epicentre (marked by a dark red circle) to the EAF junction, outlines a NE-SW oriented plane dipping westward at approximately 80 degrees (Fig. 3 Inset b, black dashed line). This plane crosses the Narlı Basin and borders the complex NFZ to the west. The epicentre of the $M_w$ 7.0 sub-event, located at the basin's entrance near the step-over between the Yesemek segment and the NFZ, highlights the faulting complexity that is likely mechanically significant. Therefore, the kinematics of the Narlı basin requires a more detailed analysis, as this relatively small region provides a crucial setting to understand the interactions of three major faults/three plates.

The geometry of the Narlı Basin is influenced by the northward motion of the Arabian Plate and the westward escape of the Anatolian Plate (Fig. 1). The basin is confined in the west by the rigid Amanos block, while in the east, it is bordered by a relatively wide deformation zone of the Arabian plate. The oblique orientation of the EAF relative to the Amanos block and its intersection

with the DSF causes the basin to expand northeastward, resulting in a rhomboidal-shaped basin structure. This kinematic process is illustrated in Fig. 6 with simplified segments outlining the basin, inferred from the fault network (represented by red dashed lines). The basin is bordered by the EAF to the north and the Amanos Fault to the west, both characterized by distinct morphological features. To the east, the basin is confined by two nearly orthogonal linear segments. The NE-SW oriented segment, approximately 15 km long, starts at the epicentre of the $M_w$ 7.0 sub-event, near where the 1822 rupture nearly terminated, and extends to the NNE-SSW oriented segment, which is characterized by a sharp topographic expression. Notably, the epicentre of the $M_w$ 7.0 sub-event was situated at a kink in the basin, where aftershock seismicity indicates a small stepover structure to the south (blue arrow in Fig. 3, inset c). The NNE-SSW oriented eastern boundary of the basin connects to the East Anatolian Fault within a diffuse zone of the Arabian Plate.

As a result of this fault network, the deformation has developed asymmetrically in the basin: thicker sedimentation occurs in the west along the margin of the Amanos Fault, while it is shallower in
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Seismicity from AFAD catalogue (http://tdvm.afad.gov.tr) between 2000 January 1 and 2023 February 6, in and around the Narlı basin and Karasu rift. The colours show the temporal evolution of seismicity. The events occurred during day time (06 hr–19:00 hr) are removed to eliminate quary blasts (see also Fig. S1 in Supplementary materials). Thick red and black lines indicate the surface breaks of the 2023 February 6, M<sub>w</sub> 7.8 Kahramanmaraş earthquake (MTA, 2023, http://yerbilimleri.mta.gov.tr/anasayfa.aspx). Green lines indicate the surface ruptures from the 1822 M 7.4 earthquake (Karabacak et al. 2023). Thin black contour in the inset figures show 650 m elevation. Dark grey areas in inset figures show the elevations greater than 650 m and white regions less than 650 m. C0–C3 are the zones of seismic clusters activated prior to M<sub>w</sub>7.8 earthquake. The dark red circle shows the epicentre of the M<sub>w</sub>7.0 sub-event.</figcaption></figure>

the east (Karataş & Boulton 2019). Additionally, rotated fault blocks (Tatar et. al. 2004), secondary faults and fault splays (Emre et al. 2013) are present in the eastern part of the basin. The M<sub>w</sub>7.0 sub-rupture in 2023 follows a remarkably straight path across the basin, despite the complexities observed at the surface. Overall, the Narlı Basin is a dynamic tectonic environment shaped by the interaction of strike-slip faults, leading to extensional processes along the EAF. This setting results in significant geological and seismic activity, contributing to the basin's ongoing evolution.

## Geology of the northern Karasu Valley-Narlı basin

The stratigraphic units of northern Karasu region can be categorized under paleotectonic and neotectonic periods. The paleotectonic period, spanning from the Late Cretaceous to the Miocene, is associated with the closure of the northward-subducting southern Tethys, corresponding to the closure of the Bitlis-Zagros Ocean (Şengör & Yılmaz 1981). The onset of the neotectonic period in the region is marked by the Late Miocene propagation of the Dead Sea Fault and its linkage with the East Anatolian Fault zone (Hempton 1987).

In the west of the Karasu valley, the Amanos Mountains, comprise rock units ranging from Precambrian to Eocene in age, while the mountain range in the southeast consists of Cretaceous to Miocene rock units (Rojay et al. 2001). The Karasu Rift formed on a pre-Pliocene basement composed of two primary rock series: Paleozoic
<figure><img src="image_6.png" /><figcaption>**Figure 5.** A simplified geology map of northern Karasu valley (modified from 1/250 000 scale geological map of MTA). The black solid lines are the fault traces from Emre *et al.* (2013). The surface projection of the M<sub>w</sub> 7.0 sub-rupture is indicated by black dashed lines.</figcaption></figure>

crustal units associated with a Mesozoic allochthonous ophiolitic complex and over 1000-m-thick, folded Upper Miocene–Lower Pliocene sequence (Tekeli *et al.* 1983). Around 2 Ma, a shift in the local tectonic regime occurred, transitioning from regional N–S compression to more recent E–W extension, as evidenced by volcanism, sedimentation and faulting (Tekeli *et al.* 1983; Çapan *et al.* 1987; Perincek & Çemen 1990; Yu’ru’r & Chorowicz 1998).

Quaternary volcanic activity was concentrated along regional faults such as the DSF and AF, particularly at intersections where internal cross-basin faults connect with these continental transform faults (Tatar *et al.* 2004) (Fig. 5). These volcanic fields comprise volcanic cones, lava flows and ash-fall deposits (e.g. Parlak *et al.* 1997, 1998; Alıcı *et al.* 2001; Rojay *et al.* 2001; Yurtmen *et al.* 2000; Kavak *et al.* 2009). The volcanic outcrops are observed above the Quaternary alluvial cover and occasionally in contact with dolomitic limestones (Fig. 5).

Petrographic and geochemical studies suggest that the Karasu volcanic rocks originated from the asthenospheric mantle, with varying degrees of crustal contamination (Çapan *et al.* 1987; Rojay *et al.* 2001; Yurtmen *et al.* 2000). It has been proposed that tensional forces along the main transform fault induced decompression of the lithospheric mantle at greater depths, creating pathways for basaltic eruptions (Çapan *et al.* 1987; Yurtmen *et al.* 2000). Deep fractures, such as those associated with the Narlı strike-slip fault zone, and the complex block movements within the Karasu Valley (Tatar *et al.* 2004), facilitated the extension necessary to generate magma and transport it to the surface (Yu’ru’r & Chorowicz 1998).

## Seismicity and focal mechanisms in the Narlı basin

The temporal evolution of seismic activity within a 200 km radius from the epicentre of the Kahramanmaraş earthquake is shown for
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Topographic map of the northern Karasu Basin is depicted along with the surface traces of the major faults, shown as thick black lines. The surface projection of the $M_w$ 7.0 sub-rupture is indicated by light dashed red dashed lines. Thick dashed red lines represent a simplified fault network associated with the regional faults, highlighting active force systems (red and blue arrows) driving basin evolution. Blue beach balls illustrate the earthquake source mechanism solutions for events with $M_w$ 3.2+ from 2008 to 2023 February 6, while the green beach ball represents the source mechanism solution for the $M_w$ 7.0 sub-event. Red and blue triangles mark the locations of two accelerometric stations on either side of the surface-projected rupture. The inset shows the North–South component of displacement recordings at the two accelerometric stations (thick lines) and the Up-Down component of displacement recordings (thin lines).</figcaption></figure>

the five years leading up to the 2023 event (Fig. 7a). The cumulative number of earthquakes generally follows a steady rate, with occasional fluctuations due to the occurrence of two M5+ earthquakes and the 2020 January 24, M6.8 Sivrice earthquake, either within or near the study area. Although the epicentre of the $M_w$ 6.8 Sivrice earthquake lies outside the 200 km radius, these fluctuations, linked to larger events are distant from the zone where the $M_w$ 7.8 earthquake originated, reveal a slight acceleration in seismic activity in the months before the $M_w$ 7.8 earthquake. This increase becomes more pronounced as the focus narrows on the area closer to the future epicentre (Figs 7b–d). A more detailed examination of the zone with the most intense activity shows that this increase occurred in at least three bursts, beginning about 200 d before the 2023 earthquake.

The increased seismicity rate shown in Fig. 7 occurred in the northeastern Narlı Basin, the most seismically active region within the Karasu Basin. More precise locations of seismic activity since 2012, shown in Fig. 6 (details in Fig. S2 of Supplementary Materials), with horizontal uncertainties of less than ~1.5 km, reveal that the seismicity is more localized than depicted in Fig. 4. Instead of

being diffuse, it aligns closely with the surface traces of the faults within the Narlı Basin.

To better understand the tectonic characteristics of the seismic activity, we analyse focal mechanism solutions of earthquakes. In addition to the source mechanisms from Karabulut et al. (2023), we derived solutions for 19 earthquakes using regional waveforms with the Grond program (Heimann et al. 2018), part of the Pyrocko package (Heimann et al. 2017). Grond utilizes regional surface waves and applies a Bayesian bootstrapping approach to produce solutions with associated uncertainties. Further details about the data, methodology and the list of source mechanisms can be found in the Supplementary Material (Section S4).

The source mechanism solutions with $M_w$ 3.2 + since 2012, presented in Fig. 6 indicate predominantly left-lateral strike-slip motion with varying normal components and dip angles, occurring near the basin boundaries. Few of the events inside the basin show nearly N-S oriented normal faulting mechanisms.

The 2012 $M_w$ 5.2 event was the largest in the basin during the instrumental period, with its aftershock distribution defining a NE–SW fault approximately 8 km in length (grey circles in Fig. 8) and
<figure><img src="image_8.png" /><figcaption>**Figure 7.** (a–c): Time evolution of seismic activity at decreasing distance ranges (R) from the epicentre of *M*<sub>w</sub>7.0 earthquake in the 5 yr preceding the earthquake. The occurrence time of the largest earthquakes in the region is indicated with vertical dashed lines. (d) Focusing on the zone at the junction between the epicentre fault and the East Anatolian.</figcaption></figure>

characterized by left-lateral strike-slip mechanisms (Events 12–15 in Fig. 6). The *M*<sub>w</sub>5.2 earthquake related activity occurred along the southern extension of the surface deformations observed after the *M*<sub>w</sub>7.8 rupture in the Narlı Basin (thick black solid lines in Fig. 8). The green hexagons in Fig. 8 represent events triggered by the 2020 *M*<sub>w</sub> 6.7 Sivrice earthquake (~200 km east), with the largest having a magnitude of *M*<sub>w</sub>3.5 (Event 4 in Fig. 6) and lasting about two weeks. This triggered activity did not cluster but was mostly dispersed across the northeastern part of the Narlı Basin. The activity level remained low (represented by magenta hexagons in Fig. 8) for the following two years until 2022 July 10.

Increased seismicity and localized activity started on 2022 July 10, occurring simultaneously at multiple sites within the Narlı Basin. Given the swarm-like nature of this activity, we conducted a cluster analysis to better understand the seismic events from July 10 up until the onset of the *M*<sub>w</sub>7.8 earthquake on 2023 February 6. This analysis involved applying an STA/LTA trigger algorithm and manually selecting the arrival times. The detected events were used as a templates to extend the event catalogue (Fig. S3 in Supplementary Materials).

A total of four clusters was identified based on waveform similarity where 80 per cent correlation with the template event is required to be identified as part of a cluster. The inset in Fig. 8 shows the seismic activity within the first 48 hr after the first event

was detected in Cluster C0. The largest event during this period, with a magnitude of *M*<sub>w</sub>3.2, also occurred in Cluster C0 (Event 16 in Fig. 6). In the following days, the zone of Cluster C1 became the primary active area, evolving over several days before activity began in Cluster C2 on the EAF. Cluster C3 was activated ~30 d after the initiation of the activity in Cluster C1 and continued until about 15 d before the *M*<sub>w</sub> 7.8 earthquake. It is notable that the duration of the seismic activity is longest in C2 (~100 d) and shortest in C3 (~10 d).

Clusters C1 and C3, which cover approximately 5 and 3 km in length, respectively, are oriented NW-SE and are located to the east of the aftershock zone of the 2023 *M*<sub>w</sub>7.8 rupture (light grey zone and red dashed line in Fig. 8). The events in these clusters (C1 and C3) occurred at depths between 8 and 10 km. Cluster C2, situated on the EAF segment near its termination close to the Amanos Fault, displays two distinct clusters of seismic activity. The larger magnitude events in this cluster predominantly occur at depths between 3 and 6 km and align with the surface trace of the EAF in a NE-SW direction. Additionally, a N-S oriented line of smaller magnitude events is observed at depths between 5 and 10 km.

The NW-SE oriented planes of the source mechanism solutions in clusters C1 (Events 26–28) and C3 (Events 1–3) are aligned with the seismicity (Fig. 6). This NW-SE orientation follows a distinct
<figure><img src="image_9.png" /></figure>

**Figure 8.** Relocated seismic activity during the selected time periods in the Narlı Basin is illustrated. A light grey stripe marks the aftershock zone of the $M_w$ 7.0 sub-event. Different symbols and colours are used to show seismic activities for various time periods. The red thick lines depict the surface break of the $M_w$ 7.8 earthquake along the Amanos and Pazarcik faults. Thick green line shows the Yesemek fault. Thick black lines show the surface breaks during the $M_w$ 7.0 sub-rupture in the Narlı Basin. The seismic events detected prior to the 2022 activations are displayed in the figure (black circles, green and purple hexagons). The colour scale represents the time evolution of seismic clusters starting from 2022 July 10. The inset figure, in the upper left corner, highlights the events that occurred during the first 48 hr from the initiation of the seismic activity. The black contour represents the 650 m elevation line. Dark grey areas indicate elevations greater than 650 m, while white regions represent elevations less than 650 m. The surface projection of the $M_w$ 7.0 sub-rupture is indicated by light dashed red dashed lines.

morphological trend that is oblique to the eastern boundary of the basin (thick red dashed line in Fig. 6). In Cluster C2, NE striking left-lateral focal mechanism solutions are consistent with both the alignment of the seismicity and the surface trace of the EAF (Events 5–11 in Fig. 6).

Overall, the seismic activity leading up to the 2023 $M_w$ 7.8 event was not concentrated on the ruptured faults but rather on smaller-scale structures, primarily on the northeastern side of the Narlı Basin. The activation of these events frequently happened almost simultaneously. These nearly synchronized events or clusters occurred at distances much larger than their rupture sizes, resulting in minimal interactions among them.

It is crucial to search for any evidence of deformation at the basin scale through these seismic observations, especially since there is currently no evidence from geodetic monitoring. Several questions arise and need to be further discussed. Why did the rupture begin approximately 25 km south of these clusters? Why did the $M_w$ 7.0 sub-event follow a straight path from the epicentre to the junction of the EAF? What explains the quiescence periods between the activations of the seismic clusters in 2022? Does the increased seismicity

rate in localized clusters provide any clues into the initiation of the catastrophic failure of the fault system, which had been inactive for over 500 yr?

## Seismic clusters prior to $M_w$ 7.8 rupture

Localized seismic clusters often offer insights into active tectonic deformation processes. In the Anatolian region, these clusters typically occur at fault complexities, have extended durations and are influenced by both local and regional tectonic stress perturbations (Karabulut et al. 2022). The distinctive characteristics of these clusters provide evidence on a variety of underlying physical processes. For instance, local clusters with typical main shock–aftershock sequences differ from those in creeping zones. The presence of seismic multiplets within these clusters, along with rupture properties determined from spectral analysis, such as slip and stress drops, provides evidence on the evolution of the fault zones and underlying rheology.

Here, we further examine the nature of three seismic clusters in the Narlı Basin that began around six months before the 2023
earthquake. We utilized continuous recordings from broad-band stations (refer to their locations in Fig. 3) in the region surround-ing the basin to identify similar activations. Both LTA/STA and template detection methods were applied to 10 yr of continu-ous recordings. The event catalogues for the three clusters were used as templates to search for comparable events within these recordings.

While the number of detected events in the three clusters in-creased, similar events were found to be limited to periods starting from 2022 July 10 (Fig. 8). Each cluster in Fig. 8 contains a large magnitude event ($M_w$3.5+), but their activations start with smaller magnitude events. Cluster 1 and Cluster 2 are approximately 25 km apart, whereas Cluster 1 and Cluster 3 are ~3 km apart.

The seismicity rate of Cluster 1 exhibits a rapid decay within about 20 d, after which the activation in Cluster 2 begins with a sharp increase in the rate followed by an $M_w$4.4 event (Fig. 9). There is no evidence of cross-triggering between Cluster 1 and Cluster 2. The second activation in Cluster 1, marked by a magnitude $M_w$4.5 event, begins 100 d after the first activation. Activation in Cluster 3 nearly starts with an $M_w$3.9 event, and a weak cross-triggering between Cluster 1 and Cluster 3 is likely. The activities of both Cluster 1 and Cluster 3 stop nearly simultaneously after 15 d of evolution. It is worth noting that the seismic activity in the clusters starts when the previous seismic activation ceases (inset in Fig. 9).

The three clusters also exhibit variability in the detected wave-forms. We constructed family trees for each cluster based on wave-form similarities, using CC coefficients greater than 0.8. Fig. 10 displays the waveform families identified in each cluster. Clusters 1 and 3 show similar behaviour, each containing eight families of sim-ilar waveforms. In contrast, Cluster 2 does not display any notable sub-clustering.

A notable feature of the activity in Cluster 1 is that it does not follow a classic main shock-aftershock sequence but instead occurs in multiple bursts separated by periods of relative quiescence. Three particularly intense periods of bursts, lasting from several days to weeks, are evident (green curve in Fig. 9). Each burst consists of numerous smaller bursts, often clustered closely in time and last-ing only a few hours. Cluster 3 remains active only during the late stage of Cluster 1 and exhibits high variability in waveform similar-ities. The source mechanism solutions for large magnitude events in Clusters C1 and C3 are similar, and their strike orientations align with the seismicity (Fig. 6). This suggests that both seismic clus-ters are part of the same shear zone, separated by an approximately 3 km long aseismic zone. In contrast, Cluster 2 shows no significant sub-clustering and exhibits characteristics of swarm-type activity. Several events with magnitudes of $M_w$3.5 + occurred in this clus-ter, with the seismicity rate decaying to background levels within ~90 d.

## Spectral analysis

The multiplet families in Clusters 1 and 3 could offer insights into the underlying physical processes of the seismic clusters activated before the rupture in 2023. We evaluate the seismic waveforms in the multiplet families using Madariaga's extended fault model (1976) and Boatwright's spectrum model (1978). According to these models, the source dimension is related to the corner frequency (fc) by the equation $fc = 0.32$ v/a, where a is the radius of the fault, and v is the rupture velocity, assumed to be 0.9 Vs (shear wave velocity) in the source region.

The P wave spectrum is calculated after removing the instrument response and trend from the waveform. A time window is used, starting 0.28 s before and ending 1 s after the onset of the P wave. The signal-to-noise ratio (S/N) is determined using a 5-s window before and after the P wave onset. Events with an S/N greater than 2 are chosen for analysis. The P wave spectrum is then smoothed with a 5-point moving average filter. Seismic moments (M0) were derived from the low-frequency portion of the spectra, with magnitudes within the multiplet family ranging from 1.5 to 3.9. The corner frequency (fc) was determined by fitting the spectra with an $\omega^{-2}$ decayed source spectrum, representative of all events in the multiplet family. The best-fitting spectrum provides a corner frequency of 6.0 Hz and a quality factor (Q) of 400.

We also computed the relative spectra by dividing the spectrum of each event to the smallest magnitude event (~$M_w$1.7) in the multiplet family. The relative spectra provides a better resolution in discriminating the corner frequencies and high frequency decays due to variations in source radius. Fig. 11 displays the relative spec-tra of the largest multiplet family identified in Cluster 3 (Fig. 9). The inset shows the normalized signals from the P time windows used to compute the spectra, with maximum amplitudes varying by more than three orders of magnitude. The relative spectra of the events in the multiplet family are flat to ~6.0 Hz and show a tendency of decreasing higher frequencies with increasing magnitudes. There-for, we separately estimated the corner frequencies of the events with magnitudes greater than $M_w$3.0. The events with magnitudes less than $M_w$3.0 in the multiplet family have similar source sizes (~200 m) but varying stress drops (0.001–5.5 MPa). The corner fre-quency of $M_w$3.9 event was computed as 4.0 Hz and source radius as 320 m (see also Fig. S6 in Supplementary Materials for spectra of events in the Cluster 1).

Two major mechanical parameters—induced slip ($\Delta u$) and static stress drop ($\Delta \sigma$)—are determined from the spectral estimates of seismic moment (M0) and corner frequencies. The formulas used are $\Delta u = M0/(\pi \mu a^2)$, where $\mu$ is the shear modulus (35 GPa), and $\Delta \sigma = 7M0/(16a^3)$, where a is the source radius. The stress drops obtained in the multiplet families of range from 0.001 to 5.5 MPa, and the slips range from 0.01 to 36 cm. The radius for the events with $M_w < 3.0$ is ~200 m, and ~360 m for the event with $M_w$3.9+.

Using the computed seismic moments (M0) and corner frequen-cies (fc), we calculated the cumulative slip history for the multiplet families with $M_w < 3.0$ in Clusters 1 and 3, as shown in Fig. 12 (red and green curves). The cumulative slip during three bursts with magnitudes < $M_w$3.0 exhibits similar jumps of ~15 cm during activations. When the events with $M_w$3.0 + are included, the cu-mulative slip jumps become more pronounced, with the maximum cumulative slip exceeding 1.0 m. A notable feature in Fig. 12 is the decreasing duration of quiescence periods between successive seismic activations as the $M_w$7.0 sub-event approaches. The first quiescence period lasts around 100 d, the second approximately 65 d and the third about 35 d, occurring just before the $M_w$7.0 sub-event, which marks the initial phase of the $M_w$7.8 earthquake.

We acknowledge that the cumulative slip computed for each cluster is assumed to be on to the overlapping asperities. It is crucial to justify the calculation of cumulative slip shown in Fig. 12, as the events may not necessarily originate from the same or overlapping rupture zones. The average uncertainties in horizontal locations are about 1.0 km, which is significantly larger than the rupture size of the events. For the three events in Cluster 3 with magnitudes greater than $M_w$3.5, which have reliable locations with horizontal uncertainties of less than 0.7 km (red stars in Cluster 3 of Fig. 6), we can infer their relative positions. These events occur within a ~2 km
<figure><img src="image_10.png" /><figcaption>**Figure 9.** Cumulative seismicity of three clusters detected using LTA/STA and template matching methods between 2008 and 2023 February 6. The locations of clusters C1, C2 and C3 are shown in Fig. 5. The inset figure provides a zoomed-in view of the cumulative seismicity from 2022 January 1, to 2023 February 6. Vertical dashed lines mark the largest events in each temporal activation.</figcaption></figure>

zone, and they occur at similar depths. The estimated rupture sizes of these events are around 360 m, indicating that their rupture zones may not overlap. Therefore, the cumulative slip curves (blue and black curves) shown in Fig. 12 represent upper bounds considered in our interpretation.

However, events within a multiplet family may present a different scenario. To investigate their relative shifts, we applied time stretching of similar waveforms within a given multiplet. This technique, typically used in coda wave interferometry to detect small changes in wave velocities (Snieder et al. 2002), was repurposed here to compare the relative arrival times of each phase for different events (Bouchon et al. 2011). We calculated the average normalized waveforms in the cluster to serve as the master event. Specifically, we compared the events recorded at station KHMR (refer to Fig. 3 for its location) with the master event by computing the cross-correlation function using a 3-s moving window along both waveforms, after aligning them with the *P* arrival time (Fig. 13). The correlation coefficients between *P* and *S* phases are high (>0.8) and remain nearly constant. We measured time-shifts of the *S* waves relative to the aligned *P* waves and found a shift of ±0.015 s, indicating an approximate ±120 m shift in their positions relative to the master event. This analysis suggests that the source areas of the events within the multiplet family are more likely to overlap. Similar computations for other families in Cluster 1 yielded similar results.

## DISCUSSION

The simplified fault network associated with the major faults bordering the broader northern Karasu Valley (depicted as thick red dashed lines in Fig. 6), along with the fault that ruptured during the 2023 M<sub>w</sub>7.8 event crossing the basin (shown as light red dashed lines in Fig. 6), highlights an active pull-apart structure. The crust's rheology and the forces exerted along the boundaries of

this transtensional basin, driven by regional faults, play a key role in shaping this dynamic environment. In this compact and complex geodynamic setting, the northern part of the Narlı basin experiences greater deformation, as it marks the junction of the EAF and DSF.

Although there is an extensional component south of the Narlı Basin along both the Amanos Fault and the Karasu Valley, the Narlı Basin forms a broader deformation zone compared to the Karasu Basin due to the intersection of the DSF with the EAF and Amanos Fault. In a way, it could be described as an ‘incompatibility basin,’ where extension occurs as a result of an unstable triple junction or fault intersection (Şengör et al. 2019). The NFZ in Narlı Basin is characterized by sub-parallel normal faults dipping westward, separated by relay ramps (Duman & Emre 2013). The nearly east-west orientated tensional axis of the earthquake source mechanism solutions indicates crustal extension in that direction (Fig. 6). These features indicate low normal stress on the fault segments in the Narlı Basin.

The Karasu Basin shares notable geometric and mechanical similarities with the well-studied Dead Sea Basin, located ~500 km to the south along the same Arabian–African plate boundary (Fig. 1). Both basins are elongated, narrow depressions bounded by two strike-slip faults and normal faults. Similar to the Karasu Basin, the Dead Sea Basin is also made up of multiple sub-basins (Smith et al. 2008). This well-documented pull-apart basin helps understand the present observations. The near concomitant start of the seismic activity on the opposite edges of the pull-apart basin (inset in Fig. 8) may be explained by the presence of cross-basin faults which are a characteristic of this type of basin. Citing from Garfunkel & Ben-Avraham (1996) about the Dead Sea Basin: ‘longitudinal strike-slip and normal faults are the most prominent elements controlling the basin structure, but transverse faults that extend across the basin are also important’.
<figure><img src="image_11.png" /><figcaption>**Figure 10.** Temporal occurrences of the detected seismic events and their magnitudes in three clusters (C1: Top, C2: Middle, C3: Bottom). The multiplet families of waveforms with a cross-correlation greater than 0.8 from each cluster are represented in different colours. The inset shows the Gutenberg–Richter plot for the events in each cluster.</figcaption></figure>

Peacock & Anderson (2012) have shown that in pull-apart basins fluids can be channellized efficiently through the fractures and opening fissures present in the rock mass because of the reduction in compressive stresses in these tectonic environments. Citing from their theoretical investigation of pull-apart basins’ hydraulic properties: ‘*Fluid flow through a pull-apart may be closer to that of flow through a pipe*’. And ‘*Pull-aparts would provide easy migration pathways.*’

The burst-like activity of the clusters and the migration of seismicity observed show that fluids were present and circulating in the basin during the period of activity.

The seismic clusters, which started at 2022 July 10, is an accelerated response of the deformation in the northern part of Narli Basin. The synchronization of the seismic clusters, progressively
<figure><img src="image_12.png" /><figcaption>**Figure 11.** Relative P-wave spectra of the largest multiplet family of waveforms ($M_w$ < 3.9) with a cross-correlation greater than 0.8 from cluster 3. A time window of 0.28 s before and 1.0 s after the P-wave onset was used to compute the spectra. The inset figure on the left shows the logarithmic time (minute) of the occurrence of the events relative to the first event in the family and the inset figure on the right displays the velocity waveforms of the P waves, normalized by their maximums around 0.55 s. The colour bar indicates the magnitude of the events.</figcaption></figure>

developing from small to large scale, at distances with no signif-
icant stress interactions, indicates a critically stressed zone at its
final stage of the catastrophic failure started almost a decade ago
from the easternmost extent of the EAF (Fig. 1).

The initiation of seismic swarms is influenced by aseismic slip and diffusion of high-pressure fluids (Ross et al. 2020). Studies have shown that the co-seismic response of the crust to moderate to large earthquakes creates high-permeability structures, which serve as pathways for fluid diffusion (Miller et al. 2004; Miller 2013; Ross et al. 2020). The enhanced permeability enables the upward migration of high-pressure fluids from crustal or mantle degassing, linking these processes to the initiation of seismic swarms. Post-seismic geochemical evidence, such as the infiltration of mantle-derived CO₂ into aquifers in seismogenic regions, further substantiates this connection (Frondini et al. 2019; Chiodini et al. 2020). Decarbonation caused by frictional heating in the slip zones of carbonate-hosted faults has been proposed as a likely mechanism for the release of high-pressure fluids like CO₂ during earthquakes (e.g. Chiodini et al. 2020 and references therein).

Volcanic activity in the Karasu basin, interacting with the con-
tinental faults, such as AF and DSF, creates pathways for fluids to
ascend into the seismogenic crust. The analysis of the geochemical

fluids circulating in the Amik Basin of the Karasu valley show that regional degassing in this area occurs due to deep mantle-derived volatiles reaching the surface through fracture networks within the active DSF (Yüce *et al.* 2014). Groundwater throughout the region consistently shows evidence of deep-sourced fluids of either crustal or mantle origin.

The escalation of seismic activity in the Narlı Basin preceding the $M_w$ 7.8 earthquake can be attributed to increased upper crust permeability, driven by regional stress changes that originated nearly a decade ago at the eastern terminus of the East Anatolian Fault. Extended and abundant aftershock sequences are expected in extensional pull-apart regions and dilational zones, as these areas lack the tectonic mechanisms required to close fracture networks (Guznatilake & Miller 2022).

The occurrence of the repeating events in two of the seismic clusters (C1, C3) in the northern proximity of the fault that ruptured with $M_w$ 7.0 sub-event, points to an aseismic process, which can be considered as an emerging physical response of a large-scale deformation. The seismic repeaters, which help to understand spatial and temporal behaviours of a fault, suggests rapid healing of ruptured
<figure><img src="image_13.png" /><figcaption>**Figure 12.** Bottom: Magnitudes of detected events in Clusters 1 and 3 from 2022 July 10 to 2023 February 2. The circles show the events of the Cluster 1 and the crosses for the Cluster 3. The magnitudes greater than $M_w$3.0 are shown with different colours in each case. Top: Cumulative slip for the multiplet families shown in Fig. 9, calculated from each event using Madariaga's extended model (1976). The red and green curves show the cumulative slip for the events with $M_w < 3.0$ for Cluster 1 and Cluster 3, respectively. The blue and black curves represent the total cumulative slip when events with magnitudes greater than 3.0 are included. The red star shows the time of the $M_w$7.0 sub-rupture.</figcaption></figure>

asperities in a deformation zone. This implies a fast reloading process, enabling the asperities to regain strength more quickly than the rate of tectonic loading.

One question arising from this observation is whether the observed seismic activity related to a possible aseismic process influences the weakening of the fault, contributing to the Narlı rupture. The seismicity and focal mechanism solutions shown in Fig. 6 suggest that clusters 1 and 3 are situated within a zone approximately 10 km long, oriented about 35 degrees relative to the segment of $M_w$7.0 sub-rupture. This 10 km zone may be continuously creeping, with seismic activations sampling the creep at various asperities within the zone. As a result, the accumulated slip exceeding ~1.0 m in each cluster is represent the total amount of fault creep (Fig. 12). This value is smaller than the horizontal displacement of ~2.5 m observed at the SM stations during the $M_w$7.0 sub-rupture (inset in Fig. 6). The three distinct periods of activity in clusters 1 and 3, with decreasing quiescence periods (100, 65 and 30 d) leading up to the $M_w$7.0 sub-rupture, may indicate an accelerated response of the aseismic deformation.

The synchronization of seismic events may be the evidence of fluid migration and pore pressure changes throughout the fault system in the basin. The information we have from the seismicity prior to the main shock, is that by December 2022/January 2023 the seismic activity had reached the fault segment which was about to break. During these periods of quiescence, fluids may have accumulated within the fault zones, gradually increasing pore pressure until critical thresholds were reached, triggering the next cluster of seismic activity. Details of what happened after may only be guessed. One possible scenario is that the combination of the basin extension and the arrival of fluids made this fault segment ripe for rupture. Another possibility is that once the fault was reached, it provided a direct path for fluids to the epicentre.

The seismic clusters are located at the transition zone between high and low Pn velocities (Fig. S7). The low Pn velocities, associated with elevated temperatures and Quaternary volcanism in the Narlı Basin, suggest an active physical system facilitating crust-mantle fluid interactions. Additionally, the nearly simultaneous activation of seismic swarms on two distinct fault branches (Amanos and Dead Sea Faults), located about 40 km apart, suggests that the
<figure><img src="image_14.png" /><figcaption>**Figure 13.** (a) Cross-correlation coefficients of sliding window (3 s) between the master event and the vertical component waveforms from a multiplet family in the cluster 3 observed at KHMR station (see Fig. 3 for station location). The average of the normalized waveforms is selected as master event. Inset figure shows the time occurrences of the events relative to the first event. (b) Time-shifts between two waveforms for *S* wave. (c) Superimposition of the waveforms after normalization with the maximum amplitude and alignment with *P* waves.</figcaption></figure>

fluids may originate from a common reservoir at the crust–mantle boundary.

The final question to address is why the rupture initiated approximately 25 km south of cluster 3. The epicentre of the *M*<sub>w</sub> 7.0 sub-event is near the northern end of the 1822 event (Karabacak *et al.* 2023). Consequently, the *M*<sub>w</sub> 7.0 sub-rupture closed the seismic gap along the DSF, extending from the end of the 1822 rupture to the EAF junction. Notably, the *M*<sub>w</sub> 7.0 epicentre is located where the Narlı basin significantly widens east–west from the 15–20 km wide Karasu Valley. This geometric expansion of the basin boundaries affects the kinematics driving the basin’s evolution, making this area a significant zone for rupture initiation or termination.

The contrast between this complex and diffuse faulting zone spread along the eastern margin of the basin and the straight and direct short-cut path that rupture was to follow inside the basin may provide a clue. The tendency of large earthquake ruptures

encountering pull-apart basins is to bypass the complex faulting of the basin margin to cut a direct path through the floor of the basin (Armijo *et al.* 2005; Kuşcu 2009; Komura & Sugimoto 2021; Güvercin 2024). The largest earthquake prior of *M*<sub>w</sub> 7.0 sub-event was the 2012 *M*<sub>w</sub> 5.2 event which revealed ~8 km of rupture zone. The observed localized seismic activities since 2000 in the basin do not indicate a long and continuous fracture zone even though a long surface deformation was mapped following the *M*<sub>w</sub> 7.8 earthquake (thick black line in Fig. 8). Furthermore, the absence of aftershock seismicity within the NFZ indicates that the deformation there is superficial and likely aseismic (Fig. 3, inset b). Therefore, the NFZ did not provide the right environment for allowing rupture to grow.
## CONCLUSIONS

The $M_w$ 7.0 rupture in the northern Karasu Basin was likely triggered by a combination of tectonic, geometric and fluid-driven factors, particularly at the termination point of the 1822 event. Seismic clustering and decreasing quiescence periods prior to the event reflect an accelerated deformation process within the basin. The typical ‘nu- cleation’ processes (late repeaters) observed at cluster C1 and C3 are rather remote from the epicentre zone, suggesting an initiation to the fault instability from the reduction of a strong barrier along the fault at the north of the basin that was responsible for its late strength. The $M_w$ 7.0 sub-rupture likely initiated at a highly proba- ble location in the basin where the 1822 event terminated, probably due to a significant geometric barrier and high stress concentration. The timing of the event aligns with the observed seismic clustering and decreasing quiescence periods in the northern clusters, which suggest large-scale and synchronous deformation of the basin.

Despite complex faulting and ongoing deformation in the months preceding the earthquake, the rupture occurred along a direct path through the basin, bypassing the more complex faulting along the basin margin. The presence of fluids, high heat flow and fault zone characteristics significantly influenced the fault’s reloading and rupture mechanics. Overall, this event highlights the role of both seismic and aseismic processes in fault rupture dynamics in pull-apart basins.

## ACKNOWLEDGMENTS

We are grateful to Stephen Miller for his comments and suggestions that significantly helped in improving the manuscript.

## SUPPORTING INFORMATION

Supplementary data are available at *GJI RAS* online.

Please note: Oxford University Press is not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the paper.

## DATA AVAILABILITY

The seismic data used in this study were obtained from the repositories of Boğaziçi University, Kandilli Observatory and Earthquake Research Institute, Regional Earthquake-Tsunami Monitoring Center (https://www.fdsn.org/networks/detail/KO/), the Disaster and Emergency Management Authority (https://doi.org/10.7914/SN/TU) and RESIF (http://dx.doi.org/10.15778/RESIF.XW2007).

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