Browsing by Author "Yavuz, Evrim"

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Focal Mechanisms and Bouguer-Gravity Anomalies of the 2025 Earthquake Cluster in the Santorini-Amorgos Region (Southern Aegean Sea, Greece): Evidence for Shallow Extensional Magmatism
(Springer, 2025) Toker, Mustafa; Yavuz, Evrim; Balkan, Emir
Understanding clustered earthquake sequences is essential for seismic hazard assessment, as it involves constraining faulting styles and nodal planes of potential ruptures. This study investigates the nature of a dense earthquake sequence (similar to 3000 events) initiated on January 27, 2025, in the Santorini-Amorgos region of the Southern Aegean Sea (SAS), a tectonically active Volcanic Island Arc (VIA). We analyzed 23 shallow crustal earthquakes (Mw >= 4.5, depth <= 10 km) that occurred between February 2-9, 2025, using full-waveform, low-frequency Centroid Moment Tensor (CMT) inversion from regional seismograms. The inversion was complemented by high-resolution Bouguer gravity anomaly data derived from the EIGEN-6C4 satellite gravity model to assess subsurface density variations. The focal mechanisms consistently indicate NE-SW striking, high-angle (>= 45 degrees) normal faults with NW- and SE-dipping planes and centroid depths <= 10 km. Integration of CMT results with gravity anomalies (90-100 mgal) suggests a migrating zone of shallower extensional magmatism (SEM) driving the sequence. These findings reveal a Precursory Seismic Cluster (PSC) and provide new constraints on the seismotectonic and magmatic processes shaping seismic hazard in the region.
Multiple Segmentation and Seismogenic Evolution of the 6th February 2023 (Mw 7.8 and 7.7) Consecutive Earthquake Ruptures and Aftershock Deformation in the Maras Triple Junction Region of Se-Anatolia, Turkey
(Elsevier, 2023) Toker, Mustafa; Yavuz, Evrim; Utkucu, Murat; Uzunca, Fatih
On 6th February 2023 (UTC), two consecutive and catastrophic earthquakes with moment magnitudes (Mw) 7.8 and 7.7 struck the Maras Triple Junction (MTJ) region in SE Anatolia along with dozens of aftershocks, causing numerous casualties and significant building damage, and generating the most complex and longest surface ruptures ever observed in Turkey. The main driving mechanisms of this complex double event are still unresolved and remain controversial, even though they are likely linked with conventional fault activations, recurrence intervals and seismic gaps. Here, the aim was to gain insight into the source regimes and rupture processes of both events and their relationship with resolved fault focal solutions for the observed aftershocks, and to present an interpretation that accounts for the most puzzling aspects of the fault rupture models. In line with this, the coseismic slip distributions of these two events were examined by joint analyses of centroid moment tensor (CMT) and finite-fault source inversions using regional and teleseismic broadband observations. Inversion results indicate that both earthquakes were left-lateral strike-slip events, and the main ruptures extended mainly from close to NNE to SSW and E to W, with maximum slips of -6.5-10 m, mostly confined to a shallow depth range of <= -10-15 km and extending to the surface, indicating bilateral source processes with an average rupture velocity of -3.5-5.5 km/s. The estimated total seismic moment range was 4.94-8.22 x 1020 N m, associated with -352-152 km long (along strike) and - 25 km wide (along dip) fault planes at focal depth of -10 km. Regional CMT results indicate nearly pure normal-slip and left-lateral normal oblique-slip focal mechanisms and shallow centroid depths (<= -15 km) for the early aftershock distribution that are obviously complementary with the coseismic bilateral rupture propagations. This result highlights that double pull-apart branching of focal mechanisms for aftershock occurrence implies interacting fault ruptures embedded in the MTJ area, where two sub-/ supershear-rupturing faults meet, thus explaining multiple segmentation and seismogenic evolutions of two interrelated mainshocks, i.e. "triple junction earthquakes". The results reveal that the MTJ tends to migrate to the SSW and likely drives the SSW-stepping of the left-lateral strike-slip shear (-136 km). This accounts for the peak slips, long co-seismic fault ruptures and the associated faulting styles. Hence, the co-seismic faulting apparently distributed across the MTJ may reflect triple junction migration, and thus large extension at the core of the Anatolian-Arabian plates, leading to very high seismic hazard in similar junction regions of the country.
New Insight Into the 24 January 2020, Mw 6.8 Elazig Earthquake (Turkey): an Evidence for Rupture-Parallel Pull-Apart Basin Activation Along the East Anatolian Fault Zone Constrained by Geodetic and Seismological Data
(Ist Nazionale Di Geofisica E Vulcanologia, 2021) Irmak, Tahir Serkan; Toker, Mustafa; Yavuz, Evrim; Senturk, Erman; Guvenaltin, Muhammed Ali
In this study, we investigated the main features of the causative fault of the 24 January 2020, M-w, 6.8 Elazig earthquake (Turkey) using seismological and geodetic data sets to provide new insight into the East Anatolian Fault Zone (EAFZ). We first constrained the co-seismic surface deformation and the rupture geometry of the causative fault segment using Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1A/B satellites) and teleseismic waveform inversion, respectively. Also, we determined the centroid moment tensor (CMT) solutions of focal mechanisms of the 27 aftershocks using the regional waveform inversion method. Finally, we evaluated the co-seismic slip distribution and the CMT solutions of the causative fault as well as of adjacent segments using the 27 focal solutions of the aftershocks, superimposed on the surface deformation pattern. The CMT solution of the 24 January 2020Elazig earthquake reveals a pure strike-slip focal mechanism, consistent with the structural pattern and left-lateral motion of the EAFZ. The rupture process of the Elazig event indicated that the rupture is started at 12 km around the hypocenter, and then propagated bilaterally along the NE-SW but mainly toward the southwest. The rupture slip has initially propagated toward the southwest (first 10 s) and northeast (4 s), and again toward the southwest (9 s). Maximum displacement is calculated as 1.3 m about 20 km southwest of the hypocenter at 6 km depth (centroid depth). The rupture stopped to down-dip around 20 km depth toward the southwest. The distribution of the slip vectors indicates that the rupture continued mostly through a normal oblique movement. Most of the moment release was released SW of the hypocenter and the rupture reached up to around 50 km. The focal mechanisms of analyzed 27 aftershocks show strike-slip, but mostly normal and normal oblique-slip faulting with an orientation of the tensional axes (NNE-SSW), indicating a normal oblique-slip, "transtensional" stress regime, parallel-subparallel to the strike of the EAFZ, consistent with SW-rupture directivity and coseismic deformation pattern. Finally, based on the co-seismic surface deformation compatible with the distributional pattern of normal focal solutions, normal and normal oblique-slip focals of the aftershocks evidence the rupture-parallel pull-apart basin activation as a segment boundary of the left-lateral strike-slip movement of the EAFZ.