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The M w 7.8 earthquake in Turkey on 6 February 2023 was extraordinary for various reasons. It originated in depth of only 10 km, ruptured along a fault plane around 300 km long and the surface was covered by an extensive network of high-quality seismic instruments. The strong motions resulted in a vast number of tragic casualties and huge material losses in Turkey and Syria. However, abundant and proximate seismic observations of this event and numerous aftershocks give an opportunity to deepen the understanding of earthquake processes. In this study, we carried out an assessment of coseismic changes of seismic velocity using passive image interferometry. We used data from one strong-motion and twenty-four broad-band sensors. We observed coseismic drops of seismic velocity, which reached up to −1.79 per cent at a location directly at the ruptured East Anatolian Fault Zone. Along the M w 7.8 earthquake fault, we observe frequency dependence of the velocity changes. At frequencies above 0.5 Hz, the velocity drops seem to be higher at locations close to the ruptured faults than in the more distant areas. [ABSTRACT FROM AUTHOR]

This article investigates the stress drop variability of shallow normal-faulting earthquakes in western Anatolia through strong-motion simulations. For this purpose, source characteristics of three moderate to large magnitude events are constrained by the empirical Green's function simulation in a broadband frequency range. Recordings of ten strong-motion stations in 78-km epicentral distance range are utilized for the simulation. Estimated strong-motion generation areas (SMGAs) are 22 km2, 66 km2, and 110 km2 where rise times are 0.6 s, 0.7 s, and 0.6 s, respectively, for the 2011 Simav (Mw 5.8), 2017 Lesvos (Mw 6.3), and 2017 Bodrum-Kos (Mw 6.6) earthquakes. Those values are found to be consistent with global source-scaling relationships. One particular observation is that stress drop ratios between the mainshock and aftershock for all events are relatively large compared with those previously calculated for strike-slip events in Turkey. Stress drops of SMGAs for the Simav and Bodrum earthquakes are in the range of 25 MPa, and this value drops to 19 MPa for the Lesvos earthquake. To further investigate the stress drop variation of earthquakes in Western Anatolia, an earthquake source database (fc-Mo) offered by Yamanaka et al. (IAG-IASPEI 2017, S07-1-03, 2017) is utilized. Brune (Journal of Geophysical Research, 76:5002, 1971) stress drop values of > 360 small to moderate earthquakes (Mw 3.0–6.0) are calculated with the given corner frequency and seismic moment information assuming a constant shear wave velocity. Results indicate that the majority of the earthquakes have a stress drop value < 5 MPa. This value changes to between 5 and 57 MPa for the remaining earthquakes. These high stress drop values support the former findings stating that normal-faulting earthquakes may release higher stress than strike-slip earthquakes. This indicates that the regional stress regime in western Turkey may cause relatively larger stress release during the normal-faulting mainshocks. [ABSTRACT FROM AUTHOR]

We investigate the crustal structure beneath the western part of the North Anatolian fault zone (NAFZ), an area where at least five damaging earthquakes occurred during the twentieth century. This study is based on local earthquake tomography using the data from aftershocks of the Izmit event (17 August 1999, M 7.4) recorded by stations of permanent and temporary networks. We derive the distribution of VP, VS, and the VP/VS ratio based on the iterative inversion for both VP - VS and VP - VP/VS using the LOTOS code. Innovatively, in this study we perform an inversion for frequency-dependent S-wave attenuation (1/QS). The reliability of the results is assessed through synthetic tests. The distributions of the resulting seismic parameters (VP, VS, VP/VS, and QS) highlight important geodynamical features in the study area. The low-velocity and high-attenuation patterns mostly correlate with the fracturing zones of the NAFZ. Low velocities are also observed beneath the main sedimentary basins (e.g., Adapazarl, Dtizce, and Kuzuluk). High-velocity and low-attenuation patterns correlate with blocks presumed to be rigid (Kocaeli, Armutlu, and Almacik blocks). The rupture traces of the largest earthquakes in this area pass generally in the transition areas between high and low velocities, while moderate and weak seismicity is mostly concentrated in low-velocity areas. Based on these results we propose and discuss the role that the Almacik block could have played in producing the largest earthquakes in the study area in the twentieth century. [ABSTRACT FROM AUTHOR]

Throughout the last century, the North Anatolian Fault (NAF) produced mostly west migrating sequence of M>7 earthquakes, and in this period of time, the NAF did not break beneath the Marmara Sea. The Main Marmara Fault (MMF), which is the north branch of the North Anatolian Fault in the Marmara Region, is the most active branch in this area. The interseismic behavior of the MMF beneath the Marmara Sea is crucial with regards to assessments for the earthquake risk of the Marmara Region which has a population over 24 million. As a result, the prediction of the locking depth and the fault slip rate have great importance as they provide a comprehension concerning the seismic moment deficit, that can be released at the time of a future event.In this study, we modeled the interseismic locking of the MMF by making use of the interseismic GPS velocities and implementing a 3D finite-element analysis. Our kinematic model has a realistic 3D fault geometry, in which each and every fault section of MMF is confined to a fault slip rate down below a given locking depth which ranges from 2.5 to 15 km. We improve the fits to GPS velocity data just by changing the interseismic locking depth of each segment and placing more complexity to the locking structure in cases where the data needs. Our preliminary models reveal that a change in locking depth is required in between the Ganos Segment at the western end of the Marmara Sea and Central Segment. A model with a locking depth of 10 km for the Ganos Segment and 2.5 km for the Central Segment fits the GPS velocities well. We outrule the likelihood of similar locking patterns for the two segments. This result is consistent with the earlier work utilizing 1D profiles demonstrating that there is no significant strain accumulation beneath the Central Segment and also seismicity studies show that the background seismicity is very different in these two segments. Under the Princes' Islands Segment, the data is more compatible when we add some shallow creep. For the Izmit Segment, we predict that the locking depth is around 8 km, yet further analysis is needed. Our results show that in most cases, the variation in the seismic behavior of each segment is in accordance with its interseismic behavior in relation to the fault locking. [ABSTRACT FROM AUTHOR]