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The tectonically active southwestern part of Turkey is dominated by the Aegean Extensional Province. The primary aim of this study is to evaluate the seismic hazard for the cities in SW Turkey using a probabilistic approach. As part of this research, a new earthquake database based on a unified moment magnitude scale was created, which contains shallow crustal earthquakes from 1000 to 2021. The catalog's foreshock and aftershock occurrences were excluded depending on the space–time windows, and a catalog completeness analysis was conducted. The uncertainty in magnitude determination was taken into account. The seismic sources were defined as homogeneous area source zones, taking into consideration the active fault zones. The activity rate and the Gutenberg–Richter b parameter as earthquake hazard parameters for each seismic source have been evaluated by the Kijko–Smit maximum likelihood estimation method. An \"efficacy test,\" which uses the average log likelihood value (LLH), was performed to find the suitable ground motion prediction equations for SW Turkey. The maximum magnitude (Mmax) of each seismic source was calculated based on the regional rupture characteristics and the Kijko–Sellevoll methods. Seismic hazard maps for SW Turkey were developed for Peak ground acceleration (PGA), spectral acceleration with periods of 0.2 and 1 s and for bedrock with a hazard level of 10% probability of exceedance in 50 years by using Geographical Information System software. Based on this study, the PGA values on the bedrock in SW Turkey range from 0.253 to 0.572 g. As part of this research, seismic hazard curves and uniform hazard spectrums were created for the settlement areas of the cities in SW Turkey. In addition to this PSHA, PGA values at the settlement areas with events at probable future rupture locations were estimated for different site amplification classes.

Katırcıoğlu, F. Muhtar; map collection; catalog.

Turkey is located in one of the most seismically active regions in the world. Characterizing seismic source zones in this region requires evaluation and integration of geological, geophysical, seismological and geodetical data. This first seismotectonic database for Turkey presented herein was prepared, under the framework of the National Earthquake Strategy and Action Plan—2023. The geographic information system (GIS)-based database includes maps of active faults, catalogues of instrumental and historical earthquakes, moment tensor solutions and data on crustal thickness. On the basis of these data, 18 major seismotectonic zones were delineated for Turkey and the surrounding region. The compilation and storage of the seismotectonic data sets in a digital GIS will allow analyses and systematic updates as new data accrete over time.

Earthquakes trigger other earthquakes and build up clusters in space and time that in turn create a bias in seismic catalogues. Therefore, declustering is considered as a prerequisite in seismic studies, particularly for probabilistic seismic hazard analysis, not only to eliminate the bias but also to decouple mainshocks and triggered events. However, a declustering process is not a straightforward task due to the complex nature of earthquake phenomena. There exist several declustering methods that mostly employ subjective rules to distinguish between background seismicity and offsprings. Eventually, the final declustered catalogues usually deviate significantly according to the employed method. This issue is raising some concerns, such as how to select the most suitable declustering algorithm, or to assess how this selection affects seismic hazard assessment. In consequence, the main goal of this paper is to quantify the sensitivity of seismic hazard assessments to different declustering techniques. Accordingly, the recently compiled Turkish earthquake catalogue was declustered by making use of three declustering algorithms. A total of six declustered catalogues, two catalogues per method, one by implementing the default input parameters, and one by altering the free input parameters of the employed methods, were produced. The clusters of selected earthquakes were studied in terms of the spatial–temporal distribution of earthquake sequences. A sensitivity analysis was conducted through the major steps of seismic hazard assessment for Istanbul metropolitan city. The seismicity of Istanbul and surroundings was modeled on the basis of four areal source zones. Comparative studies showed that, while the selected declustering algorithm did not significantly affect the completeness periods of moderate to large size earthquakes, it considerably altered those of small magnitude events (e.g. Mw 4.3–5.2) and consequently the recurrence parameters of the source zones. Depending on the declustering algorithm and input parameters, the activity rate was observed to vary up to a factor of two. The differences in the declustered catalogues obtained from different declustering approaches resulted in considerable variations in seismic hazard estimations. The hazard maps at return periods of 475 and 2475 years indicated that peak ground acceleration values may vary up to 20% at some locations. Moreover, the differences in 5% damped elastic spectral accelerations at T = 0.2 for the return periods of 475 and 2475 years are about 18 and 12%, respectively, on the southern shores of Istanbul where the highest hazard levels are observed.

The alarm-based forecasting model for earthquakes called moment ratio (MR) is retrospectively tested on Turkey and adjacent area seismicity. This model uses the ratio of the mean inter-event time over the variance as a precursory alarm function to forecast future earthquakes in a given region. In a former study, the MR model was successfully tested in forecasting large earthquakes with magnitude M ≥ 7, occurred in Japan. In this study, it is tested on Turkey and adjacent area seismicity using lower magnitude thresholds, namely by learning from M ≥ 5 events, to forecast earthquakes with magnitude M ≥ 6. For this purpose, a composite earthquake data file is compiled using Kandilli Observatory and Earthquake Research Institute Regional Earthquake and the Tsunami Monitoring Center (KOERI-RETMC) provided catalogs, for the period 1900–2016, and the SHARE European Earthquake Catalog (SHEEC) for the historical period 1000–1899. In this catalog, earthquakes are listed using surface magnitude scale Ms. The time periods used in training and testing are selected by taking into consideration the completeness of the magnitude. Finally, Molchan error diagrams are used to evaluate the forecasting performance of the MR method in practice using a retrospective test. Obtained results are presented as standard MR forecasting maps showing the overall forecasts and optimal maps showing high alarm areas with minimal miss and alarm rates. In addition, the relative intensity (RI) forecasting method is applied to compare different results. Results show MR forecasts outscoring random guessing with good performance compared to RI forecasts. The forecasting maps point to a small high alarm area situated along the Hellenic arc subduction zone east of Crete Island.