Şeşetyan, Karin

Abstract. Earthquake hazard analyses rely on seismogenic source models. These are designed in various fashions, such as point sources or area sources, but the most effective is the three-dimensional representation of geological faults. We here refer to such models as fault sources. This study presents the European Fault-Source Model 2020 (EFSM20), which was one of the primary input datasets of the recently released European Seismic Hazard Model 2020. The EFSM20 compilation was entirely based on reusable data from existing active fault regional compilations that were first blended and harmonized and then augmented by a set of derived parameters. These additional parameters were devised to enable users to formulate earthquake rate forecasts based on a seismic-moment balancing approach. EFSM20 considers two main categories of seismogenic faults: crustal faults and subduction systems, which include the subduction interface and intraslab faults. The compiled dataset covers an area from the Mid-Atlantic Ridge to the Caucasus and from northern Africa to Iceland. It includes 1248 crustal faults spanning a total length of ∼95 100 km and four subduction systems, namely the Gibraltar, Calabrian, Hellenic, and Cyprus arcs, for a total length of ∼2120 km. The model focuses on an area encompassing a buffer of 300 km around all European countries (except for Overseas Countries and Territories) and a maximum of 300 km depth for the subducting slabs. All the parameters required to develop a seismic source model for earthquake hazard analysis were determined for crustal faults and subduction systems. A statistical distribution of relevant seismotectonic parameters, such as faulting mechanisms, slip rates, moment rates, and prospective maximum magnitudes, is presented and discussed to address unsettled points in view of future updates and improvements. The dataset, identified by the DOI https://doi.org/10.13127/efsm20 (Basili et al., 2022), is distributed as machine-readable files using open standards (Open Geospatial Consortium).

In the frame of a comprehensive investigation of historical earthquakes of Anatolia, we propose a re-appraisal of four major earthquakes/sequences occurred after 1000 AD (1114/1115, 1269, 1513/1514 and 1544), which could be considered as predecessors of the earthquakes of February 6, 2023. The main purpose is to provide reliable parameter values for the investigated earthquakes. Our investigation consisted of: retrieving and analysing the main primary historical sources; identifying the localities mentioned and assessing macroseismic intensity; determining earthquake parameters (location, magnitude and – where possible – the source azimuth) with the repeatable and transparent “Boxer” method, after properly calibrating the relevant coefficient by considering recent earthquakes of the Anatolian region. Our investigations show that the 1114 earthquake can be considered as a predecessor of the main 2023 earthquake, although the latter ruptured a larger area; the earthquake of 1544 may be a predecessor of the second event of 2023, February; and that the background of the 1513/1514 earthquake is so poor that a lot of care is required while handling the currently available parameters. In conclusion, we also compare our results with the findings of paleoseismological investigation and discuss how they contribute to understanding the rupture history of the East Anatolian Fault.

The 2020 update of the European Seismic Hazard Model (ESHM20) is the most recent and up-to-date assessment of seismic hazard for the Euro-Mediterranean region. The new model, publicly released in May 2022, incorporates refined and cross-border harmonized earthquake catalogues, homogeneous tectonic zonation, updated active fault datasets and geological information, complex subduction sources, updated area source models, a smoothed seismicity model with an adaptive kernel optimized within each tectonic region, and a novel ground motion characteristic model. ESHM20 supersedes the 2013 European Seismic Hazard Model (ESHM13; Woessner et al., 2015) and provides full sets of hazard outputs such as hazard curves, maps, and uniform hazard spectra for the Euro-Mediterranean region. The model provides two informative hazard maps that will serve as a reference for the forthcoming revision of the European Seismic Design Code (CEN EC8) and provides input to the first earthquake risk model for Europe (Crowley et al., 2021). ESHM20 will continue to evolve and act as a key resource for supporting earthquake preparedness and resilience throughout the Euro-Mediterranean region under the umbrella of the European Facilities for Seismic Hazard and Risk consortium (EFEHR Consortium).

When large earthquakes occur, it is natural enough to wonder about their likely predecessors (if any). This is why, after the earthquakes of February 2023, we began a review of the historical seismic record of Eastern Anatolia Our investigation concentrated on six major earthquakes/sequences occurred after 1000 AD (1003, 1114/1115, 1269, 1344, 1513/1514 and 1544). The earthquakes of 1822, 1872 and 1893 are better known and do not require priority investigation. The geopolitical history of this region is very complex, with many changes of rulers along the centuries, but its long-term seismic history is rather well known. Many earthquakes are on record both before and since the date - some 2100 years ago - when the region became a Roman province. Many of its main towns (Aleppo and Antakya for instance) have a long-term history that includes eyewitness observations of many strong earthquakes. Historical earthquake records for this area were collected and studied several times. The latest studies are Soysal et al. (1981), Ambraseys and Finkel (1995), Guidoboni & Comastri (2005), Sbeinati et al. (2005), Tan et al. (2008). N.N. Ambraseys wrote many papers on this subject and compendiated his work in Ambraseys (2009). However, time, epicentral location and size of many earthquakes are debatable and earthquake catalogues propose contrasting values for the same events. Our work consisted of: a) retrieving and analyzing the main historical sources for each earthquake; b) identifying the localities mentioned in the sources and assessing macroseismic intensities from the original information; c) determining earthquake parameters (Io, Mw and - whenever possible - source azimuth) with the “Boxer” method (Gasperini et al., 1999), after properly calibrating the relevant coefficient by using recent earthquakes of the Anatolian region. Fig. 3 presents the seismological results of this work: Mw (with uncertainty equal to 0.3) and the “boxes” obtained with the Gasperini et al. (1999) procedure and representing the surface projection of the possible earthquake sources: the epicentre is in the middle of the “box”. The epicentral location and Mw calculated for the main earthquake of 1114 are close to those of the first event of 6 February, 2023. The “box” seems to match the Pazarcik segment of the EAFZ (we refer hereafter to the fault definitions used by Duman and Emre, 2013 and by Duman et al., 2018). The 1269 earthquake was less energetic than the 1114 one, and its parameters are less well constrained. Its “box” suggests the Amanos segment as the likeliest source, with the Toprakkale segment as an alternative candidate. The 1344 earthquake is rather well known and was indeed a very large one. On account of its location it was not considered in the debate on the 2023 earthquake source. However, the identification of its source would be helpful for the understanding of seismicity in this region. As for the 1513/1514 earthquake, the first interpretation by Ambraseys (1988) was -and still is – considered as the absolute truth by literature, leading to a strong connection with the Pazarcık segment. Unfortunately, this interpretation is founded on poor information, as later stated by Ambraseys (2009) and confirmed by us. Though we cannot provide reliable epicentral location and magnitude estimates, we believe that the Toprakkale or Karataş segments could represent a more appropriate option for the source. Similar considerations can be proposed for the 1544 earthquake, whose informative background is also very weak. It could be located in the area where the recent M7.6 took place (Çardak fault), with Mw around 6.9.

The region where the 2023 February 6 earthquakes took place is known to have been very active in the past; it is part of the contact between the Eurasian and the Arabic plate, an area where seismic activity was relatively low in the 20th century (Figure 1). It is a “border zone” also from the historical point of view; therefore, although many earthquakes are known to most earthquake catalogues and compilations, the relevant information is often rather poor and affected by chronological and place-names problems. As a consequence, earthquake parameters given in the catalogues are often based on a poor dataset; the situation is even more complicated by chronological issues and careless compilations, which lead to earthquake duplications. In the frame of a larger investigation effort underway (Sesetyan et al., 2020; Stucchi et al. 2022) we first considered the available information from the main earthquake catalogues and compilations in the time-window 1000-1900. The volume by Ambraseys (2009) summarizes and sometimes updates the knowledge already proposed in previous works such as Ambraseys and Jackson (1988), Ambraseys (1989), Ambraseys and Finkel (1995). We also considered the works by Soysal et al. (1981), Guidoboni et al. (2019), Sbeinati et al. (2005), some recent historical earthquake investigations and the parametric catalogues by Shebalin and Tatevossian (1997), Tan et al. (2008), Sesetyan et al. (2013). Next, we assessed - when possible - macroseismic intensities at the mentioned localities and, from them, we assessed earthquake parameters by making use of the so-called “Boxer” method by Gasperini et al. (1999). For some of the main earthquakes in the region we briefly summarise here the earthquake parameters from our investigation and from the main catalogues. In the Appendix we summarise the available information.

Questo articolo traduce e compendia parte dei dati contenuti nel sito della Boğaziçi University Kandilli Observatory and Earthquake Research Institute, Department of Earthquake Engineering. Nella pagina relativa ai Rapporti sul terremoto a Kahramanmaraş – Gaziantep del 6 febbraio 2023 (ore 04:17 locali) c’è un Report preliminare dedicato ai forti terremoti storici (Large historical earthquakes of the earthquake-affected region: a preliminary report) a firma di K. Sesetyan, M. Stucchi, V. Castelli e A.A. Gomez Capera, pubblicato il 17 febbraio 2023.

The 2020 update of the European Seismic Hazard Model (ESHM20) is the most recent seismic hazard model of the Euro-Mediterranean region. It was built upon unified and homogenized datasets including earthquake catalogues, active faults, ground motion recordings and state-of-the-art modelling components, i.e. earthquake rates forecast and regionally variable ground motion characteristic models. ESHM20 replaces the 2013 European Seismic Hazard Model (ESHM13), and it is the first regional model to provide two informative hazard maps for the next update of the European Seismic Design Code (CEN EC8). ESHM20 is also one of the key components of the first publicly available seismic risk model for Europe. This chapter provides a short summary of ESHM20 by highlighting its main features and describing some lessons learned during the model’s development.

N.N. Ambraseys left us a wealth of papers and volumes on a number of topics; many of them concern the historical earth- quake investigation. One of the last works is the 2009 volume (Ambraseys in Earthquakes in the Eastern Mediterranean and the Middle East: a multidisciplinary study of 2000 years of seismicity, Cambridge, Cambridge, UK, 2009), where he summarizes the results of more than thirty years of investigation through archives and libraries, covering earthquakes of a large area, from Albania to Caucasus. For each earthquake, a short summary of the main effects is supplied, together with the list of the sources used. Such information is intended as material for assessing location and size of the earthquakes, task that the author accomplished only in a very preliminary way for a few earthquakes, only. In addition to exhaustive descriptions of the most known earthquakes and the relevant historical sources, the volume contains information on a large number of earthquakes, so far unknown to the current earthquake catalogues. This paper intends to represent a homage to his immense work, partially showing the potential of his volume. We briefly present here some case histories, including the preliminary location and size of the earthquakes – known and unknown—around Anatolia. We add some examples of how he was able to prove that some alleged earthquakes are actually to be considered as fake or very doubtful. We also present the damage information supplied for some known and unknown earthquakes, and how they can be used for assessing location and size of them.

Executive Summary The 2020 European Seismic Hazard Model (ESHM20) provides an update of the earthquake hazard assessment of the Euro-Mediterranean region. ESHM20 has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No.s 730900 of the SERAProject (www.sera-eu.org) The ESHM20 follows the same principles as the ESHM13, with state-of-the art procedures homogeneously applied for the entire pan-European region, without the country-borders issues. The model was built upon recently compiled datasets (i.e. earthquake catalogues, active faults, ground shaking recordings), information (tectonic and geological) and models (seismogenic sources, ground shaking). A fully probabilistic framework was adopted in the hazard model implementation and all datasets and inputs are fully cross-border harmonized. The newly developed seismogenic source model encompass fully harmonized and cross borders seismogenic sources following the recent national earthquake hazard models. The inherent uncertainties in characterizing the earthquake rupture forecast are handled by a complex logic tree, consisting of two main models (branches): an area source-based model and a hybrid fault-smoothed seismicity model. The ground motion characteristic model is built upon the most complete ground shaking recordings in Europe and aims at capturing the effects of source and attenuation path of the expected ground shaking at a site. The regional variability of the ground shaking as constrain by observations is captured by a novel approach of spatial clustering and the overall uncertainties are handled in a backbone logic tree. The master logic tree combines the earthquake rate forecast with the backbone ground motion models into a computational model for assessing the earthquake ground shaking at across the entire Euro- Mediterranean region. Full sets of hazard results (i.e. hazard curves and maps, uniform hazard spectra) for various combinations of return periods and descriptive statistics (mean, median and quantiles) are available. The ESHM20 development process involved several regional workshops where the scientists and experts were consulted and their feedback was acknowledged and considered on the model outcome. Finally, ESHM20 provides the basis to derive informative hazard maps for two key engineering parameters defining the anchoring points of the seismic design spectra for the next version of the European Seismic Design Code (CEN-EC8). The European Facilities for Earthquake Hazard and Risk (EFEHR) will maintain and further develop this model in collaboration with the GEM Foundation and the European Plate Observing System (EPOS). The source data, input models, software and outputs of ESRM20 are thus being openly released with a Creative Commons CC-BY International 4.0 license (https://creativecommons.org/licenses/by/4.0/). This license allows re-users to distribute, remix, adapt, and build upon the material in any medium or format, so long as attribution is given to the creator. The license allows for commercial use. Each product is released with a clear notice on how it should be cited in order to abide by the license. ESHM20 is online available at the www.hazard.EFEHR.org.