Instrumental Earthquakes Research Articles

Comment on “Active Crustal Deformation in the Trans‐Mexican Volcanic Belt as Evidenced by Historical Earthquakes During the Last 450 Years” by G. Suárez et al.

Comment on “Active Crustal Deformation in the Trans‐Mexican Volcanic Belt as Evidenced by Historical Earthquakes During the Last 450 Years” by G. Suárez et al.

A database of potential paleoseismic evidence in Switzerland

Paleoseismological evidence is indispensable for the identification of large prehistoric earthquakes and the extension of the temporal coverage of historical and instrumental earthquake catalogues. In the geological record, diverse traces of past earthquakes are found. This study presents a database of potential primary and secondary evidence for seismic activity in the past 20,000 years in Switzerland. The database includes data from sedimentological, archaeological, speleological and geomorphological research. This unique dataset allows identifying periods during which the geologic record reveals enhanced occurrence of evidence that are further discussed as potentially earthquake-triggered. For the most recent 700 years, an increased occurrence of evidence features was found. This clustering is an effect of the historical earthquakes. Furthermore, periods with enhanced occurrence are identified at six phases during the past 20,000 years. Even though dating uncertainties for the geological record are large (e.g. 14C calibration range) and an unequivocal attribution of earthquakes as trigger mechanism for secondary evidence is not possible, the database reflects the natural-hazard potential of a region and represents valuable information for seismic hazard assessment. Furthermore, despite the uncertainty regarding the definition of the trigger mechanism, we propose that the database can be used to validate and improve earthquake-hazard models.

Comparing calibration coefficients constrained from early to recent macroseismic and instrumental earthquake data in Greece and applied to eighteenth century earthquakes

The purpose of this paper is to determine calibration constants of instrumental Greek earthquakes in order to calculate the basic seismic parameters of historical earthquakes (magnitude, epicenter, focal depth) via macroseismic data. Two different approaches are adopted for calibration procedure. The first implements the macroseismic estimation of earthquake parameters (MEEP procedure) and is based on macroseismic data points (MDPs). The second approach calculates macroseismic magnitude based on isoseismal areas, using both linear and multiple regression techniques. The datasets used for analysis comprise of 121 instrumental earthquakes with 7247 MDPs and 123 isoseismal maps. Validation of the results is performed using six instrumental earthquakes in order to verify the calibration parameters. Finally, calibration constants are successfully applied for parameters calculation of eight eighteenth century events. Thus, the application of the results to historical earthquakes contributes to the improvement of the seismic picture of Greece.

A Reassessment of the Magnitude of the 1755 Lisbon Earthquake

ABSTRACTThe 1755 Lisbon earthquake is still poorly understood due to its offshore location and complex macroseismic intensity pattern. Gutenberg and Richter (1949) tentatively assigned a magnitude between 8¾ and 9 judging from an estimated perceptibility radius of 2500 km. More recent attempts to estimate the magnitude from isoseismal areas led to results in the 8.5–8.7 range. These values have been adopted in several studies of the seismic hazard of southwest Iberia.In this article, I use the available macroseismic data—a total of 1206 data points from Portugal, Spain, and Morocco—to reassess the magnitude of the 1755 Lisbon earthquake. Because a training set of instrumental earthquakes was not available, I apply the technique of Bakun and Wentworth (1997) in conjunction with the ground-motion model of Atkinson and Wald (2007), which was selected through comparison with the 1969 M 7.8 Gorringe bank earthquake data. I obtain a moment magnitude of 7.7±0.5, significantly lower than previous results. The epicentral location obtained with this analysis is offshore southwest Iberia—as expected given the tsunami effects—but poorly constrained.Based on the pattern of intensity data outliers, I suggest that the source was complex and spatially distributed, with part of the rupture taking place onshore or inshore. I propose an explanation for the large tsunamigenic power of the earthquake, which invokes the basal rupture of the Gulf of Cadiz accretionary prism. Finally, I discuss the implications for hazard assessment of the type of complex rupture proposed.

Evolution in seismic monitoring system and updating seismic zones of Egypt

ABSTRACT On 3 September 2015, an unexpected earthquake with magnitude MB = 4.5 occurred in a completely silent area, located at 40 km south El Alamein city north western desert, where there are ~ 20 million pieces of unexploded ordnance. This study aims to: i) review and interpret the widespread seismic activity in Egypt during the last three decades, and ii) delineate the active seismic source zones map of Egypt based on realistic assumptions. These assumptions include only the real and accurate seismic events from the whole earthquake catalogue of Egypt. The idea based on the accuracy of the monitoring system, magnitude size of earthquakes, and detailed structure elements map of Egypt. The seismic catalogue of Egypt was updating to the end of 2019 including the historical and instrumental earthquakes since −2200 BC. The seismicity map of Egypt was outlined along five interval periods based on the significant evolution in the seismic monitoring system. The advanced monitoring period in the detection and location of seismic events in entire Egypt was supposed to begin in 2003 where the ENSN almost accomplished. Seismicity of advanced monitoring period 2003–2019 can be used as a tracer of real seismicity boundaries entire Egypt. Active seismic source zones map was delineated based on the advanced monitoring period and the unified Mw ≥ 4.5 instrumental catalogue from 1900 to 2019. To validate the seismic zones model, it compared with the structural elements map and given a rational matching. The new local seismic spots entire Egypt was discussed because of the invention in the seismic monitoring system entire Egypt and attributed to the misdetection and/or dislocation events due to the lack of monitoring system before the accomplished of ENSN. In conclusion, a validated active seismic zones map of Egypt was delineated using real and accurate seismic and geologic data based on powerful assumptions.

Seismic liquefaction features in the Kashmir Karewas: Natural seismograms of the paleoearthquakes

Karewas occur within the seismically active Kashmir Valley. Natural seismograms in the form of seismites exist within the Karewa sediments. Seismites were identified and characterized to know their genesis. The magnitude of the paleoearthquakes were ranging from 6.0 to 7.4 with paleointensity IO = 8.5. LSI values computed for the paleomagnitudes were ranging from 16 to 63 mm. PGA computed for the paleoearthquakes were ranging from 0.18 g to 0.77 g using cyclic stress method. PGA computed for the simulated historical and instrumental earthquakes using GMPE were of the order of 1.11 g, 0.86 g and 0.83 g. FS, VS and Dr values indicate that the sediments of the source stratum near the paleoliquefaction sites are still liquefiable and have retained their liquefaction potential since Early Pleistocene. FS values of the paleoearthquakes indicate that the geological conditions were appropriate to liquefy sediments as FS = 1 and are still susceptible to liquefaction as advocated by the FS values of the simulated historical and instrumental earthquakes. Empirical relationships that account for the age of soil deposits show significant influence on PGA values of the paleoearthquakes and no effect on FS values of the paleoearthquakes as FS = 1. However, significant influence was observed on FS values of the historical and instrumental earthquakes.

Earthquake damage and related factors associated with the 2016 ML = 5.8 Gyeongju earthquake, southeast Korea

The Gyeongju area covers the intersection of two major young structural features in Korea: the Yangsan and Ulsanfaults. More than 60 Quaternary fault sites have recently been reported along these fault zones, which are thus considered major active tectonic features in southeast Korea. An earthquake of local magnitude ML = 5.8 struck the Gyeongju area on September 12, 2016; the largest instrumental earthquake recorded in South Korea. We performed detailed investigation for severely damaged buildings and houses in villages around the epicenter, and determined the characteristics and controlling factors of the earthquake damage. The distribution of damaged buildings is relatively scattered around the epicenter, which may be related to the relatively deep focal depth of approximately 13–15 km. The radius of the reported damage area affected by ground motion is approximately 17 km from the epicenter, which is almost equal to the focal depth. Old buildings with traditional styles are more seriously damaged than modern buildings, suggesting that the damage intensity depends on the building structure, material properties, and seismic design. Interestingly, in a small village, the degree of building damage is clearly divided by a small stream. Based on an electrical resistivity survey for the local geological condition, we found that the degree of building damages also strongly depends on the local unconsolidated alluvium thickness. Moreover, the orientation of tilted or damaged buildings is closely related to the general trend of the related faults indicating the propagation direction of ground motion. Although focal depth, building style, and foundation condition are the main controlling factors to the building damages caused during the 2016 Gyeongju earthquake, other minor factors could also be involved. This information will be useful to design proper construction codes for reinforced buildings and for hazard studies against future earthquakes in potential earthquake prone areas.

Temporal and Spatial Patterns of Seismic Activity Associated with the Dead Sea Transform (DST) during the Past 3000 Yr

AbstractHistorical reports of earthquakes occurring before the twentieth century along the Dead Sea Transform (DST) are available for the past 3000 yr. Most of them are organized in various catalogs, reappraisals, and lists. Using a comprehensive and consistent compilation of these reports, the historical seismicity associated with the DST as a complete tectonic unit was examined. The compilation, supported by paleoseismic and archeoseismic evidence, resulted in 174 reliable historical earthquakes and 112 doubtful ones. The reliable earthquakes, along with 42 post‐nineteenth century instrumental earthquakes, are an up‐to‐date evaluation of the DST seismicity starting from the mid‐eighth century B.C.E. until 2015 C.E. Additionally, the scenario of historical earthquakes such as the 363 C.E. and 1033 C.E. events was resolved. The characterization of temporal and spatial patterns of DST seismicity, classifying them into four geographical zones, raised that most of the northern destructive earthquakes are clustered while clustering at the central and southern zones is less abundant.

Active Crustal Deformation in the Trans‐Mexican Volcanic Belt as Evidenced by Historical Earthquakes During the Last 450 Years

AbstractThe Trans‐Mexican Volcanic Belt (TMVB) is oblique to the subduction zone. Background seismicity is very low. Nevertheless, two large instrumental earthquakes were recorded: the 1912 Acambay (Mw6.9) and the 1920 Jalapa (Mw6.4) earthquakes. Macroseismic data were systematically searched for crustal earthquakes on the TMVB contained in historical accounts. The macroseismic reports were inverted for the best fitting magnitude and epicenter. The western end of the TMVB shows the highest level of seismic activity. An earthquake is reported in 1568 (Mw7.2 ± 0.2) and a second large event took place in 1875 (Mw7.0 ± 0.2). Two other earthquakes are reported in the western TMVB in 1611 and 1749 (Mw6.4). The 1858 earthquake (Mw7.6 ± 0.3), in the central TMVB occurred on faults located to the west of the 1912 Acambay event. The eastern TMVB is less active. An earthquake in 1575 (Mw5.7) occurred east of the city of Puebla. The 1887 (Mw6.2 ± 0.2) and the 1920 Jalapa earthquakes are the larger events in this region. The seismicity observed from the instrumental and historical seismicity catalogs show that active tectonic deformation occurs throughout the TMVB and that it is not confined to specific regions where instrumental earthquakes have been located and active faults mapped. The number of large earthquakes in the TMVB, suggest unique large‐scale deformation of this complex volcanic belt. The presence of large earthquakes in this tectonic province, where 40% of the population of Mexico live, underlines the importance of reevaluating hazard in this densely populated region.

An Earthquake Catalog for the Lebanese Region

ABSTRACTThe present work aims at establishing an earthquake catalog for seismic hazard assessment in Lebanon. This catalog includes two different parts: historical earthquakes and instrumental earthquakes. The first part of the article describes the work done on the period 31 B.C.E. to the end of the nineteenth century. Numerous studies published in the last 30 yr, devoted to preinstrumental earthquakes in Lebanon, had not been included in any parametric earthquake catalog. A thorough and critical review of these studies was devised to check their respective interpretations of available earthquake records in terms of seismic parameters (date, location, and size) and to select for each earthquake the most reliable interpretation. The second part provides the details on the selection of instrumental solutions for the period 1900–2015 and for magnitudes≥4. From global instrumental earthquake catalogs, we build a unified earthquake catalog for Lebanon and bordering regions. A selection scheme is applied for the choice of the best location and the best magnitude among solutions available. The number of events in the catalog is relatively small, and all earthquakes can be checked one by one. The earthquake catalog is homogenized in moment magnitude. For 89% of the events, an Mw proxy was calculated from the original magnitude, applying conversion equations. The merging of the historical and instrumental periods highlights a specificity of this zone: the instrumental seismicity (1900–2015) corresponds to a relatively quiet period for Lebanon. The historical part, covering 2000 yr, includes similar periods of quiescence, as well as much more active periods with destructive earthquakes.

Catalogue of the Geological Effects of Earthquakes in Spain Based on the ESI-07 Macroseismic Scale: A New Database for Seismic Hazard Analysis

This paper summarizes the content and scope of the “Catalogue of Earthquake Geological Effects in Spain”. The catalogue has been published by the Geological Survey of Spain (IGME) and constitutes the first official publication (in Spain) on seismic hazard containing geological information. The catalogue gathers the 51 stronger earthquakes that have occurred in Spain since the Neolithic period to the present and classifies earthquakes with geological or archaeological seismic records in paleoseismic, ancient, historical and instrumental earthquakes. The catalogue offers a variety of parametric information, quality indexes (Qe, Qi, Qg), and Environmental Seismic Intensity Scale (ESI-07) based description of environmental damage structured in individual “event files”. Sixteen of the 51 catalogued events present full information files (full event files), with individualized analyses of the geological and geoarchaeological data as well as graphic information with hybrid ESI-EMS intensity maps, ShakeMaps (seismic scenarios) and complementary kmz files (Google Earth) for each of the sixteen selected earthquakes; among which is the well-known AD 1755 Lisbon earthquake-tsunami. These selected earthquakes present individual environmental earthquake effects (EEE) or earthquake archaeoseismological effects (EAE) files for each catalogued effect containing specific site geo-information and graphic data (photos, graphs, maps, etc.). The second edition of the catalogue record 1027 EEEs and 187 EAEs, of which 322 effects have individual files.

Seismicity of the Arctic in the Early Twentieth Century: Relocation of the 1904–1920 Earthquakes

Abstract The parameters of earthquake hypocenters in the Arctic at the beginning of the twentieth century, published by researchers in the first half of the twentieth century, are still used today for building maps of epicenters of instrumental earthquakes. However, they are based on bulletins that did not use data from all seismic stations operating during that period, and on approximate ideas about the propagation of seismic waves in the Earth. We relocated earthquakes recorded within the Arctic region beginning from the early twentieth century with a view to creating a relocated catalog. For the relocation, we collected all available seismic bulletins from the global network using data acquired for the International Seismological Centre–Global Earthquake Model (ISC-GEM) catalog, the EuroSeismos project, the Geophysical Survey of the Russian Academy of Sciences, and the Russian State Library. The relocation was performed using a modified method of generalized beamforming and the ak135 velocity model. The relocation procedure was applied to 18 of 25 earthquakes in the Arctic region. The new coordinates of some earthquakes turned out to be significantly different from those that were determined previously. As a result, this may have a significant impact on the final seismic hazard assessment of the territory of the Severnaya Zemlya and Franz Josef Land archipelagoes, which are characterized by weak seismicity. Most of the relocated earthquake epicenters are confined to major seismic zones in the Arctic, namely, mid-ocean ridges, the Svalbard Archipelago, and the Laptev Sea shelf. One earthquake, that of 14 October 1914 with magnitude Mw(ISC-GEM)=6.6, occurred in the shelf of the Barents Sea in the “continent–ocean” transition zone near the Franz Victoria graben.

A seismic quiescence before the 2017 Mw 7.3 Sarpol Zahab (Iran) earthquake: Detection and analysis by improved RTL method

A major earthquake, with magnitude Mw 7.3, struck Sarpol Zahab (Kermanshah province, Iran) on November 12, 2017, causing extended damage and casualties. The epicenter was located in the Northwestern part of the Zagros mountain range, an active belt originated by the Arabia-Eurasia collision.We explore seismicity preceding this earthquake, by using the Iranian Seismological Center instrumental earthquake catalog (IGTU), with the aim to identify possible anomalies in background seismicity that can be related with this and other future large events. For this purpose, we used a method for intermediate term forecasts of large earthquakes, namely the Region Time Length (RTL) algorithm, which analyzes declustered catalogs and is sensitive to quiescences that may precede major earthquakes. RTL has been progressively refined and has been applied in several regions worldwide during the last decades. To decluster the earthquake catalog we used a quite novel approach, based on the nearest-neigbour distances between events in the space-time-energy domain, a method that preserves the background seismicity while removing the clustered component.The retrospective application of RTL algorithm to the area surrounding the mainshock epicenter highlights two significant quiescences: one preceding the Sarpol Zahab Mw 7.3 earthquake, and the other occurring before a Mw 5.7 earthquake, which struck the same region on November 2013. The quiescences duration ranges from few months to one year and is compatible with earlier results from different regions of the world.In addition, we applied an enhanced variant of RTL algorithm, which allows us drawing maps for the whole study region and that shows only quiescences consistently detected for different choices of the free parameters, and hence more stable. The resulting map for Northwestern Iran, calculated for the time span 1 June 2017–11 November 2017, evidences two broad quiescence regions, oriented NW-SE along the Zagros belt. One, located to the north, evidences a significant seismic anomaly corresponding to the Sarpol Zahab earthquake, which disappeares immediately after the event. The second one, located in the southeastern part of the study region, persists up to the end of the available catalog (October 4, 2018).

Prediction of the Maximum Expected Earthquake Magnitude in Iran: From a Catalog with Varying Magnitude of Completeness and Uncertain Magnitudes

This paper concerns the problem of predicting the maximum expected earthquake magnitude \(\mu\) in a future time interval \(T_{\text{f}}\) given a catalog covering a time period \(T\) in the past. Different studies show the divergence of the confidence interval of the maximum possible earthquake magnitude \(m_{ \hbox{max} }\) for high levels of confidence (Salamat et al. 2017). Therefore, \(m_{ \hbox{max} }\) should be better replaced by \(\mu\) (Holschneider et al. 2011). In a previous study (Salamat et al. 2018), \(\mu\) is estimated for an instrumental earthquake catalog of Iran from 1900 onwards with a constant level of completeness \(\left( {m_{0} = 5.5} \right)\). In the current study, the Bayesian methodology developed by Zoller et al. (2014, 2015) is applied for the purpose of predicting \(\mu\) based on the catalog consisting of both historical and instrumental parts. The catalog is first subdivided into six subcatalogs corresponding to six seismotectonic zones, and each of those zone catalogs is subsequently subdivided according to changes in completeness level and magnitude uncertainty. For this, broad and small error distributions are considered for historical and instrumental earthquakes, respectively. We assume that earthquakes follow a Poisson process in time and Gutenberg–Richter law in the magnitude domain with a priori unknown \(a\) and b values which are first estimated by Bayes’ theorem and subsequently used to estimate \(\mu\). Imposing different values of \(m_{ \hbox{max} }\) for different seismotectonic zones namely Alborz, Azerbaijan, Central Iran, Zagros, Kopet Dagh and Makran, the results show considerable probabilities for the occurrence of earthquakes with \(M_{w} \ge 7.5\) in short \(T_{\text{f}}\) , whereas for long \(T_{\text{f}}\), \(\mu\) is almost equal to \(m_{ \hbox{max} }\).

The ISC-GEM Earthquake Catalogue (1904–2014): status after the Extension Project

Abstract. We outline the work done to extend and improve the ISC-GEM Global Instrumental Earthquake Catalogue, a dataset which was first released in 2013 (Storchak et al., 2013, 2015). In its first version (V1) the catalogue included global earthquakes selected according to time-dependent cut-off magnitudes: 7.5 and above between 1900 and 1918 (plus significant continental earthquakes 6.5 and above); 6.25 between 1918 and 1959; 5.5 between 1960 and 2009. Such selection criteria were dictated by time and resource limitations. With the Extension Project we added both pre-1960 events below the original cut-off magnitudes (if enough station data were available to perform relocation and magnitude recomputation) and added events with magnitude 5.5 and above from 2010 to 2014. The project ran over a 4-year period during which a new version of the ISC-GEM Catalogue was released each year via the ISC website (http://http://www.isc.ac.uk/iscgem/, last access: 10 October 2018). For each year, not only have we added new events to the catalogue for a given time range but also revised events already in V1 if additional data became available or location and/or magnitude reassessments were required. Here we recall the general background behind the production of the ISC-GEM Catalogue and describe the features of the different periods in which the catalogue has been extended. Compared to the 2013 release, we eliminated earthquakes during the first 4 years (1900–1903) of the catalogue (due to lack of reliable station data), added approximately 12 000 and 2500 earthquakes before 1960 and between 2010 and 2014, respectively, and improved the solution for approximately 2000 earthquakes already listed in previous versions. We expect the ISC-GEM Catalogue to continue to be one of the most useful datasets for studies of the Earth's global seismicity and an important benchmark for seismic hazard analyses, and, ultimately, an asset for the seismological community as well as other geoscience fields, education and outreach activities. The ISC-GEM Catalogue is freely available at https://doi.org/10.31905/D808B825.

The 1952 Kern County, California earthquake: A case study of issues in the analysis of historical intensity data for estimation of source parameters

Seismic intensity data based on first-hand accounts of shaking give valuable insight into historical and early instrumental earthquakes. Comparing an observed intensity distribution to intensity-prediction models based on modern calibration events allows the magnitude to be estimated for many historic earthquakes. Magnitude estimates can also potentially be refined for earthquakes for which limited instrumental data are available. However, the complicated nature of macroseismic data and the methods used to collect and interpret the data introduce significant uncertainties. In this paper, we illustrate these challenges and possible solutions using the 1952 Kern County, California, earthquake as a case study. Published estimates of its magnitude vary from MW 7.2–7.5, making it possibly the second largest in California during the 20th century. We considered over 1100 first-hand reports of shaking, supplemented with other data, and inferred the magnitude in several ways using intensity prediction equations, yielding a preferred intensity magnitude MI 7.2 ± 0.2, where the uncertainty reflects our judgement. The revised intensity distribution reveals stronger shaking on the hanging wall, south of the surface expression of the White Wolf fault, than on the footwall. Characterizing the magnitude and shaking distribution of this early instrumental earthquake can help improve estimation of the seismic hazard of the region. Such reinterpreted intensities for historic earthquakes, combined with U.S. Geological Survey (USGS) Did You Feel It? data for more recent events, can be used to produce a uniform shaking dataset with which earthquake hazard map performance can be assessed.

Seismotectonic and seismicity of Makran, a bimodal subduction zone, SE Iran

The present study combined spatial distribution and mechanism of seismicity, seismicity parameters, geomorphology in satellite imagery and tectonic features in Makran inland to show that the Makran low angle thrust fault is divided into two separate segments. Depth dispersal of earthquakes, especially relocated events, shows that the sinking plate is steeper in the west than in the east of Makran. Significant facts were identified from the variation of Moho depth between west and east, slip rates of the inland faults and crustal shortenings in the north of Makran. The Sistan suture zone plays a significant role in dividing the overriding plate into a non-deforming Helmand block and a less deforming Lut block. These blocks have different resistances against the northward subduction. Large and great events, such as the 1989 Chabahar (MW 5.5) and 1945 Gwadur (MW 8.1), occurred in the south of subduction fronts of the western and eastern Makran, respectively. They occurred in shallow depth and were the pieces of evidence of weak and strong coupling between the oceanic and continental crusts on a low angle and a very low angle reverse faults, respectively. Spectral analysis and depth determination of the instrumental and pre instrumental earthquakes, respectively, helped to separate some plate boundary and intraplate events. This separation, geometry and non-characteristic behavior of the subduction zone, revealed from spatial and temporal distribution of the earthquakes, probably dedicate a mature Aleutian type for the subduction zone in the east. Missing great (M > 6.1) earthquakes, less slip rate of the continental faults and steeper dipping of the sinking plate in the west probably indicate a Kurile type for the Iranian part of subduction. Makran is a bimodal subduction, because geomorphological and seismic pieces of evidence indicate a NNW surficial and right lateral fracture zone in the continental crust and a NW deep and left lateral fracture zone in the oceanic crust, respectively. The fractures and geometry of subductions showed that the eastern Makran has seismotectonically a wider area for potential coupling of a larger earthquake than the western Makran.

Probabilistic Seismic Hazard Analysis Using the New Correlation Relationships for Magnitude Scales

Amol is one of the oldest cities located in north of Iran, Mazandaran province, and its history dates back to the pre-Islamic period. Amol is a city with an area about 3000 square kilometers, a population exceeding 370,000, and includes the old and famous neighborhoods that have a religious, commercial, and service with a long history background. Considering the importance of buildings constructed in this city and the need for their preservation and restoration on one hand, and the occurrence of many severe earthquakes in the past centuries, as well as the recent earthquakes of the last century, on the other hand, encourage us to study the seismicity of this city. Therefore, in this paper, by considering the historical and instrumental earthquakes recorded within a radius of 150 km around this city and the seismic mechanism of the faults located in this region, probabilistic seismic hazard analysis of the area is studied. Then, using the probabilistic relations of the seismic hazard analysis of the Kijko 2000 computer program, the seismicity parameters and the return periods of the earthquake magnitudes are obtained for the area, and at the end, the horizontal peak ground acceleration is zoned for this city.

Los mayores sismos en Argelia en la época moderna: las fallas de El Asnam y Zemmouri-Boumerdès

Algeria has experienced many destructive earthquakes during the last few centuries (e.g., Ayadi and Bezzeghoud, 2015). The city of El Asnam (formerly Orleanville, today Chlef) was severely damaged in 1954 and 1980 by magnitude 6.7 and 7.3 earthquakes, respectively. On October 10, 1989, a magnitude 5.9 earthquake struck the Mont Chenoua-Tipasa coastal area approximately 150 km west of Zemmouri, which is where the May 21, 2003 earthquake occurred (Mw6.8). Many other large historical and instrumental earthquakes have severely damaged the coastal cities of Algeria over the last few centuries (i.e., around Algiers, Oran, Mascara, Djidjelli, Constantine and Bejaia). These earthquakes suggest active deformation of the margin in conjunction with the clear offshore extent of active coastal faults. According to several studies, the main active geological structures around El Asnam, Algiers, Zemmouri and Boumerdes have experienced several disastrous earthquakes. Many other earthquakes have occurred in and around the Chlef and Mitidja Basins, underlining the seismic activity in the area. In this chapter, we highlight the main characteristics of the two largest earthquakes that have occurred in Algeria in the modern period: the El Asnam earthquakes of September 9 (Mw6.7) and October 10 1980 (Mw7.3), and the Zemmouri-Boumerdes earthquake of May 21, 2003, (Mw6.8).

When probabilistic seismic hazard climbs volcanoes: the Mt. Etna case, Italy – Part 2: Computational implementation and first results

Abstract. This paper describes the model implementation and presents results of a probabilistic seismic hazard assessment (PSHA) for the Mt. Etna volcanic region in Sicily, Italy, considering local volcano-tectonic earthquakes. Working in a volcanic region presents new challenges not typically faced in standard PSHA, which are broadly due to the nature of the local volcano-tectonic earthquakes, the cone shape of the volcano and the attenuation properties of seismic waves in the volcanic region. These have been accounted for through the development of a seismic source model that integrates data from different disciplines (historical and instrumental earthquake datasets, tectonic data, etc.; presented in Part 1, by Azzaro et al., 2017) and through the development and software implementation of original tools for the computation, such as a new ground-motion prediction equation and magnitude–scaling relationship specifically derived for this volcanic area, and the capability to account for the surficial topography in the hazard calculation, which influences source-to-site distances. Hazard calculations have been carried out after updating the most recent releases of two widely used PSHA software packages (CRISIS, as in Ordaz et al., 2013; the OpenQuake engine, as in Pagani et al., 2014). Results are computed for short- to mid-term exposure times (10 % probability of exceedance in 5 and 30 years, Poisson and time dependent) and spectral amplitudes of engineering interest. A preliminary exploration of the impact of site-specific response is also presented for the densely inhabited Etna's eastern flank, and the change in expected ground motion is finally commented on. These results do not account for M > 6 regional seismogenic sources which control the hazard at long return periods. However, by focusing on the impact of M < 6 local volcano-tectonic earthquakes, which dominate the hazard at the short- to mid-term exposure times considered in this study, we present a different viewpoint that, in our opinion, is relevant for retrofitting the existing buildings and for driving impending interventions of risk reduction.