José A. Peláez

Attenuation of Intensity with Epicentral Distance in the Iberian Peninsula

We have classified the attenuation of the Medveded, Sponheuer, and Karnik (MSK) intensity into five types (each as a function of the epicentral intensity I o) based on the mean radii of 254 isoseismal maps, mainly historical earthquakes in the Iberian Peninsula. Geographically representing each earthquake with its corresponding attenuation tendency, it can be seen that those with low attenuation lie west of the Peninsula and those with high attenuation in the south and east. This regionalization seems to be due as much to the seismotectonic characteristics (different crustal types and size of the earthquakes) as to the different construction types in each region. These attenuation values are similar to those of southern Europe, but much higher than those found in the United States. From the point of view of seismic-hazard evaluation, these laws represent an improvement with regard to those used so far. We have extended previous attenuation studies to the whole of the Iberian Peninsula, and, in some points, differences of attenuation assignment of almost two degrees of intensity have been corrected.

Stress fields in the Iberian-Maghrebi region

This study concerns the present stressfield between the Eurasian and Africanplates in the Iberian-Maghrebi region(Portugal, Spain, Morocco, Algeria andTunisia). In addition to an up-to-datecatalogue of earthquakes in this area, acatalogue of the focal mechanisms composedof 486 solutions of fault planes,standardized in terms of notation andinformation type, was used. These data wereused applying the right-dihedron method ofAngelier and Mechler (1977), to obtaindifferent zones with homogeneous stress.The results obtained for shallowearthquakes (h < 30 km) coincide, in themajority of cases, with the general stressfields proposed by numerous authors forthis region, according to which there isNW-SE compression. However, the stressorientation appears to vary in certainareas, perhaps perturbed by the opening ofthe Atlantic Ocean, the approach of Iberiaand Africa, or the extension of the AlboranSea.For the intermediate earthquakes (30 < h< 150 km) no general pattern was found,and the P and T axes seem to be randomlyoriented for the depth intervalsconsidered. For the very deep earthquakes(h > 600 km), however, the P axis lies ina NNW-SSE direction, dipping towards theSSE, while the T axis is subhorizontal in aNE-SW direction.The determinations from the focalmechanisms highlight the existence of aregional stress field with a subhorizontalcompression axis trending NW-SE. Superimposed are others that specificallyaffect particular sectors; these arerelated to the opening of the AtlanticOcean, the extension of the BeticCordillera and the Alboran Sea, and eventhe present compression between the Iberianand European plates.

An Earthquake Catalogue (2200 B.C. to 2013) for Seismotectonic and Seismic Hazard Assessment Studies in Egypt

The instrumental earthquake catalogues show the overall seismicity of the Earth. However, examining and inspecting the historical records plus the instrumental recorded events is necessary to understand long-term seismicity. Moreover, regional seismic and focal mechanism solutions catalogues provide critical information for different seismological investigations, including seismic hazard assessments and seismotectonic studies. The present work aims at preparing new and up-to-date unified and Poissonian earthquake catalogue for Egypt including the focal mechanism solutions data, so that the earthquake information can be reached from a single source. A catalogue for earthquakes that occurred in Egypt and its vicinity during the period 2200 B.C. - 2013 was compiled for achieving a unified magnitude scale. Data were obtained from different sources, local, regional and international (e.g., the regional catalogue of Ambraseys et al., 1994; the International Seismological Center and the Annual Bulletin of the Egyptian National Seismic Network). The initial compilation spanning a region from 21° to 38°N and from 22° to 38°E, and includes all the events having an assigned magnitude of 3.0 and above for international sources and any magnitude for local sources on any magnitude scale. Earthquake magnitudes are reported in different scales and come from a variety of sources. The initial compiled catalogue comprised a total of 64613 earthquakes (historical and instrumental events). In addition, a focal mechanism solutions database was collected. This database contains 688 fault-plane solutions gathered from different published and unpublished sources, covering the time period from 1940 until the end of 2013. For establishing a common magnitude, namely an equivalent moment magnitude, two new relationships correlating surface-wave and body-wave magnitudes with moment magnitude were derived. After this process, the catalogue was cut off below moment magnitude 3.0, because these magnitudes are not significant for seismic hazard studies. All non-Poissonian (dependent) events were removed using the approach proposed by Gardner and Knopoff (1974). A total of 16642 events represent the final Poissonian catalogue including main shocks with a moment magnitude above or equal to 3.0. Finally, the degree of completeness for the entire catalogue was checked for the different magnitude values. In addition, separately from the Eastern Mediterranean region, the completeness for the Egyptian territory sub-catalogue was also checked. The results derived in each case have been interpreted in the framework of the establishment and development of the international and national networks.

Spanish National Strong Motion Network. Recording of the Huelva Earthquake of 20 December, 1989

In recent years a network of 30 accelerographs has been installed through the zones of highest seismic activity of Spain. For the first time, digital strong motion records have been obtained in Spain, with a maximum horizontal acceleration value of 0.06 g. A comprehensive study is made of the strong motion recordings of an earthquake which occurred in southwest Spain, on December 20, 1989. The isoseismal map is drawn and the data confirm the main attenuation directions in the area observed in other shocks.

A review of seismic hazard assessment studies and hazard description in the building codes for Egypt

Reduction of damage in earthquake-prone areas requires modern building codes that should be continuously updated to reflect the improvement in our understanding of the physical effects of earthquake ground shaking on buildings and the increase in the quality and amount of seismological and tectonic studies, among other factors. This work reviews the published seismic hazard assessments available for Egypt as well as the seismic actions included in the building codes, in order to show the state-of-the-art of the seismic hazard assessment studies for the country. The review includes the history and development of seismic hazard assessments and the adoption of seismic building codes in Egypt. All the previous studies were analyzed in order to conclude that a new seismic hazard assessment according to the state-of-the-art is desirable, as well as a change in the hazard description for the actual Egyptian building code.

Deformation style and controlling geodynamic processes at the eastern Guadalquivir foreland basin (Southern Spain)

The tectonic structure of the Guadalquivir foreland basin becomes complex eastwards evolving from a single depocenter to a compartmented basin. The deformation pattern within the eastern Guadalquivir foreland basin has been characterized by combining seismic reflection profiles, boreholes and structural field data to output a 3D model. High-dipping NNE-SSW to NE-SW trending normal and reverse fault arrays deform the Variscan basement of the basin. These faults generally affect Tortonian sediments, which show syntectonic features sealed by the latest Miocene units. Curved and S-shaped fault-traces are abundant and caused by the linkage of nearby fault segments during lateral fault propagation. Preexisting faults were reactivated either as normal or reverse faults depending on their position within the foreland. At Tortonian time, reverse faults deformed the basin forebulge, while normal faults predominated within the backbulge. Along-strike variation of the Betic foreland basin geometry is supported by an increasing mechanical coupling of the two plates (Alboran Domain and Variscan basement) towards the eastern part of the cordillera. Thus, subduction would have progressed in the western Betics, while it would have failed in the eastern one. There, the initially subducted Iberian paleomargin (Nevado-Filabride Complex) was incorporated into the upper plate promoting the transmission of collision-related compressional stresses into the foreland since the middle Miocene. Nowadays, compression is still active and produces low-magnitude earthquakes likely linked to NNE-SSW to NE-SW pre-exiting faults reactivated with reverse oblique-slip kinematics. Seismicity is mostly concentrated around fault tips that are frequently curved in overstepping zones.

A Review of the 1170 Andújar (Jaén, South Spain) Earthquake, Including the First Likely Archeological Evidence

The origin of the town of Andujar (figures 1 and 2), in southern Spain, is likely a Roman settlement, as suggested by certain archaeological evidence in its historical center (figure 2). Andujar was probably founded to control a significant strategic route on the edge of the Guadalquivir River and ending in Cordoba. The town was a flat settlement, without natural shelters, presumably defended in this epoch by a defensive wall or fortification, although no evidence remains of it.

A Seismogenic Zone Model for Seismic Hazard Studies in Northwestern Africa

Known seismic activity in the northern Algeria-Morocco region (Fig. 1), especially during the last 50 years, includes several damaging earthquakes. In particular, in the El Asnam region (nowadays Cheliff) have been located the most destructive and damaging earthquakes recorded in northern Algeria.

Forecasting Moderate Earthquakes in Northern Algeria and Morocco

In this work, we have studied the correlation between locations of MW ≥ 5.0 earthquakes and locations of 5.0 > MW ≥ 4.0 events for Northern Algeria and Morocco. A preliminary study shows that it can be observed a relatively good agreement between locations for these two data sets, that is, minor earthquake locations could be used to forecast future places where will happen moderate and moderate to strong earthquakes. Then, we propose a time-independent forecasting model based on the spatially smoothed seismicity rate of MW ≥ 4.0 earthquakes. Initially, the area under study was divided into square cells. The number of earthquakes with magnitude MW ≥ 4.0 that have taken place at a given cell is counted and smoothed. Finally, a time-independent forecasting model is proposed from the computation of MW ≥ 5.0 and MW ≥ 6.0 earthquake probabilities for each cell for different exposure times. Probabilities are computed considering that seismicity follows both a Poisson process and the Gutenberg-Richter magnitude-frequency relationship.

Seismic hazard and deaggregation assessment at the nuclear power plants (NPP) sites in Spain using an updated seismological database

In this paper we present the results obtained in the seismic hazard reassessment at the sites, in the free field, of the seven Spanish nuclear power plants (NPPs), six of them are still operative till date. The computation was done using the spatially-smoothed seismicity approach, and an updated seismic catalog. In most cases, the results do not greatly differ from those previously reported. In addition, at two of these locations where the hazard levels were higher, we carried out a seismic deaggregation study. We were thus able to determine the characteristics of the seismicity responsible for seismic hazard in terms of distance, magnitude and azimuth. The results obtained show that seismic hazard at the Cofrentes NPP is entirely due to close-range seismicity (< 70 km), whereas hazard at the Almaraz NPP is due to earthquakes located from 190 to 350 km away. A clear limitation of this work is the fact that we cannot include into our seismic hazard assessment, at this stage, paleoseismic information for the Spanish NPP sites.