Carlos López Casado

Seismic hazard estimate at the Iberian Peninsula

Abstract — Seismic hazard at the Iberian Peninsula has been evaluated by using a methodology which combines both zonified and non-zonified probabilistic methods. Seismic sources are used when considering zones where certain calculation parameters may be considered homogeneous, as in zonified methods, while, on the other hand, earthquakes are considered wherever it has taken place, as in non-zonified methods. The methodology which is applied in this paper has been originally used to calculate the seismic hazard maps in the United States. In our case, it has been necessary to adapt the method to the specific features of the seismicity in the Iberian Peninsula and its geographical surroundings, not only with respect to its distribution and characteristics, but also with respect to the properties of the seismic catalog used.¶Geographically, the main feature of the result is the fact that it reflects both historical seismicity and current seismic clusters of the region. Despite the smoothing, maps show marked differences between several seismic zones; these differences becoming more noticeable as exposure time increases. Maximum seismic hazard is found to be in the southwestern region of the Peninsula, especially in the area of the Cape St. Vicent, and around Lisbon. The uncertainty of the results, without considering that due to the attenuation laws, as deduced from the other evaluation parameters, is quite stable, being more sensitive to the parameters b and mmax of the Gutenberg-Richter relation.

Deaggregation in Magnitude, Distance, and Azimuth in the South and West of the Iberian Peninsula

We present the results of the seismic deaggregation in 15 of the most important cities with the greatest seismic hazard in the south and west of the Iberian Peninsula (Spain and Portugal). The deaggregation was carried out based on the calculation of the seismic hazard in the zone, taking into account the peak horizontal acceleration with 10% probability of exceedance in 50 years (return period of 475 years). We first performed a deaggregation study in terms of magnitude and distance in order to subsequently carry it out in azimuth. The aim of both studies is to determine the relative contribution of the different seismic foci and sources to the seismic hazard in a given location. Due to the lack of enough seismotectonic data in the study region, we could not obtain information about the contribution of specific active faults and we have not been able to include a characteristic earthquake model. However, by starting from a calculated hazard using smoothed background seismicity, it is possible to determine the contribution of the different seismic foci of the region to the seismic hazard at each location. The results reveal that there are cities where the hazard is entirely, or almost entirely, due to the local seismicity (e.g., in Portugal: Lisbon, 87%; Coimbra, 82%; in Spain: Almero ´a, 99%; Cordoba, 99%; Granada, 99%). We have also determined that there are cities where seismic foci at 200 km away or more can be the most important or at least contribute significantly to the hazard (e.g., Beja and Faro in Portugal and Cadiz and Huelva in Spain).

Seismic hazard in Northern Algeria using spatially smoothed seismicity. Results for peak ground acceleration

Seismic hazard in terms of peak ground acceleration (PGA) has been evaluated in northern Algeria using spatially smoothed seismicity data. We present here a preliminary seismic zoning in northern Algeria as derived from the obtained results. Initially, we have compiled an earthquake catalog of the region taking data from several agencies. Afterwards, we have delimited seismic areas where the b and mmax parameters are different. Finally, by applying the methodology proposed by Frankel [Seismol. Res. Lett. 66 (1995) 8], and using four complete and Poissonian seismicity models, we are able to compute the seismic hazard maps in terms of PGA with 39.3% and 10% probability of exceedance in 50 years. A significant result of this work is the observation of mean PGAvalues of the order of 0.20 and 0.45 g, for return periods of 100 and 475 years, respectively, in the central area of the Tell Atlas. D 2003 Elsevier B.V. All rights reserved.

Use of Active Fault Data versus Seismicity Data in the Evaluation of Seismic Hazard in the Granada Basin (Southern Spain)

In this article we evaluate the seismic hazard in the Granada Basin (southern Spain), using for the first time the slip rate of known active faults. Our study, as an attempt to compute seismic hazard using active fault data in low to moderate seismicity regions, relies on a complete database of these faults containing information relevant to their seismic potential. We obtain peak ground acceleration values above 0.4g for a return period of 475 years. This result is compared with previous evaluations carried out on the basis of the historical seismicity of the area and the application of the well-known theorem of total probability. In these cases, maximum values of 0.2g are obtained. We explain the discrepancies found between the slip rate-derived and seismicity-derived estimates of seismic hazard as owing to the different strikes of the faults in relation to the directions of the main stresses affecting the Granada Basin, in the context of the Betic Cordilleras, some of them with evidence of aseismic slip.

Soil liquefaction potential induced by the Andalusian earthquake of 25 December 1884

On 25 December 1884, an earthquake of epicentral intensityI0 = IX in the MSK scale caused great damage ‘in a large area in the provinces of Granada and Málaga, in the south of Spain. The reports of the Spanish, Italian and French Commissions that studied the earthquake described ground phenomena in seven different sites which can be identified as soil liquefaction.By means of dynamic penetration tests carried out in the above sites, the corresponding soil profiles (based on SPT data and water table depth) were established, and the occurrence of liquefaction was proved in five out of seven of these sites. Also, the intensities at such locations and the magnitude of the earthquake were estimated.From the geotechnical data and the cyclic stress ratio induced by the earthquake, liquefaction conditions were confirmed in all the five sites which presumably liquefied. Then, possible values of the minimum ground surface accelerations necessary for the onset of liquefaction at each location were calculated. The results obtained were completed with data reported in six liquefaction case studies from Japan and the United States, from which design charts relating soil acceleration with normalized SPT values for different intensity levels were drawn.Finally, by using standard attenuation curves, the above data were translated into epicentral distances, and good agreement with the known epicentral area was found. As a result, a consistent approach for liquefaction hazard and source location problems has been developed. The proposed method combines in its formulation historical evidence and earthquake engineering techniques.

How Distant Earthquakes Contribute to Seismic Hazard in Mainland Portugal

The used method to compute the seismic deaggregation is that proposed by Bernreuter (1992) for assessing the control earthquake. This approach has been recommended by the SSHAC (1997) as a means of simplifying the understanding of the results obtained in a seismic hazard analysis. We begin by calculating the seismic hazard, using spatially smoothed seismicity, for the Iberian Peninsula (Pelaez and Lopez Casado, 2002). This methodology was proposed by Frankel (1995), and was used here with certain modifications. The seismicity of the region was delimited in seismic sources, and the b andmmax parameters of the truncated Gutenberg-Richter relationship were smoothed. The attenuation relationships in the Iberian Peninsula and its surrounding areas were also regionalised (Lopez Casado et al., 2000a); this ensures a decrease in the uncertainties in seismic hazard evaluations. Five types of intensity attenuation relationships are used for eleven specific regions, with mean attenuation coefficient (absorption coefficient) values that range from 0.001 to 0.083 km. The regionalization is clearly correlated with the seismotectonic characteristics of each region.When necessary, the relationship betweenmacroseismic intensity and horizontal peak ground acceleration proposed byMurphy and O’Brien (1977) is adopted. We were unable to include a characteristic earthquake model, since in the study region it was impossible to specifically associate the earthquakes with active faults at the moment. Instead, we incorporated a historical seismicity model, including the most destructive earthquakes in the area. We have not considered necessary to include a uniform background zone due to the historical extension and the spatial quality of the used catalog. Moreover, including this model would imply a hazard decrease in the most active zones. In short, four models were initially considered: those with a seismicity of M