Josep Batlló

The SHARE European Earthquake Catalogue (SHEEC) 1000–1899

In the frame of the European Commission project “Seismic Hazard Harmonization in Europe” (SHARE), aiming at harmonizing seismic hazard at a European scale, the compilation of a homogeneous, European parametric earthquake catalogue was planned. The goal was to be achieved by considering the most updated historical dataset and assessing homogenous magnitudes, with support from several institutions. This paper describes the SHARE European Earthquake Catalogue (SHEEC), which covers the time window 1000–1899. It strongly relies on the experience of the European Commission project “Network of Research Infrastructures for European Seismology” (NERIES), a module of which was dedicated to create the European “Archive of Historical Earthquake Data” (AHEAD) and to establish methodologies to homogenously derive earthquake parameters from macroseismic data. AHEAD has supplied the final earthquake list, obtained after sorting duplications out and eliminating many fake events; in addition, it supplied the most updated historical dataset. Macroseismic data points (MDPs) provided by AHEAD have been processed with updated, repeatable procedures, regionally calibrated against a set of recent, instrumental earthquakes, to obtain earthquake parameters. From the same data, a set of epicentral intensity-to-magnitude relations has been derived, with the aim of providing another set of homogeneous Mw estimates. Then, a strategy focussed on maximizing the homogeneity of the final epicentral location and Mw, has been adopted. Special care has been devoted also to supply location and Mw uncertainty. The paper focuses on the procedure adopted for the compilation of SHEEC and briefly comments on the achieved results.

The 1531 Lisbon Earthquake: A Tsunami In the Tagus Estuary?

Abstract On 26 January 1531, a strong‐magnitude earthquake heavily impacted Lisbon downtown. Immediately after the earthquake, the eyewitnesses reported large waves in the Tagus estuary, mainly north of the city and along the northern bank of the river. Descriptions include large impacts on ships anchored in the estuary and even morphological changes in the riverbed. We present a synthesis of the available information concerning both the earthquake and the water disturbance as a basis for the discussion of the probable tectonic source and the magnitude of the associated river oscillations. We hypothesize that the initial disturbance of the water can be attributed to the coseismic deformation of the estuary riverbed, and we use a nonlinear shallow water model to simulate the tsunami propagation and inundation. We show that the Vila Franca de Xira fault is the most probable source of the 1531 event. The largest inundation effects of the model correlate well with the historical descriptions: the impact is relevant in the inner Tagus estuary, but inundation in downtown Lisbon is small.

Lateral variations of the local magnitude at Ebre station, Northeastern Iberian Peninsula

Since 1984, a seismic surveillance at the Baix Ebre Region (NE of the Iberian Peninsula) has been carried out from a seismic network consisting of two digital and one analog stations. Large discrepancies between the recorded amplitudes and the magnitude estimates given by different agencies have been observed. To explain these discrepancies, that consist of a large reduction of amplitude and a strong dependence of the azimuth, a local magnitude formula for the area has been computed, using the analog records obtained at EBR short-period station. Global results show an average station correction factor between the computed local magnitude and the reported magnitude of the order of 1, presenting large variations around the mean value with a clear dependence on the azimuth. A detailed analysis of this variation around the mean value reveals that earthquakes can be classified according to their location in different geological units, thus obtaining for each zone a distinct station correction factor that can fluctuate as much as 0.6 around the average. These large lateral variations of the magnitude are in good agreement with previous studies of coda-Q attenuation.

Los terremotos del Bajo Valle del Tajo (Portugal): Lisboa 26 de Enero de 1531 y Benavente 23 de Abril de 1909

The Lower Tagus Valley (LTV) has been the source of several local earthquakes that affected the region of Lisbon, in the central part of Portugal. In spite of absence of strong earthquakes during the last 100 years, past events have produced large damage in this area. We present the state of the art concerning the last two major earthquakes, which occurred in 1531 and 1909. The effects of both events are described, based on coeval reports already interpreted by several authors. The source parameters determined by different authors are also presented. The characterization of the seismogenic potential of the LTV is very important to better estimate the seismic hazard and risk of the region of Lisbon and Tagus Valley.

Normal Faulting in the 1923 Berdún Earthquake and Postorogenic Extension in the Pyrenees

The 10 July 1923 earthquake near Berdun (Spain) is the largest instrumentally recorded event in the Pyrenees. We recover old analog seismograms and use 20 hand-digitized waveforms for regional moment tensor inversion. We estimate moment magnitude Mw 5.4, centroid depth of 8 km, and a pure normal faulting source with strike parallel to the mountain chain (N292°E), dip of 66° and rake of -88°. The new mechanism fits into the general predominance of normal faulting in the Pyrenees and extension inferred from Global Positioning System data. The unique location of the 1923 earthquake, near the south Pyrenean thrust front, shows that the extensional regime is not confined to the axial zone where high topography and the crustal root are located. Together with seismicity near the northern mountain front, this indicates that gravitational potential energy in the western Pyrenees is not extracted locally but induces a wide distribution of postorogenic deformation.