F. Carrilho

GEOALGAR Project: First Results on Seismicity and Fault-plane Solutions

—The project GEOALGAR, initiated in May 2000, is devoted to the geodynamic monitoring and seismic characterization of the Algarve region. A brief description of the project goals, as well as the first results concerning the analysis of the recent seismic digital data, from 1999 and 2000, are presented. After simultaneous inversion of the seismic data and the velocity model parameters, the relocation of the hypocenters was performed for two selected areas. Twenty-five earthquakes were used for Area 1 and 125 earthquakes for Area 2, selected from the period 1999–2000 and with magnitude ML ≥ 2.0. The results show that there are two main regions where there are more events: the Monchique region (inland) and another one in the area of Guadalquivir Bank (ranging from 36.4°N, 08°W to 36.8°N, 7.2°W); for Area 1, the hypocentral corrections are relatively small, with the focus slightly deeper than that in the old solution; for the more regional events, the hypocenters corrections are bigger, with the focus becoming more shallow. Fault-plane solutions for the recent events were also estimated, showing that best solutions are dominated by strike-slip movement consistent with a stress controlled by a horizontal compression in the NW-SE to NNW-SSE direction, with two exceptions showing reverse mechanisms, with maximum horizontal stress orientation slightly rotated to a N-S direction. These results are also in agreement with those presented in previous studies performed by different authors. The new epicentral locations show a more organized spatial distribution that could indicate a possible correlation with some known tectonic features. However the fault-plane solutions are considerably more difficult to correlate with the neotectonic features.

Geophysical study of the Ota-VF Xira-Lisbon-Sesimbra fault zone and the lower Tagus Cenozoic basin

This paper focuses on the interpretation of seismic reflection, gravimetric, topographic, deep seismic refraction and seismicity data to study the recently proposed Ota–Vila Franca de Xira–Lisbon–Sesimbra (OVLS) fault zone and the lower Tagus Cenozoic basin (LTCB). The studied structure is located in the lower Tagus valley (LTV), an area with over 2 million inhabitants that has experienced historical earthquakes which caused significant damage and economical losses (1344, 1531 and 1909 earthquakes) and whose tectonic sources are thought to be local but mostly remain unknown. This study, which is intended as a contribution to improve the seismic hazard of the area and the neotectonics of the region, shows that the above-proposed fault zone is probably a large crustal thrust fault that constitutes the western limit of the LTCB. Gravimetric, deep refraction and seismic reflection data suggest that the LTCB is a foreland basin, as suggested previously by some authors, and that the OVLS northern and central sectors act as the major thrusts. The southern sector fault has been dominated by strike-slip kinematics due to a different orientation to the stress field. Indeed, geological outcrop and seismic reflection data interpretation suggests that, based on fault geometry and type of deformation at depth, the structure is composed of three major segments. These data suggest that these segments have different kinematics in agreement with their orientation to the regional stress field. The OVLS apparently controls the distribution of the seismicity in the area. Geological and geophysical information previously gathered also points that the central segment is active into the Quaternary. The segment lengths vary between 20 and 45 km. Since faults usually rupture only by segments, maximum expectable earthquake magnitudes and other parameters have been calculated for the three sectors of the OVLS fault zone using empirical relationships between earthquake statistics and geological parameters available from the literature. Calculated slip rates are compatible with previous estimates for the area (0.33 mm yr –1 ). A more accurate estimation of the OVLS throw in the Quaternary

A Regional Approach for Earthquake Early Warning in South West Iberia: A Feasibility Study

Abstract A Spanish ALERT‐ES project was set up to study the feasibility of setting up an earthquake early warning system to warn of potentially damaging earthquakes that can occur in the Cape of San Vicente (SV)–Gulf of Cadiz (GC) area, located in the south west of the Iberian Peninsula, such as the 1755 Lisbon earthquake. Four events, located close to the epicenters of the largest earthquakes in the area, were simulated using different seismic software packages (Earthworm, SeisComP3, and PRobabilistic and Evolutionary early warning SysTem [PRESTo]) and the errors were analyzed. In addition, a study about the blind zone and the lead time at six selected targets was carried out. The results show a blind zone in the southwest corner of Portugal for SV earthquakes and also a blind zone in the coastal area, from Portimao to Cadiz, for the GC earthquakes.

Calibration of Local Magnitude MLin the Azores Archipelago Based on Recent Digital Recordings

The development of the digital seismic network in the Azores Archipelago during recent years made it possible to obtain the amplitudes (waveform) of recorded motion in a large set of stations. With this new data, maximum amplitudes of the Wood Anderson seismograph are computed, for each station/component, which, together with epicentral distances, allows for the estimation of local magnitude ML.

Improving the Seismic Hazard Evaluation of the Lisbon and Lower Tagus Valley Area

SUMMARY The Lisbon and surrounding area of the Lower Tagus Valley has experienced in the past the effects of several moderate sized earthquakes that caused significant damage and destruction. They have been attributed to local sources, though in some cases the source remains to be located. The lack of outcrops in the flat lying Quaternary terrains, the low slip-rates of the area in connection with sedimentation and erosion rates that erase surface ruptures are among the causes of a poor association between faults and seismicty, which has opened way to geophysical studies. Seismic, potential-field and seismicity data have been recently used to improve the seismic hazard evaluation of the area. In this work we complement these studies with DTM and PSInSAR data with the purpose of obtaining an accurate evaluation of the seismic hazard of the study area. The correlation of improved epicentre locations with major fault zones located from the above mentioned data shows that we have progressed in the understanding of the earthquake sources in the region. Some structures show no apparent relationship with present-day seismicity but some are known to be active into the Quaternary. Further geophysical and geological studies are required to understand the causes.