Paula Teves-Costa

Seismotectonics of Portugal and its adjacent Atlantic area

Abstract A study of focal mechanisms of earthquakes and stress indicators are presented for Portugal and its adjacent Atlantic margins. Recently Riberio et al. (1996) published a work on this subject. In this study, we introduce new focal mechanisms of nine earthquakes. We use these results together with 12 other focal mechanisms and the main tectonic features in the region to define more accurately the type of mechanism and the stress pattern of the region. All the focal mechanisms used in this study differ partially but are compatible with the regional stress field. The stress indicators resulting from focal mechanisms and other geological and geophysical data show that the Portugal continental and its Atlantic margins are under horizontal pressure in NNW–SSE direction, with a greater proportion of strike-slip and reverse-oblique mechanisms for the whole area.

Seismic behaviour estimation of thin alluvium layers using microtremor recordings

Abstract The microtremor spectral ratio proposed by Nakamura (A method for dynamic characteristics estimation of subsurface using microtremor on ground surface. QR of RTRI, 1989, 30(1), 25–33.) developed a new interest on the use of microtremor measurements for the estimation of site effects. The Lisbon region is an earthquake-prone area and several descriptions of destructive earthquakes are reported in the historical data, which suggest the existence of site effects in the town of Lisbon. Several seismic hazard studies were already performed, as well as some microzonation studies based on records from blasts. In order to characterize the seismic behaviour of alluvium layers existing in the town, two sets of microtremor measurements were performed. The data were processed according to the Nakamura technique and the results are presented here. These results were correlated with the thickness of the alluvium deposits and compared with the spectral ratios obtained using records from blasts, as well as with 1D theoretical models and with 2D theoretical simulations obtained by the use of the Aki-Larner technique. These comparisons seem to prove that the use of microtremor measurements could give a good contribution for the microzonation techniques, specially in urban zones, due to the simplicity and rapidity of their acquisition.

Source Parameters of Old Earthquakes: Semi-Automatic Digitization of Analog Records and Seismic Moment Assessment

Recovering seismic information contained in old analog records could increase our knowledge of seismic source characteristics and the seismicity of a region. This is particularly important in zones with low to moderate seismicity.To extract the available information, it is necessary to digitize the seismic records. This is not an easy task especially owing to the generally poor quality of the original records, with illegible or missing parts. However some exceptions were found in the records of a few seismological stations in Germany, Sweden and France.This paper presents an example of the recovering of source parameters and can be divided in two parts: the first one presents a simple semi-automatic technique for digitization of old analog seismic records, developed using commercial software on a PC; the second part describes the methodology of assessing the seismic moment, using empirical and theoretical relations, as well as the seismic source dimensions.The earthquake selected to illustrate this procedure is the 23 April 1909 Benavente (Portugal) earthquake. This earthquake occurred in the Lower Tagus Valley region and caused great destruction in the meizoseismal area. It is the biggest earthquake that occurred during this century in the central part of the country and its magnitude has been estimated between 6.6 and 7.6.The digitization procedure allowed the recovery of seismic information contained in old analog records, in particular, the seismic moment estimation. The results obtained indicated that 7.6 was a very high value for the magnitude of the 23 April 1909 earthquake, suggesting that the magnitude reported in the Portuguese catalogue is overestimated. The estimated moment magnitude is 6.0.

From Non-invasive Site Characterization to Site Amplification: Recent Advances in the Use of Ambient Vibration Measurements.

A series of investigations has been carried out over the last decade in Europe aimed at deriving quantitative information on site amplification from non-invasive techniques, based principally on surface wave interpretations of ambient noise measurements. The present paper focuses on their key outcomes regarding three main topics. First, methodological, hardware and software developments focusing on the acquisition and the processing of both single point and array microtremor measurements, led to an efficient tool with in situ control and processing, giving rise to robust and reproducible results. A special attention has been devoted to the derivation and use of the Rayleigh wave ellipticity. Second, the reliability of these new tools has been assessed through a thorough comparison with borehole measurements for a representative – though limited – set of sites located in Southern Europe, spanning from stiff to soft, and shallow to thick. Finally, correlations between the site parameters available from such non-invasive techniques, and the actual site amplification factors as measured with standard techniques, are derived from a comprehensive analysis of the Japanese KIKNET data. This allows to propose alternative, simple site characterization providing an improved variance reduction compared with the “classical” VS30 classification. While these results could pave the road for the next generation of building codes, they can also be used now for regulatory site classification and microzonation studies, in view of improved mapping and estimation of site amplification factors, and for the characterization of existing strong motion sites.

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.

Earthquake Damage Scenarios in Lisbon for Disaster Preparedness

Research on the earthquake history of the town of Lisbon has been improved in order to assure the scientific background to launch an effective earthquake disaster preparedness program. The characterization of the seismology of the area as well as its geology were fundamental in preparing a microzonation of the seismic risk of the building stock. Furthermore, to evaluate the seismic risk, the dynamics of the population, the property value and the vulnerability of the building typology were considered and integrated in a seismic impact model. This knowledge has been used to prepare the first version of the seismic risk emergency plan for the town of Lisbon, and many efforts are carried out to improve it in order to assure the adequate disaster preparedness.

Comment on “Lisbon 1755: A Case of Triggered Onshore Rupture?” by Susana P. Vilanova, Catarina F. Nunes, and Joao F. B. D. Fonseca

The work published recently by Vilanova et al. (2003) in the Bulletin of the Seismological Society of America proposes that the accounts of destruction and other effects reported in the Lisbon area as a result of the 1 November 1755 earthquake are best explained by a local rupture on the Lower Tagus Valley fault (ltvf), triggered by the static stress change produced by the main offshore source located in the Gorringe area. Because of the potential impact of this hypothesis on the seismic hazard of the Lisbon area, we discuss and complement the evidence presented by Vilanova and co-workers, concluding that the “local rupture model” should remain, for the moment, as an unsubstantiated speculation. Vilanova et al. (2003) (referred to herein as “the authors” or “the paper”) compare the intensities reported for the 1755 earthquake in Lisbon and Algarve to conclude that Lisbon intensities are abnormally high for the same distance, in particular, when compared with what was observed for the 28 February 1969 earthquake. They interpreted this as the result of local rupture. In Figure 1 we re-plot the macroseismic intensity as a function of the distance to the Gorringe Bank source presumed by the authors. We can see that “Lower Tagus Valley intensities” do not show any abnormal behavior. Similar plots could be made for all source areas proposed by other authors (Baptista, 1998; Baptista et al. , 1998a, 1998b; Zitellini et al. , 1999, 2001; Terrinha et al. , 2003) with minor changes to the conclusion we reached previously. The conclusions reached by the authors were based on their figure 8A, from which intensity values for the Algarve and Portuguese west coast corresponding to distances greater than 350 km are missing. Figure 1. Modified Mercalli (MM) intensities for the 1755 earthquake, plotted as a …

Damage seismic scenarios for Angra do Heroísmo, Azores (Portugal)

Angra do Heroísmo, located in Terceira Island, is the main town of the Central Group of Azores Archipelago and it is classified as World Heritage by Unesco. The preservation of this patrimony, located inside a seismic region, includes the implementation of a policy to protect it against earthquakes. The development of credible damage scenarios could help this objective, as they can be used in prevention measures to minimize the impact of future earthquakes. Two methods were used for seismic damage estimation: the capacity spectrum method and a new method, based on the direct response of a 1D non-linear oscillator to simulate the building seismic performance. Both methods seem to overestimate the damages for the near sources, but this could be due to the empirical model used to estimate the input ground motion. Comparison with the damage observed after the January 1st, 1980 earthquake show a large discrepancy with an underestimation bias, which can be explained, mainly, by the difference of building performance and on the plausible hypothesis of 2D site effects.

SHALLOW CRUSTAL MODELS IN THE LISBON AREA FROM EXPLOSION DATA USING BODY AND SURFACE WAVE ANALYSIS

Abstract Knowledge of the shallow velocity structure is very important to understand the possible site effects which could greatly contribute to the irregular damage distribution observed in Lisbon for the 1755 earthquake. Taking advantage of small charge blasts in the Tagus River bed in front of Lisbon, seismic records were obtained in a scattered array extending to a 8.5 km distance from the shotpoint. Geological considerations allowed the grouping of traces in wide profiles for three zones; West, East and South. The presence of a distinct group of Rayleigh waves made possible the joint modelling of first arrivals and surface waves for the West and East zones. The P and S-wave velocity models inferred show the existence of two low velocity layers (in the West zone), which is in agreement with the known geology for the area. The East zone structure could not be resolved in detail from refraction data only, but surface wave analysis allow the identification of the first low velocity layer. For the South zone, the study of first arrivals revealed a structure similar to the West zone, although velocity values and interface depths are slightly different. The interpretation of these models was done according to the age and lithology of the different geological formations. The final models obtained represent a significant improvement in detail over previous regional studies for the Tagus Valley basin ( Mendes-Victor et al., 1980 ) and over the preliminary interpretation of this data presented in Mendes-Victor (1987) .

Soil characterization for seismic damage scenarios purposes: application to Angra do Heroísmo (Azores)

Angra do Heroismo, the main town of Terceira Island in the Azores Archipelago, was hit in 1980 by a 7.2 magnitude earthquake that caused great destruction in the central part of the town. Taking into consideration the high seismic hazard of the region and the cultural and social importance of Angra do Heroísmo, the elaboration of damage scenarios is of particular importance to implement measures for preserve and protect the town against future earthquakes. The first step is to perform microzonation studies in order to characterize the soil seismic behaviour. Taking into consideration the available geologic, geotechnical and geophysical information, a detailed soil characterization was performed based on the results from numerical modelling and the analysis of microtremor experimental measurements. Nine different soil profiles were identified, characterized and classified. Discussion on the detailed soil classification and the Eurocode 8 soil classification is presented. This study shows that even with an available code, microzonation studies must be developed in order to identify differences on soil behaviour inside the interested area. It shows also that the use of experimental measurements presents a great help on soil characterization. The obtained detailed classification will be used on the estimation of damage scenarios for Angra do Heroísmo.