Geophysical methods applied to fault characterization and earthquake potential assessment in the Lower Tagus Valley, Portugal

Tectonophysics 418 (2006) 277 – 297 www.elsevier.com/locate/tecto

Geophysical methods applied to fault characterization and earthquake potential assessment in the Lower Tagus Valley, Portugal João Carvalho a,⁎, João Cabral b , Rui Gonçalves c , Luís Torres a , Luís Mendes-Victor d b

a Instituto Nacional de Engenharia, Tecnologia e Inovação, Estrada da Portela-Zambujal, 2720-461 Amadora, Portugal Faculdade de Ciências de Lisboa, Dep. Geologia and LATTEX, Ed. C6, 2° Piso, Campo Grande, 1749-016 Lisboa, Portugal c Instituto Politécnico de Tomar, Quinta do Contador, 2300-313 Tomar, Portugal d Instituto Geofísico Infante D. Luís, Rua Escola Politécnica 51, 1050 Lisboa, Portugal

Received 14 September 2005; received in revised form 2 February 2006; accepted 25 February 2006 Available online 24 April 2006

Abstract The study region is located in the Lower Tagus Valley, central Portugal, and includes a large portion of the densely populated area of Lisbon. It is characterized by a moderate seismicity with a diffuse pattern, with historical earthquakes causing many casualties, serious damage and economic losses. Occurrence of earthquakes in the area indicates the presence of seismogenic structures at depth that are deficiently known due to a thick Cenozoic sedimentary cover. The hidden character of many of the faults in the Lower Tagus Valley requires the use of indirect methodologies for their study. This paper focuses on the application of highresolution seismic reflection method for the detection of near-surface faulting on two major tectonic structures that are hidden under the recent alluvial cover of the Tagus Valley, and that have been recognized on deep oil-industry seismic reflection profiles and/or inferred from the surface geology. These are a WNW–ESE-trending fault zone located within the Lower Tagus Cenozoic basin, across the Tagus River estuary (Porto Alto fault), and a NNE–SSW-trending reverse fault zone that borders the Cenozoic Basin at the W (Vila Franca de Xira–Lisbon fault). Vertical electrical soundings were also acquired over the seismic profiles and the refraction interpretation of the reflection data was carried out. According to the interpretation of the collected data, a complex fault pattern disrupts the near surface (first 400 m) at Porto Alto, affecting the Upper Neogene and (at least for one fault) the Quaternary, with a normal offset component. The consistency with the previous oil-industry profiles interpretation supports the location and geometry of this fault zone. Concerning the second structure, two major faults were detected north of Vila Franca de Xira, supporting the extension of the Vila Franca de Xira–Lisbon fault zone northwards. One of these faults presents a reverse geometry apparently displacing Holocene alluvium. Vertical offsets of the Holocene sediments detected in the studied geophysical data of Porto Alto and Vila Franca de Xira–Lisbon faults imply minimum slip rates of 0.15–0.30 mm/year, three times larger than previously inferred for active faults in the Lower Tagus Valley and maximum estimates of average return periods of 2000–5000 years for M 6.5–7 co-seismic ruptures. © 2006 Elsevier B.V. All rights reserved. Keywords: Fault zones; Tagus River Valley; Seismic reflection profiles; Refraction methods; Seismic potential assessment

1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (J. Carvalho). 0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.02.010

The study region is located in the central-western zone of the Portuguese mainland and includes the

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densely populated and highly developed area of Lisbon. It is sited a few hundred kilometers north of the Iberia– Africa plate boundary (the Azores–Gibraltar fracture zone), in the Lower Tagus Valley (LTV). It includes the Lower Tagus Cenozoic Basin (LTCB), a tectonic depression filled with Cenozoic sediments that hosts the lower reach of the Tagus River (Fig. 1) and the Arruda sub-basin, which is part of the Mesozoic Lusitanian Basin. Earthquakes that caused severe damage and many casualties have periodically affected this region. The seismic activity comprises relatively distant events, as the 1 November 1755 earthquake, one of the largest historical earthquakes ever described (estimated magnitude ≥ 8.5, Martins and Mendes-Víctor, 1990). This major event was probably generated by N–S- to NNE– SSW-trending offshore structures located southwest of the Portuguese coastline (Zitellini et al., 1999, 2001; Baptista et al., 2003; Gràcia et al., 2003). Besides the effects of the earthquakes generated in the southwestern offshore area, directly connected to the Iberia–Africa plate boundary, the study region experiences a significant intraplate seismicity, attested by the occurrence of moderate to large historical earthquakes, as in 1344, 1531 and 1909, with estimated magnitudes

ranging from 6 to 7 (Mezcua, 1982; Moreira, 1985; Oliveira, 1986; Martins and Mendes-Víctor, 1990; Moreira, 1991) (Fig. 2). The earthquakes in 1344 and 1531 are poorly located due to the scarcity of historical descriptions, being positioned in the LTV based upon the destruction that they produced in the Lisbon area. Actually, the 1531 event caused severe damage and many casualties in the town of Lisbon, reaching an intensity of VIII–IX MM (Mezcua, 1982; Moreira, 1984; Oliveira, 1986; Henriques et al., 1988; Justo and Salwa, 1998) (Fig. 2). The 1909 event caused serious damage and several fatalities in the epicentral area, where liquefaction phenomena were observed. According to Moreira (1984), a maximum intensity of IX (MM) affected an area of 600–700 km2 centred approximately 20 km NE of Lisbon. Recently, Teves-Costa et al. (1999) calculated a moment magnitude of 6.0 for the 1909 earthquake based on analog seismograms recorded at the Strasbourg and Uppsala seismic stations. Despite its history of destructive earthquakes, the seismicity in the study region has been monitored by a sparse national network up to 1991, which precludes a good earthquake location. The few available focal depths (mostly between 10 and 20 km) indicate that

Fig. 1. Location map and simplified geology (after Oliveira et al., 1992) of the Lower Tagus Valley Cenozoic Basin. 1–oil-industry seismic reflection profiles used for structural mapping of the basin by Carvalho (2003); 2–deep wells; 3–localities mentioned in the text; A–profile Ar10-81; B–profile T16.2.

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Fig. 2. Epicentre distribution of instrumental seismicity in the Lower Tagus Valley Region and adjacent areas for the period 1915–2004, from the International Seismological Centre, On-line Bulletin, http://www.isc.ac.uk/Bull, Internatl. Seis. Cent., Thatcham, United Kingdom, 2001. Symbol dimension is proportional to magnitude: (1) M ≤ 2; (2) 2 < M ≤ 3; (3) M > 3 (<4). Seismic intensities (I MM) and isoseismal lines for the earthquakes in 1531 (dashed lines, adapted from Oliveira and Sousa, 1991) and 1909 (doted lines, intensity in italic, adapted from Moreira, 1984). L–Lisbon.

the seismic activity in the LTV area extends through the upper crust and that it is usually generated by faults in the Paleozoic basement, which rupture well below the Cenozoic sedimentary cover of the Tagus basin. The low instrumental seismicity, poor earthquake location and depth at which the earthquakes are generated in the study region do not allow establishing the relationship between the earthquakes and the faults recognized at the ground surface. The geometry of the Tertiary sedimentary basin also plays an important role on local energy enhancement and site effects, masking the relationship between the historical events location based on seismic intensity studies and the earthquake sources. This earthquake activity represents a serious threat for the densely populated region, stressing the need to identify and characterize the regional seismogenic faults for performing a thorough seismic hazard assessment. However, the low slip rates and the presence of a thick sedimentary cover that conceals faults in the underlying basement rocks complicate their study. Recent studies from several authors show the importance of local seismic sources to the seismic hazard of the LTV area (e.g., Peláez et al., 2002; Vilanova et al., 2003; Cabral et al., 2004; Vilanova and

Fonseca, 2004). Stich et al. (2005) have calculated a focal mechanism solution for the 1909 Benavente earthquake and obtained almost pure reverse faulting with nodal planes oriented (051°, 52°SE) and (242°, 38° NW), indicating that this event was generated by a ENEtrending fault in the Lower Tagus fault system. Due to the poor epicenter location and the likely interference of rupture directivity and site effects, it is difficult to accurately locate the earthquake and to correlate it with known regional structures. The Vila Franca de Xira– Lisbon fault, or the southern, hidden sector of the Azambuja fault (Carvalho, 2003; Cabral et al., 2003, 2004) is the nearest candidates, although they trend NNE–SSW. An alternative, as proposed by Stich et al. (2005), is that the Benavente earthquake was generated by an ENE–WSW-trending blind thrust beneath the Tagus Valley sedimentary basin. The buried character of many of the active and/or potentially active regional tectonic structures requires the use of geophysical methodologies for their knowledge. Accordingly, reprocessing and reinterpretation of seismic reflection data acquired for oil exploration in the LTV and surrounding areas was carried out in an attempt to improve knowledge regarding the deep structure of

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the LTCB, in particular to locate and characterize hidden faults that may be the source of the significant regional seismicity (Cabral et al., 2003; Carvalho, 2003). Two regional structures, the Vila Franca de Xira– Lisbon and the Porto Alto fault zones, were selected as priority targets for detailed investigation, based upon their near-surface expression on the oil-industry seismic reflection profiles, their significance in the Cenozoic basin structural pattern and the apparent relationship to the regional seismicity, as well as for easy accessibility. This work discusses the acquisition and analysis of geophysical data (high-resolution seismic reflection profiles, vertical electrical soundings [VES] and the refraction interpretation of reflection data) on these two fault zones in order to characterize near-surface faulting. 2. Geological framework 2.1. The Lower Tagus Cenozoic Basin The LTCB is a complex tectonic depression where up to 2000 m of Cenozoic sediments have been preserved. The lowermost deposits consist of 200–400 m of continental Paleogene sediments, which accumulated in response to secondary tension and subsidence related to NNE–SSW convergence of Iberia and Eurasia during the Upper Eocene and the Oligocene (Carvalho et al., 1983–85). The Paleogene structuring was largely obliterated by the following Miocene tectonics, when the LTCB evolved as a compressive foredeep basin related to tectonic inversion of the former Mesozoic extensional Lusitanian Basin, located to the West, under the action of a NW–SE compression (Ribeiro et al., 1990; Rasmussen et al., 1998). More than 800 m of continental and shallow-marine Miocene sediments have accumulated in depocenters. Less intense compressive tectonics continued up to the present. Subsidence slowed during the Pliocene, when fluvial sediments of a “pre”-Tagus River were deposited. Entrenchment of the fluvial network and the presence of Quaternary terraces provide evidence of reversal of the regional subsidence to uplifting since the Early Pleistocene (Cabral, 1995). According to data from borehole breakouts and earthquake focal mechanisms (Ribeiro et al., 1996; Borges et al., 2001), the maximum horizontal compressive stress presently trends NW–SE to WNW–ESE. The Neogene–Quaternary evolution of the LTCB was controlled by the NNE–SSW structural trend of the Lusitanian Basin. Tectonic inversion of former normal faults generated SE-verging reverse faults that place Mesozoic rocks of the Lusitanian Basin to the NW, over

Cenozoic sediments of the LTCB, to the SE (Fig. 1). Fault-controlled subsidence resulted in the accumulation of relatively flat-lying Neogene sediments. However, geological, gravimetric and seismic reflection data point to some complexity, evidencing structural lows and highs that are delimited by WNW–ESE to NW–SE transverse faults, in addition to the NNE–SSW-trending “longitudinal” faults (Cabral, 1995; Lomholt et al., 1996; Rasmussen et al., 1998; Cabral et al., 2003; Carvalho, 2003). The structural contour map of the base of Neogene (Fig. 3) obtained from the oil-industry seismic reflection data (Carvalho, 2003) shows that deep important faults occur below the Quaternary cover, such as the Pinhal Novo, Vila Franca de Xira–Lisbon and Azambuja faults (see location in Fig. 3). These NNE–SSW- to N–Strending, en echelon faults show significant Neogene vertical offsets and are linked by a system of WNW– ESE to NW–SE faults that apparently work as transfer structures inherited from the Mesozoic extensional regime, as the Porto Alto fault zone (Cabral et al., 2003; Carvalho, 2003). Albeit the likely influence of rupture directivity and site effects on the distribution pattern of the shaking intensity, a WNW–ESE structural trend is also suggested by the elongation of isoseismal lines of local earthquakes along that direction (earthquakes in March 19, 1914, M = 4.7, 15 km NE of Benavente, and in September 23 and 25, 1914, M = 5.3, near Benavente, according to Mezcua, 1982; Oliveira, 1986). The same direction is denoted by and the presence of a conspicuous WSW–ENE-elongated negative gravimetric anomaly in the area (Cabral et al., 2003; Carvalho, 2003). The study of these transversely trending structures is important because apparently, they exert some control on the location of the seismicity in the Tagus estuary region. However, as they are hidden under thick Holocene alluvia of the Tagus River, they have not been recognized in the field. Only their approximate location and deep geometry are constrained on the oilindustry seismic reflection data. 2.2. The Porto Alto fault zone One of the above-referred NW–SE to WNW–ESE major transverse structures was identified in oil-industry seismic section T16-2 (Fig. 4), consisting of a deepseated fault zone that extensively displaces the Cenozoic sediments. The recognition of a similar deformation zone in another seismic section located 6 km to the west strongly suggested that it was the same WNW–ESE-

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Fig. 3. Depth structural map of the base of Neogene showing the geometry of the Lower Tagus Cenozoic Basin and major faults, obtained from the interpretation of the oil-industry seismic reflection profiles (adapted from Carvalho, 2003). 1–faults after Oliveira et al. (1992); 2–shoreline. A–Porto Alto fault zone; B–Vila Franca de Xira–Lisbon fault zone; C–Azambuja fault zone; D–northern end of Pinhal Novo fault zone.

trending structure—the Porto Alto fault zone. It dips to the south, downthrowing the southern block with normal fault geometry and confining a Neogene depocenter. The lack of geomorphologic and outcrop evidence for active surface faulting in this area of the Tagus alluvial plain must not be taken as evidence of tectonic inactivity. It may be explained by the young age and large thickness of the alluvial sediments and by the relatively low fault slip rates. Data from several boreholes drilled for underground water exploitation show that the Upper Pleistocene to Holocene alluvial cover reaches a depth of up to 60 m in this area (Fig. 5). This results from a time of deeper incision of the Tagus River during the last glacial period, followed by aggradation in response to a rapid baselevel rise since final wurmian times through the Holocene (roughly during the last 15 ka). Average sedimentation rates were estimated from two cores drilled in the alluvial plain, 40 km (Fonte Bela, 7.40 m deep) and 55 km (Quinta da Boavista, 3.70 m deep) upstream from the Porto Alto site (Ramos et al., 2002). Three 14C ages were obtained at different depths in each core, giving an irregular sedimentation

rate, comprised between 0.1 mm/year and 3 mm/year. Considering the age of the deeper sample (3400 ± 40 years BP, at 740 cm in Fonte Bela core), a rate of 2.1 mm/year is estimated for the last 3.4 ka, while considering the older sample (4020 ± 40 BP, at 163 cm in Quinta da Boavista core), a lower rate of 0.4 mm/year is obtained for the last 4 ka. A rough estimation of an average sedimentation rate of the alluvial sediments can also be obtained considering 60 m of sedimentation roughly in 15 ka, giving a longer term deposition rate of 4 mm/year. The young age of the alluvial sediments and the relatively fast sedimentation rate imply that only the coseismic rupture of very recent earthquakes generated by the Porto Alto fault zone could be registered at the alluvial plain. Also, for a MW 6.2 earthquake, inferred from an estimated fault rupture length of 10 km (Cabral et al., 2003), the estimated average and maximum coseismic offsets of the ground surface are only 0.30 m and 0.42 m, respectively, taking the regressions of magnitude on average and maximum surface displacements by Wells and Coppersmith (1994). These correspond to very low scarps, easily obliterated during the frequent large floods that used to occur in the LTV.

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Fig. 4. Profile T16.2 from the oil-industry showing the fault zone investigated by high-resolution profiles PA1 and PA2. See Fig. 1 for profile location.

Albeit the lack of evidence of surface faulting on the alluvial plain, the observation on the oil-industry lowresolution seismic sections that the Porto Alto fault zone apparently affects the uppermost seismic horizons, was the criterion for performing high-resolution seismic reflection lines across the surface projection of this structure, in order to verify its relationship with the shallower sedimentary units and the Holocene alluvia in particular. 2.3. The Vila Franca de Xira–Lisbon fault zone The Vila Franca de Xira–Lisbon fault zone (Figs. 1 and 3) consists of a NNE-trending complex fracture zone known from the surface geology, deep seismic reflection data and also from aeromagnetic data

interpretation (Domzalski, 1969). It was generated in the Mesozoic as a large normal fault zone, bounding the Arruda half-graben of the Lusitanian Basin at its eastern side (Rasmussen et al., 1998), and was later tectonically inverted, moving with a predominant reverse-slip component during the Neogene. The fault zone is partially hidden under recent alluvium of the Tagus River plain along most of its length. For 5 km southwards of Vila Franca de Xira, it outcrops as a steeply dipping reverse fault thrusting Jurassic rocks of the Lusitanian Basin, at the west, over tilted Upper Miocene sediments of the Tagus Basin, at the east. Further south, it is intersected by a transverse NW–SE fault system (probably the Porto Alto fault system) and is apparently offset eastwards under the sediments of the Tagus alluvial plain, through a left-

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Fig. 5. Well locations and logs available in the Porto Alto and Vila Franca de Xira areas, drilled for geotechnical studies and water supply. a: Holocene; P–Pleistocene; N–Neogene; J–Jurassic. Seismic profile locations are also shown here and in Fig. 6.

lateral stepover. Seismic reflection data show that this thrust fault is just an outcropping branch of a rather complex normal fault system, which has suffered a variable degree of inversion. The extension of the Vila Franca de Xira fault zone southwards as far as Lisbon, as a blind thrust under the recent alluvia of the Tagus River plain, is suggested by the eastward-dipping monocline that affects the Tertiary sediments in this area. Geophysical data also show the presence of a steep eastward-dipping monocline and of a fault at depth near the western bank of the Tagus River south of Vila Franca de Xira (Domzalski, 1969; Walker, 1983; GPEP, 1986; Carvalho, 2003). The extension of the fault zone northwards of Vila Franca de Xira is indicated by the presence of outcropping faults affecting Jurassic rocks, and it is also suggested by data from a few oil-industry seismic profiles. The area is only covered by low-quality, nondigitally available profiles and this does not allow verifying if the fault affects the Cenozoic sediments in this sector. Although the fault zone shows a significant geomorphic expression, particularly near the village of Vila Franca de Xira where it is located along a steep slope at the western bank of the Tagus estuary, no evidence of

fault displacements affecting the Holocene alluvial deposits has been found so far. However, following a similar reasoning as for the Porto Alto structure, if we assume that the Vila Franca de Xira–Lisbon fault zone can generate a maximum earthquake of MW 6.7, for an estimated fault rupture length of 25 km (Cabral et al., 2003), the estimated average and maximum co-seismic displacements of the ground surface are only 0.67 m and 1.08 m, respectively (Wells and Coppersmith, 1994). The small vertical offset produced on the alluvial plain, easily obliterated by the river dynamics during floods, as well as the considerable thickness of the recent alluvial sediments and the low fault slip rate, may thus explain the lack of evidence for recent surface faulting. 3. Seismic reflection and geoelectric surveys 3.1. Seismic field parameters The Porto Alto fault zone was investigated with two overlapping high-resolution seismic reflection profiles with a total length of 800 m (PA1 and PA2), acquired near Porto Alto, across the estimated surface projection of the main fault identified on the T16-2 deep seismic reflection line. The aim was to better

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constrain the near-surface fault location and geometry and to determine its relationship with the uppermost sedimentary units in order to evaluate the fault activity. For this latter purpose, a shorter profile of nearly 100 m (PA3) was acquired over a segment of profile PA2, where the major fault was located. Two other parallel profiles were acquired later (PA4 and PA5). Fig. 6 shows the detailed layout of the seismic reflection profiles and VES acquired near Porto Alto. The Vila Franca de Xira–Lisbon fault zone was investigated with a unique 400-m-long profile (VF1), acquired on the Tagus alluvial plain near the village of Vila Franca de Xira, at a gap between the oil-industry seismic profiles. All the seismic lines were located over Quaternary alluvial sediments. The acquisition geometry for profiles PA1, PA2 and VF1 was the same: a fixed 50-Hz geophone spread of 48 channels with a 7.5-m shot and receiver spacing. Additional off-end shots were fired (5 for VF1, 2 for PA2). Each of these profiles has a total length of about 400 m. Profiles PA3, PA4 and PA5, acquired later to study near-surface faulting detected in

profile PA2, used a shot and receiver spacing of 2.5 m and reached a maximum length of about 120 m. A maximum coverage (fold) of 48 was attained in the center of each profile. PA3 receivers were located between CMP 195 and 227 of profile PA2. Due to a malfunction of the seismic source, the profile was terminated after 37 shotpoints, which means that subsoil coverage ends between CMP 222 and 223 of profile PA2. Profiles PA4 and PA5 were acquired parallel to PA3, to the northwest of this profile, spaced 40 m apart (Fig. 6). A seismic data acquisition system of 48 channels (two connected RAS-24) with an A/D converter of 24 bits and an accelerated weight drop of 250kg as a seismic source was used. A total length of 1200 ms was recorded for each shot, with a sampling rate of 0.5 ms. The area of the Porto Alto profiles is a rural zone with low cultural noise, which allowed a good seismic signal up to 1 s for the longer profiles (PA1 and PA2). For the shorter profiles (PA3 to PA5), no viable reflections are seen below 250–300 ms. Fig. 7 shows an example from profile PA3 of a typical raw shot gather, processed shot

Fig. 6. (a) General location of the high-resolution seismic reflection profiles (1), overlying an intra-Neogene fault map (interpreted from oil-industry seismic reflection profiles after Carvalho, 2003). (b) Detailed layout of the high-resolution seismic profiles and VES of Porto Alto. A–Porto Alto fault zone; B–Vila Franca de Xira–Lisbon fault zone; C–Azambuja fault zone.

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gathers and respective amplitude spectra, as well as the amplitude spectra of the final migrated section. The seismic signal (reflections) has a frequency range between 30 and 100 Hz. The area of profile VF1 is highly populated so cultural noise was always present in the data and the signal-to-noise ratio is poor below 600– 700 ms. Using a 70-Hz value for frequency and the P-wave velocities from Table 1, one obtains a horizontal resolution for the Porto Alto profiles of 10 m at about 100 ms and of 39 m at nearly 300 ms. As the CMP spacing is 3.75 m for profiles PA1 and PA2 and 1.25 m for the shorter profiles (PA3 to PA5), each Fresnel Zone is sampled about three times for the upper part of the longer profiles, but at intermediate and lower parts of all profiles, it is always sampled more than six times, which is quite satisfactory (Steeples, 1997). 3.2. Vertical electrical soundings VES were acquired over all the seismic profiles in order to complement the information (see Fig. 6 for location). A Schlumberger array was used, with an AB spacing (current injection) ranging from 400 to 1200 m. VES-3 was acquired over profile PA1, VES-4 over line PA2 and VES-5 over lines PA2 to PA5, while VES-6 and VES-7 were acquired at the far ends of profile VF1. Due to the lack of lateral space to implement the field layout due to the presence of cultivated terrain, it was not possible to acquire a VES in the middle part of the latter profile.

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Velocity analysis followed either fitting hyperbolas to the shot gathers or using constant velocity panels. Three iterations of residual statics (surface-consistent) and velocity analysis were then applied. Parameters for automated statics were chosen from the analysis of common-receiver point and common shotpoint stacks. To avoid cycle skips and considering that usable frequencies of up to 100 Hz (see Fig. 7) are present in the data, maximum allowable shifts used were up to 10 ms. After determination of the velocity functions, NMO correction with stretch mute (60%) and stack of all CMP gathers were performed. Data were then DMO corrected due to some inclination of the reflectors, velocity analysis repeated and FK migration applied, in order to correct reflectors dip and improve seismic resolution. Finally, some poststack processing was performed on the data, to improve S/N ratio and eliminate migration grins and smiles. FK filters, time-variant bandpass and coherency filters or a spatial noise filter (Hornbostel, 1991) were applied to eliminate incoherent noise present in the stack. All sections were depth converted using interval velocities obtained from the stacking velocities. Table 1 shows the interval velocities used for the Vila Franca de Xira and Porto Alto profiles. For profile PA1, an average between the two CMP used for the velocity analysis is presented, since similar velocities were found along the profile. The velocities in the first CMP of profile PA2 where velocity analysis was carried out (not shown) show similar values to profile PA1. The velocities encountered for profiles PA3 to PA5 are also quite similar to the ones presented in Table 1 for profile PA1.

3.3. Seismic reflection data processing 3.4. Refraction processing of the reflection data Data processing was undertaken with Seismic Processing Workshop (SPW™) and consisted of a standard flow. After data conversion from SEG-2 to SPW digital format, geometry introduction, trace editing, spherical divergence correction, frequency filtering (Butterworth bandpass in the range 35– 300 Hz with a 16 dB/oct rolloff), first arrival mute and deconvolution (typical parameters used were around a 20 ms prediction leg operator and an inverse filter length of about 160 ms) were applied. Due to the presence of possibly aliased surface waves (see Fig. 7c), surgical mute followed by amplitude trace equalization (trace constant equalization with a 650-ms window) was applied to the shot gathers. Elevation corrections were not necessary because differences in elevation greater than 2 m were not found along each profile (400 m in length). Examination of first arrivals showed that refraction statics were not required.

Refraction processing of first arrival data was performed with commercial software using the method of Haeni et al. (1987). This method uses the classical delay times method for a first model and, through minimum square minimization of the residuals between observed times and ray tracing, produces an optimized model. Velocities are calculated by a weighted average (according to the number of data points used in each determination) of a simple linear regression of timedistance curves and the velocity function of Palmer (1981). 3.5. Geoelectric data processing The apparent resistivity curves were inverted using an in-house software that uses a derivative inversion algorithm and the Levenberg–Marquardt convergence.

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Fig. 7. (a) Example of a typical raw field record from profile PA3 and respective amplitude spectra of 4 intermediate offset traces (11.25–18.75m). (b) The same field record and amplitude spectra after pre-processing and deconvolution. (c) After surface wave surgical muting and amplitude equalization. (d) Amplitude spectra of traces in the migrated section of profile PA3 (Fig. 9).

J. Carvalho et al. / Tectonophysics 418 (2006) 277–297 Table 1 Interval velocities obtained from the stacking velocities used in depth conversion of high-resolution seismic reflection profiles PA1/PA2–Porto Alto

VF1–Vila Franca de Xira

Average PA1

CMP 120

CMP 247-PA2

CMP 170

TWT Velocity TWT Velocity TWT Velocity TWT Velocity (ms) (m/s) (ms) (m/s) (ms) (m/s) (ms) (m/s) 70 100 200 300 450 600 850

375 603 1340 1634 2230 2848 2890

100 160 250 280 320 500 850

451 1118 1731 2291 1676 1761 2499

230 350 500 740 850

867 2320 2837 3068 3922

70 140 400 650 850

908 1746 4290 4101 4330

The rms errors for the inversion of each VES are generally acceptable. 4. Data interpretation 4.1. General considerations As there were no well logs available along the seismic lines, it was not possible to directly tie the seismic horizons with the geological units present in the area. However, with the help of seismic stratigraphy (Mitchum et al., 1977; Roksandiæ, 1978), seismic attributes, data from nearby wells, geological outcrop information and the interpretation from the oil-industry profiles (Rasmussen et al., 1998; Cabral et al., 2003; Carvalho et al., 2005), it was possible to infer the geological units present in the seismic sections. In fact, we could use the logs from several wells a few hundred meters deep, drilled in the study region for engineering or water supply purposes and that are located in the vicinity of the seismic profiles (within 4– 8 km from the Porto Alto profiles and only a few hundred meters from both ends of the Vila Franca de Xira profile VF1, Fig. 5) for constraining the geological units in the profiles. The results from the VES were also analyzed to supply further information, and a refraction interpretation, based on the first arrivals of reflection data, was also undertaken. Though a specific refraction survey is expected to provide more detailed results (Phillips et al., 1997), valuable information can be obtained for the near-surface from the performed analysis. Assuming a dominant frequency of 70 Hz for the reflection data (Fig. 7) and an average P-wave velocity of 1300 m/s (Table 1), an average vertical resolution between 2.3 m and 4.6 m is obtained depending on the noise level, according to the criteria usually found in the

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literature (e.g., Widess, 1973; Yilmaz, 1987). A value between the two numbers is probably realistic for these profiles due to a good signal-to-noise ratio. To help the interpretation, trace-to-trace complex seismic attributes (Taner et al., 1979), namely, the instantaneous phase, amplitude strength and instantaneous frequency were calculated from the stacked data. The first one enhances tenuous reflections while the others allow identification of seismic sequences. Both attribute sections showed very similar structural aspects but the latter provided information of the several seismic patterns present in the stacked data. For fault marking and seismic pattern analysis, besides visual analysis of time and depth sections and respective complex seismic attribute sections, amplitude derivatives and differences of the referred seismic sections were used. The interpretation of the six reflection profiles is presented in (Figs. 8, 9, and 13), with a variable area-wiggle display to enhance structural aspects. The complex seismic attribute “reflection strength” of the Porto Alto profiles is shown in Fig. 10, with a variable density color display. 4.2. The Porto Alto profiles Migrated sections PA1 and PA2 (Fig. 8) show a parallel configuration of the seismic reflectors, except for profile PA1 between approximately 150 and 250 ms, where a clinoform configuration is observed. In the first 100 ms, the reflectors present strong amplitude and excellent continuity. From well information located 4– 8 km away (Fig. 5), the base of the Quaternary is found at a constant depth of 50–60 m, which corresponds approximately to 145–162 ms twt, considering the interval velocities (Table 1). This agrees with the interpretation of profiles PA1 and PA2 that present a seismic interface at about the same depth. This seismic pattern generally rises from SW to NE but, due to faulting, its base is nearest the surface at the southwestern end of profile PA2. Results from three VES acquired over profiles PA1 and PA2 support these findings. Fig. 11 shows the fitting of the apparent resistivities theoretical curves to the measured points. All VES were interpreted with a three-layer model in which resistivities range approximately from 0.5 to 105 Ω m, typical of clastic Tertiary and Quaternary water-saturated sediments (Dobrin, 1976). The first layer, above the water table, is always more resistive. VES-5 presents higher resistivity values because, due to technical problems, the sounding had to be repeated a few months later in much dryer soil conditions. VES-3, -4 and -5 indicate an upraise of geoelectric horizons to

288 J. Carvalho et al. / Tectonophysics 418 (2006) 277–297 Fig. 8. Migrated and interpreted depth-converted stack of profiles PA1 and PA2. Location of VES are also shown. Solid lines–interpreted faults. QB–base of Quaternary. BPL–base of Pliocene or Upper Miocene boundary.

J. Carvalho et al. / Tectonophysics 418 (2006) 277–297 Fig. 9. Migrated (left) and depth-converted (right) stacks of profiles PA3, PA4 and PA5. Depth conversion was done with interval velocities obtained from stacking velocities stacking. Solid lines– interpreted faults. QB–base of Quaternary.

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Fig. 10. Complex seismic attribute “reflection strength” for the Porto Alto profiles. Interpretation from Figs. 8 and 9 is overlaid, showing consistency of data interpretation.

the northeast, with VES-4 and -5 presenting similar depths of the geoelectric horizons. Below the inferred unconformity at the base of the Quaternary and the sector showing the clinoform configuration, a zone of intermittent, coarsely spaced reflectors is observed on the profiles up to 400 ms (about 362 m, using interval velocities from Table 1), after which the spacing of the reflectors decreases and their continuity improves. From the seismic to well tie produced for the interpretation of the oil-industry seismic reflection profiles in the study region (Rasmussen et al., 1998; Carvalho, 2003; Carvalho et al., 2005),

the upper geologic horizon interpreted in the wells was an interface near the top of Upper Miocene sediments. This interface roughly corresponds to a seismic horizon that can be picked throughout the Tagus estuary area (intra-Neogene horizon). On the two oil exploration sections that are closest to the high-resolution profiles PA1 and PA2 (nearest points located 750 m and 200 m, respectively), the intra-Neogene horizon was found roughly at 500 ms (490 m, using stacking velocities). Therefore, and accordingly to the lithological logs of wells in the surrounding area, the zone of coarsely spaced reflectors

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Fig. 11. Apparent resistivity curves from the vertical electrical soundings acquired over seismic reflection profiles PA1 (VES-3), PA2 (VES-4 and -5), PA3 (VES-5) and VF1 (VES-6 and -7). The dashed line represents the true resistivity vs. depth model, with the uppermost layer omitted. The dotted lines are displayed to help visualization. Note higher resistivity in all layers of VES-5 compared to VES-3 and VES-4 (see text for explanation).

observed on profiles PA1 and PA2 up to a depth of approximately 400 ms (362 m) probably corresponds to Pliocene and/or Miocene poorly consolidated sandy

and clayey sediments. Below that depth, the zone of improved reflector's continuity and tighter spacing probably corresponds to the presence of more

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consolidated Miocene sandy limestones, clays and sandstones. The base of the Tertiary sediments in the oil-industry profiles is at a depth over 1.3 s twt and, therefore, is not imaged in the seismic reflection profiles PA1 and PA2. The Porto Alto profiles PA1 and PA2 show a large deformation zone in accordance with the oil-industry profile T16-2 (Fig. 4), marked by a complex system of sub-vertical faults and faults showing normal offset. The two southernmost faults in profile PA2 seem to affect the unconformity surface at the base of the Late Pleistocene to Holocene alluvial fill. The vertical offsets of the faults detected in profiles PA1 to PA2 are difficult to estimate. Lateral geological facies variations and the alternation of thin layers of clays, sandstones and silts that compose Neogene and Quaternary formations do not produce a continuous relevant seismic marker that may allow a measurement of fault throws. Due to expected small co-seismic ruptures and the low slip rates estimated for the active faults in the study area, the vertical offsets produced by these faults on the Quaternary sediments are estimated to be of only a few meters. Considering that the vertical seismic resolution is around 2–5 m, it is very probable that the fault offsets affecting the Quaternary sediments are close to the limit of the vertical seismic resolution and so are quite difficult to detect in the time and depth seismic sections. Brittle deformation may also be masked by the plasticity of the watersaturated alluvium. Upper Miocene slip rates were probably greater and the accumulated fault throws may be more easily observed, though geological unit's thickness and alternation associated with lateral facies variations still make them difficult to be detected. The constancy of stacking velocities inside each profile shows that the detected faults are not the effect of lateral velocity variations. Lateral velocity differences are found only from profile PA1 to PA2 (see Table 1) and also inside profile PA2 (not shown here). Refraction interpretation of section PA3 (Fig. 12) confirmed the rise of the horizons to the SW, in accordance with PA2 interpretation. It also agrees with VES results that present a rise of geoelectric horizons in VES-5 comparably to VES-3 and VES-4. P-wave velocities found also agree with the interval velocities obtained during data processing shown in Table 1. The seismic interface detected at an average depth of 17 m has several ondulations (2–3 m of vertical offset in 20– 25 m of horizontal distance) that might correspond to stratigraphic structures, lateral velocity variations or faults.

Using both refraction and reflection information from profiles PA3 to PA5, it was possible to better clarify the period of activity of the faults detected in profiles PA1 and PA2. Two faults are interpreted on the sections PA3 to PA5 (Fig. 9), with the northernmost affecting the base of the Quaternary. A significant difference in the seismic character of the reflectors can be observed on all profiles at around 100–200 ms (Figs. 8 and 9). Regardless of using stacking velocities from the Porto Alto profiles (Table 1) or velocities obtained from the refraction interpretation (Fig. 12) an average depth of 40–50 m is obtained for that change in the seismic character of the reflectors. This boundary very likely corresponds to the unconformity at the base of the Upper Pleistocene to Holocene alluvial fill of the Tagus River wurmian paleo-valley, cut on the underlying Neogene sediments, which is found at an average depth of 50–60 m in nearby wells (Fig. 5). Some differences are found in the two faults interpreted in the three closely spaced parallel profiles. This distinct fault pattern may reflect intricate shallow fault geometry due to the complexity of fault propagation towards the surface in the poorly consolidated sediments of the Upper Neogene sedimentary fill. The two faults appear to coincide with the deep-rooted normal faults seen at the southwestern end of profile PA2 around CMPs 196 and 206. In accordance with the proposed interpretation and depth conversion shown in Fig. 9, the northeastern fault on profiles PA3, PA4 and PA5 seems to extend up into the quaternary alluvium, while the other fault seems to stop near the base of the Quaternary sediments, affecting only Neogene sediments. This is confirmed by the instantaneous amplitude seismic attribute and the refraction interpretation of these profiles (Figs. 10 and 12). The small disturbance produced by the northeastern fault on the Quaternary alluvia is close to the vertical resolution of the seismic reflection data, implying a minimum vertical throw of 2.3–4.6 m, while about 5 m are inferred from the refraction interpretation. VES data also support a similar vertical throw. 4.3. The Vila Franca de Xira Profile The migrated section of profile VF1 (Fig. 13) shows an eastward steeply dipping monocline, with deepening of the seismic interfaces eastwards, towards the Tagus River, followed by a rise of the seismic reflectors, which design a small basin in the center of the profile. Strong deformation is also visible on the depth-converted profile, suggesting the upthrow of the western block of a major reverse fault.

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Fig. 12. Refraction interpretation of the reflection data from profiles PA3 to PA5. Letters on the seismic interface represent emerging rays from the respective shot. Their dispersion around the interface reflects the quality of the solution. P-wave velocities are indicated for each layer. Also shown is the location of the vertical electrical sounding.

Two distinct seismic patterns are observed, whose frontier stands between 80 m (100 ms) to the west and 130 m (160 ms) to the east. It may correspond to the unconformity between the Tertiary and the underlying Jurassic sediments, which outcrop 400 m to west. The

upper pattern of reflectors presents a better continuity and stronger amplitude, in accordance with the greater lithological variations that compose the Tertiary units. Seismic reflectors of lesser amplitude and moderate continuity are observable at greater depths.

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Though Jurassic sediments outcrop 400 m to the west of the profile, it is difficult to estimate at what depth these sediments are under profile VF1 location. Available wells located in the vicinity (Fig. 5) show that 250–300 m to the west of the seismic section Jurassic sediments are at a depth of about 25 m and that Tertiary deposits are absent. In the area to the east of the seismic profile, the wells reach depths of several hundreds of meters but reports and well logs often only describe lithologies, without any reference to their age. In such cases, well stratigraphy was inferred from the lithological sequences and SPT data in the framework of the regional geology. The well data, however, suggest that the Tertiary sediments already reach a thickness of hundreds of meters at a short distance (to the east) of the seismic profile, suggesting the existence of a large fault.

Two major faults are observed on profile VF1, the westernmost presenting reverse offset. These structures cut Jurassic and Tertiary sediments, but the quality of the data do not allow confirming if they reach the near surface. Data from wells (Fig. 5) show that the base of the Quaternary varies from around 20 m to the west to about 50 m to the east. On the western side of the profile, the Quaternary is underlain by Jurassic units while to the east it is underlain by Tertiary sediments. This reinforces the presence of a major fault in between (Fig. 13). VES data acquired at both ends of the profile do not indicate significant differences in the depth of the geoelectric horizons that might signal the presence of major faulting at the investigated depths (30–40 m at the most), though a slight upraise to the west (towards VES-6) of around 5 m was found. VES-6 and -7 show a similar behavior to the Porto Alto VES, although compared to VES-3 and

Fig. 13. Migrated and interpreted depth-converted sections of profile VF1, acquired near Vila Franca de Xira, with VES location indicated. Solid lines–interpreted faults. BT–base of Tertiary; QB–base of Quaternary; J–Jurassic.

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-4, the third layer is more resistive. This might indicate a decrease in clay content or the presence of a sandstone or limestone layer. This higher resistivity layer probably corresponds to Jurassic rocks, as they outcrop a few hundreds of meters to the W of profile VF1 and, as shown by the wells, dip towards the E. Refraction interpretation of reflection data (not shown here) also shows significant dips (6°) of the base of Quaternary sediments in the western and central part of the profile, with upraise at the westernmost part of the profile (about 5 m), in accordance with the seismic reflection profile and VES interpretation. This upraise to the west can also be explained by a preHolocene relief and not necessarily by faulting. P- and S-wave high-resolution reflection profiles with a tighter receiver, shot and source-nearest offset spacing, such as PA3 to PA5, are needed to further investigate if the Vila Franca de Xira–Lisbon fault affects the Quaternary alluvial sediments. 5. Conclusions The aim of this work was to investigate the tectonic activity of two regional deep fault zones that are known to exist in the Lower Tagus Valley from geological, seismic reflection and potential field data, and that may be a source of the significant seismicity that affects the study region. One is the NNE–SSW-oriented, reverse fault zone that borders the Tagus estuary from Lisbon to north of Vila Franca de Xira. The other is a NW–SEoriented normal fault zone located south of Porto Alto. Both fault zones are known (from outcrop and/or seismic data) to offset Upper Miocene sediments. It was intended to characterize near-surface faulting produced by these regional structures. As deep oil-industry seismic profiles provide poor near-surface resolution, there was the need to apply high-resolution seismic reflection in sites where the geological structures are concealed by Holocene alluvium that covers most of the study area. The two faults were selected according to their inferred importance in the regional seismotectonics framework and to acquisition constraints. Several high-resolution seismic reflection profiles and vertical electrical soundings were therefore acquired and their data were processed and interpreted. A 7.5-m receiver and shot spacing was adopted in order to locate the fault zones, and acquisition with tighter receiver and shot spacing was used to improve near-surface imaging. To assist the seismic reflection interpretation, analysis of first arrivals data, VES data, well data, amplitude derivatives and differences of the stacked sections and complex seismic attributes were useful tools.

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The analysis of the reflection data, the refraction interpretation of reflection data and the geoelectric results confirm the presence of the Vila Franca de Xira– Lisbon fault zone to the north of Vila Franca de Xira, establishing the total fault length to more than 20 km in the Tertiary rocks. A complex fault system was detected near Porto Alto, apparently with a normal offsetting component. This confirms the inferred strike of the fault zone previously established on the sparse oil-industry profiles. The interpretation of the collected data strongly supports the possibility that at least one of the detected faults in the Porto Alto fault zone affects the Upper Pleistocene to Holocene alluvial sediments. Due to its length (at least 10 km) and proximity to Lisbon and other smaller cities, it might represent a serious hazard to the region. The results presented here are in agreement with previous work of several authors, which point to the importance of local seismogenetic sources in the seismic hazard estimation of the study area (Peláez et al., 2002; Vilanova et al., 2003; Vilanova and Fonseca, 2004). The capability for detecting fault offsets on the seismic lines is constrained by the vertical resolution of the seismic reflection data, which is between 2.3 m and 4.6 m. Accordingly, if we consider the estimated average co-seismic displacements of approximately 0.3 m and 0.7 m for the Porto Alto and the Vila Franca de Xira fault zones, respectively, and admitting dip–slip movement on the faults (the most favourable kinematics for accumulating vertical offset), we conclude that a minimum of 8–15 events for the Porto Alto and 3–7 events for the Vila Franca de Xira fault zones are needed in order to detect some vertical offset of the ca. 15 ka years old unconformity surface at the base of the Quaternary alluvial cover in the Tagus Valley. These values imply minimum fault slip rates of 0.15 and 0.30 mm/year, and maximum estimates of the average return period for the co-seismic ruptures in the two active faults of approximately 2000 and 5000 years, respectively. Considering a vertical throw of approximately 5 m of the unconformity surface at the base of the Quaternary alluvia by the Porto Alto fault zone, as indicated by seismic refraction and VES data, we get a similar slip rate of 0.33 mm/year, although 17 MW = 6.2 events are needed to accumulate that vertical offset, with an average return period of approximately 900 years. Potential aseismic fault slip was not considered in the return period estimations as, although it may occur, it is not possible to evaluate in the study region. Accordingly, the obtained recurrence values should be taken as conservative estimates because it was assumed that fault slip occurs entirely by co-seismic slip during maximum

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(characteristic) events. Although slightly larger, the obtained slip rates are of the same order of magnitude as those previously inferred for active faults in the Lower Tagus Valley (Cabral, 1995; Cabral et al., 2003). The dip–slip hypothesis considered in the above calculations may lead to underestimation of the slip rates needed for accumulating the vertical offsets to the detection threshold of the seismic data and, accordingly, overestimate the return period of the co-seismic ruptures. In fact, there probably exists some left-lateral transpression in the LTV fault system (Fonseca and Long, 1991; Cabral et al., 2003; Vilanova and Fonseca, 2004). However, the present regional orientation of the maximum horizontal compressive stress (SHmax.), which trends NW–SE to WNW–ESE according to data from borehole breakouts and earthquake focal mechanisms (Ribeiro et al., 1996; Borges et al., 2001) points to a relatively small strike–slip component on the NNE– SSW-trending faults in the LTV. The historical earthquake record in the LTV (1344, 1531 and 1909) indicates much shorter average return periods for the 6–7 magnitude regional earthquakes than those inferred from the geological record. This may be explained by time clustering of the earthquakes due to the interaction between adjacent faults of the Lower Tagus fault system. Accordingly, M 6–7 earthquakes probably occur in clusters, where events are separated by recurrence times of the order of 102 years. As stress is released along the fault system, the next earthquake set probably occurs separated by recurrence times of the order of 103 years, in agreement with the geological data. A major question is whether the stress accumulated in the LTV fault system is already entirely relaxed, and the regional moderate to large earthquakes will recur after the long time period needed for tectonic stress loading, or the fault system is not yet entirely relaxed and so the next event will occur within the much shorter time range indicated by the historical seismicity. Albeit the large uncertainties involved in the above calculations, we may conclude that the geophysical exploration methods used in this study, namely, highresolution seismic reflection, seismic refraction and VES, with their inherent vertical resolution, seem adequate to detect fault activity within the last ca. 15 ka for the fault zones in the study region, although they are in the limit of detection which precludes a detailed characterization. Acknowledgments The Portuguese Foundation for Science and Technology (FCT) and the European Community supported

the SHELT (Seismic Hazard Evaluation of the Lower Tagus Valley) project (POCTI/CTA/11178/2001) and the first author with a 4-year Ph.D. grant. The support of the Geophysics Division of the former Instituto Geológico e Mineiro, a SHELT partner, is also acknowledged. The authors thank those who contributed to the final results presented here: Luís Matias for discussions on the interpretation of the seismic reflection profiles, Elsa Silva for the same reason and additionally for calculating the seismic attributes of the Porto Alto seismic profiles. The authors are also indebted to the field crew: J. Leote, L. Pinto, R. Castro and R. Rocha. We thank João Lopo Mendonça for providing borehole data from the Tagus alluvial plain and Pedro Falé for graphical improvements to most of the figures and R. Caranova for the epicenter map. We additionally acknowledge William Stephenson and Thomas Pratt for their significant improvements to the original version of the manuscript.

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