Eos, Vol. 82, No. 26, June 26,2001

EOS.TRANSACTIONS, AMERICAN GEOPHYSICAL UNION

VOLUME 82 NUMBER 26

JUNE 26, 2001

PAGES 285-296

Source of 1755 Lisbon Earthquake and Tsunami Investigated PAGES 285, 290-291

On November 1,1755, the city of Lisbon was completely devastated by the combined effect of a tremendous earthquake, tsunami waves, and fire. The 1755 Lisbon earthquake was the most destructive cataclysm recorded in western Europe since the Roman Republic, with an estimated earthquake magnitude M w -8.5 [Martins and Mendes Victor, 1990] and estimated tsunami magnitude of Mt= Mw= 8.5.The earth­quake was felt as far away as Great Britain and Finland.The tsunami hit many coastal cities along southwest Iberia and North Africa, causing heavy destruction in Tanger and Casablanca.

However, even though the earthquake's epicenter is known to have been offshore, the exact location remains controversial. Many authors inferred the source area to be around the Gorringe Bank (Figure 1). Others located the source area closer to the coast, along the continental slope of southwestern Iberia. Multi-channel seismic data and study of earth­quake distribution in the area suggest that the Marques de Pombal (Figure 1) was a likely source location for the 1755 event, and this location could be useful in the study of tsunami generation.This advancement in understand­ing the 1755 source location resulted from a research cruise taken in the autumn of 1998 (Figure 1,BIGSETS lines) and from the outcome of a new seismic network operating since 1995 in southwestern Portugal (Figure 1).

Earthquake Source

The occurrence of other historical tsunamis in southwestern Iberia goes back at least to 60 B.C. [Campos, 1991],demonstrating the potential threat to the area.The location's seis-micity results from tectonic activity along the Europe-Africa plate boundary (inset of Figure 1), which trends roughly east-west, connecting the Azores-Triple Junction to the Gibraltar Strait with mainly north-northwest-south-southeast-trending compressive stresses near Cape San Vincente and demonstrates a conver­gence rate of 4 mm/yr in the last 3 million years. Between the Gorringe Bank and Gibral­tar, the plate boundary consists of diffuse, active compressive deformation distributed over a 200-300-km-wide area [Sartori et al,

1994] .The seismicity pattern reflects this kind of plate interaction, with the presence of scat­tered hypocenters spanning from shallow to intermediate depth [Buforn et aL, 1988]. Focal mechanisms show a mixture of thrust and strike-slip motion (Figure 2).

In this regional frame, the location of the 1755 Lisbon earthquake and tsunami is still a debated question. Early determinations, based on iso-seismal maps, suggested it was in the vicinity of the Gorringe Bank, a huge, uplifted block of oceanic mantle bounding the Tagus Plain on

39 N

the south (Figure l).This interpretation was reinforced by the small, tsunamigenic earth­quake that occurred on February 28,1969, south of the Gorringe Bank (Figure 2). Other studies, based on numerical modeling [Bap-tista et al, 1998b] and geological investigations [Zitellini et al, 1999], favor a localization along the southern Portuguese margin.

Recently, Baptista et al. [1998a] revisited all the historical data regarding the tsunami generated by the 1755 Lisbon earthquake to determine travel time, polarity of first move­ment, maximum run-up height, period, num­ber of waves, duration of sea disturbance, and extent of flooding reported from many localities along the coasts of Portugal, Spain, Morocco, the Azores, and Madeira. Baptista et al. [1998b] used these tsunami parameters

38 N

37 N

36 N

35 N 12W 11 W 8 W 7 W 6 W

Fig. 1. Bathymetric map of southwestern Iberia (data from GEBC097 Digital Atlas Web site: www.nbi.ac.uk, color bar scale in meters) with location of seismic stations and MCS lines. Inset map sketches the main elements of plate boundaries (data from NOAA Global Relief Data); the stippled patch indicates absence of well-defined plate boundary. MAR: Mid-Atlantic Ridge; TR: Terceira Ridge; GF: Gloria Fault. Solid arrows show relative plate kinematics; shaded box outlines area of figures I and 2 in the plate boundary framework. (This figure and figure 2 were produced using GMT software [Wessel and Smith, 1991].) Original color image appears at the back of this volume.

Eos, Vol. 82, No. 26, June 26, 2001

Fig. 2. Bathymetric map of southwestern Iberia with location of earthquake epicenters recorded by the new seismic network from January 1995 to March 2000; the contour intervals are as in Figure 1. The focal mechanisms of main earthquakes before March 1995 are from Hay ward et al. [1999]. MP: Marques de Pombal; HSF: Horseshoe Fault; GB: Guadalquivir Bank. Original color image appears at the back of this volume.

to derive the location and geometry of the tsunami source through backward ray-tracing and shallow water simulation.They concluded that the 1755 tsunami probably originated along the continental margin between the Gorringe Bank and the Iberian coast, with an elongated and shallow L-shaped, double rup­ture area trending north-northwest-south-southwest to northwest-southeast, centered near Marques de Pombal (Figure 1), with an estimated error of 16 min.

They also considered the Gorringe Bank a very unlikely location for the 1755 event, because the simulation leads to wrong travel time for almost all stations.This localization was obtained by trial and error.

The enormous quantity of energy required to generate a great earthquake like this one requires huge, multiple ruptures about 10,000-20,000 km2; that is, with dimensions of 100-200 km x 70-100 km and uplift of 10-20 m.

A regional MCS survey, carried out in 1992 (Figure 1,AR92 lines) showed that besides Gorringe Bank, there were other active, com­pressive, tectonic structures of regional signifi­cance (Figure 2): the Horseshoe fault (HSF), the Guadalquivir Bank (GB),and a large hill of tectonic origin located at 37°N, \0°W,about 100 km offshore Cape San Vincente; which here is named "Marques de Pombal" (Figure 1; "MP" in Figure 2) in honor of the secretary of state under the Portuguese monarchy who was one of the most active promoters of the recon­struction of Lisbon after the 1755 earthquake.

All these structures share active tectonic uplift due to contraction and relevant lateral continu­ity Among these tectonic structures, the Marques de Pombal potentially satisfies the requirements needed to be the 1755 generator; that is, it is a relevant, active, tectonic feature and is located in an area compatible with Baptista et al£ [1998b] numerical modeling.The tectonic structure

found at Marques de Pombal is a remarkable, active thrust (Marques de Pombal Fault;"MPTF" in Figure 3) imaged by multi-channel seismic (MCS) line AR92-10 (Figure 3a).Figure 3b shows an interpretative sketch based on the same line, depth-converted, with earthquake hypocenters. The seismological data plotted in the same fig­ure was acquired by a network of 14 seismic stations (Figure 1) with seven short-period stations located near Cape San Vincente that has operated since March 1995. In Figure 3a, the hanging wall of MPTF shows an antiform-like shape between 60-110 km. Between 60-80 km, the MPTF-plane (Figure 3) reflections are recog­nizable down to 8 sTWT (about 11 km depth), with a mean apparent south-southeast dip of 20°. At 110-150 km, a second active structure characterized by complex folding is observed. The folding pattern of the latter points out to a north-northwest-dipping plane of a deeper back-thrust fault (BTF of Figure 3).

Eos,Vol. 82, No. 26,June 26,2001

J N N W J Marques de Pombal (MP)

Cape San Vincente ; s Canyon M

Back-Thrust Fault f (BTF)

Marques de Pombal , ;| Thrust F a u l t ( M P T F ) | l € l S : r i V e r t i c a l Exaggera t ion At S e a Bo i l r \

. .. - " :i

I N o Vert ical Exaggera t ion

Fig. 3.AR92-10 seismic line a) after post-stack time migration, and b) tectonic sketch of the same line, depth-converted, with projected earthquake hypocenters whose coordinates are within 30 km from AR92-10 line. For location of line AR92-10, see Figures 1 and 2; for epicenter locations, see Figure 2. The dimension of circles is proportional to the magnitude. In the upper diagram, A: syn-compressional sedimentary sequence; B: a pre-compressional sedimentary sequence; and C: acoustic basement made up of continental crust thinned during the Eurasia-North America Mesozoic rifting.

BIGSETS MCS Survey and Results

During the fall of 1998, a high-resolution MCS and sampling survey (BIGSETS lines, Figure 1) was carried out to investigate in detail the Marques de Pombal zone and to look for other possible, undetected tsunami-genic sources in the area. MCS lines measur­ing 2,715 km, chirp sonar, magnetic data, and six gravity cores were collected during the cruise, onboard the RV Urania of the Italian Consiglio Nazionale delle Ricerche.The survey was carried out on behalf of the European Community project Big Sources of Earthquake and Tsunami (BIGSETS) in southwestern Iberia. One of the main goals of BIGSETS was to measure some key parame­ters of the Marques de Pombal (MP) structure (Figure 3) such as size, orientation, and displacement to better constrain it as a possi­ble source of the 1755 Lisbon earthquake.

Most of the MCS lines were shot parallel to the present north-northwest-south-southeast slip vector (Figure l),and five lines were acquired perpendicular to the strike of the MP antiform.The high-resolution chirp sonar data were collected to search for mass wast­ing deposits that could be related to the 1755 or earlier events.The campaign found that there are no other major, large contractional features besides those previously found, except for a hill of tectonic origin ("A" in Fig­ure 2), which is too close to the city of Lisbon to justify the travel time expected for the 1755 event. Moreover, the chirp sonar profiles do not show important gravitative movement of the recent sediments.Therefore, it is very unlikely that the tsunami was enhanced by significant mass wasting in the area.

MCS lines shot perpendicular to the frontal ramp of the MPTF (Figure 4a, b, c) indicate that the thrust fault breaches the sea bottom only at the center of the structure (Figure 4b). The MPTF becomes a blind fault both at its northern and southern tips (Figures 4a and c, respectively). Here, displacements along the fault are accommodated by folding in the

uppermost sedimentary sequence.The maxi­mum uplift of about 1100 m is located where the thrust fault emerges at the surface with a strike of N20°E (Figure 4d).The MPTF can be followed for at least 50 km along-strike (Figure 4d),and the deformed area, associated with the emplacement of the MP structure, is at least 100 km in length (longer side of shaded area of Figure 4d).The constraints derived from the MCS lines allow us to infer a true fault plane dip of about 24° in the first 11 km.The geometry of the hanging wall anticline suggests

a tectonic transport toward west-northwest; that is, the Marques de Pombal structure is mainly a dip-slip thrust.The trend of MPTF may be related to pre-existing Mesozoic north-south-trending rifting faults that were reactivated by the present-day stress field.

On the basis of the empirical relationship between this fault rupture area and earthquake magnitude, the imaged segment of the MPTF alone is undersized to reach the estimated magnitude of the 1755 event.The MPTF and the BTF merge, with opposing dips, to the south-southeast and to the north-northwest, respectively into the earthquake cluster located almost along line AR92-10 (Figure 2) at around 16-18 km depth (Figure 3b).This cluster of hypocenters may represent an intra-crustal weak layer where the two-fault system accom­modates. This fact led us to suggest that dur­ing the 1755 earthquake, the MPTF-BTF fault system may have ruptured simultaneously Extrapolating the MPTF at depth and assum­ing a similar, opposite dip plane for BT^we can infer the dimension of the rupture area of the MPTF-BTF fault system, placing a decolle-ment surface at 16-18 km depth. Considering that the fault plane flattens at depth, a rough estimate of the fault width gives 70 km, as sketched in Figures 3b and 4.The resulting seismic moment M0 = uSD gives M0 = 3.15 x 1021Nm, where u = 3 x 1010Pa is the crustal rigidity, S = 7000 km2 is the fault area (100 km x 70 km; see Figure 4d),and D is the mean displacement of 15 m, estimated from tsunami modeling [Baptista et al, 1998b]. Using M w = 2/31ogM0-6.03, we obtain Mw = 8.3.

Fig. 4. Seismic lines in (a) BS20, (b) BS22, and (c) BS24 after post-stack time migration showing details of the Marques de Pombal thrust fault, and in (d) location of seismic lines with respect to the Marques de Pombal thrust fault (MPTF) and the Backthrust Fault (BTF). Shaded area depicts part of the source area of the 1755 event as derived from MCS and seismological data.

Eos, Vol. 82, No. 26, June 26, 2001

The estimated energy associated with the MPTF-BTF rupture area is of the same order of magnitude as the 1755 Lisbon earthquake. This geological and seismological evidence led us to conclude that the MPTF-BTF system is a satisfactory candidate for the source of the earthquake. Ribeiro [1994] estimated a returning period ranging from 300 years up to 1,500 years, in southwestern Iberia, for events of magnitude 8.5-9.0, comparable to those responsible for the 1755 earthquake. Neverthe­less^ number of lower-magnitude events were recorded in the area during the last 5 years, with 8 events above magnitude 4.0.

The monitoring of MPTF-BTF by means of ocean bottom seismometers—coupled with water pressure sensors, tide gauges, a detailed bathymetry and the knowledge of the geometric constrains derived from MCS—could provide not only a prototype of a tsunami local warn­ing network, but also new insights into the tsunami generation problem, at least for low-magnitude events.

Acknowledgments

We thank Vincenzo Lubrano and the crew of the R/V Urania for skillfully performing the BIGSETS survey The research was funded by

the European Community Environment and Climate Program 1994-1998,Technologies to Forecast, Prevent and Reduce Natural Risks (contract n.ENV4-CT97-0547).Istituto Geologia Marina contribution No. 1248.

Authors

N Zitellini, L. A. Mendes, D. Cordoba, J. Danobeitia, R. Nicolich, G. Pellis,A. Ribeiro, R. Sartori, L. Torelli, R.Bartolome, G Bortoluzzi, A. Calafato, FCarrilho, L. Casoni, EChierici, C. Corela,A.Correggiari,B. Delia Vedova, E. Gracia, PJornet, M. Landuzzi, M. Ligi,A. Magagnoli, G Marozzi, L. Matias, D. Penitenti, P Rodriguez, M. Rovere, PTerrinha, L. Vigliotti and A. Zahinos Ruiz For more information, contact Nevio Zitellini, Istituto Geologia Marina, C.N.R.,Via Gobetti 101, 40129 Bologna, Italy; E-mail: nevio@igm.bo.cnr.it.

References

Baptista, M. A., S. Heitor, J . M. Miranda, P Miranda, and L. Mendes Victor,The 1755 Lisbon tsunami; evalua­tion of the tsunami parameters, J. Geodyn., 25, 143-157 ,1998a .

Baptista, M. A., PM. A. Miranda, J . M. Miranda, and L. Mendes Victor, Constraints on the source of the

1755 Lisbon Tsunami inferred from numerical modelling of historical data on the source of the 1755 Lisbon Tsunami, J Geodyn., 25,159-174,1998b.

Buforn, A. Udias, and M. A. Colombas, Seismicity source mechanisms and tectonics of the Azores-Gibraltar plate boundary, Tectonophys., 152, 8 9 - 1 1 8 , 1 9 8 8 .

Campos M.L.,Tsunami hazard on the Spanish coasts on the Iberian Peninsula, Science of Tsunami Hazards, vol. 9,n.l, pp. 8 3 - 9 0 , 1 9 9 1 .

Hayward, N., A, B.Watts, G. K.Westbrook,and J . S. Collier, A seismic reflection and GLORIA study of compressional deformation in the Gorringe Bank region, eastern North Atlantic, Geophys. J. Int., 138, 8 3 1 - 8 5 0 , 1 9 9 9 .

Martins, L., and V L. A. Mendes Victor, Contribuicao para o estudo da sismicidade de Portugal continen­tal, Univ. De Lisboa, Instituto Geofisico do Infante D.Luis, publicacation n . 1 8 , 1 - 7 0 , Lisbon, 1990.

Ribeiro, A., Deformable plate tectonics of the Azores-Gibraltar boundary - Where will the next 1755 earthquake strike again?, Gaia, 9,109-113,1994.

Sartori, R., L.Torelli, N. Zitellini, D. Peis, and E. Lodolo, Eastern segment of the Azores-Gibraltar line (central-eastern Atlantic): An o c e a n i c plate boundary with diffuse compressional deformation, Geology, 22,555-558,1994.

Wessel, P, and W H. FSmith, Free-software helps map and display data, Eos, Trans. AGU, 72,445,1991.

Zitell ini ,N.,EChierici ,R.Sartori ,and L.Torelli,The tectonic source of the 1755 Lisbon earthquake and tsunami, Ann. di Geofis., 4 2 , 4 9 - 5 5 , 1 9 9 9 .

AGU/AGI Showcase ODP

PAGE 285

On Wednesday, June 13, lawmakers and their staffs jammed a Capitol Hill exhibit of research programs supported by the National Science Foundation (NSF). Sponsored by the Coalition for National Science Funding (CNSF),the exhibit is intended to demonstrate to members of Congress—who often wonder where the money they appropriate goes—the exciting research programs funded by NSF and their results.

AGU joined with the American Geological Institute (AGI) in sponsoring an exhibit high­lighting the Ocean Drilling Project (ODP). Frank Rack and Brecht Donoghue of the Joint Oceanographic Institutions (JOI) explained to interested legislators and congressional staff members that ODP is an international partner­ship of scientists and research institutions

on Capitol Hill

organized to explore the evolution and struc­ture of Earth by coring the ocean bottom in water over 8 kilometers deep. Since January 1985, ODP has recovered more than 160,000 meters of core.

At least 15 members of Congress and over 70 congressional staff members examined more than 30 displays during the two-hour reception.

AGU has co-sponsored CNSF exhibits 5 years in a row on topics including seismology ice core research, and fluctuations of the atmos­phere and oceans in the North Atlantic. Sum­maries of previous CNSF exhibits are available on AGU's Web site at http://www.agu.org/sci_ soc/policy/sci_pol.html.

Author Peter Folger Manager, Public Affairs, AGU

Congressman Vern Ehlers (R-Mich.) at the Ocean Drilling Project booth during the CNSF exhibit on Capitol Hill. Rep. Ehlers is one of two physicists in Congress; he chairs the Environment, Technology, and Standards Sub­committee of the House Science Committee.

Support for a Tropical Lidar in Latin America PAGES 285,289

The vertical profile of stratospheric aerosols is a crucial parameter to monitor the effects of volcanic eruptions on climate. These effects include climate change and ozone depletion. Satellites provide the best instruments for pro­ducing global coverage for these profiles, but existing and planned satellite missions all have limitations. Ground-based lidar observations are needed for calibration and validation of satellite observations, as well as for filling the gaps when satellite observations are not available.

SAGE (Stratosphere Aerosol and Gas Exper­iment) II has been a very successful satellite mission, providing global, high-quality meas­urements of ozone, nitrogen dioxide, water vapor, and multi-wavelength aerosol extinc­tion from the mid-troposphere to as high as the lower mesosphere. The mission was initially conceived for 2 years' duration, but it has lasted more than 16 years and is still providing information; although in the past several months, with only half the designed frequency. Unfortunately, it is expected to end soon, resulting in a lack of global aerosol

measurements. Furthermore, the orbit only allows observations at any one latitude once per 40 days, and dense aerosol clouds, like those in the tropics after the 1991 Mt.Pinatubo eruption, preclude any observations.

The next satellite mission carrying such an instrument (SAGE III) is scheduled later this year on Meteor-3M,but only with high-latitude coverage. SAGE III with mid-latitude and tropical coverage is not expected before 2004 in the International Space Station.

ICESat,scheduled to be launched in 2002,will have a vertically pointing lidar that will provide some aerosol data, but only in very small foot­prints, which will produce sampling problems for stratospheric aerosols. HIRDLS on EOS Aura

Eos, Vol. 82, No. 26, June 26, 2001

12W 11W 10W 9 W 8 W 7 W 6 W

Page 285 Fig. 1. Bathymetric map of southwestern Iberia (data from GEBC09 7 Digital Atlas Web site: www.nbi.ac.uk, color bar scale in meters) with location of seismic stations and MCS lines. Inset map sketches the main elements of plate boundaries (data from NOAA Global Relief Data); the stippled patch indicates absence of well-defined plate boundary. MAR: Mid-Atlantic Ridge; 77?: Terceira Ridge; GF: Gloria Fault. Solid arrows show relative plate kinematics; shaded box outlines area of figures I and 2 in the plate boundary framework. (This figure and figure 2 were produced using GMT software [Wessel and Smith, 1991].)

Eos, Vol. 82, No. 26, June 26,2001

39°N

38°N

1 4>

Earthquakes hypoc enter deplh and magnilude (M)

Odep1h<l0km • M<3 • I0km<dep1h<20km # 20 km < deplh < 30 km

• 3<M<3.5

# 30 km < depth < 50 km • 3.5<M<4 # deplh > 50 km # M>4

Focal Mechanism

« & 0 strike-slip reverse normal

6°W

Fig. 2. Bathymetric map of southwestern Iberia with location of earthquake epicenters recorded by the new seismic network from January 1995 to March 2000; the contour intervals are as in Figure 1. The focal mechanisms of main earthquakes before March 1995 are from Hay ward et al. [1999]. MP: Marques de Pombal; HSF: Horseshoe Fault; GB: Guadalquivir Bank.

Page 290