Types Of Focal Mechanisms Research Articles

Stress Tensor Inversion for the Chi-Chi Earthquake Sequence and its Implications on Regional Collision

We study the stress regime associated with the 1999 Chi-Chi, Taiwan, earthquake sequence. Broadband waveforms of 115 events recorded by the Broadband Array in Taiwan for Seismology (BATS) are analyzed to determine their focal mechanisms, depths, and seismic moments. These solutions are then used to invert for the reduced stress tensor using the algorithm of Angelier (1998). We perform the stress inversion in three different ways. First, we aim at defining, in the first approximation, a uniform stress field by taking the entire dataset as a whole. The inversion result shows a stress regime with the maximum compression along the direction of N119°E, remarkably consistent with the direction of the relative motion between the Philippine Sea plate and Eurasia. This solution is stable and varies little in terms of the orientations of stress axes (within 8°) and the ratio of stress differences (between 0.26 and 0.44). In the second approximation, we aim at explaining more subtle features of our observations by considering differently built subsets. We divide the dataset according to different seismogenic structures as identified by Kao and Chen (2000). Our results yield rather similar results (axes of maximum compressive stress fall in the range of N99-138°E in terms of trends, and in the range of 1-17° in plunges) for all subsets except a small group in the footwall. Furthermore, we observe a systematic clockwise change in the direction of compression from south to north, representing the more general fan-shaped pattern of stress trajectories that characterizes the whole collision zone. Finally, the inversion is performed for the three main types of faulting, that is, reverse, strike-slip, and normal types. The axis of maximum compression for the reverse mechanisms has the same direction as that for the strike slip, whereas the normal type subset yields a N54°E extension that trends obliquely to the compression. The fact that the stress regime associated with the Chi-Chi aftershock sequence is similar to that derived from geological and seismic data in the region implies that the stress regime corresponding to the regional collision in Taiwan is responsible for the occurrence of the Chi-Chi earthquake. Moreover, systematic, secondary variation in the state of stress is observed for different types of focal mechanisms and seismogenic structures. Such variation may play an important role in producing local stress concentration that leads to the nucleation of disastrous earthquakes in central Taiwan. Manuscript received 12 January 2001.

Global test of seismic static stress triggering model

Seismic static stress triggering model is tested using Harvard centroid moment tensor (CMT) solution catalogue of 1976–2000 and concept of “earthquake doublet”. Result shows that seismic static stress triggering effect does exist in the view of global earthquakes, but the effect is very weak. Dividing the earthquakes into thrust focal mechanism, normal focal mechanism, strike-slip focal mechanism, we find that non-strike-slip focal mechanism earthquakes have significant triggering effect, whereas, the triggering effect in strike-slip focal mechanism earthquakes is not obvious. Divided the subsequent events delay time of “earthquake doublet” into 5 classes of t≥1, t<1, t≥10, t<10, 1≤t≤10 (t is in unit of d), then seismic static stress triggering effect does not change with delay time in short time period after earthquakes. The research on seismic static stress triggering in different regions of the world indicates that triggering effect is significant in subduction belts. Seismic static stress triggering model is tested by using “earthquake doublets” in China and its adjacent region. The result indicates that seismic static stress triggering effect cannot be observed easily in China and its adjacent region due to the seismic focal mechanism type (most of the earthquakes are strike-slip earthquakes).

The dynamic friction term in the spring-block models for earthquakes: a constraint from seismic moment and energy catalogue

In the spring-block models of earthquakes, one of the key factors is the dynamic friction term which determines the complexity of the faulting process. Generally, two kinds of friction, namely velocity-dependent friction and slip-dependent friction, are used in the modelling. But until now there has still been a lack of information on which kind of friction term is more suitable for modelling the phenomenology of earthquakes. Based on the numerical studies of Shaw (1998 Bull. Seismol. Soc. Am. 88 1457), we have examined the ratio of the broadband radiated energy and the scalar seismic moment of shallow earthquakes worldwide from 1987 to 1998. The result shows that for earthquakes with strike-slip mechanisms, velocity-dependent friction seems to be predominant, while for thrust and normal events, slip-dependent friction seems predominant. This suggests that in the spring-block models for earthquakes, the type of focal mechanism has to be accounted for and different types of earthquakes require different dynamic friction terms in the corresponding spring-block model.

Microseismic activity and seismotectonics of Heraklion Area (central Crete Island, Greece)

A seismological network of 10 portable analogue stations was installed in the area of Heraklion (central Crete) from September to December 1995. During this period, more than 1000 events were recorded by at least 4 stations with magnitudes ranging from 0.5 to 4.6 and depths up to 70 km. Analysis of 336 well located events revealed high seismic activity. In the onshore area seismicity is shallow (<20 km) and concentrated along the eastern margin of the Heraklion Basin and in the Messara graben to the south. Seismicity decreases rapidly from east to west, with practically no events located along the western boundary of Heraklion Basin. Epicenter distribution indicates that microseismicity is closely associated to the tectonics of the region. Significant seismic activity was also observed in the southern offshore area, restricted north of the Hellenic Trench and related to the subduction process. The determination of different types of focal mechanisms in the area indicates that the investigated region is characterized by complex tectonics related to the southward subduction of the African plate and the northward extension of the Aegean lithosphere.

Rupture extent of the 1755 Lisbon earthquake inferred from numerical modeling of tsunami data

The occurrence of tsunamis, affecting the Portuguese coasts, has been reported since the year 60 BC and the Gorringe bank region has been assumed as the most prone area for tsunami generation in the southwestern Iberian area. The tsunami generated by the 1755.11.01 earthquake is the largest one known in this area, having deeply affected the coasts of Iberia and Morocco. The earthquake, itself, was felt all over Europe, its estimated magnitude being 8.3/4 and its MSK intensity at the epicenter I 0=XI-XII. The great similarity between the isoseismal maps of this event and those of the recent 1969.02.28 earthquake lead several authors to locate the epicenter of the 1755 tsunami close to the Gorringe Bank (cf. Fig 1) and to infer the same type of focal mechanism (e.g. Machado, 1966, Martinez Solares et al., 1979, Levret, 1991) as the one deduced for the 1969 event (Fukao, 1973). The correct identification of the tsunamigenic sources is the essential task for the determination of tsunami risk at the Iberian area. The question we try to answer in the present study is whether or not there is only one major tsunami generating area in the western Iberia as it has been assumed until now. The results obtained here suggest that the 1755 tsunami was probably originated on the continental Iberian shelf, implying an epicentre area located between the Gorringe Bank and the Iberian coasts (cf. Fig. 1), closer to the coast. A complex source is proposed in order to justify tsunami observations in Morocco and seismic intensity values along the lberian and north Morocco coasts

Variation of stress field in the source region around a strong shock: an example

An earthquake with magnitude 6.6 occurred on November 16, 1983 in the Kaoiki, Hawii. The hierarchical clustering analyses were made by stages for the focal mechanism solutions of 74 earthquakes withM>-3.0 in this area from 1977 to 1985, it was found that each type of focal mechanism is roughly with invariable percentage, and that the strike-slip events including the mainshock, which did not appear in other stages, occurred in the stage aroundM=6.6 mainshock. The dominant types in the other stages were depressed in the period around the mainshock. This is a believable precursor phenomena. Orientation of the principal axes of stress field was inversed by fitting slip vector based on the focal mechanisms in the different periods. The results show that the stress field around the main shock withM=6.6 was more stable, its maximum and minimum principal axes is nearly horizontal in EW and NS direction respectively, the middle one is nearly vertical. The middle principal axis of stress field in the other periods appeared to be horizontal and the corresponding seismic rupture horizontally slip with a small dip. The significant difference between the stress field of source region in the period around the strong shock and that in the other periods shows that the seismogeny and occurrence of the strong shock were controlled by the variation of stress field. The characteristic environment factors of Kaoiki region and its effects were discussed.

特集 火山活動と地殻応力場 ダイクの貫入と群発地震

A kinematic mechanism of earthquake swarms induced by dyke intrusions was investigated for the seismo-volcanic activity off the east coast of the Izu Peninsula in July 1989. In this activity an intensive earthquake swarm composed of tectonic earthquakes continued for one week with large crustal deformation. From the crustal deformation data Okada and Yamamoto revealed that a dyke with 3 km width intruded from 7 to 1 km depth in the first day of the intensive swarm and then thickned to be 1 to 1.5 m thick. Relocated hypocenters show that the seismically active region moved from deep to shallow during the priod of the ascent of the dyke, suggesting the earthquakes occurred around the top of the intruded dyke. On the basis of the seismic data and the Okada and Yamamoto's dyke model, we examined three possible explanations or the cause of dyke-induced-earthquake swarm, that is, the stress change due to dyke intrusion, open crack-shear crack interaction (Hill's model) and the effect of pore-fluid-pressure increase, In order to explain the focal depth migration, the stress change due to dyke intrusion is the most favorable cause of the earthquake swarm, because the dyke intrusion increases differential stress above the top of the dyke and decreases that below the top. The observed focal mechanism type is consistent with the one expected from the ambient tectonic stress and the stress change due to the dyke intrusion.

The seismic source parameters of the 1991 Costa Rica Aftershock Sequence: Evidence for a transcurrent plate boundary

The April 22, 1991, Valle de la Estrella, Costa Rica earthquake (Ms = 7.6) was a back‐arc thrusting event associated with the underthrusting of the Caribbean plate beneath Central America. A network of three PASSCAL‐type, portable instruments was deployed to monitor the aftershock activity in southern Costa Rica 2 to 6 weeks after the main shock. The waveforms recorded on three‐component midperiod seismometers were used to recover source information for 15 small aftershocks (magnitudes between 3.2 and 4.4) with a linear moment tensor inversion method. We conducted several tests to investigate the effects of unknown structure and event mislocation on source parameter recovery. The longer‐period waveforms, in general, are less sensitive to the effects of the structural details so that the essential source information can be successfully extracted from the waveform data. The earlier part of the seismic wave‐forms has proven to be the most important carrier of the source information. A gross crustal model can be used to describe the structure for the source study. The small changes in the waveform character resulting from the mislocation of the events, or inexact Green's functions generated from the oversimplified crustal model, do not prohibit us from the recovery of the source orientation at local distances. In contrast, the determination of the focal depth is subject to uncertainty because of the lack of detailed structural information. Our focal mechanisms are generally in good agreement with P wave first‐motion fault plane solutions determined from a local short‐period network. The aftershocks show a clear spatial segmentation based on focal mechanism type. Most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock. In contrast, a cluster of aftershocks northwest of the main shock showed dominantly left‐lateral, strike‐slip motion on a northeasterly striking nodal plane. This suggests that a diffuse deformation zone exists in central Costa Rica and is characterized by left‐lateral strike‐slip motion. This diffuse, transcurrent deformation zone coincides with several geologic and geophysical features, and perhaps is a result of the slower subduction rate of the buoyant Cocos Ridge, than its adjacent segments along the Middle America Trench (MAT). The diffuse transcurrent boundary may intersect with the North Panama Deformed Belt (NPDB) near limon, Costa Rica, and is very likely a plate boundary for the proposed Panama block.

Seismotectonics of the eastern Swiss Alps and evidence for precipitation-induced variations of seismic activity

One of the most striking features revealed by a regional study of seismic activity in the eastern part of the Swiss Alps, conducted with a dense network of seismic stations between 1986 and 1988, is the restricted, around 13 km vertical extension of the seismogenic zone (defined by approximately 300 located earthquakes) compared with the earthquake distribution down to the Moho, both north and south of the Alpine body. Studies of focal mechanisms, station diagrams and very precise relative locations of clustered earthquakes all reveal a consistent picture of the stress field in this region: σ Hmax and σ Hmin are oriented northwest-southeast and northeast-southwest, respectively. All three types of focal mechanisms are present. Because of an increase in seismic activity following an exceptionally long period of intense precipitation, the possibility was investigated whether under some circumstances, as in the well known cases of reservoir-induced seismicity, very intense precipitation coupled with seasonal snow melt could trigger shallow seismic activity. Some weak indications supporting this hypothesis were found.

A seismotectonic model for western Hawaii based on stress tensor inversion from fault plane solutions

For 57 fault plane solutions of earthquakes with magnitude ML≥3.0 located west of the southwest rift zone of Mauna Loa volcano in Hawaii, which occurred between 1972 and 1988, the dominant focal mechanisms (44 events) are decollement type with one nodal plane nearly horizontal (dip ≤ 30°). The average slip vector of the upper crust on the decollement plane points in an azimuth of 260° toward the ocean, away from Mauna Loa's southwest rift zone at an angle of about 150° with respect to the NE‐SW oriented rift. Two other types of focal mechanisms are present: normal faults (four events) and strike‐slip faults with normal component (nine events). The orientation of the principal stresses was derived by minimizing the sum of the misfits between the theoretical and observed fault geometry for each focal mechanism. After subdividing the data set into three different regions we found that the stress tensor is not homogeneous in west Hawaii. The orientation for the stress tensor is best resolved in the area located between latitudes 19.27°N and 19.4°N and longitudes 155.7° and 155.9° W where the greatest principal stress directions within the 95% confidence limits are nearly vertical with some spreading to the west. Their plunge varies between 53° and 85°. The intermediate and least principal stresses are mostly horizontal and have similar magnitudes. Their plunge varies between 1° and 36°. The area between latitudes 19.4° and 19.6°N and longitudes 155.7° and 156°W is characterized partly by near‐vertical and partly by east‐west oriented greatest principal stresses. The least principal stresses are approximately horizontal and have magnitudes similar to the intermediate principal stresses. In the two areas where the stress tensor was well resolved, the principal strain directions differed from the principal stress directions by 30° in the plunge, suggesting that the faulting in western Hawaii takes place on a weak plane, but this result could not be established at the 95% confidence level. The tectonic model proposed for west Hawaii is similar to the model for the Kalapana region. The strain is accumulated through magmatic intrusions in the southwest rift zone of Mauna Loa, and the earthquakes occur along a zone of weakness composed of oceanic sediment at about 10 km depth. The west flank of Mauna Loa slips in the direction away from the rift.

Aftershock focal mechanisms of the Spitak earthquake

The focal mechanisms of foreshock, main shock, and 190 aftershocks of the Spitak earthquake, which occurred on December 7, 1988, were obtained using the signs of the first motion of body waves. The focal mechanisms of the aftershocks with magnitude M > 4 are completely representative ones and about 70% of the focal mechanisms of the aftershocks with M > 3.3 have been determined. The change in types of focal mechanisms of the aftershocks was observed from the second day of the main shock onwards. The prevailing types of motion on the first day (December, 7, 1988) were normal and strike-slip motions. The majority of the aftershocks beginning December, 8th were characterized by reversed and strike-slip motions. Vertical motions along a subvertical fault plane were observed during the aftershocks. Some characteristic types of focal mechanisms in different parts of the aftershock zone have been selected. Peculiarities of the aftershock distribution and types of focal mechanisms for each part of the zone are discussed. The result of this investigation may create a basis for a model of the aftershock processes.

A discussion on the amplitude ratios of PP and P waves for earthquakes with different focal mechanisms

Using the WKBJ approximation method we calculate the synthetic teleseismograms of P and PP waves to match the observed ones of six large Chinese earthquakes with known focal mechanisms: Tibet earthquake of July 14, 1973; Haicheng earthquake of February 4, 1975; Songpan earthquakes of August 16, 1976, August 21, 1976 and August 23, 1976 and Nignhe earthquake of November 15, 1976. The focal mechanism of the Tibet earthquake is discussed to examine the technique used in the calculation. We note that the amplitude ratios of PP and P waves (A PP/A P) have different characteristics for dip—slip events and strike—slip events within certain epicentral distances. We calculate the synthetic teleseismograms of P and PP waves for the strike—slip and dip—slip events with fault angles of 330°, 240° and 0°, focal depths of 8 km, 17 km and 24 km, at the assumed station with an azimuth of 310° and epicentral distances from 40°; to 80°. The diagrams of maximum amplitude ratios of PP and P waves (A PP/A P) versus distances are given. The possibility to use the (A PP/A P) values to give an approximate estimation for the focal mechanism type is discussed. This work may be useful for determining the focal mechanism type for those earthquakes which have only few records such as the Chinese earthquakes from the 1930s to 1960s.

Focal Mechanism Analyses of Aftershocks of the 1984 Western Nagano Prefecture Earthquake.

Focal mechanisms around Ontake Volcano, central Japan, were investigated by using the data set obtained by a seismic observation project of the 1986 Joint Seismological Research in Western Nagano Prefecture (JSR'86). The data set included P-wave first motions of aftershocks of the 1984 Western Nagano Prefecture Earthquake (M=6.8) mainly. A total of 556 events was selected for the focal mechanism analysis. The P-wave data from the surrounding university routine networks were also utilized additionally. Four hundred two events were successfully analyzed. Based on the types of focal mechanisms and on the spatial distribution of the events, we classify these events into the following 4 groups: (1) events of strike-slip fault type with a common nodal plane parallel to the ENE-WSW striking nodal plane in the fracture zone of the 1984 main shock; (2) events of reverse fault type in the area of the preexisting earthquake swarm to the northeast of the main shock, in a possible relation with an earthquake swarm activated by the main shock; (3) events of reverse fault type that occurred near the largest aftershock (M=6.2), but are not the continued aftershock sequence of the largest aftershock; (4) events of strike-slip fault type at depths to the easternmost part where no activity had been observed before the operation of JSR'86. The plural types of focal mechanisms obtained and the spatial distribution of foci suggest a complex seismo-tectonic structure that consists of at least 2 kinds of fracturings around Ontake Volcano.

Scaling differences between large interplate and intraplate earthquakes

A study of large intraplate earthquakes with well determined source parameters shows that these earthquakes obey a scaling law similar to large interplate earthquakes, in which M sub o varies as L sup 2 or u = alpha L where L is rupture length and u is slip. In contrast to interplate earthquakes, for which alpha approximately equals 1 x .00001, for the intraplate events alpha approximately equals 6 x .0001, which implies that these earthquakes have stress-drops about 6 times higher than interplate events. This result is independent of focal mechanism type. This implies that intraplate faults have a higher frictional strength than plate boundaries, and hence, that faults are velocity or slip weakening in their behavior. This factor may be important in producing the concentrated deformation that creates and maintains plate boundaries.

Determination of the homogeneous magnitude system magnitudes in seismological practice

Routine determination of network magnitudes by means of the Eurasian homogeneous magnitude system (HMS) is described and tested on a selection of recent shallow earthquakes. The HMS earthquake magnitudes appear to be more accurate and more reliable than the conventional earthquake magnitudes used in the present seismological practice. A preliminary investigation of correlations between magnitudes determined from different wave types shows a possible dependence of the correlation on the type of focal mechanism of earthquakes.

北海道および東北地方における稍深発地震の発震機構

Precise focal mechanism study of earthquakes occurring in Hokkaido and northern Honshu is made by using 35 stations of microearthquake observation networks of Tohoku University and of Central Research Institute of Electric Power Industry.In the northern Honshu and at the junction between the Northeastern Japan Arc and the Kurile Arc, the upper seismic plane of the double-planed deep seismic zone is characterized by down dip compression and the lower seismic plane by down dip extension. The events occurring in the shallower part of the upper seismic plane (shallower than 60km in the northern Honshu and shallower than 70km at the junction) have low-angle thrust fault type.The upper seismic plane is not down dip compression but has various types of focal mechanisms beneath Hokkaido, the southwestern end of the Kurile Arc, although the shallower part of it has low-angle thrust fault type as the case of northern Honshu. This upper seismic plane disappears at the depths greater than about 120km. The lower seismic plane is characterized by down dip exension, which is the same as northern Honshu. In Hokkaido, the lower seismic plane with down dip extension type extends to the deeper part down to about 300km depth. On the contrary, in nortern Honshu, it is the upper seismic plane with down dip compression type that extends to the deeper part down to at least 200km depth.The characteristics of the double-planed deep seismic zone in the southwestern end of the Kurile Arc may suggest relatively strong slab pull force superimposed on the stress system generating the double-planed deep seismic zone.

Triple seismic zone and the regional variation of seismicity along the Northern Honshu Arc

The regional variation of seismicity along the northern Honshu arc, Japan, is studied using accurate focal depths and focal mechanism types. We use focal depths determined from pP‐P time intervals reported in the ISC bulletins. For submarine earthquakes, depths are corrected by considering the pP phase reported in the bulletins as the pwP phase (the reflection from the ocean surface). Out of more than 600 well‐located earthquakes selected from the ISC bulletins, we determine the types of the focal mechanisms of 184 events using P wave first motion data. Based on historical seismicity of great and large earthquakes, we divide the zone of thrust type earthquakes at the plate interface into two regions: the shallow thrust zone (0–40km), where great earthquakes (MS∼8.0) occur, and the deep thrust zone (40–60km), where large earthquakes (MS∼7.4) occur. The activity of great or large earthquakes shows a variation along the arc; in some regions, both the shallow and deep thrust zones are active, and in other regions, only one of the thrust zones is active. The seismicity of recent moderate size earthquakes (mb&gt;4) combined with the focal mechanism type shows a variation along the arc, which reflects the variation of the activity of great or large earthquakes. Where large earthquakes do not occur in the deep thrust zone, neither thrust type nor down‐dip compression/tension type events occur in and beneath the deep thrust zone. Where large earthquakes do occur in the deep thrust zone, we find a number of thrust type earthquakes. Further, in the latter case, in some regions, the down‐dip compression and tension type events of the double seismic zones extend seaward just beneath the deep thrust zone and form a triple‐planed structure of seismicity (the triple seismic zone). This study confirms the hypothesis of previous workers (Seno and Pongsawat, 1981) on the causal relation between the strong seismic coupling of two converging plates at the deep thrust zone and extension of the double seismic zone; i.e., the presence or absence of activity within the slab beneath the deep seismic zone occurs when the deep thrust zone has a strong or weak coupling, respectively. Here, the weak coupling could be interpreted as either aseismic slip or as low stress buildup since the last large event occurred at the deep thrust zone. Triple seismic zones are found offshore of Miyagi prefecture, where the deep thrust zone has been broken recently in 1978, and offshore of Fukushima prefecture. We expect a future large earthquake at the deep thrust zone offshore of Fukushima prefecture because the presence of the triple seismic zone suggests stress has been accumulating and 40 years have passed since the deep thrust zone was ruptured in 1938.

Faulting mechanisms for two oceanic earthquake swarms

A study of the relative excitation of long-period surface waves from earthquake swarms near the Delfin basin in the northern Gulf of California and near the Queen Charlotte Islands fault suggests that a prominent mechanism of faulting for these swarm-type events was strike slip. Both swarms were apparently located on fracture zones near active spreading centers. Earlier studies have suggested that normal faulting may dominate oceanic earthquake swarms and that this type of faulting may be related to the occurrence of swarms. However, the present observations imply that the mechanisms for generating swarm-type earthquakes are probably due to factors other than the nature of the faulting involved. The suggested faulting mechanisms were determined by an examination of the relative amplitudes of Love and Rayleigh waves with periods between 10 and 50 s for some 60 events, assumed to be shallow (h < 15 km), that occurred in the northern Gulf of California. These surface waves were recorded at a new very long period seismograph station at Albuquerque, New Mexico. Bathymetry and seismicity of the northern Gulf of California suggest that faulting is occurring at short spreading centers (dip-slip faulting) and along transform faults (strike-slip faulting). Theoretical radiation patterns and relative amplitudes of Love and Rayleigh waves determined for both of these fault mechanisms were compared with the observed surface wave amplitudes recorded at Albuquerque. This technique provides a means of discriminating between the two possible types of focal mechanisms. The observations suggest that strike slip, possibly with a small component of dip slip, was the dominant faulting mechanism for many of these swarm-type events. Further support for this suggestion is provided by the observation of S-P times at a local (Δ < 100 km) short-period seismograph station, and thus the epicentral locations of these events are restricted to a portion of a mapped transform fault. This surface wave technique was also applied to an earthquake swarm that occurred near the Queen Charlotte Islands fault, and a similar conclusion was drawn.

微小地震観測網から求めた和歌山地方の地震の発震機構 (第1報)

Focal mechanisms of 82 earthquakes that occurred during the period from 1966 to 1969 in the Wakayama region (well known as to be one of the seismically most active region) have been investigated from the polarities of the first motions of P waves recorded at the microearthquakes observation network that spread over Kinki District. In order to get clean pictures of the radiation patterns of the first motions, all recording stations were projected on the Wulf net grids, according to the emergent angles determined from the velocity structure of this region. For 75 earthquakes, the observed first motions of P waves showed quadrantal distributions, and the azimuthes and dips of the maximum pressure axes and the maximum tension axes have been estimated for those earthquakes. But the radiation patterns of P waves of 7 earthquakes did not seem consistent with the quadrant type mechanisms, suggesting that the possibility that they might be explained as to be different types of focal mechanisms.

The viscous boundary layer at the free surface of a rotating baroclinic fluid : Hide R., 1964. Tellus, 16 (4): 523–529.

The occurrence of tsunamis, affecting the Portuguese coasts, has been reported since the year 60 BC and the Gorringe bank region has been assumed as the most prone area for tsunami generation in the southwestern Iberian area. The tsunami generated by the 1755.11.01 earthquake is the largest one known in this area, having deeply affected the coasts of Iberia and Morocco. The earthquake, itself, was felt all over Europe, its estimated magnitude being 8.3/4 and its MSK intensity at the epicenter I0=XI-XII.The great similarity between the isoseismal maps of this event and those of the recent 1969.02.28 earthquake lead several authors to locate the epicenter of the 1755 tsunami close to the Gorringe Bank (cf. Fig 1) and to infer the same type of focal mechanism (e.g. Machado, 1966, Martinez Solares et al., 1979, Levret, 1991) as the one deduced for the 1969 event (Fukao, 1973).The correct identification of the tsunamigenic sources is the essential task for the determination of tsunami risk at the Iberian area. The question we try to answer in the present study is whether or not there is only one major tsunami generating area in the western Iberia as it has been assumed until now.The results obtained here suggest that the 1755 tsunami was probably originated on the continental Iberian shelf, implying an epicentre area located between the Gorringe Bank and the Iberian coasts (cf. Fig. 1), closer to the coast. A complex source is proposed in order to justify tsunami observations in Morocco and seismic intensity values along the lberian and north Morocco coasts