Seismotectonics of the Hindukush and Baluchistan arc

A seismicity map of that part of the Pakistan-Afghanistan region lying between the latitudes 28° to 38°N and longitudes 66° to 75°E is given using all available data for the period 1890–1970. The earthquakes of magnitude 4.5 and above were considered in the preparation of this map. On the basis of this map, it is observed that the seismicity pattern over the well-known Hindukush region is quite complex. Two prominent, mutually orthogonal, seismicity lineaments, namely the northvestern and the north-eastern trends, characterize the Hindukush area. The northwestern trend appears to extend from the Main Boundary Fault of the Kashmir Himalaya on the southeast to the plains of the Amu Darya in Uzbekistan on the northwest beyond the Hindukush. The Sulaiman and Kirthar ranges of Pakistan are well-defined zones of intermontane seismicity exhibiting north-south alignment.Thirty-two new focal-mechanism solutions for the above-mentioned region have been determined. These, together with the results obtained by earlier workers, suggest the pre-dominance of strike-slip faulting in the area. The Hazara Mountains, the Sulaiman wrench zone and the Kirthar wrench zone, as well as the supposed extension of the Murray ridge up to the Karachi coast, appear to be mostly undergoing strike-slip movements.In the Hindukush region, thrust and strike-slip faulting are found to be equally prevalent. Almost all the thrust-type mechanisms belonging to the Hindukush area have both the nodal planes in the NW-SE direction for shallow as well as intermediate depth earthquakes. The dip of P-axes for the events indicating thrust type mechanisms rarely exceeds 35°. The direction of the seismic slip vector obtained through thrust type solutions is always directed towards the northeast. The epicentral pattern together with these results suggest a deep-seated fault zone paralleling the northwesterly seismic zone underneath the Hindukush. This NW-lineament has a preference for thrust faulting, and it appears to extend from the vicinity of the Main Boundary Fault of the Kashmir Himalaya on the southeast of Uzbekistan on the northwest through Hindukush. Almost orthogonal to this NW-seismic zone, there is a NE-seismic lineament in which there is a preference for strike-slip faulting.The above results are discussed from the point of view of convergence of the Indian and Eurasian plates in the light of plate tectonics theory.     

Stress State, Seismicity and Subduction Geometries of the Descending Lithosphere Below the Hindukush and Pamir

The paper presents an analysis of spatial distribution of 6600 earthquake events which occurred during the period 1964 to 1999 between latitude 34 to 40°N and longitude 68 to 76°E. This large volume event is reported in the International Seismological Centre (ISC) catalog. In addition to this a total of 248 focal mechanism solutions are considered to derive a generalised predominant stress prevailing in the descending lithosphere below the Hindukush and Pamir regions.

The analysis of spatial distribution shows that the epicentres of the events at shallow level (depth<70 km) are sparsely distributed throughout except for a cluster at the northern end of both the Hindukush and Pamir. The concentration of epicentres at intermediate-depth level between 71 and 170 km below the Hindukush takes a strip-like pattern. It trends along SW-NE, and narrows at the northeastern end of the Hindukush. At deeper level (depth>170 km) the epicentres below the Hindukush are mainly concentrated in a triangular-shaped zone, and the mean points of concentration of the epicentres appear to be shifted towards southwest at increasing depth. The distribution of epicentres at the intermediate and deeper layers of the Pamir is observed to be diffused except a cluster of few events in each layer appears to be shifted towards south-southeast at increasing depth. The distribution of hypocentres changes its concentration from lesser to considerably higher at about 70 km depth, and further takes a minimum at about 170 km depth below the Hindukush and Pamir.

The present study further involves in analysing the composite/group effect of stresses associated with the descending lithosphere below the Hindukush and Pamir after deriving the best-fit generalized predominant directions of stresses. It shows that the intermediate-depth seismic zone below the Hindukush is acted upon by maximum compressive stresses (P axes) from two directions while the deeper-depth zone from three directions, and may convincingly be correlated with the changing shape of the respective seismic zones. Another interesting phenomenon observed here is the change in direction of maximum compressive stresses in clockwise fashion from intermediate to deep seismic zones below the Hindukush. At shallow depths below the Pamir the maximum and minimum (T axes) compressive stresses are acting almost along NNW-SSE and ENE-WSW and are oriented horizontally. T-axes for few events at these depths show almost vertical orientation. The observed down-dip extension is predominantly parallel with the descending lithosphere below the Hindukush. The entire analysis along with the observed scattering of P- and T-axes of some events at intermediate-depths might be indicating a slight contortion of the middle layer below the Hindukush. The spatial distribution of seismicity and the generalised stress pattern of both the regions infer the existence of two-isolated subducting lithosphere. It perhaps has created the eastward expulsion or lateral extrusion of Tibet along the major strike-slip faults like Karakorum, Altyn-Tagh, Kunlun and Red River. Finally, the whole analysis confirms the existence of shield-like continental rigid slab at depths greater than 170 km below the Hindukush.     

Complex seismic signatures and narrow azimuthal study of Hindukush earthquakes

About eighty earthquakes, exclusively from the Hindukush region, which were recorded at Gauribidanur seismic array (GBA) have been used in the present study. Short periodP-wave recordings upto 36 seconds were processed using adaptive cross-correlation filtering technique. The main objective of this exercise was to examine the signal complexities and seismic ray direction anomalies of these earthquakes which have identical epicentral distances and a narrow azimuthal range from GBA but different focal depths from 10 to 240 km. Slowness anomalies of the order of 1–4 sec/deg and azimuthal anomalies upto 6° have been found in this case. These deviations have been attributed to the upper mantle region between source and the receiver. Analysis of the data reveals that most of the events occurring at shallower depths had complex signatures as compared to the deeper events. The structure near the source region, complicated source functions and scattering confined to the crust-upper mantle near source are mainly responsible for complexity of the observed signals as the transmission path of the ray tubes from turning point to the recording stations is practically the same.     

Seismic ray direction anomalies and relative residuals of earthquakes recorded at Gauribidanur array

Slowness and azimuthal anomalies provide valuable information about lateral inhomogeneities within the crust and mantle of the earth. Over 300 earthquakes (distance range 14°–36° and azimuth 0°–360°) recorded at Gauribidanur seismic array (GBA) in southern India, were analysed using adaptive processing techniques. Slowness anomalies upto 1·3 sec/deg and azimuthal anomalies upto 8° have been observed in the present analysis. Slowness anomaly patterns for Java trench, Mid-Indian oceanic ridge earthquakes are more consistent as compared to the events originating in the Himalayan and Hindukush regions. A significant feature of the azimuthal anomaly pattern was the distinct absence of any positive anomalies from earthquakes occurring in mid-oceanic ridge. These anomalies have also been analysed as a function of epicentral distance and are mainly attributed to the transition zones occurring between 400–700 km depth ranges in the Indian upper mantle regions. Relative residuals between the stations of GBA have very little dependence on azimuth and distance. An anomalous structure beneath the array in the direction of the Java trench region (azimuth 116–126°) has been postulated on the basis of large systematic slowness vectors observed.     

The devastating Muzaffarabad earthquake of 8 October 2005: New insights into Himalayan seismicity and tectonics

The recent earthquake of 8 October 2005 in the Muzaffarabad region in western Himalaya destroyed several parts of Pakistan and the north Indian state of Jammu and Kashmir. The earthquake of magnitude 7.6 claimed more than 80,000 lives, clearly exposing the poor standards of building construction — a major challenge facing the highly populated, earthquake prone, third world nations today. In this paper, we examine variations in the stress field, seismicity patterns, seismic source character, tectonic setting, plate motion velocities, GPS results, and the geodynamic factors relating to the geometry of the underlying subsurface structure and its role in generation of very large earthquakes. Focal mechanism solutions of the Muzaffarabad earthquake and its aftershocks are found to have steep dip angles comparable to the Indian intra-plate shield earthquakes rather than the typical Himalayan earthquakes that are characterized by shallow angle northward dips. A low p-value of 0.9 is obtained for this earthquake from the decay pattern of 110 aftershocks, which is comparable to that of the 1993 Latur earthquake in the Indian shield — the deadliest Stable Continental Region (SCR) earthquake till date. Inversion of focal mechanisms of the Harvard CMT catalogue indicates distinct stress patterns in the Muzaffarabad region, seemingly governed by an overturned Himalayan thrust belt configuration that envelops this region, adjoined by the Pamir and Hindukush regions. Recent developments in application of seismological tools like the receiver function technique have enabled accurate mapping of the dipping trends of the Moho and Lithosphere–Asthenosphere Boundary (LAB) of Indian lithosphere beneath southern Tibet. These have significantly improved our understanding of the collision process, the mechanism of Himalayan orogeny and uplift of the Tibetan plateau, besides providing vital constraints on the seismic hazard threat posed by the Himalaya. New ideas have also emerged through GPS, macroseismic investigations, paleoseismology and numerical modeling approaches. While many researchers suggest that the Himalayan front is already overdue for several 8.0 magnitude earthquakes, some opine that most of the front may not really be capable of sustaining the stress accumulation required for generation of great earthquakes. We propose that the occurrence of great earthquakes like those of 1897 in Shillong and 1950 in Assam have a strong correlation with their proximity to multiple plate junctions conducive for enormous stress build up, like the eastern Himalayan syntaxis comprising the junction of the India, Eurasia plates, and the Burma, Sunda micro-plates.     

Intraplate earthquakes in Northern Svalbard

A temporary network of microearthquake seismographs deployed in the northern part of Svalbard recorded numerous earthquakes during the summer of 1982. Most of the earthquakes occurred in concentrated zones in restricted portions of three fault complexes in central Nordaustlandet, but minor activity also occurred to the east and west of these complexes. Within the uncertainty of the epicentral locations, most earthquakes which occurred on land in regions not covered by glaciers could be associated with segments of mapped faults. There are numerous fault segments and even entire faults, however, along which no recorded earthquake occurred. The concentrated spatial locations of these earthquakes along preferred portions of mapped faults, and the absence of throughgoing lines of activity, suggest that these are intraplate earthquakes occurring along existing faults in response to an applied stress field. Composite fault-plane solutions for the three most active regions each include one nodal-plane which can be associated, at least roughly, with a trend of seismicity and trend of a mapped fault. The directions of greatest principle stress for the three fault plane solutions vary between N90°W and N55°W, directions which agree approximately with maximum stress directions determined for a region about 200 km to the south. The consistency of the directions of maximum principle stress obtained for earthquakes throughout a broad region of Svalbard suggests that the earthquakes there occur in response to a stress field which is of plate tectonic origin.     

Coupling of the Crustal and upper mantle deformations in the Mongolia-Siberian Mobile Area

The complex analysis of parameters characterizing the modern deformations of the Earth’s crust and upper mantle in the territory of the Mongolia-Siberian Area is made. Directions of principal tension axes of stress-tensors, calculated with the use of earthquake source mechanisms have been taken as parameters of modern deformations at the level of the middle crust; directions of axes of horizontal strains in the geodesic network by the GPS data have been taken as such parameters at the level of the Earth’s surface. The strain parameters for the mantle depths are the data on seismic anisotropy derived from the published sources about the results of studies on splitting of transversal waves from distant earthquakes. Seismic anisotropy is interpreted as the ordered orientation of olivine crystals, which appears with great strains resulting from the flow of the mantle material. It has been shown that directions of extensional strain axes (minimal compression) by geodesic and seismological data coincide with anisotropy directions in the upper mantle in the region whose median value is 310°–320°. The observed mechanical coupling of the crust and the upper mantle of the Mongolia-Siberian Mobile Area shows the participation of the lithospheric mantle in the formation of neotectonical structures and enables us to distinguish the principal processes determining the Late Cenozoic tectogenesis in this territory. One of the leading mechanisms for the neotectonical and modern deformations of the Mongolia-Siberian Region is the large-scale NW-SE material flow in the upper mantle causing both motion of the entire northern part of the continent and divergence of the Eurasia and the Amurian Plate. Lithospheric deformations in the western part of the region are related to collision-induced compression, while those in the central part are caused by interaction of these large-scale tectonic processes.     

Focal mechanism solutions and dynamic parameters of earthquakes in the eastern Himalayas and northern Burma

Focal-mechanism solutions of four earthquakes in the eastern Himalayas and northern Burma are determined using the first motion of compressional waves. Two possible solutions thus obtained for each event reveal steeply dipping fault with predominantly strike-slip motion. The stress directions inferred from the focal mechanism solutions are interpreted in the light of predictions of the plate tectonics theory, viz., the underthrusting of the Indian plate in the Burma region in an easterly direction.Dynamic parameters (seismic moment, apparent stress and average dislocations) are obtained using the corrected spectra of Love waves. The earthquakes are found to possess low seismic moment and apparent stress values. A comparison of these estimates with values for intraplate earthquakes is given. It is suggested that these earthquake might be a consequence of a nonhomogeneous rupture process.     

Tectonics and earthquake mechanism of the shallow earthquake seismic belt,the Himalaya

The foci distribution of upper mantle earthquakes occurred between 1961 and 1972 in Himalayan, Tibetan and Hindukush regions, were studied and compared. The V-shaped pockets above a vertical plane have shown a relationship with the dip-slip type or normal faulting at the borders of the continental plates. The left-lateral and right-lateral displacements of Kirthar-Sulaiman shear zone and Brailly fault respectively found by the focal mechanism indicate a relative difference in motion between the Kashmir region and Nepal region and the uniform and right angle displacement of Tibetan Plateau has produced the Saradah depression. On the basis of seismic and geological evidences a simple tectonic model is proposed.     

Evidence for right-lateral strike-slip environment in the Kutch basin of northwestern India from moment tensor inversion studies

The Kutch region located in northwestern part of India is an ancient rift basin that was active until Cretaceous period. The region falls close to the India–Arabia and the India–Eurasia plate boundaries and has experienced devastating earthquakes in the past, namely the 1819 Allah Bund earthquake, the 1956 Anjar earthquake and the 2001 Bhuj earthquake. To understand the tectonics of this region with respect to the adjacent plate boundaries, we invert seismic waveform data of 11 earthquakes in this region recorded by a network of the Institute of Seismological Research (ISR) during 2007–2009. The study yields focal mechanism solutions of reverse fault and strike-slip type mechanism. The inferred fault planes correlate well with the local trends of the known tectonic faults while the principal stress directions derived from stress inversion based on a linearized least squares approach, trend agreeably with the ambient stress field directions. A consistently right-lateral sense of shear is found on all the local faults as derived from each of the matching planes of the focal mechanism solutions computed in the present study. It is inferred that in the Kutch region a right-lateral strike-slip environment prevails along predominantly EW to NW-SE oriented deep-seated pre-existing faults in an otherwise compressive stress regime. This, in conjunction with the left-lateral movements along the Girnar mountain in southern Saurashtra, inferred from previous studies, indicates a westward escape of the Kutch–Saurashtra block as a consequence of the northward collision of the Indian plate with respect to the Eurasian landmass.     

Application of pattern recognition algorithm in the seismic belts of Indian convergent plate margin — CN algorithm

The earthquake catalogue from 1964 to August 1991 is used to identify the times of increased probabilities (TIPs) of the earthquake mainshocks of magnitudes greater than or equal to 6·4 and are associated with the Indian convergent plate margins, in retrospect. In Pakistan and Indo-Burma regions, the analysis was repeated for magnitude threshold 6·2 and 7·0 respectively. All the earthquakes (except one in the Hindukush region and one in Indo-Burmese region) in Pakistan, Hindukush-Pamir, Himalaya and Indo-Burmese regions were preceded by the special activation and hence were predicted. Approximately 23 ± 10% of the total time (1970 to August 1991) is occupied by the TIPs in all the regions. The reasons for failure to predict the two earthquakes in these regions are discussed. Our analysis gives a better picture of the regionalization and the size of the space-time volume for the preparation of an earthquake. The high success ratio of the algorithm proves that it can be applied in this territory for further prediction in the real time, without any significant changes in its parameters.     

Reflections on the rigidity of the Lut Block and recent crustal deformation in eastern Iran

Imagery from the first Earth Resources Technology Satellite passes over east-central Iran are interpreted with special emphasis on geomorphology of the structural trends in the area. Several units of arcuate structure are named and discussed. Newly found dislocations and other lineaments within the Lut Block, in addition to its internal seismicity and Quaternary volcanic activity, support the argument against the rigidity of the block.Neotectonic and seismotectonic maps of the area are prepared and discussed. These maps reveal a complicated and mostly radial pattern of the pre-Neogene compressive directions. The post-Neogene directions of compression are, however, consistent with a NE—NNE trend throughout the area. These axes of maximum horizontal shortening are deduced from the trend of normal and en-echelon Neogene folds, the pattern and sense of movement of faults active in Quaternary, geodetic measurements, and the source mechanism of earthquakes.It is concluded that the plate-tectonic concept cannot readily be used to explain the recent deformation taking place in continental area structurally as complicated as eastcentral Iran.     

Implications of north pacific plate tectonics in central Alaska: Focal mechanisms of earthquakes

The focal mechanisms for 86 selected earthquakes (3.0 m_b 5.5) located in central Alaska have been investigated from P-wave first motions; the data were gathered by local seismic networks. The results show a depth-dependent characteristic to the fault-plane solutions. For earthquakes having focal depths shallower than 60–70 km, the focal mechanisms indicate either strike-slip or normal faults, while for earthquakes with foci at intermediate depths the focal mechanisms correspond to thrust faults. The nature of the seismicity indicates the hinge line of the Pacific lithospheric plate under the study area to be striking N17°E from Cook Inlet towards interior Alaska. The comparison of the focal mechanisms with the seismicity shows that the strike-slip and normal faults are the predominant processes of stress release along the shallow section of the plate. The earthquakes with intermediate foci systematically occur along the inclined section of the plate. If the gently dipping nodal planes for these earthquakes are chosen as the fault planes, the focal mechanisms correspond to underthrust motions at the foci. In these, the slip vectors are oriented either to the west or north with the resultant being in the N30°W direction. The tension axes for the underthrust solutions are also found to be parallel to the local dip of the plate, indicating that the subducted plate in interior Alaska is undergoing gravitational sinking.     

Synthesis of seismicity studies for western Alaska

An examination of the Alaskan earthquake catalogs revealed that from 1928 through 1965, twelve earthquakes of magnitudes (M) in the range 5.6 to 7.3 were located in and around Seward Peninsula region of western Alaska. Moreover, a number of earthquakes of M < 5.0 were found to distribute over the same area. Further investigation of the seismicity employing a local seismographic network in the above area showed a higher level of onshore and offshore seismic activity than had been previously recognized. A number of clusters of earthquakes have been identified. Some of them are located in the epicentral areas of past strong earthquakes (M > 5.5) and some are located in areas traversed by mapped faults. The nature of the seismicity as identified with the local network data is primarily crustal over the entire study area. Investigation of focal mechanisms of isolated strong earthquakes or clusters of small earthquakes show normal faulting as the dominant mode of strain energy release in the western part of Alaska. Moreover, in areas lying, approximately, south and north of Kotzebue Sound, the principal components of horizontal stresses tend to orient in the NW-SE and WNW-ESE directions, respectively.     

Earthquake evidence for compressive stress in the southeast Australian crust

Earthquakes in SE Australia are usually caused by compressive stresses acting in the crust, and are associated with steeply dipping faults. Sometimes the faulting is predominantly strike‐slip, as for the Bowning earthquakes of 1977 and some of the Dalton/Gunning earthquakes; and sometimes it is high‐angle thrust faulting, as for the 1961 Robertson and 1973 Picton earthquakes. No surface expression of the faults associated with any recent earthquakes in SE Australia has been reported.

The directions of the pressure axes, from all the earthquakes for which focal mechanisms have been determined, do not form a consistent pattern. This suggests that the faulting associated with earthquakes in SE Australia is dominated by the geometry of pre‐existing crustal faults or zones of weakness.

In situ stress measurements have not been made near the epicentral areas of the larger recent earthquakes, because of the absence of competent, near‐surface rocks coupled to the crust. However, in the western part of the Lachlan Fold Belt the in situ stress results indicate that the maximum pressure axis is approximately E‐W. The evidence from the focal mechanisms does not preclude the persistence of this stress regime farther to the east, and a regional compressive stress in the crust with an azimuth of about 120° is consistent with most of the earthquake focal mechanisms and the in situ stress measurements throughout SE Australia.     

中国岩石圈应力场与构造运动区域特征

笔者系统分析了1918—2005年间中国大陆及其周缘发生的3130个中、强地震的震源机制解,根据其特征进行了岩石圈应力场构造分区,首次得到区域应力场的压应力轴和张应力轴空间分布的统计数字结果。在此基础上研究了应力场的区域特征、探讨了其动力学来源以及构造运动特征。总体结果表明,中国大陆及其周缘岩石圈应力场和构造运动可以归结为印度洋板块、太平洋板块、菲律宾海板块与欧亚板块之间相对运动,以及大陆板内区域块体之间的相互作用的结果。印度洋板块向欧亚板块的碰撞挤压运动所产生的强烈的挤压应力,控制了喜马拉雅、青藏高原、中国西部乃至延伸到天山及其以北的广大地区。在青藏高原周缘地区和中国西部的大范围内,压应力P轴水平分量方位位于20～40°,形成了近NE方向的挤压应力场。大量逆断层型强震集中发生在青藏高原的南、北和西部周缘地区,以及天山等地区。而多数正断层型地震集中发生在青藏高原中部高海拔的地区,断层位错的水平分量位于近东西方向。表明青藏高原周缘区域发生南北向强烈挤压短缩的同时,中部高海拔地区存在着明显的近东西向的扩张运动。中国东部的华北地区受到太平洋板块向欧亚板块俯冲挤压的同时,又受到从贝加尔湖经过大华北直到琉球海沟的广阔地域里存在着的统一的、方位为170°的引张应力场的控制。华北地区大地震的震源机制解均反映出该区地震的发生大体为NEE向挤压应力和NNW向张应力的共同作用结果。台湾纵谷断层是菲律宾海板块与欧亚板块之间碰撞挤压边界。来自北西向运动的菲律宾海板块构造应力控制了从台湾纵谷、华南块体,直到中国南北地震带南段东部地域的应力场。地震的震源机制结果还表明,将中国大陆分成东、西两部分的中国南北地震带是印度洋板块、菲律宾海板块与太平洋板块在中国大陆内部影响控制范围的分界线。     

The 1980, 1997 and 1998 Azores earthquakes and some seismo-tectonic implications

We have studied the focal mechanisms of the 1980, 1997 and 1998 earthquakes in the Azores region from body-wave inversion of digital GDSN (Global Digital Seismograph Network) and broadband data. For the 1980 and 1998 shocks, we have obtained strike–slip faulting, with the rupture process made up of two sub-events in both shocks, with total scalar seismic moments of 1.9 × 10¹⁹ Nm (M_w = 6.8) and 1.4 × 10¹⁸ Nm (M_w = 6.0), respectively. For the 1997 shock, we have obtained a normal faulting mechanism, with the rupture process made up of three sub-events, with a total scalar seismic moment of 7.7 × 10¹⁷ Nm (M_w = 5.9). A common characteristic of these three earthquakes was the shallow focal depth, less than 10 km, in agreement with the oceanic-type crust. From the directivity function of Rayleigh (LR) waves, we have identified the NW–SE plane as the rupture plane for the 1980 and 1998 earthquakes with the rupture propagating to the SE. Slow rupture velocity, about of 1.5 km/s, has been estimated from directivity function for the 1980 and 1998 earthquakes. From spectral analysis and body-wave inversion, fault dimensions, stress drop and average slip have been estimated. Focal mechanisms of the three earthquakes we have studied, together with focal mechanisms obtained by other authors, have been used in order to obtain a seismotectonic model for the Azores region. We have found different types of behaviour present along the region. It can be divided into two zones: Zone I, from 30°W to 27°W; Zone II, from 27°W to 23°W, with a change in the seismicity and stress direction from Zone I. In Zone I, the total seismic moment tensor obtained corresponded to left-lateral strike–slip faulting with horizontal pressure and tension axes in the E–W and N–S directions, respectively. In Zone II, the total seismic moment tensor corresponded to normal faulting, with a horizontal tension axis trending NE–SW, normal to the Terceira Ridge. The stress pattern for the whole region corresponds to horizontal extension with an average seismic slip rate of 4.4 mm/yr.     

Stress fields and fault reactivation angles of the 2000 western Tottori aftershocks and the 2001 northern Hyogo swarm in southwest Japan

We estimated the stress fields of the aftershocks of the 2000 western Tottori earthquake (M_w 6.6) and the northern Hyogo swarm (max M_w 5.2) by a stress tensor inversion of moment tensor solutions reported from the National Research Institute for Earth Science and Disaster Prevention (Japan). The maximum principal stress direction of the western Tottori sequence was estimated as N107°E with a strike–slip regime. In the northern Hyogo swarm, the orientations of the principal stress directions could not be well constrained by the observed data, but after examining the detailed characteristics of the solution, we obtained a most probable solution of N113°E for the σ₁ direction. These solutions are consistent with the maximum horizontal directions roughly estimated from the strike directions of large earthquakes occurring geographically between these two seismic activities. We measured the angle between each fault–slip direction and maximum principal stress direction to investigate the frictional properties of earthquakes. The distribution of the angles was forward modeled to estimate the coefficient of friction and the stress ratio, assuming uniformly distributed fault orientations. For the western Tottori sequence, a homogeneous stress field with a coefficient of friction less than 0.4 was estimated. A high stress level was also suggested because very little change occurred in the stress field during the mainshock. For the northern Hyogo sequence, the coefficient of friction was estimated to be between 0.5 and 1.0.     

Seismotectonics of Portugal and its adjacent area in the Atlantic

New elements on the seismicity of Portugal and new focal-mechanism solutions of earthquakes with epicentres situated off the coast of the Portuguese mainland and in the Azores region are presented. Historical seismicity data show that in the territory of the Portuguese mainland there are active faults that are responsible for earthquakes that have caused important damage and many casualties. However, most of the intraplate earthquakes with epicentres situated in the Portuguese mainland or near the shore are normally of small magnitude and this renders difficult their interpretation in the light of focal mechanisms. A solution for one earthquake, with magnitude 5 and epicentre at the Nazaré submarine canyon, is presented.Southwestwards of Cape St. Vincent there is an important seismic zone responsible for high-magnitude earthquakes such as that of 1 November 1755. This zone is situated in the region where the extension of the Messejana fault into the ocean joins with the Azores-Gibraltar fault.The seismicity of the area situated between the western coast of the Portuguese mainland and the Azores increases approximately along the 15°W meridian, from the latitude of the Azores-Gibraltar fault up to 44°N. Focal mechanisms of earthquakes with epicentres situated along this line show very similar solutions.The interpretation of the focal mechanism solutions of the earthquakes with epicentres situated in the studied area shows that the stress field trends approximately NW-SE. It is assumed that this stress field results from the interaction of the Eurasian and African plates; however, this direction is not maintained in the Azores region.