3-D velocity structure beneath the crust and upper mantle of Aegean Sea region

The region of the Aegean Sea and the surrounding areas in the Eastern Mediterranean lies on the boundary zone between the Eurasian and the African plates. It is a zone of widespread extensive deformation and, therefore, reveals a high level of seismicity.Three-dimensional velocity structure, beneath the crust and upper mantle of the region between 33.0°N–43.0°N and 18.0°E–30.6°E, is determined.The data used are arrival times ofP-waves from 166 earthquakes, recorded at 62 seismological stations. In total, 3973 residual data are inverted.The resultant structure reveals a remarkable contrast of velocity. In the top crustal layer, low velocities are dominant in Western Turkey and on the Greek mainland, while a high velocity zone is dominant in the Ionian Sea and in the southern Aegean Sea.In the upper mantle, high velocity zones dominate along the Hellenic arc, corresponding to the subducting African plate and in the northern part of the region, corresponding to the subducting African plate and in the northern part of the region, corresponding to the margin of Eurasian plate.A low velocity zone is dominant in the Aegean Sea region, where large-scale extension and volcanic activity are predominant, associated with the subduction of the African plate.     

Body-wave wave-form modelling and source parameters for the indochina border earthquakes

Wave-form modelling of body waves has been done to study the seismic source parameters of three earthquakes which occurred on October 21, 1964 (M _b =5.9), September 26, 1966 (M _b =5.8) and March 14, 1967 (M _b =5.8). These events occurred in the Indochina border region where a low-angle thrust fault accommodates motion between the underthrusting Indian plate and overlying Himalaya. The focal depths of all these earthquakes are between 12–37 km. The total range in dip for the three events is 5°–20°. TheT axes are NE-SW directed whereas the strikes of the northward dipping nodal planes are generally parallel to the local structural trend. The total source durations have been found to vary between 5–6 seconds. The average values of seismic moment, fault radius and dislocation are 1.0–11.0×10²⁵ dyne-cm, 7.7–8.4km and 9.4–47.4 cm, respectively whereas stress drop, apparent stress and strain energy are found to be 16–76 bars, 8.2–37.9 bars and 0.1–1.7×10²¹ ergs, respectively. These earthquakes possibly resulted due to the tension caused by the bending of the lithospheric plate into a region of former subduction which is now a zone of thrusting and crustal shortening.     

Active deformation in the Vrancea region,Rumania

Recent and historical seismicity as well as reliable fault plane solutions are used to study the active deformation caused by the occurrence of intermediate depth (60–170 km) earthquakes of the Vrancea region, Rumania. In this area, located in the southeastern part of the Carpathian arc, the westward subduction of the Carpathian trench has terminated, leaving continental lithosphere, at present, at the arc. The principalT axis of the intermediate depth events trends N159°E and has a plunge of 74°, which is the same as the dip of the subducted plate. TheP axis has a trend of 314° and a shallow plunge of 15°. The analysis of the moment tensor of six focal mechanisms showed that the dominant mode of deformation of the subducted lithosphere is a down-dip extension at a rate of about 2 cm/yr, based on seismicity data.     

Seismicity and stress state in the South China Sea, Indochina and their vicinity

The distribution of earthquakes from 1973 to 1982 in the South China Sea, Indochina and their vicinity was studied using the data from I. S. C. It was found that the earthquakes are mainly concentrated along the boundaries of plates. Beside, some of shallow eartqhuakes are distributed in west part of Burma and the boundary between Burma and China, a few of earthquakes occurred in South China Sea. The features of Benioff zone along the boundaries between India plate, Philippine Sea plate and Eurasia plate were studied. The plate do not coupled well under the Java trench and the Philippine trench. The subducted India plate under Burmese range, Andaman—Nicobar arc moves NNE. The fault plane solutions of earthquakes were studied using the first motions of P wave. The stress state on subduction zones and within the area were deduced from the fault plane solutions and the fault movement. It was found that the direction of principal compression axis of stress is in the NNE in west part of Burma, in S—N in south and middle part of Bruma and Thailand, and in NNE or S—N in the South China Sea. It was also found that the stress state has close relation with the interaction of plates. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 129–137, 1991.     

Apparent stress and stress drop for intraplate earthquakes and tectonic stress in the plates

The magnitude of shear stress in the lithosphere is bounded from below by the apparent stress and stress drop during intraplate earthquakes. Apparent stresses and stress drops for a number of mid-plate earthquakes are calculated from the earthquake magnitude, SH wave amplitude spectra, and estimates of the length of the fault zone. Apparent stresses vary between 0.1 and 2 bars, ifm _b is used as a measure of seismic energy, and stress drops lie between 2 and 70 bars. There is no systematic difference in either apparent stress or stress drop between these intraplate events and typical plate boundary earthquakes. These bounds on intraplate shear stresses are consistent with the inference from current models of plate tectonic driving forces that regional stress differences in the plates are typically on the order of 100 bars. The highest stress drops measured for midplate earthquakes under this model represent nearly total release of local tectonic stress.     

Subduction and back-arc activity at the Hikurangi convergent Margin,New Zealand

The Hikurangi Margin is a region of oblique subduction with northwest-dipping intermediate depth seismicity extending southwest from the Kermadec system to about 42°S. The current episode of subduction is at least 16–20 Ma old. The plate convergence rate varies along the margin from about 60 mm/a at the south end of the Kermadec Trench to about 45 mm/a at 42°S. The age of the Pacific lithosphere adjacent to the Hikurangi Trench is not known.The margin divides at about latitude 39°S into two quite dissimilar parts. The northern part has experienced andesitic volcanism for about 18 Ma, and back-arc extension in the last 4 Ma that has produced a back-arc basin onshore with high heaflow, thin crust and low upper-mantle seismic velocities. The extension appears to have arisen from a seawards migration of the Hikurangi Trench north of 39°S. Here the plate interface is thought to be currently uncoupled, as geodetic data indicate extension of the fore-arc basin, and historic earthquakes have not exceededM _s=7.South of 39°S there is no volcanism and a back-arc basin has been produced by downward flexure of the lithosphere due to strong coupling with the subducting plate. Heatflow in the basin is normal. Evidence for strong coupling comes from historic earthquakes of up to aboutM _s=8 and high rates of uplift on the southeast coast of the North Island.The reason for this division of the margin is not known but may be related to an inferred increase, from northeast to southwest, in the buoyancy of the Pacific lithosphere.     

Seismic Sources on the Iberia-African Plate Boundary and their Tectonic Implications

—The plate boundary between Iberia and Africa has been studied using data on seismicity and focal mechanisms. The region has been divided into three areas: A; the Gulf of Cadiz; B, the Betics, Alboran Sea and northern Morocco; and C, Algeria. Seismicity shows a complex behavior, large shallow earthquakes (h < 30 km) occur in areas A and C and moderate shocks in area B; intermediate-depth activity (30 < h < 150 km) is located in area B; the depth earthquakes (h 650 km) are located to the south of Granada. Moment rate, slip velocity and b values have been estimated for shallow shocks, and show similar characteristics for the Gulf of Cadiz and Algeria, and quite different ones for the central region. Focal mechanisms of 80 selected shallow earthquakes (8 m_b 4) show thrust faulting in the Gulf of Cadiz and Algeria with horizontal NNW-SSE compression, and normal faulting in the Alboran Sea with E-W extension. Focal mechanisms of 26 intermediate-depth earthquakes in the Alboran Sea display vertical motions, with a predominant plane trending E-W. Solutions for very deep shocks correspond to vertical dip-slip along N-S trends. Frohlich diagrams and seismic moment tensors show different behavior in the Gulf of Cadiz, Betic-Alboran Sea and northern Morocco, and northern Algeria for shallow events. The stress pattern of intermediate-depth and very deep earthquakes has different directions: vertical extension in the NW-SE direction for intermediate depth earthquakes, and tension and pressure axes dipping about 45 ° for very deep earthquakes. Regional stress pattern may result from the collision between the African plate and Iberia, with extension and subduction of lithospheric material in the Alboran Sea at intermediate depth. The very deep seismicity may be correlated with older subduction processes.     

Modeling the dynamics of the block structure and seismicity of the Caucasus

Based on the morphostructural zoning scheme of the Caucasus, the block structure reflecting the real fault geometry and the block formation of the region is constructed. Several dozens of numerical experiments are conducted for simulating the dynamics of the block structure and the arising seismicity. The modeling relies on the following principles. It is assumed that the structure is composed of perfectly rigid blocks separated by infinitely thin fault planes. On the fault planes and on the blocks' bottoms, the blocks viscoelastically interact with each other and with the underlying medium. At each time instant, the translational displacements and rotations of the blocks are calculated from the condition of the quasi-static equilibrium of the entire block structure. The earthquakes occur in accordance with the dry friction model at the time instants when within a certain segment of the fault the stress-to-pressure ratio exceeds the given threshold. The modeling yields the synthetic catalog of the Caucasian earthquakes the spatial distribution of which reflects a set of characteristic features of the real seismicity. The similarity is observed in the magnitude–frequency diagrams of the synthetic and real seismicity. The comparison of the positions of the epicenters of the strong synthetic earthquakes with the results of recognizing the highly seismically active areas in the Caucasus demonstrates the presence of such epicenters in a few highly active areas where, according to the observations, strong earthquakes have not occurred to date.     

Classical focal mechanism techniques for body waves

The initial motion of primary body waves and polarization directions of secondary body waves have been applied successfully to the study of the mechanism operating at the earthquake focus. Equal area plots of these body wave characteristics resulted in radiation patterns that were compared to patterns due to theoretical focal mechanism sources. Such an approach indicated that a double couple force is the source mechanism operating at earthquake foci. This can be physically represented by faulting at the focus. This seems likely because of the relation between earthquakes and fault motion in shallow earthquakes. The possibility of other mechanisms operating in deep focus earthquakes has not been ruled out.The technique of solving for fault plane orientation/motion in equal area plots ofP-wave first motion andS-wave polarization, has been a powerful tool in areas where fault motions cannot be directly observed in studying tectonics. Such an approach has been used to test the theory of global tectonics. This approach has resulted in the confirmation of the suggested mechanics of plate motion, and the results of plate motion, such as spreading of the sea floor from mid-oceanic ridges and underthrusting of lithospheric plates at the sites of oceanic trenches.     

Mainshocks and aftershocks of the 2002 molise seismic sequence, southern Italy

In October and November 2002, the Molise region (southern Italy) was struck by two moderate magnitude earthquakes within 24 hours followed by an one month long aftershocks sequence. Soon after the first mainshock (October 31st, 10.32 UTC, M_w 5.7), we deployed a temporary network of 35 three-component seismic stations. At the time of occurrence of the second main event (November 1st, 15.08 UTC, M_w 5.7) the eight local stations already installed allowed us to well constrain the hypocentral parameters. We present the location of the two mainshocks and 1929 aftershocks with 2 < M_L < 4.2. Earthquake distribution reveals a E-trending 15 km long fault system composed by two main segments ruptured by the two mainshocks. Aftershocks define two sub-vertical dextral strike-slip fault segments in agreement with the mainshock fault plane solutions. P- and T-axes retrieved from 170 aftershocks focal mechanisms show a coherent kinematics: with a sub-horizontal NW and NE-trending P and T-axes, respectively. For a small percentage of focal mechanisms (∼ 10%) a rotation of T axes is observed, resulting in thrust solutions. The Apenninic active normal fault belt is located about 80 km westward of the 2002 epicentral area and significant seismicity occurs only 20-50 km to the east, in the Gargano promontory. Seismic hazard was thought to be small for this region because neither historical earthquake are reported in the Italian seismic catalogue or active faults were previously identified. In this context, the 2002 seismic sequence highlights the existence of trans-pressional active tectonics in between the extensional Apenninic belt and the Apulian foreland.     

Possible creep-related tilt precursors obtained in the Central Apennines (Italy) and in the Southern Caucasus (Georgia)

Daily averaged tilt component data from two sites of the Central Apennines (Italy) and of the Southern Caucasus (Georgia), respectively, revealed intermediate-term tilts as possible precursors to earthquakes (M=3.0÷4.7) which occurred in the above-mentioned seismic areas within a distance of 50 km from the sites. A good temporal correlation as well as a fair spatial correspondence between these residual tilts (with amplitude and duration of some microradians and months, respectively) and main shocks were pointed out, by removing both secular trends and seasonal thermoelastic effects from the raw tilts. An attempt was made to justify the above-mentioned results, based on the assumption that the observed intermediate-term preseismic tilts are the manifestation of aseismic creep episodes of comparable duration in the fault materials of thrust faults close to the tilt sites. The mechanism refers to a strain field slowly propagating from the preparation focal area to the tilt site, through crustal blocks separated by weak transition zones. This propagation is thought to be the cause of the local aseismic fault slip recorded by the tiltmeters. Previously, both discrete structures and strain propagation effects were revealed in the Central Apennines and are thought also to exist in the Southern Caucasus. As in the past, the rheological properties of fault materials are revealed as viscoelastic ones. In fact, creep equations obtained by applying several viscoelastic models on our data, proved to fit quite well some of the observed tilt precursors, producing viscosity and rigidity values very similar to those reported in literature.Professor Petr Viktorovich Manjgaladze died during the writing of this paper     

Paleotectonic structures and their influence on recent seismo-tectonics in the south Kuril subduction zone

Abstract Bathymetric data from south of Hokkaido obtained during a cruise of R/V Hakuho-Maru are summarized, and their correlation with earthquake occurrence is discussed. There are structural lineations on the seaward slope of the Kuril Trench, oblique to the Kuril Trench axis and parallel to the magnetic lineations in the Pacific plate. The structural lineations comprise horst-grabens generated by normal faulting. This suggests that Cretaceous tectonic structures originating at the spreading centre affect present seismotectonics around the trench axis. The structural-magnetic relation is compared to the case of the Japan Trench. North-east of the surveyed area, there are two major fracture zones (Nosappu Fracture Zone and Iturup Fracture Zone) that divide the oceanic plate into three segments. If the fracture zones (FZ) and the zone of paleo-mechanical weakness, represented by magnetic lineations, can control the direction of normal faults at a trench, the extent of the resulting topographic roughness on the seaward slope of the trench would be different across an FZ because of the differences in ages. By studying recent large earthquakes occurring in the south Kuril region, it is shown that several main-aftershock distributions for large earthquakes in this region are bounded by the Nosappu FZ and the Iturup FZ. Two models (Barrier model and Rebound model) are presented to interpret earthquake occurrence near the south Kuril Islands. The Barrier model explains seismic boundaries seen in several examples for earthquake occurrence in the south Kuril regions. The fracture zone forming the boundary of two segments with different magnetic lineations is also the boundary of two different normal fault systems on their ocean bottom, and the difference in sea-bottom roughness between two normal fault systems should affect the seismic coupling at a plate interface. Due to the difference of seismic coupling, earthquake occurrence is controlled by an FZ and then the FZ acts as a seismic boundary (Barrier model). Existing normal faults created by plate bending of subducting oceanic plate should rebound after its subduction (Rebound model). This rebound of normal faults may cause intraplate earthquakes with a high-angle reverse-fault mechanism such as the 1994 Shikotan Earthquake. The energy released by an intraplate earthquake generated by normal-fault rebounding is not directly related to that of interplate earthquakes such as low-angle thrust earthquakes. It is a reason why large earthquakes occurred in the same region during a relatively short period.     

Relocation of shallow earthquakes in southern Alaska using Joint Hypocenter Determination method

A subset of 2660 shallow earthquakes (0–50 km) that occurred from 1988 to 1996 in south central Alaska between 155°W and 145°W and 59°N and 63°N was relocated using the joint hypocenter determination (JHD) method. Both P- and S-wave observations recorded by the regional seismic network were used. Events were relocated in twenty different groups based on their geographic location and depth using two velocity models. As a result of the relocation, the majority of the hypocenters shifted downward, while the epicenter locations did not change significantly. The distribution of the shallow subduction zone earthquakes indicates the existence of two seismically independent blocks, with one block occupying the northeastern part and the other occupying the central and western parts of the study area. The boundary between the blocks is marked by a 15 to 20 km wide seismicity gap to the southeast of 149.5°W and 62°N. The analysis of the fault plane solutions for shallow subduction zone earthquakes shows that an overwhelming majority of the solutions represent normal, oblique-normal or strike-slip faulting with predominant WNW-ESE orientation of T-axes. This indicates a down-dip extensional regime for the subducting slab at shallow depths. Very few earthquakes yielded fault plane solutions consistent with thrusting on a contact zone between the overriding and subducting plates. This result may be an indication that currently either the strain energy is not released at the contact zone or it is associated with aseismic motion.     

Reevaluation of the earthquakes of 10 March and 19 May 1951 in southern Spain

Damage and parameters of the earthquakes of 10 March and 19 May 1951 in southern Spain have been reevaluated. Data available do not allow accurate depth determinations and previous estimates of larger depths are not confirmed, so depths have been fixed at 30 km for both shocks. Magnitudes (Ms) have been determined as 5.4 and 5.6, respectively. Intensities estimated at 22 and 29 sites from contemporary documentary sources give maximum values of VI–VII and VI (EMS Scale), lower than previous estimates. The focal mechanism for the May shock is right-lateral strike-slip with a normal component of motion, with planes with strikes 273° and 169°; seismic moment 1.9 × 10¹⁶ Nm and dimension 6 km (radius of circular fault). Shocks are located near the boundary between the Iberian plateau and the Guadalquivir Basin and may be related to faults connected with this boundary.     

Fault density,earthquakes, and the topography of crustal stratification interfaces: Central Magadan Oblast

This study is concerned with quantitative estimation of the relationship between earthquakes and tectonic crustal fragmentation based on a correlation analysis of fault density with seismicity parameters (the number and energy of earthquakes per unit area) for the Sredne-Yamskoi seismic junction and adjacent area. The highest level of seismic activity and the highest probability of earthquake occurrence with energy classes K ≥ 12 within areas that have a continental crust with a well-pronounced granite layer occur in those areas with the mean fault density. Within areas with a thinner granite layer in the crust, the most likely seismic events are K ≥ 12 earthquakes that occur in areas with lower fault density. We estimated the relationship between the degree of crustal fragmentation and the topography of stratification interfaces in the crust as identified by new interpretative gravimetry. Zones with the lowest degree of fragmentation tend to be areas where the top of the crystalline basement lies deeper.     

Rupture process of large earthquakes in the northern Mexico subduction zone

The Cocos plate subducts beneath North America at the Mexico trench. The northernmost segment of this trench, between the Orozco and Rivera fracture zones, has ruptured in a sequence of five large earthquakes from 1973 to 1985; the Jan. 30, 1973 Colima event (M _s 7.5) at the northern end of the segment near Rivera fracture zone; the Mar. 14, 1979 Petatlan event (M _s 7.6) at the southern end of the segment on the Orozco fracture zone; the Oct. 25, 1981 Playa Azul event (M _s 7.3) in the middle of the Michoacan gap; the Sept. 19, 1985 Michoacan mainshock (M _s 8.1); and the Sept. 21, 1985 Michoacan aftershock (M _s 7.6) that reruptured part of the Petatlan zone. Body wave inversion for the rupture process of these earthquakes finds the best: earthquake depth; focal mechanism; overall source time function; and seismic moment, for each earthquake. In addition, we have determined spatial concentrations of seismic moment release for the Colima earthquake, and the Michoacan mainshock and aftershock. These spatial concentrations of slip are interpreted as asperities; and the resultant asperity distribution for Mexico is compared to other subduction zones. The body wave inversion technique also determines theMoment Tensor Rate Functions; but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes. An appendix describes theMoment Tensor Rate Functions methodology in detail.The systematic bias between global and regional determinations of epicentral locations in Mexico must be resolved to enable plotting of asperities with aftershocks and geographic features. We have spatially shifted all of our results to regional determinations of epicenters. The best point source depths for the five earthquakes are all above 30 km, consistent with the idea that the down-dip edge of the seismogenic plate interface in Mexico is shallow compared to other subduction zones. Consideration of uncertainties in the focal mechanisms allows us to state that all five earthquakes occurred on fault planes with the same strike (N65°W to N70°W) and dip (15±3°), except for the smaller Playa Azul event at the down-dip edge which has a steeper dip angle of 20 to 25°. However, the Petatlan earthquake does prefer a fault plane that is rotated to a more east-west orientation—one explanation may be that this earthquake is located near the crest of the subducting Orozco fracture zone. The slip vectors of all five earthquakes are similar and generally consistent with the NUVEL-predicted Cocos-North America convergence direction of N33°E for this segment. The most important deviation is the more northerly slip direction for the Petatlan earthquake. Also, the slip vectors from the Harvard CMT solutions for large and small events in this segment prefer an overall convergence direction of about N20°E to N25°E.All five earthquakes share a common feature in the rupture process: each earthquake has a small initial precursory arrival followed by a large pulse of moment release with a distinct onset. The delay time varies from 4 s for the Playa Azul event to 8 s for the Colima event. While there is some evidence of spatial concentration of moment release for each event, our overall asperity distribution for the northern Mexico segment consists of one clear asperity, in the epicentral region of the 1973 Colima earthquake, and then a scattering of diffuse and overlapping regions of high moment release for the remainder of the segment. This character is directly displayed in the overlapping of rupture zones between the 1979 Petatlan event and the 1985 Michoacan aftershock. This character of the asperity distribution is in contrast to the widely spaced distinct asperities in the northern Japan-Kuriles Islands subduction zone, but is somewhat similar to the asperity distributions found in the central Peru and Santa Cruz Islands subduction zones. Subduction of the Orozco fracture zone may strongly affect the seismogenic character as the overlapping rupture zones are located on the crest of the subducted fracture zone. There is also a distinct change in the physiography of the upper plate that coincides with the subducting fracture zone, and the Guerrero seismic gap to the south of the Petatlan earthquake is in the wake of the Orozco fracture zone. At the northern end, the Rivera fracture zone in the subducting plate and the Colima graben in the upper plate coincide with the northernmost extent of the Colima rupture zone.     

Large earthquakes in the macquarie ridge complex: Transitional tectonics and subduction initiation

While most aspects of subduction have been extensively studied, the process of subduction initiation lacks an observational foundation. The Macquarie Ridge complex (MRC) forms the Pacific-Australia plate boundary between New Zealand to the north and the Pacific-Australia-Antarctica triple junction to the south. The MRC consists of alternating troughs and rises and is characterized by a transitional tectonic environment in which subduction initiation presently occurs. There is a high seismicity level with 15 large earthquakes (M>7) in this century. Our seismological investigation is centered on the largest event since 1943: the 25 MAY 1981 earthquake. Love, Rayleigh, andP waves are inverted to find: a faulting geometry of right-lateral strike-slip along the local trend of the Macquarie Ridge (N30°E); a seismic moment of 5×10²⁷ dyn cm (M _w=7.7) a double event rupture process with a fault length of less than 100km to the southwest of the epicenter and a fault depth of less than 20km. Three smaller thrust earthquakes occurred previous to the 1981 event along the 1981 rupture zone; their shallow-dipping thrust planes are virtually adjacent to the 1981 vertical fault plane. Oblique convergence in this region is thus accommodated by a dual rupture mode of several small thrust events and a large strike-slip event. Our study of other large MRC earthquakes, plus those of other investigators, produces focal mechanisms for 15 earthquakes distributed along the entire MRC; thrust and right-lateral strike-slip events are scattered throughout the MRC. Thus, all of the MRC is characterized by oblique convergence and the dual rupture mode. The true best-fit rotation pole for the Pacific-Australia motion is close to the Minster & Jordan RM2 pole for the Pacific-India motion. Southward migration of the rotation pole has caused the recent transition to oblique convergence in the northern MRC. We propose a subduction initiation process that is akin to crack propagation; the 1981 earthquake rupture area is identified as the crack-tip region that separates a disconnected mosaic of small thrust faults to the south from a horizontally continuous thrust interface to the north along the Puysegur trench. A different mechanism of subduction initiation occurs in the southernmost Hjort trench region at the triple junction. newly created oceanic lithosphere has been subducted just to the north of the triple junction. The entire MRC is a soft plate boundary that must accommodate the plate motion mismatch between two major spreading centers (Antarctica-Australia and Pacific-Antarctica). The persistence of spreading motion at the two major spreading centers and the consequent evolution of the three-plate system cause the present-day oblique convergence and subduction initiation in the Macquarie Ridge complex.     

A seismic gap in the eastern margin of the Sea of Japan as inferred from the time-space distribution of past seismicity

Abstract Based on the spatial distribution of the focal areas from previous large earthquakes, a significant seismic gap was found off the coast of the Akita Prefecture at the eastern margin of the Sea of Japan. We forecast that the 75 km segment of plate boundary may be ruptured by a large thrust earthquake with a magnitude of 7.5 in the near future. The time of occurrence is uncertain, but the recurrence interval of large earthquakes in this region suggests that it will be no later than the end of next century. The time sequence of large earthquakes in the eastern margin of the Sea of Japan shows a systematic shortening of the earthquake interval for the past 250 years. This feature is simulated successfully by a simple mechanical model that represents a plate boundary by segmented blocks, each of which is composed of a spring friction slider system. We further propose to revise the conventionally accepted plate boundary between the Eurasian and Okhotsk plates in the eastern margin of the Sea of Japan; the observed data of seismic activity and recent crustal deformation indicate that the southern part of the plate boundary traverses the land region from Niigata to Otari along the Shinanogawa seismic belt. The fault geometries of previous large earthquakes show that the mode of plate convergence in the eastern margin of the Sea of Japan is a collision type rather than a simple eastward subduction as was postulated in early studies.     

Regional focal mechanisms for earthquakes in the Aegean area

The distribution of the focal mechanisms of the shallow and intermediate depth (h>40 km) earthquakes of the Aegean and the surrounding area is discussed. The data consist of all events of the period 1963–1986 for the shallow, and 1961–1985 for the intermediate depth earthquakes, withM _s 5.5. For this purpose, all published fault plane solutions for each event have been collected, reproduced, carefully checked and if possible improved accordingly. The distribution of the focal mechanisms of the earthquakes in the Aegean declares the existence of thrust faulting following the coastline of southern Yugoslavia, Albania and western Greece extending up to the island of Cephalonia. This zone of compression is due to the collision between two continental lithospheres (Apulian-Eurasian). The subduction of the African lithosphere under the Aegean results in the occurrence of thrust faulting along the convex side of the Hellenic arc. These two zones of compression are connected via strike-slip faulting observed at the area of Cephalonia island. TheP axis along the convex side of the arc keeps approximately the same strike throughout the arc (210° NNE-SSW) and plunges with a mean angle of 24° to southwest. The broad mainland of Greece as well as western Turkey are dominated by normal faulting with theT axis striking almost NS (with a trend of 174° for Greece and 180° for western Turkey). The intermediate depth seismicity is distributed into two segments of the Benioff zone. In the shallower part of the Benioff zone, which is found directly beneath the inner slope of the sedimentary arc of the Hellenic arc, earthquakes with depths in the range 40–100 km are distributed. The dip angle of the Benioff zone in this area is found equal to 23°. This part of the Benioff zone is coupled with the seismic zone of shallow earthquakes along the arc and it is here that the greatest earthquakes have been observed (M _s 8.0). The deeper part (inner) of the Benioff zone, where the earthquakes with depths in the range 100–180 km are distributed, dips with a mean angle of 38° below the volcanic arc of southern Aegean.