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1.
From an analysis of many seismic profiles across the stable continental regions of North America and northern Europe, the crustal and upper mantle velocity structure is determined. Analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques. TheP wave velocity structure beneath the Canadian Shield is virtually identical to that beneath the Baltic Shield to a depth of at least 800 km. Two major layers with a total thickness of about 42 km characterize the crust of these shield regions. Features of the upper mantle of these region include velocity discontinuities at depths of about 74 km, 330 km, 430 km and 700 km. A 13 km thickP wave low velocity channel beginning at a depth of about 94 km is also present.A number of problems associated with record section interpretation are identified and a generalized approach to seismic profile analysis using many record sections is described. TheS wave velocity structure beneath the Canadian Shield is derived from constrained surface wave data. The thickness of the lithosphere beneath the Canadian and Baltic Shields is determined to be 95–100 km. The continental plate thickness may be the same as the lithospheric thickness, although available data do not exclude the possibility of the continental plate being thicker than the lithosphere.  相似文献   

2.
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.  相似文献   

3.
Source mechanism of intermediate and deep earthquakes in southern Spain   总被引：1，自引：0，他引：1  
Focal mechanisms of 10 intermediate-depth earthquakes (30相似文献   

4.
A three-dimensional model for the central Fennoscandian Shield was constructed for analysing the thermal, the rheological and the structural conditions in the lithosphere. The mesh covers a rectangular area in the southern Finland with horizontal dimensions of 500 km × 400 km and a depth extent of 100 km. Structural boundaries are derived from the several deep seismic soundings carried out in the area. Constructed model is first used in the calculation of the thermal and the rheological models and secondly in analysing the stress and the deformational conditions with the obtained rheology. Thermal and structural models are solved with the finite element method. The calculated surface HFD is between 40 and 48 mW m−2 in the Proterozoic southern part and below 40 mW m−2 in the older and northern Archaean part of the model. The calculated rheological strength shows a layered structure with two individual rheologically weak layers in the crust and strong layer in the upper part of the lower crust. The minimum brittle–ductile transition (BDT) depth is around 10 km in the southern part of the model while in the north and north-eastern parts the BDT depth is around 45–50 km. Comparison with the focal depth data shows that as most of the earthquakes occur no deeper than the depth of 10 km are they located in the brittle regime. Resulting stress conditions and possible regions of deformation after the model is subjected to pressure of 50 MPa reveals that the stress field is quite uniformly distributed in different crustal layers and that the elastic parameters control more the state of the stress than the applied rheological structure. In the upper crust, the stress intensity has values between 42 and 45 MPa whereas in the middle crust the values are around 50 MPa. Comparison of the 3-D model with earlier 2-D models shows that some differences in the results are to be expected.  相似文献   

5.
The fundamental mode Love and Rayleigh waves generated by earthquakes occurring in Kashmir, Nepal Himalaya, northeast India and Burma and recorded at Hyderabad, New Delhi and Kodaikanal seismic stations are analysed. Love and Rayleigh wave attenuation coefficients are obtained at time periods of 15–100 seconds, using the spectral amplitude of these waves for 23 different paths along northern (across Burma to New Delhi) and central (across Kashmir, Nepal Himalaya and northeast India to Hyderabad and Kodaikanal) India. Love wave attenuation coefficients are found to vary from 0.0003 to 0.0022 km–1 for northern India and 0.00003 km–1 to 0.00016 km–1 for central India. Similarly, Rayleigh wave attenuation coefficients vary from 0.0002 km–1 to 0.0016 km–1 for northern India and 0.00001 km–1 to 0.0009 km–1 for central India. Backus and Gilbert inversion theory is applied to these surface wave attenuation data to obtainQ –1 models for the crust and uppermost mantle beneath northern and central India. Inversion of Love and Rayleigh wave attenuation data shows a highly attenuating zone centred at a depth of 20–80 km with lowQ for northern India. Similarly, inversion of Love and Rayleigh wave attenuation data shows a high attenuation zone below a depth of 100 km. The inferred lowQ value at mid-crustal depth (high attenuating zone) in the model for northern India can be by underthrusting of the Indian plate beneath the Eurasian plate which has caused a low velocity zone at this shallow depth. The gradual increase ofQ –1 from shallow to deeper depth shows that the lithosphere-asthenosphere boundary is not sharply defined beneath central India, but rather it represents a gradual transformation, which starts beneath the uppermost mantle. The lithospheric thickness is 100 km beneath central India and below that the asthenosphere shows higher attenuation, a factor of about two greater than that in the lithosphere. The very lowQ can be explained by changes in the chemical constitution taking place in the uppermost mantle.  相似文献   

6.
Two large shallow earthquakes occurred in 1942 along the South American subduction zone inclose proximity to subducting oceanic ridges: The 14 May event occurred near the subducting Carnegie ridge off the coast of Ecuador, and the 24 August event occurred off the coast of southwestern Peru near the southern flank of the subducting Nazca ridge. Source parameters for these for these two historic events have been determined using long-periodP waveforms,P-wave first motions, intensities and local tsunami data.We have analyzed theP waves for these two earthquakes to constrain the focal mechanism, depth, source complexity and seismic moment. Modeling of theP waveform for both events yields a range of acceptable focal mechanisms and depths, all of which are consistent with underthrusting of the Nazca plate beneath the South American plate. The source time function for the 1942 Ecuador event has one simple pulse of moment release with a duration of 22 suconds, suggesting that most of the moment release occurred near the epicenter. The seismic moment determined from theP waves is 6–8×1020N·m, corresponding ot a moment magnitude of 7.8–7.9. The reported location of the maximum intensities (IX) for this event is south of the main shock epicenter. The relocated aftershcks are in an area that is approximately 200 km by 90 km (elongated parallel to the trench) with the majority of aftershocks north of the epicenter. In contrast, the 1942 Peru event has a much longer duration and higher degree of complexity than the Ecuador earthquake, suggesting a heterogeneous rupture. Seismic moment is released in three distinct pulses over approximately 74 seconds; the largest moment release occurs 32 seconds after rupture initiation. the seismic moment as determined from theP waves for the 1942 Peru event is 10–25×1020N·m, corresponding to a moment magnitude of 7.9–8.2. Aftershock locations reported by the ISS occur over a broad area surrounding the main shock. The reported locations of the maximum intensities (IX) are concentrated south of the epicenter, suggesting that at least part of the rupture was to the south.We have also examined great historic earthquakes along the Colombia-Ecuador and Peru segments of the South American subduction zone. We find that the size and rupture length of the underthrusting earthquakes vary between successive earthquake cycles. This suggests that the segmentation of the plate boundary as defined by earthquakes this century is not constant.  相似文献   

7.
Intraplate seismic activity in Bolivia is mainly located in the central region (16°–19°S, 63°–67°W) which includes the East Andean Cordillera and the Sub-Andean Sierras. At this region there is a bend in the trend of the main geological structures from NW-SE in the north to N-S in the south. Focal mechanisms have been calculated for 10 earthquakes of magnitudes 4.9–5.6, using first motionP-waves from long period instruments. Their solutions correspond to reverse faulting, some with a large component of strike-slip motion. Their solutions can be grouped into two types; one with pure reverse faulting on planes with azimuth NW-SE and the other with a large strike-slip component on planes with azimuths nearly N-S or WNW-ESE. The maximum stress axis (P-axis) is practically horizontal (dipping less than 5°) oriented in a mean N56°E direction. This orientation may be related with the direction of compression resulting from the collision of the Nazca plate against the western margin of the South American continent. Wave-form analysis of long-periodP-waves for one event restricts the focal depth to 8 km in the Sub-Andean region. Seismic moments and source dimensions determined from spectra of Rayleigh waves are in the range of 1016–1017Nm and 17–24 km, respectively. The Central Bolivia region can be considered as a zone of intraplate deformation situated between the Bolivian Altiplano and the Brazil shield.  相似文献   

8.
A genetic algorithm inversion of receiver functions derived from a dense seismic network around Iwate volcano, northeastern Japan, provides the fine S wave velocity structure of the crust and uppermost mantle. Since receiver functions are insensitive to an absolute velocity, travel times of P and S waves propagating vertically from earthquakes in the subducting slab beneath the volcano are involved in the inversion. The distribution of velocity perturbations in relation to the hypocenters of the low-frequency (LF) earthquakes helps our understanding of deep magmatism beneath Iwate volcano. A high-velocity region (dVS/VS=10%) exists around the volcano at depths of 2–15 km, with the bottom depth decreasing to 11 km beneath the volcano’s summit. Just beneath the thinning high-velocity region, a low-velocity region (dVS/VS=−10%) exists at depths of 11–20 km. Intermediate-depth LF (ILF) events are distributed vertically in the high-velocity region down to the top of the low-velocity region. This distribution suggests that a magma reservoir situated in the low-velocity region supplies magma to a narrow conduit that is detectable by the hypocenters of LF earthquakes. Another broad low-velocity region (dVS/VS=−5 to −10%) occurs at depths of 17–35 km. Additional clusters of deep LF (DLF) events exist at depths of 32–37 km in the broad low-velocity zone. The DLF and ILF events are the manifestations of magma movement near the Moho discontinuity and in the conduit just beneath the volcano, respectively.  相似文献   

9.
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.  相似文献   

10.
This study is based on the detailed geometry of the Hokkaido Wadati-Benioff zone and the paleosubduction zone as delineated by Hanus and Vanek (1984). The used data includes 217 CMT Harvard solutions for earthquakes, which belong to the Wadati-Benioff zone and 13 for the paleosubduction zone. The inverse technique by Gephart and Forsyth (1984) was incorporated for determining the best fit principal stress directions σ1, σ2, σ3 and the ratio (R=σ2−σ1/σ3−σ1) for 20 km depth intervals in the Wadati-Benioff zone and for the paleosubduction zone considered as a single body. In almost all the considered depth layers, the maximum compressive stress σ1 is normal to the strike of the slab and dips less than 25°, indicating the NW-SE convergence between the Pacific and Eurasian lithospheric plates. Exceptions are in the depth layer 81–120 km, the paleosubduction zone with steeply dipping along-strike σ1, and the lower part of the subduction zone (161–220 km) where σ1 is almost horizontal and of E trend. The minimum compressive stress σ3 is mostly along-strike and of a different dip with the exception of the 21–60 km layer wher they are down-dipping. The results obtained for the depth ranges 0–20 km, 81–100 km, 121–160 km, and the paleosubduction zone indicate heterogeneous stress fields. These results show that the slab pull and the mantle resistance, acting on the slab edge, are not the main forces which control the contemporary plate tectonics in the Hokkaido region. Along-strike compression at depths 81–120 km and along-strike extension at 0–20 and 61–220 km are involved in the slab dynamics. These can be related to horizontal bending of the subducting Pacific plate.  相似文献   

11.
For small earthquakes, focal depths can be estimated jointly when epicenters are located using the arrival times of Pg and Sg waves recorded at seismic stations close to the event. However, if regional network coverage is sparse, this approach does not give accurate results. An alternative solution is the use of the regional depth-phase modeling (RDPM) method when such depth phases are available. Small, shallow earthquakes can generate Rg waves, the amplitudes of which approximately attenuate exponentially with focal depth; whereas, the amplitudes of Sg waves are, on average, less dependent on focal depth. Based on these features, a method using the maximum power spectral ratio (MPSR) between the Rg and Sg segments was developed to determine focal depth. Tests show the focal depth solutions obtained by the MPSR and RDPM methods for five events in an earthquake swarm and one event acquired by inspection are in good agreement. The error in the MPSR-determined focal depth caused by the error in the epicentral distance is in the order of 0.1 km. The error in the focal depth when using a default focal mechanism is in the order of 0.5 km. The quality factor, Q does not generate a significant error. Using the average of focal depths can provide a more reliable solution. Using an azimuth of approximately 45° from the strike direction to generate the synthetic ratio curve can reduce the error. As with any other earthquake locating technique, a reasonable regional crustal model is required when the MPSR method is used. Case studies show that the MPSR method can be used to successfully determine focal depths for events as small as m N 1.6.  相似文献   

12.
We useP andS times listed in the International Seismological Summary to relocate 23 historical earthquakes (1927–1963) reported as occurring at or below 670 km. In all cases, our relocated hypocenters are shallower than the starting depths; furthermore, all events converge to 691 km or less, with a precision estimated at ±10 km. This study upholds the results of Stark and Frohlich, who had usedpP–P times of post-WWSSN earthquakes to constrain reliable hypocentral depths to no greater than 684 km. In particular, we reject Rothé's claim that a 1963 event in the vicinity of New Guinea occurred at a depth of more than 780 km.  相似文献   

13.
Some months prior to the 1995 eruption of Mt Ruapehu (New Zealand), a series of shallow earthquake swarms occurred about 15–20 km west of the summit of Ruapehu. Several earthquakes in these swarms were felt, and the largest event was ML 4.8. Crustal earthquakes of ML≥3.0 within 20 km of the summit of Ruapehu have been rather uncommon in recent years. Furthermore, the two periods of strongest activity were both just before times when the temperature of Crater Lake showed rapid increases. The second of these rapid heating phases was immediately followed by increases in the Mg2+ ion concentration in Crater Lake, indicating that chemical interactions were occurring between fresh magmatic material and the lake water. The coincidence between seismicity and lake changes suggested a link with the following eruption. A 1-D simultaneous inversion to locate the earthquakes more accurately showed that most of the earthquakes fell into three spatial clusters, each cluster having a small horizontal cross-section. The predominant depth was about 10–16 km. The b-value of this swarm was 0.74, quite compatible with ordinary tectonic earthquakes. Each cluster of earthquakes lies close to the normal Raurimu Fault which runs predominantly north–south to the west of Ruapehu, with an east-trending branch splaying off near its northern end (see Fig. 1b). Composite focal mechanisms of events in the two more southern clusters are oblique-normal, while the other cluster to the north has an oblique-reverse mechanism. The two oblique-normal mechanisms suggest that extension has occurred on part of the fault. This stress pattern was also observed in the focal mechanism solutions of events that occurred after the eruption, when a denser network of portable seismographs covered the region. Although we cannot definitely connect the occurrence of these swarms to the eruptions later in 1995, there is a strong suggestion that the seismicity was connected to the process of magma movement, which temperature and chemical changes in Crater Lake suggest was occurring during the first half of 1995.  相似文献   

14.
Arrival times of P and S waves from local earthquakes in the Kamchatka area of the Kurile-Kamchatka Island Arc are used for calculating a spatial model of the elastic wave velocity distribution to a depth of 200 km. The lithosphere is shown to be strongly stratified in its velocity properties and laterally heterogeneous within the mantle wedge and seismic focal zone. A lower velocity layer (an asthenospheric wedge) is identified at depths of 70–130 km beneath the Eastern Kamchatka volcanic belt. The morphology of the Moho interface and the velocity properties of the crust are studied. The main tectonic structures of the region are shown to be closely interrelated with deep velocity heterogeneities. Regular patterns in the statistics of the earthquakes are analyzed in relation to variations in the elastic wave velocities in the focal layer. A mechanism of lithospheric block displacements along weakened zones of the lower crust and upper mantle is proposed.  相似文献   

15.
Interplate coupling plays an important role in the seismogenesis of great interplate earthquakes at subduction zones. The spatial and temporal variations of such coupling control the patterns of subduction zone seismicity. We calculate stresses in the outer rise based on a model of oceanic plate bending and coupling at the interplate contact, to quantitatively estimate the degree of interplate coupling for the Tonga, New Hebrides, Kurile, Kamchatka, and Marianas subduction zones. Depths and focal mechanisms of outer rise earthquakes are used to constrain the stress models. We perform waveform modeling of body waves from the GDSN network to obtain reliable focal depth estimates for 24 outer rise earthquakes. A propagator matrix technique is used to calculate outer rise stresses in a bending 2-D elastic plate floating on a weak mantle. The modeling of normal and tangential loads simulates the total vertical and shear forces acting on the subducting plate. We estimate the interplate coupling by searching for an optimal tangential load at the plate interface that causes the corresponding stress regime within the plate to best fit the earthquake mechanisms in depth and location.We find the estimated mean tangential load over 125–200 km width ranging between 166 and 671 bars for Tonga, the New Hebrides, the Kuriles, and Kamchatka. This magnitude of the coupling stress is generally compatible with the predicted shear stress at the plate contact from thermal-mechanical plate models byMolnar andEngland (1990), andVan den Buekel andWortel (1988). The estimated tectonic coupling,F tc , is on the order of 1012–1013 N/m for all the subduction zones.F tc for Tonga and New Hebrides is about twice as high as in the Kurile and Kamchatka arcs. The corresponding earthquake coupling forceF ec appears to be 1–10% of the tectonic coupling from our estimates. There seems to be no definitive correlation of the degree of seismic coupling with the estimated tectonic coupling. We find that outer rise earthquakes in the Marianas can be modeled using zero tangential load.  相似文献   

16.
A sequence of moderate shallow earthquakes (3.5M L5.3) was located within the Vercors massif (France) in the period 1961–1984. This subalpine massif has been a low seismic area for at least 5 centuries. During the period 1962–1963, 12 shallow earthquakes occurred in the neighborhood (10 km) of the Monteynard reservoir, 30 km south of the city of Grenoble. The latest fourM L4.0 earthquakes occurred in 1979–1984 either at larger distance (35 km) or greater depth (10 km) from the reservoir. Two triggering mechanisms are suggested for this sequence: (i) the direct effect of elastic loading through either increased shear stress or strength reducing by increased pore pressure at depth; (ii) the pore pressure diffusion induced by poroelastic stress change due to the reservoir filling.The weekly water levels, local balanced geological cross sections, and focal mechanisms argue for two types of mechanical connection between the earthquake sequence and the filling cycles of the Monteynard reservoir. The seismic sequence started with the 1962–1963 shallow earthquakes that occurred during the first filling of the reservoir and are typical of the direct effect of elastic loading. The 1979 deeper earthquake is located at a 10 km depth below the reservoir. This event occurred 16 years after the initial reservoir impoundment, but one month after the previous 1963 maximum water level was exceeded. Moreover the yearly reservoir level increased gradually in the period 1962–1979 and has decreased since 1980. Accordingly we suggest that the gradual diffusion of water from reservoir to hypocentral depths decreases the strength of the rock matrices through increased pore pressure. The transition between the two types of seismic response is supported by the analysis ofM L3.5 earthquakes which all occurred in the period 1964–1971, ranging between 10 and 30 km distance from the reservoir. The three other delayed earthquakes of the 1961–1984 seismic sequence (M L4 during the 1979–1984 period) are all located 35 km away from the reservoir. Based on the seismic activity, the estimates for the hydraulic diffusivities range between 0.2–10 m2/s, except for the first event that occurred 30 km north of the reservoir, the filling just started. The lack ofin situ measurements of crustal hydrological properties in the area, shared by most of the Reservoir-Induced-Seismicity cases, prevents us from obtaining absolute evidence for the triggering processes. These observations and conceptual models attest that previous recurrence times for moderate natural shocks (4.5M L5.5) estimated within this area using historical data, could be modified by 0.1–1 MPa stress changes. These small changes in deviatoric stress suggest that the upper crust is in this area nearly everywhere at a state of stress near failure. Although the paucity of both number and size of earthquakes in the French subalpine massif shows that aseismic displacements prevail, our study demonstrates that triggered earthquakes are important tools for assessing local seismic risk through mapping fault zones and identifying their possible seismic behavior.  相似文献   

17.
The method of relative seismic moment tensor determination proposed byStrelitz (1980) is extended a) from an interactive time domain analysis to an automated frequency domain procedure, and b) from an analysis of subevents of complex deep-focus earthquakes to the study of individual source mechanism of small events recorded at few stations.The method was applied to the recovery of seismic moment tensor components of 95 intermediate depth earthquakes withM L=2.6–4.9 from the Vrancea region, Romania. The main feature of the obtained fault plane solutions is the horizontality ofP axes and the nonhorizontal orienaation ofT axes (inverse faulting). Those events with high fracture energy per unit area of the fault can be grouped unambiguously into three depth intervals: 102–106 km, 124–135 km and 141–152 km. Moreover, their fault plane solutions are similar to ones of all strong and most moderate events from this region and the last two damaging earthquakes (November 10, 1940 withM W=7.8 and March 4 1977 withM W=7.5) occurred within the third and first depth interval, respectively. This suggests a possible correlation at these depths between fresh fracture of rocks and the occurrence of strong earthquakes.  相似文献   

18.
The centroid-moment tensor solutions of more than 300 earthquakes that occurred in the Himalayas and its vicinity regions during the period of 1977–1996 are examined. The resultant seismic moment tensor components of these earthquakes are estimated. The Burmese arc region shows prominent east–west compression and north–south extension with very little vertical extension. Northeast India and Pamir–Hindu Kush regions show prominent vertical extension and east–west compression. The Indian plate is subducting eastward beneath the northeast India and Burmese arc regions. The overriding Burmese arc has overthrust horizontally with the underthrusting Indian plate at a depth of 20–80 km and below 80 km depth, it has merged with the Indian plate making “Y” shape structure and as a result the aseismic zone has been formed in the region lying between 26°N–28°N and 91.5°E–94°E at a depth of 10–50 km. Similarly, the Indian plate is underthrusting in the western side beneath the Pamir–Hindu Kush region and the overriding Eurasian plate has overthrust it to form a “Y” shape structure at a depth of 10–40 km and below 60 km depth, it has merged with the Indian plate and both the plates are subducting below 60–260 km depth. Further south, the overriding Eurasian plate has come in contact with the Indian plate at a depth of 20–60 km beneath northwest India and Pakistan regions with left lateral strike slip motion.  相似文献   

19.
谢弘臻  王九洋 《中国地震》2018,34(4):704-712
对不同震中距台站的记录采用入射角法、s PL-Pg等震相到时差,对辽宁地震台网记录采用单纯形法研究了辽阳灯塔5.1级地震的震源深度。结果表明,该地震震源深度应为14km,略大于目录给出的10km。利用四川松潘台、青海湟源台的远台记录也得到同样的结果。通过对辽宁1970年以来5.0级以上地震进行分析发现,辽宁地震的震源分布存在东西两侧偏深、中部偏浅、中部地区南浅北深的统计规律,灯塔地震震源深度符合该统计规律。  相似文献   

20.
Recent improvements in the seismological networks on the Ibero-Maghrebian region have permitted estimation of hypocentral location and focal mechanisms for earthquakes which occurred at South Spain, Alboran Sea and northern Morocco of deep and intermediate depth, with magnitudes between 3.5 and 4.5. Intermediate depth shocks, range from 60 to 100 km, with greater concentration located between Granada and Málaga. Fault-plane solutions of 5 intermediate shocks have been determined; they present a vertical plane in NE-SW or E-W direction. Seismic moments of about 1015 Nm and dimensions of about 1 km have been determined from digital records of Spanish stations.P-wave forms are complex. This may be explained by the crustal structure near the station, discontinuities in the upper mantle and inhomogeneities near the source. Deep activity at about 650 km has only 3 shocks since 1954 (1954, 1973, 1990). Shocks are located at a very small region. Fault-plane solutions show a consistent direction of the pressure axis dipping 45° in E direction. For the 1990 shock seismic moment is 1016 Nm and dimensions 2.6 km. TheP-waves are of simpler form with a single pulse. The intermediate and deep activities are not connected and no activity has been detected between 100 and 650 km. The intermediate shocks may be explained in terms of a recent subduction from Africa under Iberia in SE direction. The very deep activity must be related to a sunk detached block of lithospheric material still sufficiently cold and rigid to generate earthquakes.  相似文献   

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