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1.
Two different models of the structure of the Icelandic crust have been presented. One is the thin-crust model with a 10–15 km thick crust beneath the axial rift zones, with an intermediate layer of partially molten basalt at the base of the crust and on the top of an up-domed asthenosphere. The thick-crust model assumes a 40 km thick and relatively cold crust beneath central Iceland. The most important and crucial parameter to distinguish between these different models is the temperature distribution with depth. Three methods are used to estimate the temperature distribution with depth. First, the surface temperature gradient measured in shallow wells drilled outside geothermal areas. Second, the thickness of the seismogenic zone which is associated with a 750 °C isothermal surface. Third, the depth to a layer with high electrical conductivity which is associated with partially molten basalt with temperature around 1100 °C at the base of the crust. Combination of these data shows that the temperature gradient can be assumed to be nearly linear from the surface down to the base of the crust. These results are strongly in favour of the thin-crust model. The scattered deep seismic reflectors interpreted as Moho in the thick-crust model could be caused by phase transitions or reflections from melt pockets in the mantle.  相似文献   

2.
A first generation of probabilistic seismic hazard maps of the Italian country are presented. They are based on seismogenic zoning deriving from a kinematic model of the structural tectonic units and on an earthquake catalogue with the foreshock and aftershock events filtered out. The following ground motion parameters have been investigated and mapped using attenuation equations based on strong-motion recordings of Italian earthquakes: peak ground acceleration and velocity; Arias intensity; strong motion duration; and the pseudovelocity and pseudoacceleration spectral values at 14 fixed frequencies both for the vertical and the largest horizontal component. A Poissonian model of earthquake occurrence is assumed as a default and the hazard maps are presented in terms of ground motion values expected to be exceeded at a 10% probability level in 50 years (return period 475 years) according to the requirement of Eurocode 8 for the seismic classification of national territories, as well as in terms of exceedance probabilities of selected ground motion values. Finally, as a tentative study, the use of hybrid methods (implementing both seismogenic zones and structures), renewal processes (including earthquake forecasting) and the influence of site effects (as the basis for the planning of earthquake scenarios) were explored.  相似文献   

3.
A catalogue of 1873–1972 earthquakes with M > 6.9 for the New Guinea—Solomon Islands region (130–165° E) is compiled. There are 152 events listed. Duda's (1965) results for 1900–1968 are improved for the Papua New Guinea area (141–156° E) because of the availability of historical data for that area.Although there is evidence of rapid Holocene uplift in the main seismic zones, there is little historical evidence for visible uplift or subsidence resulting directly from modern major earthquakes. Coastal subsidences commonly reported as a result of earthquakes are of smaller extent and appear to be due to settlement. However, the occurrence of tsunamigenic earthquakes does suggest that surface deformations do take place off-shore.Using Davies and Brune's (1971) method, regional fault slip rates over 5° -segments of the shallow seismic zone are determined from the seismicity catalogue. The slip rate for the island of New Guinea (Gutenberg and Richter's Region 16) is found to be at least 4.4 cm/y which is almost double the very anomalously low rate of 2.3 cm/y found by Davies and Brune (1971). If allowance is made for shear movement without seismicity and for the approximately ratio of dip-slip versus strike-slip faulting indicated by fault plane solutions, the agreement with Le Pichon's (1970) approach value of 10.7 cm/y for the Pacific—India (Australia) plates is reasonable. The fault slip rate in the area between east New Britain and Bougainville at the Pacific—Bismarck—Solomon triple junction is extremely high (20.6 cm/y at least). The smallest slip rate (1.5 cm/y) is found for westernmost New Guinea (130–135° E).Temporal cumulative summation of moments curves show a periodicity of approximately 25 years in the seismic activity at the triple junction (150–155° E). Elsewhere the rate of seismic activity is aperiodic.  相似文献   

4.
The earthquakes with magnitude M 6 which occurred in North China (30°–42°N, 105°–124°E) from 780 B.C. to 1978 A.D. have been analysed. Most of them appear in groups, each of which is confined to a definite region and period of time, called respectively the active region and active period. From 780 B.C. to 1000 A.D., groupings of earthquakes were not apparent, due to scanty data. Since 1000 A.D., 16 groups of earthquakes can be recognized. Statistics show that about 73% of the earthquakes occurred in groups. This implies that grouping of earthquakes of M 6 is a characteristic feature of seismic activity in North China. On this basis, a concept of a unified seismogenic process of major earthquakes has been proposed with the support of the geodetic data. Finally, the significance of this concept with regards to earthquake prediction has been discussed.  相似文献   

5.
To investigate the strength of frictional sliding and stability of mafic lower crust, we conducted experiments on oven-dried gabbro gouge of 1 mm thick sandwiched between country rock pieces (with gouge inclined 35° to the sample axis) at slip rates of 1.22 × 10− 3 mm/s and 1.22 × 10− 4 mm/s and elevated temperatures up to 615 °C. Special attention has been paid to whether transition from velocity weakening to velocity strengthening occurs due to the elevation of temperature.Two series of experiments were conducted with normal stresses of 200 MPa and 300 MPa, respectively. For both normal stresses, the friction strengths are comparable at least up to 510 °C, with no significant weakening effect of increasing temperature. Comparison of our results with Byerlee's rule on a strike slip fault with a specific temperature profile in the Zhangbei region of North China shows that the strength given by experiments are around that given by Byerlee's rule and a little greater in the high temperature range.At 200 MPa normal stress, the steady-state rate dependence a − b shows only positive values, probably still in the “run-in” process where velocity strengthening is a common feature. With a normal stress of 300 MPa, the values of steady-state rate dependence decreases systematically with increasing temperature, and stick-slip occurred at 615 °C. Considering the limited displacement, limited normal stress applied and the effect of normal stress for the temperatures above 420 °C, it is inferred here that velocity weakening may be the typical behaviour at higher normal stress for temperature above 420 °C and at least up to 615 °C, which covers most of the temperature range in the lower crust of geologically stable continental interior. For a dry mafic lower crust in cool continental interiors where frictional sliding prevails over plastic flow, unstable slip nucleation may occur to generate earthquakes.  相似文献   

6.
L. Faenza  S. Pierdominici   《Tectonophysics》2007,439(1-4):13-31
We present two examples of statistical analysis of seismicity conducted by integrating geological, geophysical and seismological data with the aim to characterize the active stress field and to define the spatio-temporal distribution of large earthquakes. Moreover, our data will help to improve the knowledge of the “seismogenic behavior” of the areas and to provide useful information for seismic hazard evaluation.The earthquakes are described by two non-parametric statistical procedures integrating also tectonic-physical parameters to study the spatio-temporal variability.The results show that the areas are characterized by: 1) a stress regime with mainly extensional kinematics; 2) tectonic structures mainly oriented with the active stress field (Shmin = N44° ± 18° in the southern Apennines and Shmin = N50° ± 17° in the central Apennines); 3) cluster distribution of seismicity and 4) a high probability of earthquake occurrence (M > 5.5) in the next 10 years.  相似文献   

7.
Cheng-Horng Lin   《Tectonophysics》2007,443(3-4):271
In 1999, a large earthquake (Mw = 7.6) occurred along the Chelungpu fault in the fold-and-thrust belt of western Taiwan. To shed more light on the subsurface structures and the seismogenic layers, three-dimensional velocity structures were inverted by using the travel times of both P- and S-waves from 2391 aftershocks recorded by the Central Weather Bureau during the 15 months that followed. From tomography, a typical image of the large-scale thrusting structures in the upper crust across the Chelungpu fault was obtained. In general, high velocities beneath the Western Foothills and Central Ranges are separated from low velocities beneath the Coastal Plain by an east-dipping boundary that is roughly consistent with the Chelungpu fault on the surface. The contrast in velocity on either side of the Chelungpu fault is indicative of about a 7- to 9-km vertical offset in the upper crust. The relocated hypocenter for the Chi-Chi earthquake shifts by 2.2 km toward the northwest, and its focal depth decreases by 0.7 km. A plot of focal depths versus rock velocities where the aftershocks occurred shows earthquakes are more inclined to occur in rock with a velocity of around 5.6 km/s. This strongly suggests the seismogenic layer in the fold-and-thrust belt of Taiwan is more structure-dependent than depth-dependent.  相似文献   

8.
This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10 km, defining an active layer, gently dipping north, on which the main normal faults, mostly dipping north, are rooting. It may be interpreted as a detachment zone, possibly related to the Phyllade thrust nappe. Young, active normal faults connecting the Aigion to the Psathopyrgos faults seem to control the spatial distribution of the microseismicity. This seismic activity is interpreted as a seismic creep from GPS measurements, which shows evidence for fast continuous slip on the deepest part on the detachment zone. Offshore, either the shallowest part of the faults is creeping, or the strain is relaxed in the shallow sediments, as inferred from the large NS strain gradient reported by GPS. The predicted subsidence of the central part of the rift is well fitted by the new continuous GPS measurements. The location of shallow earthquakes (between 5 and 3.5 km in depth) recorded on the on-shore Helike and Aigion faults are compatible with 50° and 60° mean dip angles, respectively. The offshore faults also show indirect evidence for high dip angles. This strongly differs from the low dip values reported for active faults more to the east of the rift, suggesting a significant structural or rheological change, possibly related to the hypothetical presence of the Phyllade nappe. Large seismic swarms, lasting weeks to months, seem to activate recent synrift as well as pre-rift faults. Most of the faults of the investigated area are in their latest part of cycle, so that the probability of at least one moderate to large earthquake (M = 6 to 6.7) is very high within a few decades. Furthermore, the region west to Aigion is likely to be in an accelerated state of extension, possibly 2 to 3 times its mean interseismic value. High resolution strain measurement, with a borehole dilatometer and long base hydrostatic tiltmeters, started end of 2002. A transient strain has been recorded by the dilatometer, lasting one hour, coincident with a local magnitude 3.7 earthquake. It is most probably associated with a slow slip event of magnitude around 5 ± 0.5. The pore pressure data from the 1 km deep AIG10 borehole, crossing the Aigion fault at depth, shows a 1 MPa overpressure and a large sensitivity to crustal strain changes.  相似文献   

9.
The Yenice–Gönen Fault (YGF) is one of the most important active tectonic structures in the Biga peninsula. On March 18, 1953, a destructive earthquake (Mw = 7.2) occurred on the YGF, which is considered to be a part of the southern branch of the North Anatolian Fault Zone (NAFZ). A 70 km-long dextral surface rupture formed during the Yenice–Gönen Earthquake (YGE).In this study, structural and palaeoseismological features of the YGF have been investigated. The YGF surface ruptures have been mapped and three trenches were excavated at Muratlar, Karaköy and Seyvan sites.According to the palaeoseismic interpretation and the results of 14C AMS dating, Seyvan trench shows that an earthquake of palaeoseismic age ca. 620 AD ruptured a different strand of the 1953 fault, producing rather significant surface rupture displacement, while there are indications that at least two older events occurred during the past millennia. Another set of trenches excavated near Gönen town (Muratlar village) revealed extensive liquefaction not only during the 1953 event, but also during a previous earthquake, dated at 1440 AD. The Karaköy trench shows no indications of recent reactivations.Based on the trenching results, we estimate a recurrence interval of 660 ± 160 years for large morphogenic earthquakes, creating linear surface ruptures. The maximum reported displacement during the 1953 earthquake was 4.2 m. Taking into account the palaeoseismologically determined earthquake recurrence interval and maximum displacement, slip-rate of the YGF has been calculated to be 6.3 mm/a, which is consistent with present-day velocities determined by GPS measurements. According to the geological investigations, cumulative displacement of the YGF is 2.3 km. This palaeoseismological study contributes to model the behaviour of large seismogenic faults in the Biga Peninsula.  相似文献   

10.
Seismotectonics of the Nepal Himalaya from a local seismic network   总被引：3，自引：0，他引：3  
The National Seismological Network of Nepal consists of 17 short period seismic stations operated since 1994. It provides an exceptional view of the microseismic activity over nearly one third of the Himalayan arc, including the only segment, between longitudes 78°E and 85°E, that has not produced any M>8 earthquakes over the last century. It shows a belt of seismicity that follows approximately the front of the Higher Himalaya with most of the seismic moment being released at depths between 10 and 20 km. This belt of seismicity is interpreted to reflect interseismic stress accumulation in the upper crust associated with creep in the lower crust beneath the Higher Himalaya. The seismic activity is more intense around 82°E in Far-Western Nepal and around 87°E in Eastern Nepal. Western Nepal, between 82.5 and 85°E, is characterized by a particularly low level of seismic activity. We propose that these lateral variations are related to segmentation of the Main Himalayan Thrust Fault. The major junctions between the different segments would thus lie at about 87°E and 82°E with possibly an intermediate one at about 85°E. These junctions seem to coincide with some of the active normal faults in Southern Tibet. Lateral variation of seismic activity is also found to correlate with lateral variations of geological structures suggesting that segmentation is a long-lived feature. We infer four 250–400 km long segments that could produce earthquakes comparable to the M=8.4 Bihar–Nepal earthquake that struck eastern Nepal in 1934. Assuming the model of the characteristic earthquake, the recurrence interval between two such earthquakes on a given segment is between 130 and 260 years.  相似文献   

11.
The preparation of the preliminary seismic hazard maps of the territory of Slovenia has been based on an expansion of the basic approach laid out by Cornell in 1968. Three seismic source models were prepared. Two of them are based mainly on the earthquake catalogue using the Poissonian probability model. A map of seismic energy release and a map of earthquake epicenter density are used to delineate seismic sources in these models. The geometry of the third model which is based on a rough estimate of seismotectonic setting is taken from the probabilistic seismic hazard analysis of a nuclear power plant in Slovenia. Published ground motion attenuation models based on strong motion records of recent strong earthquakes in Italy are used. Test maps for variable and uniform b-values are presented. The computer program, Seisrisk III, developed by the U.S. Geological Survey is used.  相似文献   

12.
We present the results of a systematic search for the identification of accelerating seismic crustal deformation in the broader northern Aegean area and in northwestern Turkey. We found that accelerating seismic deformation release, expressed by the generation of intermediate magnitude earthquakes, is currently observed in NW Turkey. On the basis of the critical earthquake model and by applying certain constraints which hold between the basic quantities involved in this phenomenon, it can be expected that this accelerating seismic activity may culminate in the generation of two strong earthquakes in this area during the next few years.The estimated epicenter coordinates of the larger of these probably impending earthquakes are 39.7°N–28.8°E, its magnitude is 7.0 and its occurrence time tc=2003.5. The second strong event is expected to occur at tc=2002.5 with a magnitude equal to 6.4 and epicenter coordinates 40.0°N–27.4°E. The uncertainties in the calculated focal parameters for these expected events are of the order of 100 km for the epicenter, ±0.5 for their magnitude and ±1.5 years for their occurrence time.  相似文献   

13.
The Great Lisbon earthquake has the largest documented felt area of any shallow earthquake and an estimated magnitude of 8.5–9.0. The associated tsunami ravaged the coast of SW Portugal and the Gulf of Cadiz, with run-up heights reported to have reached 5–15 m. While several source regions offshore SW Portugal have been proposed (e.g.— Gorringe Bank, Marquis de Pombal fault), no single source appears to be able to account for the great seismic moment as well as all the historical tsunami amplitude and travel time observations. A shallow east dipping fault plane beneath the Gulf of Cadiz associated with active subduction beneath Gibraltar, represents a candidate source for the Lisbon earthquake of 1755.Here we consider the fault parameters implied by this hypothesis, with respect to total slip, seismic moment, and recurrence interval to test the viability of this source. The geometry of the seismogenic zone is obtained from deep crustal studies and can be represented by an east dipping fault plane with mean dimensions of 180 km (N–S) × 210 km (E–W). For 10 m of co-seismic slip an Mw 8.64 event results and for 20 m of slip an Mw 8.8 earthquake is generated. Thus, for convergence rates of about 1 cm/yr, an event of this magnitude could occur every 1000–2000 years. Available kinematic and sedimentological data are in general agreement with such a recurrence interval. Tsunami wave form modeling indicates a subduction source in the Gulf of Cadiz can partly satisfy the historical observations with respect to wave amplitudes and arrival times, though discrepancies remain for some stations. A macroseismic analysis is performed using site effect functions calculated from isoseismals observed during instrumentally recorded strong earthquakes in the region (M7.9 1969 and M6.8 1964). The resulting synthetic isoseismals for the 1755 event suggest a subduction source, possibly in combination with an additional source at the NW corner of the Gulf of Cadiz can satisfactorily explain the historically observed seismic intensities. Further studies are needed to sample the turbidites in the adjacent abyssal plains to better document the source region and more precisely calibrate the chronology of great earthquakes in this region.  相似文献   

14.
We present a revision and a seismotectonic interpretation of deep crust strike–slip earthquake sequences that occurred in 1990–1991 in the Southern Apennines (Potenza area). The revision is motivated by: i) the striking similarity to a seismic sequence that occurred in 2002  140 km NNW, in an analogous tectonic context (Molise area), suggesting a common seismotectonic environment of regional importance; ii) the close proximity of such deep strike–slip seismicity with shallow extensional seismicity (Apennine area); and iii) the lack of knowledge about the mechanical properties of the crust that might justify the observed crustal seismicity. A comparison between the revised 1990–1991 earthquakes and the 2002 earthquakes, as well as the integration of seismological data with a rheological analysis offer new constraints on the regional seismotectonic context of crustal seismicity in the Southern Apennines. The seismological revision consists of a relocation of the aftershock sequences based on newly constrained velocity models. New focal mechanisms of the aftershocks are computed and the active state of stress is constrained via the use of a stress inversion technique. The relationships among the observed seismicity, the crustal structure of the Southern Apennines, and the rheological layering are analysed along a crustal section crossing southern Italy, by computing geotherms and two-mechanism (brittle frictional vs. ductile plastic strength) rheological profiles. The 1990–1991 seismicity is concentrated in a well-defined depth range (mostly between 15 and 23 km depths). This depth range corresponds to the upper pat of the middle crust underlying the Apulian sedimentary cover, in the footwall of the easternmost Apennine thrust system. The 3D distribution of the aftershocks, the fault kinematics, and the stress inversion indicate the activation of a right-lateral strike–slip fault striking N100°E under a stress field characterized by a sub-horizontal N142°-trending σ1 and a sub-horizontal N232°-trending σ3, very similar to the known stress field of the Gargano seismic zone in the Apulian foreland. The apparent anomalous depths of the earthquakes (> 15 km) and the confinement within a relatively narrow depth range are explained by the crustal rheology, which consists of a strong brittle layer at mid crustal depths sandwiched between two plastic horizons. This articulated rheological stratification is typical of the central part of the Southern Apennine crust, where the Apulian crust is overthrusted by Apennine units. Both the Potenza 1990–1991 and the Molise 2002 seismic sequences can be interpreted to be due to crustal E–W fault zones within the Apulian crust inherited from previous tectonic phases and overthrusted by Apennine units during the Late Pliocene–Middle Pleistocene. The present strike–slip tectonic regime reactivated these fault zones and caused them to move with an uneven mechanical behaviour; brittle seismogenic faulting is confined to the strong brittle part of the middle crust. This strong brittle layer might also act as a stress guide able to laterally transmit the deviatoric stresses responsible for the strike–slip regime in the Apulian crust and may explain the close proximity (nearly overlapping) of the strike–slip and normal faulting regimes in the Southern Apennines. From a methodological point of view, it seems that rather simple two-mechanism rheological profiles, though affected by uncertainties, are still a useful tool for estimating the rheological properties and likely seismogenic behaviour of the crust.  相似文献   

15.
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 (Mw = 6.8) and 1.4 × 1018 Nm (Mw = 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 (Mw = 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.  相似文献   

16.
176 vertical-component, short period observations from aftershocks of the Mw 7.7, 26 January, 2001 Kachchh earthquake are used to estimate seismic wave attenuation in western India using uniform and two layer models. The magnitudes (Mw) of the earthquakes are less than 4.5, with depths less than 46 km and hypocentral distances up to 110 km. The studied frequencies are between 1 and 30 Hz. Two seismic wave attenuation factors, intrinsic absorption (Qi− 1) and scattering attenuation (Qs− 1) are estimated using the Multiple Lapse Time Window method which compares time integrated seismic wave energies with synthetic coda wave envelopes for a multiple isotropic scattering model. We first assume spatial uniformity of Qi− 1, Qs− 1 and S wave velocity (β). A second approach extends the multiple scattering hypothesis to media consisting of several layers characterized by vertically varying scattering coefficient (g), intrinsic absorption strength (h), density of the media (ρ) and shear wave velocity structure. The predicted coda envelopes are computed using Monte Carlo simulation. Results show that, under the assumption of spatial uniformity, scattering attenuation is greater than intrinsic absorption only for the lowest frequency band (1 to 2 Hz), whereas intrinsic absorption is predominant in the attenuation process at higher frequencies (2 to 30 Hz). The values of Q obtained range from Qt = 118, Qi = 246 and Qs = 227 at 1.5 Hz to Qt ≈ 4000, Qi ≈ 4600 and Qs ≈ 33,300 at 28 Hz center frequencies, being Qt− 1 a measure of total attenuation. Results also show that Qi− 1, Qs− 1 and Qt− 1 decrease proportional to f−ν. Two rates of decay are clearly observed for the low (1 to 6 Hz) and high (6 to 30 Hz) frequency ranges. Values of ν are estimated as 2.07 ± 0.05 and 0.44 ± 0.09 for total attenuation, 1.52 ± 0.21 and 0.48 ± 0.09 for intrinsic absorption and 3.63 ± 0.07 and 0.06 ± 0.08 for scattering attenuation for the low and high frequency ranges, respectively. Despite the lower resolution in deriving the attenuation parameters for a two layered crust, we find that scattering attenuation is comparable to or smaller than the intrinsic absorption in the crust whereas intrinsic absorption dominates in the mantle. Also, for a crustal layer of thickness 42 km, intrinsic absorption and scattering estimates in the crust are lower and greater than those of the mantle, respectively.  相似文献   

17.
Focal mechanisms for three recent earthquakes in Finland are determined using P-wave polarities together with SV/P and SH/P phase amplitude ratios. The events occurred on May 11, 2000 in Toivakka, Central Finland (ML=2.4), on September 15, 2000 in Kuusamo, northeastern Finland (ML=3.5), and on May 2, 2001 in Kolari, western Finnish Lapland (ML=2.9).In order to obtain reliable estimates of the source parameters, one-dimensional crust and upper mantle velocity models are derived for the epicenter areas from deep-seismic sounding results. The starting models are modified by one-dimensional ray tracing using the earthquake observations. The events are relocated by employing P- and S-phase arrival times from the nearest seismic stations and the final velocity models. Synthetic waveforms, calculated with the reflectivity method, are used to further constrain and verify the source and structural parameters.The Toivakka earthquake indicates thrust- or reverse-faulting mechanism at a depth of 5 km. After comparison with aeromagnetic and topographic data we suggest the eastward dipping nodal plane (358°/42°) was the fault plane. The best-fitting fault plane solution of the Kolari earthquake suggests pure thrust-faulting at a depth of 5 km. The nodal plane striking 035°/30° correlates well with surface observations of the postglacial, possibly listric fault systems in the source area. The Kuusamo earthquake (focal depth 14 km) has a normal-faulting mechanism with the nodal planes trending 133°/47° or 284°/47°. Preference is given to the SW-dipping nodal plane, as it seems to coincide with topographic and magnetic lineament directions that have been active after the last ice age.The three earthquakes have occurred in old Precambrian faults and shear zones, which have been reactivated. The reactivated faults are favourably oriented in the local stress field.  相似文献   

18.
High seismic Vp velocity anomalies (8.7–9.0 km s− 1) have long been known about in regions of the uppermost mantle of the Siberian craton, often in association with kimberlite fields. Laboratory measurement of seismic properties of five xenoliths, three peridotites and two eclogites, from the Udachnaya kimberlite under confining pressures up to 600 MPa were extrapolated to uppermost mantle P–T conditions of 1500 MPa and 500 °C, however none of the velocities are high enough to explain the observations. Eclogites or peridotites are commonly considered to be the source of anomalous high velocities. We prefer a peridotitic source to an eclogitic source due to the unusual chemistry and regional uniformity of eclogitic garnets required, maximum velocity limitations on laboratory measurements of seismic properties of natural eclogites, and purported abundance of eclogites in the lithosphere. Alternatively, a highly depleted peridotite, such as dunite or harzburgite, can produce velocities high enough to match observations. Olivine petrofabrics in most peridotites, including the three peridotites used in this study, are great enough to produce the observed high velocities provided olivine petrofabrics are continuous enough and correctly oriented to be seismically detectable and the modal proportion of olivine is high. There have been suggestions by other authors that the Siberian upper mantle is highly depleted and that a lithosphere-scale shear zone exists, which may have acted to organize fabrics into segments large enough for detection. Anomalously high Vp–Vs velocity ratios of greater than 1.8 are expected parallel to the olivine [100] maxima required to be present in a high-velocity olivine-dominated upper mantle. Vp–Vs velocity ratios can serve as a means of inferring large-scale anisotropy when limited seismic data are available, as in Siberia.  相似文献   

19.
Zircon fission track (ZFT), apatite fission track (AFT) and (U–Th)/He thermochronometric data are used to reconstruct the Cenozoic exhumation history of the South China continental margin. A south to north sample transect from coast to continental interior yielded ZFT ages between 116.6 ± 4.7 Ma and 87.3 ± 4.0, indicating that by the Late Cretaceous samples were at depths of 5–6 km in the upper crust. Apatite FT ages range between 60.9 ± 3.6 and 37.3 ± 2.3 Ma with mean track lengths between 13.26 ± 0.16 µm and 13.95 ± 0.19 µm whilst AHe ages are marginally younger 47.5 ± 1.9–15.3 ± 0.5 Ma. These results show the sampled rocks resided in the top 1–1.5 km of the crust for most of the Cenozoic. Thermal history modeling of the combined FT and (U–Th)/He datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin. 1) Initial phase of rapid cooling that youngs to the north, 2) a period of relative (but not perfect) thermal stasis at ~ 70–60 °C which increases in duration from the south to the north; 3) final-stage cooling to surface temperatures that initiated in all samples between 15 and 10 Ma. The timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin. The history of South China margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the South China Sea. This rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the South China block and site of active rifting to the south. Consequently whilst the South China margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin.  相似文献   

20.
Equivalent-linear seismic analyses of MSW landfills using DEEPSOIL   总被引：4，自引：0，他引：4  
Responses of municipal solid waste (MSW) landfills during earthquakes are gaining worldwide importance due to the devastating nature of earthquake on landfills. Apart from the post-earthquake safety and serviceability issues which pose environmental and public health problems, other important concerns are related to the behavior of closed landfills during and after earthquake. In present study, one dimensional (1-D) equivalent-linear analysis was carried out to model the behaviour of MSW landfills subjected to seismic base accelerations using the DEEPSOIL software. Influence of foundation types, height and stiffness of MSW landfills and seismic base accelerations on the seismic responses in terms of surface accelerations, normalized stresses (i.e., shear stress/effective vertical stress) and spectral amplification are evaluated. The results showed that height and stiffness of landfills, type of foundation and amount of seismic base acceleration and period play important role in evaluating the seismic responses of MSW landfills. Assumption of constant unit weight and shear wave velocity for landfills underestimates maximum horizontal acceleration (MHA), normalized shear stresses and spectral amplification at the top of landfills. Landfill models with smaller heights (up to 40 m high) showed higher amplification ratio for low seismic base accelerations with mean period near to that of soil and landfill resonance for all sites. A complex behavior was observed at higher seismic base accelerations due to non-linear behavior of landfill materials.  相似文献   

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