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1.
Simple spectral theory of seismic sources was used to determine source parameters directly related to medium properties (stress drop, seismic efficiency and fracture energy) and quality factors of the Vrancea (Romania) seismic region. The results show an increase in maximum static stress drop, maximum seismic efficiency and fracture energy with depth. The seismic efficiency is magnitude independent, but the stress drop is magnitude independent only for events with ML > 3.8; below this value, the logarithm of stress drop increases quasi-linearly with magnitude. In the depth interval 50–160 km the stress drop increases with a slope of about 2–3 bar/km. The fracture energy per unit area of the fault has values of the order of 105–108 erg/cm2.The frequency independent quality factors indicate that the attenuation of P waves is generally higher than that of S waves and that Qp values are in agreement with recent tectonic models for the Vrancea region: total decoupling of the slab now sinking gravitationally is present only in the southwestern part of the Vrancea region, as suggested by the spatial position of intermediate depth hypocenters.  相似文献   

2.
Katsuyuki Abe   《Tectonophysics》1975,27(3):223-238
The source mechanism of the Saitama earthquake (36.07°N,139.40°E, Ms = 5.4) of July 1, 1968, is studied on the basis of P-wave first motion, aftershock, long-period surface-wave data and low-magnification long-period seismograms recorded in the nearfield. A precise location of the aftershocks is made using P and S—P time data obtained by a micro-earthquake observatory network. The synthetic near-field seismograms based on the Haskell model are directly compared with the observed near-field seismograms for wave form and amplitude to determine the dynamic fault parameters. The results obtained are as follows: source geometry, reverse dip slip with considerable right-lateral strike-slip component; dip direction, N6°E; dip angle 30°; fault dimension, 10 × 6 km2; rupture velocity, 3.4 km/sec in the direction S30°E; average dislocation, 92 cm; average dislocation velocity, 92 cm/sec; seismic moment, 1.9 · 1025 dyn-cm; stress drop, 100 bar. The effective stress is about the same as the stress drop. For major earthquakes in the Japanese Islands, the dislocation velocity, .D, is found to be proportional to the stress drop, σ. This relation can be expressed by .D - (β/μ)σ, where β is the shear velocity and μ is the rigidity. This result has an importance in engineering seismology because the stress drop scales the seismic motion in the vicinity of an earthquake fault.  相似文献   

3.
Theoretical study has been made investigating the seismic source spectrum generated from a coherent and incoherent rupture. An earthquake is modeled by a finite propagating rupture on a fault plane where fault heterogeneities, fault patches, are randomly distributed. The dislocation velocity of such a fracture is assumed to be approximated by a stochastic process of random impacts of particles obeying Brownian motion. The parameters of the present stochastic source are seismic moment, fault dimension, fault patch intensity, and patch fracture time. The model predicts two corner frequencies; one originates from the fault finiteness and the other from the fracturing of fault patches. The seismic source spectrum from the model consequently shows distinct frequency dependence of ω0 − ω−2 − ω−γ − ω−2 with increasing angular frequency ω, where γ is about 1.0. The seismic moment is controlled by an average dislocation on the fault and by the fault dimension. The short-period spectrum, which is much more abundant than that of the ordinary deterministic models, is controlled by the product of the fault patch intensity and the square root of their total number. The ω−2 high frequency asymptote of the theoretical spectrum is in conformity with the white acceleration spectrum usually found in the literature, and it guarantees the finite total energy of the rupture process.  相似文献   

4.
Fault dimension estimates derived from the aftershock area extent of 36 shallow depth (≤ 31 km) earthquakes that occurred in the Mediterranean Sea region have been used in order to establish empirical relationships between length, width, area and surface-wave/moment magnitude. This dataset consists of events whose aftershock sequence was recorded by a dense local or regional network and the reported location errors did not exceed on average 3–5 km. Surface-wave magnitudes for these events were obtained from the NEIC database and/or published reports, while moment magnitudes as well as focal mechanisms were available from the Harvard/USGS catalogues. Contrary to the results of some previously published studies we found no evidence in our dataset that faulting type may have an effect on the fault dimension estimates and therefore we derived relationships for the whole of the dataset. Comparisons, by means of statistical F-tests, of our relationships with other previously published regional and global relationships were performed in order to check possible similarities or differences. Most such comparisons showed relatively low significance levels (< 95%), since the differences in source dimension estimates were large mainly for magnitudes lower than 6.5, becoming smaller with increasing magnitude. Some degree of similarity, however, could be observed between our fault length relationship and the one derived from aftershock area lengths of events in Greece, while a difference was found between our regional and global fault length relationships. A calculation of the ratio defined as the fault length, derived from our relationships, to the length estimated from regional empirical relationships involving surface ruptures showed that it can take a maximum value of about 7 for small magnitudes while it approaches unity at Ms 7.2. When calculating the same ratio using instead global empirical relationships we see the maximum value not exceeding 1.8, while unity is reached at Mw 7.8, indicating the existence of a strong regional variation in the fault lengths of earthquakes occurring in the Mediterranean Sea region. Also, a relationship between the logarithms of the rupture area and seismic moment is established and it is inferred that there is some variation of stress drop as a function of seismic moment. In particular, it is observed that for magnitudes lower than 6.6 the stress drop fluctuates around 10 bar, while for larger magnitudes the stress drop reaches a value as high as 60 bar.  相似文献   

5.
This study defines the Mio-Pliocene to present-day stress regime acting at the northeastern corner of the eastern Mediterranean region along the Karasu Valley (i.e., the Amanos Range), taking in the Antakya, Osmaniye and Kahramanmaras provinces. The inversion slip vectors measured on fault planes and chronologies between striations indicate that the stress regime varied from transpressional initially to transtensional, having consistent NW- and NE-trending σHmax (σ1) and σHmin (σ3) axes, respectively; there are significantly different mean stress-ratio (Rm) values however. The older mean stress state is characterized by N151±11°E-trending σ1 and N59±12°E-trending σ3 axes, and by a mean arithmetic Rm value of 0.76, indicating that the regional stress regime is transpressional. The younger stress regime is characterized by N154±8°E-trending σ1 and N243±8°E-trending σ3 axes, and by a mean arithmetic Rm value of 0.17, indicating a transtensional character for this regional stress regime. The low R values of the stress deviators related to the recent stress state reflect normal-component slips. The earthquake focal mechanism inversions confirm that the younger stress regime continues into the Recent. The inversion identifies a transtensional stress regime representing strike-slip and an extensional stress state with a consistent NE-trending σHmin (σ3) axis. These stress states are characterized by N66°E and N249°E-trending σ3 axes, respectively. Both significant regional stress regimes induce left-lateral displacement along the southern part of the East Anatolian Fault (EAF, or Amanos Fault). The temporal change, probably in Quaternary time, within the regional stress regime—from transpression to transtension—resulted from the coeval influences of subduction processes in the west–southwest (i.e., along the Cyprus arc), continental collision in the east, and westward escape of the Anatolian block.  相似文献   

6.
To understand the generation mechanism of the Bam earthquake (Mw 6.6), we studied three-dimensional VP, VS and Poisson's ratio (σ) structures in the Bam area by using the seismic tomography method. We inverted accurate arrival times of 19490 P waves and 19015 S waves from 2396 aftershocks recorded by a temporal high-sensitivity seismic network. The 3-D velocity structure of the seismogenic region was well resolved to a depth of 14 km with significant velocity variations of up to 5%. The general pattern of aftershock distribution was relocated by using the 3-D structure to delineate a source fault for a length of approximately 20 km along a line 4.5 km west of the known geological Bam fault; this source fault dips steeply westward and strikes a nearly north–south line. The main shallow cluster of aftershocks south of the city of Bam is distributed just under the minor surface ruptures in the desert. The 3-D velocity structure shows a thick layer of high VS and low σ (minimum: 0.20) at a depth range of 2–6 km. The deeper layer, with a thickness of about 2 km, appears to have a low VS and high σ (maximum: 0.28) from 6 km depth beneath Bam to a depth of 9 km south of the city. The inferred increase of Poisson's ratio from 2 to 10 km in depth may be associated with a change from rigid and SiO2-rich rock to more mafic rock, including the probable existence of fluids. The main seismic gap of aftershock distribution at the depth range of 2 to 7 km coincides well with the large slip zone in the shallow thick layer of high VS and low σ. The large slip propagating mainly in the shallow rigid layer may be one of the main reasons why the Bam area suffered heavy damage.  相似文献   

7.
Bonini (2009, Structural controls on a carbon dioxide-driven mud volcano field in the Northern Apennines (Pieve Santo Stefano, Italy): relations with pre-existing steep discontinuities and seismicity. Journal of Structural Geology 31, 44–54) presents a 2D mechanical analysis to infer the failure conditions responsible for the seismicity distribution during an Mw = 4.6 seismic sequence nucleating during 2001 in the Northern Apennines. In my view the mechanical analysis presented in this paper has some weakness or is not well constrained, in particular: 1) the assumption of a dip angle of 50°, is not consistent with the activated structures; 2) the Pf = σ3 condition, difficult to be attained along a cohesionless fault dipping at 50°; 3) the isotropic stress state, i.e. σ2 = σ3, that is not consistent with the active or recent stress field in the area.  相似文献   

8.
The co-seismic deformations produced during the September 27, 2003 Chuya earthquake (Ms = 7.5) that affected the Gorny Altai, Russia, are described and discussed along a 30 km long segment. The co-seismic deformations have manifested themselves both in unconsolidated sediments as R- and R′-shears, extension fractures and contraction structures, and in bedrock as the reactivation of preexisting schistosity zones and individual fractures, as well as development of new ruptures and coarse crushing zones. It has been established that the pattern of earthquake ruptures represents a typical fault zone trending NW–SE with a width reaching 4–5 km and a dextral strike–slip kinematics. The initial stress field that produced the whole structural pattern of co-seismic deformations during the Chuya earthquake, is associated with a transcurrent regime with a NNW–SSE, almost N–S, trending of compressional stress axis (σ1), and a ENE–WSW, almost E–W, trending of tensional stress axis (σ3). The state of stress in the newly-formed fault zone is relatively uniform. The local stress variations are expressed in insignificant deviation of σ1 from N–S to NW–SE or NE–SW, in short-term fluctuations of relative stress values in keeping their spatial orientations, or in a local increase of the plunge angle of the σ1. The geometry of the fault zone associated with the Chuya earthquake has been compared with the mechanical model of fracturing in large continental fault zones with dextral strike–slip kinematics. It is apparent that the observed fracture pattern corresponds to the late disjunctive stage of faulting when the master fault is not fully developed but its segments are already clearly defined. It has been shown that fracturing in widely different rocks follows the common laws of the deformation of solid bodies, even close to the Earth surface, and with high rates of movements.  相似文献   

9.
Fault slip analysis of Quaternary faults in southeastern Korea   总被引：1，自引：0，他引：1  
The Quaternary stress field has been reconstructed for southeast Korea using sets of fault data. The subhorizontal direction of the maximum principal stress (σ1) trended ENE and the direction of the minimum principal stress (σ3) was nearly vertical. The stress ratio (Φ = (σ2 − σ3) / (σ1 − σ3)) value was 0.65. Two possible interpretations for the stress field can be made in the framework of eastern Asian tectonics; (1) The σHmax trajectory for southeast Korea fits well with the fan-shaped radial pattern of maximum principal stress induced by the India–Eurasia collision. Thus, we suggest that the main source for this recent stress field in southeast Korea is related to the remote India–Eurasia continental collision. (2) The stress field in Korea shows a pattern similar to that in southwestern Japan. The origin for the E–W trending σHmax in Japan is known to be related to the mantle upwelling in the East China Sea. Thus, it is possible that Quaternary stress field in Korea has evolved synchronously with that in Japan. We suggest further studies (GPS and in situ stress measurement) to test these hypotheses.  相似文献   

10.
The source parameters are determined for the Burma-India border earthquake of July 29, 1970, from body-wave spectra. We obtain seismic moment [ , ] · 1026 dyne cm, source dimension [ ] km, radiated energy [ , −ER (S) = 1.35] · 1020 ergs and the stress drop = 11 bars.  相似文献   

11.
In normal faulting regimes, the magnitudes and orientations of the maximum and minimum principal compressive stresses may be known with some confidence. However, the magnitude of the intermediate principal compressive stress is generally much more difficult to constrain and is often not considered to be an important factor. In this paper, we show that the slip characteristics of faults and fractures with complex or nonoptimal geometry are highly sensitive to variation or uncertainty in the ambient effective intermediate principal stress (σ′2). Optimally oriented faults and fractures may be less sensitive to such variations or uncertainties. Slip tendency (Ts) analysis provides a basis for quantifying the effects of uncertainty in the magnitudes and orientations of all principal stresses and in any stress regime, thereby focusing efforts on the most important components of the system. We also show, for a normal faulting stress regime, that the proportion of potential surfaces experiencing high slip tendency (e.g., Ts ≥ 0.6) decreases from a maximum of about 38% where σ′2 = σ′3, to a minimum of approximately 14% where σ′2 is halfway between σ′3 and σ′1, and increases to another high of approximately 29% where σ′2 = σ′1. This analysis illustrates the influence of the magnitude of σ′2 on rock mass strength, an observation previously documented by experimental rock deformation studies. Because of the link between fault and fracture slip characteristics and transmissivity in critically stressed rock, this analysis can provide new insights into stress-controlled fault transmissivity.  相似文献   

12.
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   

13.
This paper presents the results of a photoelastic and numerical study of stress distributions (contours and trajectories) around fault models of various geometries, submitted to a biaxial compressive load. It aims to describe typical biaxial stress behaviours and emphasize the existing differences with the well-known uniaxial compressive load case. Stress trajectories are sometimes shown by joint sets acting as markers of a paleostress field, and they can be interpreted by particular shallow tension–compression situations. At depth fractures can be reactivated, or can dilate under conditions of triaxial compression, and behaviour is essentially controlled by a high stress ratio (high σ3/σ1 ratio). In spite of the potential importance of such stress states on fracture permeability and fluid flow, and although they are frequently found at depth in a reservoir context, such stress conditions are poorly investigated, particularly in terms of stress perturbations.The presented analogue experiments consisted of compression tests done on polymethylmethacrylate (PMMA) plates; this material has mechanical properties comparable to those of brittle rocks in the upper crust, and presents birefringence. The samples contained open defects acting as faults, and the stress trajectories around these faults were investigated using a photoelastic device. Comparable numerical experiments were realised with a finite-element code (Franc 2D), using frictionless fault models.First, the effect of an increasing biaxial compressive load ratio σ3/σ1 on stress trajectories around an isolated open defect was explored. It was shown that the stress trajectories were drastically modified when σ3/σ1 increased from 0.2 to 0.4, this result being consistent with previous studies. In particular, when σ3/σ1 was superior or equal to 0.4, external isotropic points around where trajectories diverged (called repulsive isotropic points) appeared near the tips, but away from the defects. They tended to move away from the defects towards the main load direction when σ3/σ1 increased. The described isotropic points were points of stress trajectory divergence, i.e. points where stress decreased, implying that zones around them were strongly unfavourable to shear reactivation.Second, stress trajectories around fault models of various geometries (oblique isolated defect, dilational jog, compressive jog, and complex patterns) were studied, the applied biaxial compressive load ratio being 0.7. These biaxial stress trajectories were compared with similar uniaxial stress trajectories in order to explore the existing differences between the two regimes. It enabled new stress trajectory geometries to be described and interpreted. In particular, numerous external repulsive isotropic points were observed, and defect tips were shown to be zones of high convergence of stress trajectories. Furthermore, in contrast to the uniaxial compressive load case, stress trajectories and stress contours were geometrically similar within dilational and compressive jogs under biaxial compressive load. For both jog types, the centre of the overlapping zone, and the areas along the wall of a jog-defining fault and facing the overlapping tip of the other fault, proved to be zones of low mean stress, implying that fluids may migrate towards them from the tips of the faults, in response to mean stress gradients. Furthermore, the centre of the jogs exhibiting a high differential stress was a favourable area in terms of fracture reactivation, which may facilitate fluids transfer and storage.It was also observed that for both the uniaxial and biaxial compressive loads, isotropic zones were localised at the acute angle between branching defects.  相似文献   

14.
The relationship between deformation and so-called fluid paleotransfers in minor faults has been analysed in an argillaceous formation located in the Causses Basin in France. The fluid paleotransfers are related to the fault activity to a large extent. We attempt to estimate the intensity of paleo-deviatoric stress magnitudes under which the fault activity may have occurred and consequently, the change in the structural fault permeability. The paleo-deviatoric stress magnitudes were calculated with the inverse method of Etchecopar applied to calcite twinning. The measured crystals are contained within the core zone of minor faults and this study is based on a previous complete microtectonic and microstructural analysis of the faults. In this paper, analysis of calcite twinning has been applied for the first time to vein fillings associated small faults in a context of relatively weak deformation, a condition ensured by the tectonic and structural analysis. Calculation and discussion of the paleo-deviatoric stress tensors in relation to the evolution of the structural fault permeability and to the hydraulic behaviour of the faults are the aim of this paper. The analysed faults, created and active during the same tectonic event, were permeable under a (σ1–σ3) mean value of 40–50 MPa. On the other hand, the reactivation of faults during a second tectonic event implies mean (σ1–σ3) value higher than 40–50 MPa, especially for the faults that are poorly oriented with respect to the principal tectonic stress directions. The core zone of these faults remained sealed and impermeable or became permeable by development of microcracks inside the pre-existing fillings.  相似文献   

15.
In the early morning hours on Wednesday November 08, 2006 at 04:32:10(GMT) a small earthquake of ML 4.1 has occurred at southeast Beni-Suef, approximately 160 km SEE of Cairo, northern Egypt. The quake has been felt as far as Cairo and its surroundings while no casualties were reported. The instrumental epicentre is located at 28.57°N and 31.55°E. Seismic moment is 1.76 E14 Nm, corresponding to a moment magnitude Mw 3.5. Following a Brune model, the source radius is 0.3 km with an average dislocation of 1.8 cm and a 2.4 MPa stress drop. The source mechanism from a first motion fault plane solution shows a left-lateral strike-slip mechanism with a minor dip-slip component along fault NNW striking at 161°, dipping 52° to the west and rake −5°. Trend and plunging of the maximum and minimum principle axes P/T are 125°, 28°, 21°, and 23°, respectively. A comparison with the mechanism of the October, 1999 event shows similarities in faulting type and orientation of nodal planes.Eight small earthquakes (3.0  ML < 5.0) were also recorded by the Egyptian National Seismological Network (ENSN) from the same region. We estimate the source parameters and fault mechanism solutions (FMS) for these earthquakes using displacement spectra and P-wave polarities, respectively. The obtained source parameters including seismic moments of 4.9 × 1012–5.04 × 1015 Nm, stress drops of 0.2–4.9 MPa and relative displacement of 0.1–9.1 cm. The azimuths of T-axes determined from FMS are oriented in NNE–SSW direction. This direction is consistent with the present-day stress field in Egypt and the last phase of stress field changes in the Late Pleistocene, as well as with recent GPS measurements.  相似文献   

16.
The phreatic activity and the subsequent dacitic dome growth in 1998–1999 at Guagua Pichincha volcano, Ecuador, were associated with two seismic swarms: one located in the northern part of Quito (population: 1,500,000) and another one, just below the active volcano, about 15–20 km SW from the first one. Quito swarm tectonic events have high frequencies (from 1 to 10–15 Hz). We registered more than 3200 events (among which 2354 events of 1.4≤ML≤4.2) between June 1998 and December 1999 at the −2- and −17-km depth. The volcanic events below the Guagua Pichincha caldera have high (from 1 to 10–15 Hz) and low (less than 3 Hz) frequencies. Approximately, 130,000 events were registered between September 1998 and December 1999 at the +2.4- and −3.5-km depth. Here, we study the stress tensors of these two swarms deduced from the polarities of P first motions and compare them to the regional stress tensor deduced from CMT Harvard focal mechanisms. The Quito swarm stress tensor is relatively close to the regional stress tensor (the σ1 axis was oriented N117°E close to the N102°E direction of the plate motion found by the GPS measurement, and σ3 is nearly vertical). The difference may be due to the action of the closely active Guagua Pichincha volcano. The Guagua Pichincha stress tensor is very different from the regional tectonic one. The σ1 axis of the volcano is oriented N214°E, almost perpendicular to the σ1 of the swarm of Quito and σ3 is almost horizontal. Even if these two tensors are different, they can be explained in a more general tectonic scheme. The almost horizontal direction of σ3 just below the volcano is compatible with an extensional horizontal direction that may be expected in the shallow extrados part of a compressional region and consistent with an opening of the top of the Guagua Pichincha volcano. The movement of the fluids (magma, gas and/or groundwater) produced by the closely active Guagua Pichincha volcano seems to have an influence in the acceleration of the generation of seismic events.  相似文献   

17.
The earthquake magnitude is a quantity sampling the spectrum of the far-field radiation.With a suit of properly defined magnitudes in a sufficiently broad range of frequencies, the radiated spectrum can be restored and analyzed.A method is proposed for the extraction of stress drop, fault length and seismic moment from magnitudes on a routine basis. Thereby, the theoretical spectrum as predicted by the ω-square model of Aki is utilized.In applying the method to earthquakes which occurred in several parts of Asia over a time-span of 3 years, it is shown that in most cases earthquakes in a given region are characterized by the same stress drop, varying however from region to region. In one region a change of the stress drop with time is found, eventually indicating a change in the state of stress in the particular region during the time interval investigated.  相似文献   

18.
Talc is one of the weakest minerals that is associated with fault zones. Triaxial friction experiments conducted on water-saturated talc gouge at room temperature yield values of the coefficient of friction, μ (shear stress, τ/effective normal stress, σ′N) in the range 0.16–0.23, and μ increases with increasing σ′N. Talc gouge heated to temperatures of 100°–400 °C is consistently weaker than at room temperature, and μ < 0.1 at slow strain rates in some heated experiments. Talc also is characterized by inherently stable, velocity-strengthening behavior (strength increases with increasing shear rate) at all conditions tested. The low strength of talc is a consequence of its layered crystal structure and, in particular, its very weak interlayer bond. Its hydrophobic character may be responsible for the relatively small increase in μ with increasing σ′N at room temperature compared to other sheet silicates.Talc has a temperature–pressure range of stability that extends from surficial to eclogite-facies conditions, making it of potential significance in a variety of faulting environments. Talc has been identified in exhumed subduction zone thrusts, in fault gouge collected from oceanic transform and detachment faults associated with rift systems, and recently in serpentinite from the central creeping section of the San Andreas fault. Typically, talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica-saturated hydrothermal fluids. This mode of formation of talc is a prime example of a fault-zone weakening process. Because of its velocity-strengthening behavior, talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern California that are associated with ophiolitic rocks.  相似文献   

19.
Beginning with the Swabian Jura earthquake in 1911 the seismic activity in Central Europe is concentrated to this area. A comparison with other events of the same epicentral region shows that the largest earthquake in Germany has the character of a left-lateral horizontal strike slip striking N — NNE. The focal parameters can be assumed within the following intervalls: Seismic moment Mo = 1…8·1017 Nm; focal area Fo = 18…53 km2; average dislocation do = 20…53 cm and stress drop Δpo = 13…19 bar.  相似文献   

20.
The Latur earthquake (Mw 6.1) of 29 September 1993 is a rare stable continental region (SCR) earthquake that occurred on a previously unknown blind fault. In this study, we determined detailed three-dimensional (3-D) P- and S-wave velocity (Vp, Vs) and Poisson's ratio (σ) structures by inverting the first P- and S-wave high-quality arrival time data from 142 aftershocks that were recorded by a network of temporary seismic stations. The source zone of the Latur earthquake shows strong lateral heterogeneities in Vp, Vs and σ structures, extending in a volume of about 90 × 90 × 15 km3. The mainshock occurred within, but near the boundary, of a low-Vp, high-Vs and low-σ zone. This suggests that the structural asperities at the mainshock hypocenter are associated with a partially fluid-saturated fractured rock in a previously unknown source zone with intersecting fault surfaces. This might have triggered the 1993 Latur mainshock and its aftershock sequence. Our results are in good agreement with other geophysical studies that suggest high conductivity and high concentration of radiogenic helium gas beneath the source zone of the Latur earthquake. Our study provides an additional evidence for the presence of fluid related anomaly at the hidden source zone of the Latur earthquake in the SCR and helps us understand the genesis of damaging earthquakes in the SCR of the world.  相似文献   

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