首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 62 毫秒
1.
Summary. Dynamical rupture process on the fault is investigated in a quasi-three-dimensional faulting model with non-uniform distributions of static frictions or the fracture strength under a finite shearing pre-stress. The displacement and stress time functions on the fault are obtained by solving numerically the equations of motion with a finite stress—fracture criterion, using the finite difference method.
If static frictions are homogeneous or weakly non-uniform, the rupture propagates nearly elliptically with a velocity close to that of P waves along the direction of pre-stress and with a nearly S wave velocity in the direction perpendicular to it. The rise time of the source function and the final displacements are larger around the centre of the fault. In the case when the static frictions are heavily non-uniform and depend on the location, the rupture propagation becomes quite irregular with appreciably decreased velocities, indicating remarkable stick-slip phenomena. In some cases, there remain unruptured regions where fault slip does not take place, and high stresses remain concentrated up to the final stage. These regions could be the source of aftershocks at a next stage.
The stick—slip faulting and irregular rupture propagation radiate high-frequency seismic waves, and the near-field spectral amplitudes tend to show an inversely linear frequency dependence over high frequencies for heavily non-uniform frictional faults.  相似文献   

2.
Modelling dynamic rupture for complex geometrical fault structures is performed through a finite volume method. After transformations for building up the partial differential system following explicit conservative law, we design an unstructured bi-dimensional time-domain numerical formulation of the crack problem. As a result, arbitrary non-planar faults can be explicitly represented without extra computational cost. On these complex surfaces, boundary conditions are set on stress fluxes and not on stress values. Prescribed rupture velocity gives accurate solutions with respect to analytical ones depending on the mesh refinement, while solutions for spontaneous propagation are analysed through numerical means. An example of non-planar spontaneous fault growth in heterogeneous media demonstrates the good behaviour of the proposed algorithm as well as specific difficulties of such numerical modelling.  相似文献   

3.
Scaling relationships between seismic moment, rupture length, and rupture width have been examined. For this purpose, the data from several previous studies have been merged into a database containing more than 550 events. For large earthquakes, a dependence of scaling on faulting mechanism has been found. Whereas small and large dip-slip earthquakes scale in the same way, the self-similarity of earthquakes breaks down for large strike-slip events. Furthermore, no significant differences in scaling could be found between normal and reverse earthquakes and between earthquakes from different regions. Since the thickness of the seismogenic layer limits fault widths, most strike-slip earthquakes are limited to rupture widths of between 15 and 30 km while the rupture length is not limited. The aspect ratio of dip-slip earthquakes is similar for all earthquake sizes. Hence, the limitation in rupture width seems to control the maximum possible rupture length for these events. The different behaviour of strike-slip and dip-slip earthquakes can be explained by rupture dynamics and geological fault growth. If faults are segmented, with the thickness of the seismogenic layer controlling the length of each segment, strike-slip earthquakes might rupture connected segments more easily than dip-slip events, and thus could produce longer ruptures than dip-slip events of the same width  相似文献   

4.
Most seismic hazard estimations are based on the assumption of a Poisson process for earthquake occurrence, even though both observations and models indicate a departure of real seismic sequences from this simplistic assumption. Instrumental earthquake catalogues show earthquake clustering on regional scales while the elastic rebound theory predicts a periodic recurrence of characteristic earthquakes on longer timescales for individual events. Recent implementations of time-dependent hazard calculations in California and Japan are based on quasi-periodic recurrences of fault ruptures according to renewal models such as the Brownian Passage Time model. However, these renewal models neglect earthquake interactions and the dependence on the stressing history which might destroy any regularity of earthquake recurrences in reality. To explore this, we investigate the (coupled) stress release model, a stochastic version of the elastic rebound hypothesis. In particular, we are interested in the time-variability of the occurrence of large earthquakes and its sensitivity to the occurrence of Gutenberg–Richter type earthquake activity and fault interactions. Our results show that in general large earthquakes occur quasi-periodically in the model: the occurrence probability of large earthquakes is strongly decreased shortly after a strong event and becomes constant on longer timescales. Although possible stress-interaction between adjacent fault zones does not affect the recurrence time distributions in each zone significantly, it leads to a temporal clustering of events on larger regional scales. The non-random characteristics, especially the quasi-periodic behaviour of large earthquakes, are even more pronounced if stress changes due to small earthquakes are less important. The recurrence-time distribution for the largest events is characterized by a coefficient of variation from 0.6 to 0.84 depending on the relative importance of small earthquakes.  相似文献   

5.
We present a theory for the radiation of high-frequency waves by earthquake faults. We model the fault as a planar region in which the stress drops to the kinematic friction during slip. This model is entirely equivalent to a shear crack. For two-dimensional fault models we show that the high frequencies originate from the stress and slip velocity concentrations in the vicinity of the fault's edges. These stress concentrations radiate when the crack expands with accelerated motion. The most efficient generation of high-frequency waves occurs when the rupture velocity changes abruptly. In this case, the displacement spectrum has an ω-2 behaviour at high frequencies. The excitation is proportional to the intensity of the stress concentration near the crack tips and to the change in the focusing factor due to rupture velocity. We extend these two-dimensional results to more general three-dimensional fault models in the case when the rupture velocity changes simultaneously on the rupture front. Results are similar to those described for two-dimensional faults. We apply the theory to the case of a circular fault that grows at constant velocity and stops suddenly. The present theory is in excellent agreement with a numerical solution of the same problem.
Our results provide upper bounds to the high-frequency radiation from more realistic models in which rupture velocity does not change suddenly. The ω-2 is the minimum possible decay at high frequencies for any crack model of the source.  相似文献   

6.
Summary. Most crustal earthquakes of the world are observed to occur within a seismogenic layer which extends from the Earth's surface to a depth of a few tens of kilometres at most. A model is proposed in which the shear zone along a transcurrent plate margin is represented as a viscoelastic medium with depth-dependent power-law rheology. A frictional resistance linearly increasing with depth is assumed on a vertical transcurrent fault within the shear zone. Such a model is able to reproduce a continuous transition from the brittle behaviour of the upper crust to the ductile behaviour at depth. Assuming that the shear zone is subjected to a constant strain rate from the opposite motions of the two adjacent plates, it is found that there exists a maximum depth H below which tectonic stress can never reach the frictional threshold: this may be identified as the maximum depth of earthquake nucleation. The value of H is consistent with observations for plausible values of the model parameters. The stress evolution in the shear zone is calculated in the linear approximation of the constitutive equation. A change in rigidity with depth, which is also introduced in the model, may reproduce the high vertical gradient of shear stress, which has been measured across the San Andreas fault, and the fact that most earthquakes are nucleated at some depth in the seismogenic layer. A crack which drops the ambient stress to the dynamic frictional level is then introduced in the model. To this aim, a crack solution is employed without a stress singularity at its edges, which is compatible with a frictional stress threshold criterion for fracture. A constraint on the vertical friction gradient is obtained if such cracks are assumed to be entirely confined within the seismogenic layer.  相似文献   

7.
Summary. Rock stress measurements in Iceland show maximum horizontal compression perpendicular to the trend of Reykjanes Ridge crest and of its extension, the active volcanic zone of Iceland. Fault-plane solutions of dormant stage earthquakes are consistent with the measured stress orientations, but strike—slip earthquakes associated with volcanic surges and some earthquake swarms in active geothermal areas exhibit apparent reversals of mechanism and are here defined as 'stress-discordant' in the sense that they yield deduced stress orientations 90° from the regional stress field as determined by hydrofracturing and strain relief methods. It is proposed, supported by comparison with the pore-pressure induced Denver earthquakes, that the 'stress-discordant' volcanic earthquakes are triggered by increased pore pressure and probably involve stick-slip motion similar to that reported for some laboratory tests of the pore pressure effect, characterized by gradual onset and sudden stopping of each slip episode. The question is raised as to whether stress-discordant earthquakes are dominated by a stopping phase or terminal shock with consequent reversal of the deduced shear couple. A possible stopping mechanism is suggested: the dilatant stiffening of fault gouge during shear.
It is proposed that direct measurements of stress orientation be made by hydrofracturing tests at other places along the mid-ocean ridge crest and on the margins of the Red Sea and East African rifts. The Icelandic stress data indicate the need for sceptical re-examination of some fundamentals of plate tectonics theory.  相似文献   

8.
The controls on an earthquake's size are examined in a heterogeneous cellular automaton that includes stress concentrations which scale with rupture size. Large events only occur when stress is highly correlated with strength over the entire fault. Although the largest events occur when this correlation is the highest, the magnitude of the correlation has no predictive value as events of all magnitudes occur during times of high stress/strength correlation. Rather, the size of any particular event depends on the local stress heterogeneity encountered by the growing rupture. Patterns of energy release with time for individual ruptures reflect this heterogeneity and many show nucleation-type behaviour, although there is no relation between the duration of nucleation phase and the size of the event. These results support the view that earthquake size is determined by complex interactions between previous event history and dynamic stress concentrations and suggest that deterministic earthquake prediction based on monitoring nucleation zones will not be possible.  相似文献   

9.
Simulation of seismicity due to fluid migration in a fault zone   总被引：5，自引：0，他引：5  
Spatio-temporal variation of rupture activity is modelled assuming fluid migration in a narrow, porous fault zone formed along a vertical strike-slip fault in a semi-infinite elastic medium. The principle of the effective stress coupled to the Coulomb failure criterion introduces mechanical coupling between fault slip and the pore fluid. The fluid is assumed to flow out of a localized high-pressure fluid compartment in the fault at the onset of earthquake rupture. The duration of the earthquake sequence is assumed to be much shorter than the recurrence period of characteristic events on the fault. Both an earthquake swarm and a foreshock–main-shock sequence can be simulated by changing the relative magnitudes of the initial tectonic stress, pore fluid pressure, fracture strength and so on. When an inhomogeneity is introduced into the spatial distribution of fracture strength, high complexity is observed in the spatio-temporal variation of rupture activity. For example, the time interval between two successive events is highly irregular, and a relatively long quiescence of activity is sometimes observed in a foreshock–main-shock sequence. The quiescence is caused by the temporary arresting of rupture extension, due to an encounter with fault segments having locally high strengths. The frequency–magnitude statistics of intermediate-size events obey the Gutenberg–Richter relation. The calculations show the temporal variation of the b value during some foreshock sequences, and the degree of the change seems to depend on the statistical distribution of the fracture strength.  相似文献   

10.
An analysis of the Zihuatanejo, Mexico, earthquake of 1994 December 10 ( M = 6.6), based on teleseismic and near-source data, shows that it was a normal-faulting, intermediate-depth ( H = 50 ± 5 km) event. It was located about 30 km inland, within the subducted Cocos plate. The preferred fault plane has an azimuth of 130°, a dip of 79° and a rake of −86°. The rupture consisted of two subevents which were separated in time by about 2 s, with the second subevent occurring downdip of the first. The measured stress drop was relatively high, requiring a Δσ of about a kilobar to explain the high-frequency level of the near-source spectra. A rough estimate of the thickness of the seismogenic part of the oceanic lithosphere below Zihuatanejo, based on the depth and the rupture extent of this event, is 40 km.
This event and the Oaxaca earthquake of 1931 January 15 ( M = 7.8) are the two significant normal-faulting, intermediate-depth shocks whose epicentres are closest to the coast. Both of these earthquakes were preceded by several large to great shallow, low-angle thrust earthquakes, occurring updip. The observations in other subduction zones show just the opposite: normal-faulting events precede, not succeed, updip, thrust shocks. Indeed, the thrust events, soon after their occurrence, are expected to cause compression in the slab, thus inhibiting the occurrence of normal-faulting events. To explain the occurrence of the Zihuatanejo earthquake, we note that the Cocos plate, after an initial shallow-angle subduction, unbends and becomes subhorizontal. In the region of the unbending, the bottom of the slab is in horizontal extension. We speculate that the large updip seismic slip during shallow, low-angle thrust events increases the buckling of the slab, resulting in an incremental tensional stress at the bottom of the slab and causing normal-faulting earthquakes. This explanation may also hold for the 1931 Oaxaca event.  相似文献   

11.
A Bayesian approach to estimating tectonic stress from seismological data   总被引：2，自引：0，他引：2  
Earthquakes are conspicuous manifestations of tectonic stress, but the non-linear relationships between the stresses acting on a fault plane, its frictional slip, and the ensuing seismic radiation are such that a single earthquake by itself provides little information about the ambient state of stress. Moreover, observational uncertainties and inherent ambiguities in the nodal planes of earthquake focal mechanisms preclude straightforward inferences about stress being drawn on the basis of individual focal mechanism observations. However, by assuming that each earthquake in a small volume of the crust represents a single, uniform state of stress, the combined constraints imposed on that stress by a suite of focal mechanism observations can be estimated. Here, we outline a probabilistic (Bayesian) technique for estimating tectonic stress directions from primary seismological observations. The Bayesian formulation combines a geologically motivated prior model of the state of stress with an observation model that implements the physical relationship between the stresses acting on a fault and the resultant seismological observation. We show our Bayesian formulation to be equivalent to a well-known analytical solution for a single, errorless focal mechanism observation. The new approach has the distinct advantage, however, of including (1) multiple earthquakes, (2) fault plane ambiguities, (3) observational errors and (4) any prior knowledge of the stress field. Our approach, while computationally demanding in some cases, is intended to yield reliable tectonic stress estimates that can be confidently compared with other tectonic parameters, such as seismic anisotropy and geodetic strain rate observations, and used to investigate spatial and temporal variations in stress associated with major faults and coseismic stress perturbations.  相似文献   

12.
We perform a systematic parameter space study of the seismic response of a large fault with different levels of heterogeneity, using a 3-D elastic framework within the continuum limit. The fault is governed by rate-and-state friction and simulations are performed for model realizations with frictional and large scale properties characterized by different ranges of size scales. We use a number of seismicity and stress functions to characterize different types of seismic responses and test the correlation between hypocenter locations and the employed distributions of model parameters. The simulated hypocenters are found to correlate significantly with small L values of the rate-and-state friction. The final sizes of earthquakes are correlated with physical properties at their nucleation sites. The obtained stacked scaling relations are overall self-similar and have good correspondence with properties of natural earthquakes.  相似文献   

13.
Summary We consider a long strike-slip fault in a lithosphere modelled as an elastic slab. To the base of the slab a shear stress distribution is applied which simulates the viscous drag exerted by the asthenosphere. The resulant stress on the fault plane may directly fracture the lithosphere in its brittle upper portion; alternatively it may give rise at first to a stable aseismic sliding in the lower portion. In the latter case, stress concentration due to the deep aseismic slip is the relevant feature of the pre-seismic stress acting on the upper section of the lithosphere. The two cases are examined by use of dislocation theory and their observable effects compared. Different depths of the aseismic slip zone and the presence or absence of a uniform friction on the seismic fault are allowed for. If the model is applied to the San Andreas fault region, where a steady sliding condition actually seems to be present at shallow depth, it turns out that the slip amplitudes commonly associated with large earthquakes are consistent with average basal stress values which can be substantially lower than a few bars, a value often quoted as the steady state basal stress due to a velocity gradient in the upper asthenosphere.  相似文献   

14.
The Middle Durance fault system, southeastern France, is a slow active fault that produced moderate-size historical seismic events and shows evidence of at least one   M w ≳ 6.5  event in the last 29 000 yr. Based on dynamic rupture simulation, we propose earthquake scenarios that are constrained by knowledge of both the tectonic stress field and of the 3-D geometry of the Durance fault system. We simulate dynamic rupture interaction among several fault segmentations of different strikes, dips and rakes, using a 3-D boundary integral equation method. 50 combinations of reasonable stress field orientations, stress field amplitudes and hypocentre locations are tested. The probability of different rupture evolutions is then computed. Each segment ruptures mainly as a single event (44 per cent of the 50 simulations test in this paper). However, the probability that an event triggers simultaneously along three segments is high (26 per cent), leading to a potential rupture length of 45 km. Finally, 2 per cent of the simulations occur along four adjacent segments, producing the greatest total rupture length of 55 km. The simulation results show that the southernmost segment is most easily ruptured (40 per cent), because of its favourable orientation with respect to the tectonic stress and of its favourable location for interaction with the other segments. South-bound unilateral propagation is slightly preferable (41 per cent), compared to north-bound unilateral and bilateral propagation modes. Although, these rupture scenarios cannot be directly translated into probabilities of occurrence, they do provide a better insight as to which rupture scenarios are more likely, an important element to better estimate near-field strong ground motion and seismic hazard.  相似文献   

15.
Summary. The space and time characteristics of earthquake sequences, including a main shock, aftershocks and the recurrence of major shocks in a long time range, are investigated on a frictional fault model with non-uniform strengths and relaxation times, which is subjected to a time-dependent shear stress. Aftershocks with low stress drop take place successively in spaced regions so as to fill the gaps which have not yet been ruptured since the main shock, while those with high stress drop occur in and around the regions left unruptured during the main faulting. The frequency decay of aftershocks with time follows a hyperbolic law with the rates p consistent with observations. There are good linear relations in logarithmic scales for source area versus frequency and seismic moment versus frequency of the generated aftershocks. The b -value obtained in the present experiments appears slightly larger than that for observations. It was found that more heterogeneous distribution of the fault strength give smaller p and larger b -values. The recurrence of major shocks, particularly of very large shocks with high stress drop, is often preceded by a completely silent period of activity or very low activity with a small number of foreshocks. The major shocks take place successively in adjacent unruptured regions and sometimes show slow-speed migrations. These results provide explanations to various observations of earthquake sequences.  相似文献   

16.
Source models such as the k -squared stochastic source model with k -dependent rise time are able to reproduce source complexity commonly observed in earthquake slip inversions. An analysis of the dynamic stress field associated with the slip history prescribed in these kinematic models can indicate possible inconsistencies with physics of faulting. The static stress drop, the strength excess, the breakdown stress drop and critical slip weakening distance D c distributions are determined in this study for the kinematic k -squared source model with k -dependent rise time. Several studied k -squared models are found to be consistent with the slip weakening friction law along a substantial part of the fault. A new quantity, the stress delay, is introduced to map areas where the yielding criterion of the slip weakening friction is violated. Hisada's slip velocity function is found to be more consistent with the source dynamics than Boxcar, Brune's and Dirac's slip velocity functions. Constant rupture velocities close to the Rayleigh velocity are inconsistent with the k -squared model, because they break the yielding criterion of the slip weakening friction law. The bimodal character of D c / D tot frequency–magnitude distribution was found. D c approaches the final slip D tot near the edge of both the fault and asperity. We emphasize that both filtering and smoothing routinely applied in slip inversions may have a strong effect on the space–time pattern of the inferred stress field, leading potentially to an oversimplified view of earthquake source dynamics.  相似文献   

17.
Summary. Two-dimensional crack problems in elastic homogeneous isotropic media are considered which describe rupture over a fault surface characterized by non-uniform stress drop. Solutions can be found in which the stress field is finite at the crack tips and the rupture surface is not assigned a priori , but is part of the solution. These crack models are found to be consistent with the frictional stress threshold criterion for slip arrest over pre-existing fault surfaces. A crack is found to stop when its contribution to the stress field is opposite to the stress drop at the crack tips. The quasi-static propagation of a crack up to the arrest configuration is studied in terms of the minimum energy principle. The crack spontaneously propagates in such a way as to make the value of the stress intensity factor at one tip equal to the value at the other tip. Furthermore a tip propagating in a region with higher friction is found to move more slowly than the other tip propagating in a region with lower friction. Simple criteria for fracture arrest are derived, in terms of a properly averaged stress drop. Piecewise constant stress drop profiles are explicitly considered yielding a variety of solutions which can be applied to modelling asperities or barriers over a fault plane. The evaluation of the amount of the energy released during the quasi-static crack propagation shows that stopping phases cannot be efficiently radiated if the crack comes to rest in a low friction region.  相似文献   

18.
Recent high-resolution observations of crustal movements have revealed silent slip events (SSEs) with propagation velocities of around 10–15 km d−1 and with intervals of 3–14 months along the deeper parts of the Cascadia and Nankai subduction zones. This study develops 2-D and 3-D models of these short-interval SSEs considering the frictional behaviour that was confirmed experimentally by Shimamoto for the unstable–stable transition regime. To represent this frictional behaviour, a small cut-off velocity to an evolution effect is introduced in a rate- and state-dependent friction law. When the cut-off velocity to the evolution effect is significantly smaller than that to a direct effect, steady-state friction exhibits velocity weakening at low slip velocities and velocity strengthening at high slip velocities. At the deeper Cascadia and Nankai subduction interfaces, the pore pressure is inferred to be high because of the dehydration of materials in the descending plate. Under conditions where the pore-fluid pressure is nearly equal to the lithostatic pressure and the critical weakening displacement is very small, short-interval SSEs with propagation velocities and slip velocities of 4–8 km d−1 and  2 − 4 × 10−7  m s−1, respectively, can be reproduced. The propagation velocity of short-interval SSEs is in proportion to the slip velocity. The results also show that during the nucleation process of large earthquakes, the occurrence of short-interval SSEs becomes irregular because of the accelerated slips that occur at the bottom of the seismogenic zone. Our results suggest that monitoring of short-interval SSEs might be useful for forecasting the main earthquakes.  相似文献   

19.
Destructive upper crustal earthquakes in Central America are often located between active volcanic centres—a geometric relationship that we study using finite element Coulomb failure stress (CFS) models that incorporate the rheologically heterogeneous nature of the volcanic arc. Volcanoes are simulated as mechanically weak zones within a stronger crust. We find that deformation of the volcanic centres within a regional stress field dominated by dextral shear causes stress increases in surrounding crust, with a maximum CFS change between neighbouring volcanoes. This increase in CFS enhances the probability of fault slip on arc-normal faults that are located between volcanic centres; for example, the Tiscapa fault, which ruptured during the 1972 December 13,   M s   6.2 Managua earthquake. The amount of stress increase due to long-term (100 yr) volcano shearing is on the order of 0.1–0.6 bars, similar to values estimated for subduction zone earthquakes.  相似文献   

20.
The Benevento region is part of the southern Apennines seismogenic belt, which experienced large destructive seismic events both in historical and in recent times. The study area lies at the northern end of the Irpinia fault, which ruptured in 1980 with a Ms = 6.9 normal faulting event, which caused about 3000 casualties. The aims of this paper are to image lateral heterogeneities in the upper crust of the Benevento region, and to try to identify the fault segments that are expected to generate such large earthquakes. This work is motivated by the recognition that lithological heterogeneities along major fault zones, inferred from velocity anomalies, reflect the presence of fault patches that behave differently during large rupture episodes. In this paper, we define the crustal structure of the Benevento region by using the background seismicity recorded during 1991 and 1992 by a local seismic array. These data offer a unique opportunity to investigate the presence of structural discontinuities of a major seismogenic zone before the occurrence of the next large earthquake. The main result that we obtained is the delineation of two NW-trending high-velocity zones (HVZs) in the upper crust beneath the Matese limestone massif. These high velocities are interpreted as high-strength regions that extend for 30-40 km down to at least 12 km depth. The correspondence of these HVZs with the maximum intensity regions of historical earthquakes (1688 AD, 1805 AD) suggests that these anomalies delineate the extent of two fault segments of the southern Apenninic belt capable of generating M = 6.5−7 earthquakes. The lateral offset observed between the two segments from tomographic results and isoseismal areas is possibly related to transverse right-lateral faults.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号