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1.
The GRACE satellite mission has been measuring the Earth's gravity field and its temporal variations since 2002 April. Although these variations are mainly due to mass transfer within the geofluid envelops, they also result from mass displacements associated with phenomena including glacial isostatic adjustment and earthquakes. However, these last contributions are difficult to isolate because of the presence of noise and of geofluid signals, and because of GRACE's coarse spatial resolution (>400 km half-wavelength). In this paper, we show that a wavelet analysis on the sphere helps to retrieve earthquake signatures from GRACE geoid products. Using a wavelet analysis of GRACE geoids products, we show that the geoid variations caused by the 2004 December ( M w= 9.2) and 2005 March ( M w= 8.7) Sumatra earthquakes can be detected. At GRACE resolution, the 2004 December earthquake produced a strong coseismic decrease of the gravity field in the Andaman Sea, followed by relaxation in the area affected by both the Andaman 2004 and the Nias 2005 earthquakes. We find two characteristic timescales for the relaxation, with a fast variation occurring in the vicinity of the Central Andaman ridge. We discuss our coseismic observations in terms of density changes of crustal and upper-mantle rocks, and of the vertical displacements in the Andaman Sea. We interpret the post-seismic signal in terms of the viscoelastic response of the Earth's mantle. The transient component of the relaxation may indicate the presence of hot, viscous material beneath the active Central Andaman Basin.  相似文献   

2.
We explore the possible stress triggering relationship of the   M ≥ 6.4  earthquakes that occurred in Kerman Province, southern Iran since 1981. We calculated stress changes due to both coseismic sudden movement in the upper crust and the time-dependent viscous relaxation of the lower crust and/or upper mantle following the event. Four events of   M ≥ 6.4  occurred between 1981 and 2005, on and close to the Gowk fault, show a clear Coulomb stress load to failure relationship. The  2003 M = 6.5  Bam earthquake, however, which occurred approximately 95 km SW of the closest Gowk event, shows a very weak stress relation to preceding earthquakes. The coseismic static stress change at the hypocentre of the Bam earthquake is quite small (∼0.006 bars). The time-dependent post-seismic stress change could be 26 times larger or 7 times lower than that of coseismic static stress alone depending on the choice of viscoelastic crustal model and the effective coefficient of friction. Given the uncertainties in the viscoelastic earth models and the effective coefficient of friction, we cannot confidently conclude that the 2003 Bam event was brought closer to failure through coseismic or post-seismic stress loading. Interestingly, the southern Gowk segment with a similar strike to that of the Bam fault, experienced a stress load of up to 8.3 bars between 1981 and 2003, and is yet to have a damaging earthquake.  相似文献   

3.
We use GPS displacements collected in the 15 months after the 1999 Chi-Chi, Taiwan earthquake  ( M w 7.6)  to evaluate whether post-seismic deformation is better explained by afterslip or viscoelastic relaxation of the lower crust and upper mantle. We find that all viscoelastic models tested fail to fit the general features in the post-seismic GPS displacements, in contrast to the satisfactory fit obtained with afterslip models. We conclude that afterslip is the dominant mechanism in the 15-month period, and invert for the space–time distribution of afterslip, using the Extended Network Inversion Filter. Our results show high slip rates surrounding the region of greatest coseismic slip. The slip-rate distribution remains roughly stationary over the 15-month period. In contrast to the limited coseismic slip on the décollement, afterslip is prominent there. Maximum afterslip of 0.57 m occurs downdip and to the east of the hypocentral region. Afterslip at hypocentral depths is limited to the southern part of the main shock rupture, with little or no slip on the northern section where coseismic slip was greatest. Whether this results from along strike variations in frictional properties or dynamic conditions that locally favour stable sliding is not clear. In general, afterslip surrounds the area of greatest coseismic slip, consistent with post-seismic slip driven by the main shock stress change. The total accumulated geodetic afterslip moment is  3.8 × 1019 N m  , significantly more than the seismic moment released by aftershocks,  6.6 × 1018 N m  . Afterslip and aftershocks appear to have different temporal evolutions and some spatial correlations, suggesting that aftershock rates may not be completely controlled by the rate of afterslip.  相似文献   

4.
The 2004 M = 9.2 Sumatra–Andaman earthquake profoundly altered the state of stress in a large volume surrounding the ∼1400 km long rupture. Induced mantle flow fields and coupled surface deformation are sensitive to the 3-D rheology structure. To predict the post-seismic motions from this earthquake, relaxation of a 3-D spherical viscoelastic earth model is simulated using the theory of coupled normal modes. The quasi-static deformation basis set and solution on the 3-D model is constructed using: a spherically stratified viscoelastic earth model with a linear stress–strain relation; an aspherical perturbation in viscoelastic structure; a 'static' mode basis set consisting of Earth's spheroidal and toroidal free oscillations; a "viscoelastic" mode basis set; and interaction kernels that describe the coupling among viscoelastic and static modes. Application to the 2004 Sumatra–Andaman earthquake illustrates the profound modification of the post-seismic flow field at depth by a slab structure and similarly large effects on the near-field post-seismic deformation field at Earth's surface. Comparison with post-seismic GPS observations illustrates the extent to which viscoelastic relaxation contributes to the regional post-seismic deformation.  相似文献   

5.
Summary. We give the analytical formulation for calculating the transient displacement of fields produced by earthquakes in a stratified, selfgravitating, incompressible, viscoelastic earth. We have evaluated the potential of viscous creep in the asthenosphere in exciting the Chandler wobble by a four-layer model consisting of an elastic lithosphere, a two-layer Maxwell viscoelastic mantle, and an inviscid core. The seismic source is modelled as an inhomogeneous boundary condition, which involves a jump condition of the displacement fields across the fault in the lithosphere. The response fields are derived from the solution of a two-point boundary value problem, using analytical propagator matrices in the Laplace-transformed domain. Transient flows produced by post-seismic rebound are found to be confined within the asthenosphere for local viscosity values less than 1020P. The viscosity of the mantle below the low-viscosity channel is kept at 1022P. For low-viscosity zones with widths greater than about 100 km and asthenospheric viscosities less than 1018P, we find that viscoelasticity can amplify the perturbations in the moment of inertia by a factor of 4–5 above the elastic contribution within the time span of the wobble period. We have carried out a comparative study on the changes of the inertia tensor from forcings due to surface loading and to faulting. In general the global responses from faulting are found to be much more sensitive to the viscosity structure of the asthenosphere than those produced from surface loading.  相似文献   

6.
We combine Global Positioning System (GPS) measurements with forward modelling of viscoelastic relaxation and after-slip to study the post-seismic deformation of the 1997 Umbria-Marche (Central Apennines) moderate shallow earthquake sequence. Campaign GPS measurements spanning the time period 1999–2003 are depicting a clear post-seismic deformation signal. Our results favour a normal faulting rupture model where most of the slip is located in the lower part of the seismogenic upper crust, consistent with the rupture models obtained from the inversion of strong motion data. The preferred rheological model, obtained from viscoelastic relaxation modelling, consists of an elastic upper crust, underlain by a transition zone with a viscosity of 1018 Pa s, while the rheology of deeper layers is not relevant for the observed time-span. Shallow fault creep and after-slip at the base of the seismogenic upper crust are the first order processes behind the observed post-seismic deformation. The deep after-slip, below the fault zone at about 8 km depth, acting as a basal shear through localized time-dependent deformation, identifies a rheological discontinuity decoupling the seismogenic upper crust from the low-viscosity transition zone.  相似文献   

7.
The Ethiopian side of central Afar was struck in August 1989 by the largest seismic sequence (three 6.1 ≤ M s ≤ 6.3 events, 15 with M s or m b ≥ 5.0) since that of Serdo in 1969. Using the Djibouti seismological network, we relocated 297 of the events of that sequence. As most of the large events took place outside the network, we assessed the accuracy and stability of earthquake relocations by using three different velocity models and two relocation codes to try to relate individual shocks to distinct faults and surface breaks. A majority of the events apparently occurred underneath the floor of the Dôbi graben, an area about 45  km long and 15  km wide, rupturing boundary and inner floor faults, in agreement with the surface cracks and scarps that we mapped in the area. The relocation shows that the principal events propagated about 50  km northwestwards along the graben in the first 40  hr. A day and a half after the beginning of the sequence, smaller events ( M ≤ 4) started to propagate more than 55  km eastwards, towards Asal Lake. Using two three-component stations installed near the Ethiopian border, we could determine reliable depths for 21 events. The depths are compatible with a seismogenic crust about 14  km thick in the Dôbi and Hanle graben area. Although the Dôbi sequence ruptured about 50  km of the fault array extending from Serdo to Asal, where the regional stress was released by earthquakes in 1969 and 1978, respectively, a seismic gap about 50  km long still subsists along the northern part of the Gaggade region (Der'êla half-graben).  相似文献   

8.
Using the viscoelastic correspondence principle, we utilize the surface coseismic spheroidal deformation fields (i.e. vertical displacements, potential perturbations and gravity changes) of SNREI earth models caused by four typical types of point dislocation, derived by Sun & Okubo (1993 ), to deduce the fundamental formulas for spheroidal fields relevant to viscoelastic earth models. In computations, we employ a strike-slip dislocation on a vertical plane buried at the bottom of the lithosphere to estimate the maximal viscous relaxation responses to this kind of source that possibly exist on the surface of the earth. We take the seismic moment as 1022  N  m, which is characteristic of an average large earthquake. The numerical results demonstrate that, if we take the viscosity as 1019  Pa  s in the asthenosphere, and 1021  Pa  s in the other mantle layers, the rates of surface vertical displacements and gravity changes within about 2.5° for the 10 postseismic years are respectively 1.5–8.1  cm  yr−1 and 4.0–14.9  μgal  yr−1 : the viscous relaxation for this mantle viscosity profile proceeds much faster than for a constant mantle viscosity of 1021  Pa  s.  相似文献   

9.
Analytical approach for the toroidal relaxation of viscoelastic earth   总被引：1，自引：0，他引：1  
This paper is concerned with post-seismic toroidal deformation in a spherically symmetric, non-rotating, linear-viscoelastic, isotropic Maxwell earth model. Analytical expressions for characteristic relaxation times and relaxation strengths are found for viscoelastic toroidal deformation, associated with surface tangential stress, when there are two to five layers between the core–mantle boundary and Earth's surface. The multilayered models can include lithosphere, asthenosphere, upper and lower mantles and even low-viscosity ductile layer in the lithosphere. The analytical approach is self-consistent in that the Heaviside isostatic solution agrees with fluid limit. The analytical solution can be used for high-precision simulation of the toroidal relaxation in five-layer earths and the results can also be considered as a benchmark for numerical methods. Analytical solution gives only stable decaying modes—unstable mode, conjugate complex mode and modes of relevant poles with orders larger than 1, are all excluded, and the total number of modes is found to be just the number of viscoelastic layers between the core–mantle boundary and Earth's surface—however, any elastic layer between two viscoelastic layers is also counted. This confirms previous finding where numerical method (i.e. propagator matrix method) is used. We have studied the relaxation times of a lot of models and found the propagator matrix method to agree very well with those from analytical results. In addition, the asthenosphere and lithospheric ductile layer are found to have large effects on the amplitude of post-seismic deformation. This also confirms the findings of previous works.  相似文献   

10.
Focal mechanisms of small earthquakes with magnitudes of about 3 in the SE Brazilian shield are calculated using S / P amplitude ratios. Low attenuation ( Q p from 400 to 800) in the shield upper-crustal layers allowed sharp S arrivals to be recorded up to distances of 100 km. Besides P -wave polarities, SH -wave first motions were also used to constrain the nodal-plane orientations. Normal and reverse faulting mechanisms with strike-slip components were found. The inversion of four mechanisms to estimate the stress tensor indicated a strike-slip stress regime with roughly E–W-orientated σ 1 and N–S σ 3. Both the orientations and the shape factor ( φ =0.7) of the inverted stress are in excellent agreement with theoretical predictions for that part of Brazil from the driving-force model of Coblentz & Richardson (1996) . Good agreement with the nature of the stress, as well as its orientation, was also found for the model of Meijer (1995) . Both of these theoretical models include spreading stresses along the continent/ocean lithospheric transition. Because the earthquakes are more than 300 km from the continental shelf they should not be affected by the local flexural forces caused by sediment load in the marginal basins. The agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses.  相似文献   

11.
By inversion analysis of the baseline changes and horizontal displacements observed with GPS (Global Positioning System) during 1990–1994, a high-angle reverse fault was detected in the Shikoku-Kinki region, southwest Japan. The active blind fault is characterized by reverse dip-slip (0.7±0.2  m yr−1 within a layer 17–26  km deep) with a length of 208±5  km, a (down-dip) width of 9±2  km, a dip-angle of 51°±2° and a strike direction of 40°±2° (NE). Evidence from the geological investigation of subfaults close to the southwestern portion of the fault, two historical earthquakes ( M L=7.0, 1789 and 6.4, 1955) near the centre of the fault, and an additional inversion analysis of the baseline changes recorded by the nationwide permanent GPS array from 18 January to 31 December 1995 partially demonstrates the existence of the fault, and suggests that it might be a reactivation of a pre-existing fault in this region. The fact that hardly any earthquakes ( M L>2.0) occurred at depth on the inferred fault plane suggests that the fault activity was largely aseismic. Based on the parameters of the blind fault estimated in this study, we evaluated stress changes in this region. It is found that shear stress concentrated and increased by up to 2.1 bar yr−1 at a depth of about 20  km around the epicentral area of the 1995 January 17  Kobe earthquake ( M L=7.2, Japan), and that the earthquake hypocentre received a Coulomb failure stress of about 5.6 bar yr−1 during 1990–1994. The results suggest that the 1995  Kobe earthquake could have been induced or triggered by aseismic fault movement.  相似文献   

12.
3-D images of P velocity and P - to S -velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthquake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The best-fitting V P and V P / V S 1-D models were computed before the 3-D inversion. V P was measured on two rock samples representative of the investigated upper layers of the Friuli crust. The tomographic V P model was used for modelling the gravity anomalies, by converting the velocity values into densities along three vertical cross-sections. The computed gravity anomalies were optimized with respect to the observed gravity anomalies. The crust investigated is characterized by sharp lateral and deep V P and V P / V S anomalies that are associated with the complex geological structure. High V P / V S values are associated with highly fractured zones related to the main faulting pattern. The relocated seismicity is generally associated with sharp variations in the V P / V S anomalies. The V P images show a high-velocity body below 6 km depth in the central part of the Friuli area, marked also by strong V P / V S heterogeneities, and this is interpreted as a tectonic wedge. Comparison with the distribution of earthquakes supports the hypothesis that the tectonic wedge controls most of the seismicity and can be considered to be the main seismogenic zone in the Friuli area.  相似文献   

13.
Geological studies show evidence for temporal clustering of large earthquakes on individual fault systems. Since post-seismic deformation due to the inelastic rheology of the lithosphere may result in a variable loading rate on a fault throughout the interseismic period, it is reasonable to expect that the rheology of the non-seismogenic lower crust and mantle lithosphere may play a role in controlling earthquake recurrence times. We study this phenomenon using a 2-D, finite element method continuum model of the lithosphere containing a single strike-slip fault. This model builds on a previous study using a 1-D spring-dashpot-slider analogue of a single fault system to study the role of Maxwell viscoelastic relaxation in producing non-periodic earthquakes. In our 2-D model, the seismogenic portion of the fault slips when a predetermined yield stress is exceeded; stress accumulated on the seismogenic fault is shed to the viscoelastic layers below and recycled back to the seismogenic fault through viscoelastic relaxation. We find that random variation of the fault yield stress from one earthquake to the next can cause the earthquake sequence to be clustered; the amount of clustering depends on a non-dimensional number, W , called the Wallace number defined as the standard deviation of the randomly varied fault yield stress divided by the effective viscosity of the system times the tectonic loading rate. A new clustering metric based on the bimodal distribution of interseismic intervals allows us to investigate clustering behaviour of systems over a wide range of model parameters and those with multiple viscoelastic layers. For models with   W ≥ 1  clustering increases with increasing W , while those with   W ≤ 1  are unclustered, or quasi-periodic.  相似文献   

14.
Seismic phase conversions provide important constraints on the layered nature of subduction zone structures. Recordings from digital stations in North Island, New Zealand, have been examined for converted ScS ‐to‐ p ( ScSp ) arrivals from deep (>150 km) Tonga–Kermadec earthquakes to image layering in the underlying Hikurangi subduction zone. Consistent P ‐wave energy prior to ScS has been identified from stations in eastern and southern North Island, where the subducted plate interface is at a depth of between 15 and 30 km. Two ScS precursors are observed. Ray tracing indicates that the initial precursor ( ScSp 1) corresponds to conversion from the base of an 11–14 km thick subducting Pacific crust. The second precursor is interpreted as a conversion from the top of the subducting plate. The amplitude ratio, ScSp 1: ScS , increases from 0.10 to 0.19 from northern to southern North Island. This is within the range expected from a simple first‐order velocity discontinuity at an oceanic Moho. A 1–2 km thick layer of low‐velocity sediment at the top of the subducting plate is required to explain the remaining ScSp waveform. Our results imply that the abnormally thick Hikurangi–Chatham Plateau has been subducting beneath New Zealand for at least 2.9 Myr, thus explaining the high uplift rates observed across eastern North Island.  相似文献   

15.
The deformation at the core–mantle boundary produced by the 2004 Sumatra earthquake is investigated by means of a semi-analytic theoretical model of global coseismic and postseismic deformation, predicting a millimetric coseismic perturbation over a large portion of the core–mantle boundary. Spectral features of such deformations are analysed and discussed. The time-dependent postseismic evolution of the elliptical part of the gravity field ( J 2) is also computed for different asthenosphere viscosity models. Our results show that, for asthenospheric viscosities smaller than 1018 Pa s, the postseismic J 2 variation in the next years is expected to leave a detectable signal in geodetic observations.  相似文献   

16.
Summary. Attenuation of earthquake intensities with epicentral distance was studied by analysing the intensity data for 39 earthquakes in the United States. Attenuation of MM intensity ( I ) with distance (Δ km) follows a simple relation of the type log I = log I 0 - m Δ, where I 0 is the intensity at the epicentre and m is a constant. Slope m is found to be inversely proportional to the square of the focal depth. Intensity attenuation pattern in the United States in general can be represented by a unified relation I/I 0= exp [-(0.8999/ h 2+ 0.0014)Δ] where 16km ≤ h ≤ 60km. Intensities were calculated with the help of this equation and a good agreement with the observed intensities were found. A comparative study has also been made between the attenuation relations applicable to India and the United States.  相似文献   

17.
The investigation of L g attenuation characteristics in the region bounding the western branch of the East African rift system using digital recordings from a seismic network located along the rift between Lake Rukwa and Lake Malawi is reported. A set of 24 recordings of L g waves from 12 regional earthquakes has been used for the determination of anelastic attenuation, Q Lg , and regional body-wave magnitude, m b Lg , scale. The events used have body-wave magnitudes, m b , between 4.6 and 5.5, which have been determined teleseismically and listed in ISC bulletins. The data were time-domain displacement amplitudes measured at 10 different frequencies (0.7–5.0  Hz). Q Lg and its frequency dependence, η , in the region can be represented in the form Q Lg = (186.2 ± 6.5)  f  (0.78±0.05). This model is in agreement with models established in other active tectonic regions. The L g -wave-based magnitude formula for the region is given by m b Lg = log   A + (3.76 ± 0.38)  log   D − (5.72 ± 1.06), where A is a half-peak-to-peak maximum amplitude of the 1  s L g wave amplitude in microns and D is the epicentral distance in kilometres. Magnitude results for the 12 regional earthquakes tested are in good agreement with the ISC body-wave magnitude scale.  相似文献   

18.
We evaluate the stress field in and around the southern Korean Peninsula with focal mechanism solutions, using the data collected from 71 earthquakes ( ML = 1.9–5.2) between 1999 and 2004. For this, the hypocentres were relocated and well-constrained fault plane solutions were obtained from the data set of 1270 clear P -wave polarities and 46 SH / P amplitude ratios. The focal mechanism solutions indicate that the prevailing faulting types in South Korea are strike-slip-dominant-oblique-slip faultings with minor reverse-slip component. The maximum principal stresses (σ1) estimated from fault-slip inversion analysis of the focal mechanism solutions show a similar orientation with E–W trend (269°–275°) and low-angle plunge (10°–25°) for all tectonic provinces in South Korea, consistent with the E–W trending maximum horizontal stress (σHmax) of the Amurian microplate reported from in situ stress measurements and earthquake focal mechanisms. The directions of the intermediate (σ2) and minimum (σ3) principal stresses of the Gyeongsang Basin are, however, about 90 deg off from those of the other tectonic provinces on a common σ2–σ3 plane, suggesting a permutation of σ2 and σ3. Our results incorporated with those from the kinematic studies of the Quaternary faults imply that NNW- to NE-striking faults (dextral strike-slip or oblique-slip with a reverse-slip component) are highly likely to generate earthquakes in South Korea.  相似文献   

19.
A moderate earthquake of   M w= 6.8  occurred on 2003 December 10. It ruptured the Chihshang Fault in eastern Taiwan which is the most active segment of the Longitudinal fault as a plate suture fault between the Luzon arc of the Philippine Sea plate and the Eurasian plate. The largest coseismic displacements were 13 cm (horizontal) and 26 cm (vertical). We analyse 40 strong motion and 91 GPS data to model the fault geometry and coseismic dislocations. The most realistic shape of the Chihshang fault surface is listric in type. The dipping angle of the seismic zone is steep (about 60°–70°) at depths shallower than 10 km and then gradually decreases to 40°–50° at depths of 20–30 km. Thus the polygonal elements in Poly3D are well suited for modelling complex surfaces with curving boundaries. Using the strong motion data, the displacement reaches 1.2 m dip-slip on the Chihshang Fault and decreases to 0.1 m near surface. The slip averages 0.34 m, releasing a scalar moment of 1.6E26 dyne-cm. For GPS data, our model reveals that the maximal dislocation is 1.8 m dip-slip. The dislocations decrease to 0.1 m near the surface. The average slip is 0.48 m, giving a scalar moment of 2.2E26 dyne-cm. Regarding post-seismic deformation, a displacements of 0.5 m were observed near the Chihshang Fault, indicating the strain had not been totally released, as a probable result of near-surface locking of the fault zone.  相似文献   

20.
We explore a practical approach to earthquake early warning in southern California by determining a ground-motion period parameter  τ c   and a high-pass filtered displacement amplitude parameter Pd from the initial 3 s of the P waveforms recorded at the Southern California Seismic Network stations for earthquakes with M > 4.0. At a given site, we estimate the magnitude of an event from  τ c   and the peak ground-motion velocity ( PGV ) from Pd . The incoming three-component signals are recursively converted to ground acceleration, velocity and displacement. The displacements are recursively filtered with a one-way Butterworth high-pass filter with a cut-off frequency of 0.075 Hz, and a P -wave trigger is constantly monitored. When a trigger occurs,  τ c   and Pd are computed. We found the relationship between  τ c   and magnitude ( M ) for southern California, and between Pd and PGV for both southern California and Taiwan. These two relationships can be used to detect the occurrence of a major earthquake and provide onsite warning in the area around the station where onset of strong ground motion is expected within seconds after the arrival of the P wave. When the station density is high, the methods can be applied to multistation data to increase the robustness of onsite early warning and to add the regional warning approach. In an ideal situation, such warnings would be available within 10 s of the origin time of a large earthquake whose subsequent ground motion may last for tens of seconds.  相似文献   

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