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1.
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
2.
Takeshi Mikumo Miguel A. Santoyo & Shri Krishna Singh 《Geophysical Journal International》2000,140(3):611-620
A large nearly vertical, normal faulting earthquake ( M w = 7.1) took place in 1997 in the Cocos plate, just beneath the ruptured fault zone of the great 1985 Michoacan thrust event ( M w = 8.1). Dynamic rupture and resultant stress change during the 1997 earthquake have been investigated on the basis of near-source strong-motion records together with a 3-D dynamic model.
Dynamically consistent waveform inversion reveals a highly heterogeneous distribution of stress drop, including patch-like asperities and negative stress-drop zones. Zones of high stress drop are mainly confined to the deeper, southeastern section of the vertical fault, where the maximum dynamic stress drop reaches 280 bars (28 MPa). The dynamically generated source time function varies with location on the fault, and yields a short slip duration, which is caused by a short scalelength of stress-drop heterogeneities. The synthetic seismograms calculated from the dynamic model are generally consistent with the strong-motion velocity records in the frequency range lower than 0.5 Hz.
The pattern of stress-drop distribution appears, in some sense, to be consistent with that of coseismic changes in shear stress resulting from the 1985 thrust event. This consistency suggests that the stress transfer from the 1985 event to the subducting plate could be one of the possible mechanisms that increased the chance of the occurrence of the 1997 earthquake. 相似文献
Dynamically consistent waveform inversion reveals a highly heterogeneous distribution of stress drop, including patch-like asperities and negative stress-drop zones. Zones of high stress drop are mainly confined to the deeper, southeastern section of the vertical fault, where the maximum dynamic stress drop reaches 280 bars (28 MPa). The dynamically generated source time function varies with location on the fault, and yields a short slip duration, which is caused by a short scalelength of stress-drop heterogeneities. The synthetic seismograms calculated from the dynamic model are generally consistent with the strong-motion velocity records in the frequency range lower than 0.5 Hz.
The pattern of stress-drop distribution appears, in some sense, to be consistent with that of coseismic changes in shear stress resulting from the 1985 thrust event. This consistency suggests that the stress transfer from the 1985 event to the subducting plate could be one of the possible mechanisms that increased the chance of the occurrence of the 1997 earthquake. 相似文献
3.
Earthquake sequences on a frictional fault model with non-uniform strengths and relaxation times 总被引：1，自引：0，他引：1
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
4.
We perform analytical and numerical studies of scaling relations of earthquakes and partition of elastic strain energy between seismic and aseismic components using a thermodynamically based continuum damage model. Brittle instabilities occur in the model at critical damage level associated with loss of convexity of the strain energy function. A new procedure is developed for calculating stress drop and plastic strain in regions sustaining brittle instabilities. The formulation connects the damage rheology parameters with dynamic friction of simpler frameworks, and the plastic strain accumulation is governed by a procedure that is equivalent to Drucker–Prager plasticity. The numerical simulations use variable boundary forces proportional to the slip-deficit between the assumed far field plate motion and displacement of the boundary nodes. These boundary conditions account for the evolution of elastic properties and plastic strain in the model region. 3-D simulations of earthquakes in a model with a large strike-slip fault produce scaling relations between the scalar seismic potency, rupture area, and stress drop values that are in good agreement with observations and other theoretical studies. The area and potency of the simulated earthquakes generally follow a linear log–log relation with a slope of 2/3, and are associated with stress drop values between 1 and 10 MPa. A parameter-space study shows that the area-potency scaling is shifted to higher stress drops in simulations with parameters corresponding to lower dynamic friction, more efficient healing, and higher degree of seismic coupling. 相似文献
5.
Summary . The great Etorofu earthquake of 1958 November 6 is characterized by a relatively small aftershock area (70 × 150 km2 ) and an extremely large felt area. The felt area is more extensive than those of any other large earthquakes which have occurred in the southern Kurile to northern Japan arc since the beginning of this century. The mechanism is a pure thrust fault typical of most great earthquakes in island arcs. A body wave magnitude of m b = 8.2 is obtained at periods around 6 s using more than 40 observations, although an m b value of only 7.6–7.7 would be expected empirically from the observed surface wave magnitude of M s = 8.1–8.2. Both an unusually large felt area and a high m b indicate a dominance of high-frequency components in the seismic waves. A seismic moment of M o = 4.4 × 1028 dyne cm is determined from long-period surface waves from which a high stress drop of Δσ = 78 bar is obtained using a relatively small aftershock area. Historic data indicate an anomalously long time interval between the 1958 event and any earlier great earthquake from the same source region. The observed high stress drop can be interpreted as a consequence of this long intervening period through which strain built up. The dominance of the high-frequency seismic waves can then be interpreted as a result of this high stress drop. Stress drops, seismic wave spectra and recurrence intervals of great earthquakes are in this way closely related to each other. The 1958 event may represent a high strength extreme of stochastic fluctuation of fracture strength relevant to great earthquakes. 相似文献
6.
Summary. We consider the problem of the bilateral extension of a two-dimensional anti-plane shear crack that initiates spontaneously at a point and extends under the influence of cohesive forces at the edges. An approximation to the stresses in the regions beyond the edges of the crack has been found that simplifies the calculation. The exact stresses in these regions are also found iteratively. In the cases of uniformly propagating cracks, the estimates of cohesive forces obtained from the approximation are close to the exact values for high crack speeds but are significantly different for low crack speeds. It is also found that if healing is initiated due to the encounter of one end of a uniformly propagating crack with an unbreakable barrier, the static stress drop in the torn region is constant but may either overshoot or undershoot the dynamical stress drop. In these cases, the final static slip distribution is obtained by freezing the dynamic solution along a characteristic line through the location of the barrier. We find that the crack length cannot be unambiguously derived from the far field spectral properties. 相似文献
7.
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
8.
Summary. Following the classic work of Eshelby, the slip and stress distributions due to an elliptical plane shear crack are evaluated. The relation between average (or maximum) slip on the crack and the (constant) static stress drop, for faults of different aspect ratios, is found. The slip vector is not parallel to the applied stress but makes a small angle to it, except when the stress is applied along the major or minor axis of the ellipse. The stress -distribution around the crack shows that in addition to the expected stress concentration along the crack edge, there are broad regions of stress increase off the crack plane for circular and elliptical cracks, similar to those known to exist for in-plane but not for antiplane shear cracks. Whether the stress- intensity factor at the end of one axis is greater or less than that at the end of the other axis ( ka ≶ kb ), depends on the condition: √ b/a ≶ (1 − v ) where a and b are the semi-axes of the ellipse, ka and kb are the stress-intensity factors at the end of the a- and b -axes and v is Poisson's ratio. The total stress-intensity factor varies smoothly along the edge of the ellipse from one axis to the other and it is found that this variation is rather small. 相似文献
9.
Takeshi Mikumo 《Geophysical Journal International》1981,65(1):129-153
Summary. A possible mechanism for the occurrence of slow earthquakes is investigated by calculating numerical solutions for the dynamical rupture process on a quasi-three-dimensional fault with heterogeneous frictional strengths. Experimental friction laws for the dependence of sliding frictional stress on slip velocity, which are based on the cohesive properties of fault asperities, are taken into considerations.
It is found that the applied stress does not drop very rapidly with time and the rupture velocity remarkably decreases as the dependence on slip-velocity becomes smaller. These deceleration effects for the rupture propagation are greatly enhanced with increasing heterogeneities in the distribution of frictional strength and as the initial shear stress has lower levels with respect to the average strength. For these cases, the growth of rupture is extremely slow in a nucleus region with the dimension as large as 10 times the initial rupture length, and gains a terminal velocity dependent on the above factors. The displacement-time function becomes noticeably extended in these cases, and indicates a stick—slip-like phenomena in the extended time interval for a strongly heterogeneous fault.
It seems that these results could explain the characteristic features of slow earthquakes. 相似文献
It is found that the applied stress does not drop very rapidly with time and the rupture velocity remarkably decreases as the dependence on slip-velocity becomes smaller. These deceleration effects for the rupture propagation are greatly enhanced with increasing heterogeneities in the distribution of frictional strength and as the initial shear stress has lower levels with respect to the average strength. For these cases, the growth of rupture is extremely slow in a nucleus region with the dimension as large as 10 times the initial rupture length, and gains a terminal velocity dependent on the above factors. The displacement-time function becomes noticeably extended in these cases, and indicates a stick—slip-like phenomena in the extended time interval for a strongly heterogeneous fault.
It seems that these results could explain the characteristic features of slow earthquakes. 相似文献
10.
11.
The dynamic coalescence of two mode II cracks on a planar fault is simulated here using the elastodynamic boundary integral equation method. We focus on the complexity of the resultant slip rate and seismic radiation in the crack coalescence model (CCM) and on the reconstruction of a single crack model (SCM) that can reproduce the CCM waveforms from heterogeneous source parameters rather than coalescence. Simulation results reveal that localized higher slip rates are generated by coalescence as a result of stress interaction between the approaching crack tips. The synthesized seismic radiation exhibits a distinct coalescence phase that has striking similarities to stopping phases in the radiation and propagation properties. The corresponding SCM yields a singular increase in the stress drop distribution, which is accompanied by a sudden decrease in it across the point of coalescence in the CCM. This implies that the generation of high-frequency radiation is more efficient from coalescence than from stopping, although both phenomena exhibit the same strong ω−2 -type displacement spectra. 相似文献
12.
东北地区5 000 aB.P.～4 700 aB.P.左右的降温事件及对考古文化的影响 总被引：2，自引：0，他引：2
距今5000年左右世界各地普遍存在一次幅度较大的短期降温气候事件，通过对金川剖面泥炭地层14C测年，孢粉分析，认为东北地区此次降温事件结束的14C年龄在4700aB.P.左右，降温事件出现之前气候温暖湿润，降温事件过后温度持续下降，金川剖面的记录与东北地区其它类型沉积物所记录的降温事件出现的时间基本吻合，降温事件对燕辽地区古文化演变产生了深刻的影响。 相似文献
13.
地下水降深的未确知风险分析 总被引：2，自引：0，他引：2
地下水资源系统具有未确知性特征。在定义水文地质未确知参数的基础上,运用未确知数学理论建立了地下水位下降量预测的未确知数学模型。根据该模型不仅可以得到地下水位下降量的可能取值区间,也能够得到相应区间的主观可信度分布。通过将地下水位下降量超过某一给定限值定义为未确知事件,又建立了度量这一事件发生可能性的未确知可靠性计算模型。实例研究表明,所建模型对地下水降深风险的可靠性分析具有很好的适用性,研究成果有效地弥补了以往确定性模型的不足。 相似文献
14.
以虉草幼苗为试材,通过为期50 d的室内盆栽控水试验,按照土壤水分含量设置7个水分梯度：（10±0.5）%、（15±0.5）%、（20±0.5）%、（25±0.5）%、（30±0.5）%、（40±0.5）%（土壤水分饱和）和51%（淹水2 cm）,研究了不同土壤水分含量对植物的抗氧化酶活性、丙二醛含量、可溶性蛋白含量的影响。结果表明：（1）土壤水分含量在20%~35%之间时SOD （超氧化物歧化酶）活性较高,低于20%和高于35%时,SOD活性下降,且在不同土壤水分含量下试验50 d的SOD活性均大于试验25 d的。（2） CAT（过氧化氢酶）与POD （过氧化物酶）活性达极显著相关p<0.01,均随土壤水分含量的增加呈先大幅下降后平稳再小幅上升的趋势。（3） MDA （丙二醛）含量与SOD活性趋势相反,随土壤水分含量的增加不断下降,在试验25 d时淹水组略有上升。（4）可溶性蛋白在20%~40%之间时含量较高,植株生长状况较好,在干旱和淹水胁迫条件下可溶性蛋白含量呈下降趋势。由此可见：当虉草幼苗受到干旱等胁迫时,虉草幼苗会开启抗氧化酶系统以保护植株组织,可长时间的胁迫或者胁迫超过其耐受范围（土壤水分含量小于20%或处于土壤水分过饱和淹水条件）就会不同程度的破坏植物的防御系统从而影响其生长发育。 相似文献
15.
Forty-eight samples are chosen to discuss the distribution in space and time of the cooling fluctuation events during HoloCene in the tropical zone of China in this paper. The authors consider that the Neoglaciations II and III (or Cooling Event ①) have a widespread impact on the drop in temperature of 1-2 oC or less than 2 oC. The YD Event was dated at 11,300-10,200 a BP in the tropical mainland and its dating is 11,400-10,500 a BP in the sea area with a drop in temperature of 4-6 oC. The distribution of Event B and Neoglaciation I is taking a position of north, with a drop in temperature of 2.5-3.0 oC. The Cooling Event ② shows the temporality in time. The Cooling Event ③ shows the limitation of regional distribution with a drop in temperature of less than 1.5oC. The more recent the cooling event is, the smaller the drop amplitude in temperature will be. In the eastern part of tropical zone seven events are complete in all varieties but the cooling fluctuation is weaker in the western part. In Hainan Island and South China Sea the appearance of cooling fluctuations is synchronous with each other. 相似文献
16.
Forty-eight samples are chosen to discuss the distribution in space and time of the cooling fluctuation events during Holocene
in the tropical zone of China in this paper. The authors consider that the Neoglaciations II and III (or Cooling Event) have
a widespread impact on the drop in temperature of 1–2 °C or less than 2 °C. The YD Event was dated at 11,300-10,200 a BP in
the tropical mainland and its dating is 11,400-10,500 a BP in the sea area with a drop in temperature of 4–6 °C. The distribution
of Event B and Neoglaciation I is taking a position of north, with a drop in temperature of 2.5–3.0 °C. The Cooling Event
shows the temporality in time. The Cooling Event shows the limitation of regional distribution with a drop in temperature
of less than 1.5°C. The more recent the cooling event is, the smaller the drop amplitude in temperature will be. In the eastern
part of tropical zone seven events are complete in all varieties but the cooling fluctuation is weaker in the western part.
In Hainan Island and South China Sea the appearance of cooling fluctuations is synchronous with each other. 相似文献
17.
Local scouring and morphological adjustments in steep channels with check-dam sequences 总被引：2，自引：0，他引：2
This paper describes bed profile and grain size distribution adjustments in a mountain river (Maso di Spinelle River, Italian Alps) stabilized by a sequence of boulder check-dams. The control works were originally designed to simulate the geometry of natural step-pool channels, where tumbling flow is the dominant hydraulic regime. Local scouring downstream of 29 drop structures is analysed through the use of nondimensional parameters where maximum scour depth and scour length are normalised to the drop height. Prior laboratory data reveal a pattern similar to field scours, where complex interactions occur between drop height, critical flow depth, and step spacing. The linkage between scour length and depth is also discussed. There seems to be a maximum step height for impinging jets that is approximately twice the drop height; this maximum may explain the upper limit of the steepness factor found in high-gradient step-pool streams. If such a maximum upper limit is confirmed by further studies, this may aid designs of foundation heights for transverse control works in steep channels. 相似文献
18.
Volker Janssen 《The Australian geographer》2012,43(4):445-452
Animal tagging and tracking has been a fundamental tool in the quest to increase our knowledge and understanding of biogeography and ecology for about 50 years. Monitoring animal populations is also necessary for conservation purposes and to limit negative effects on the human population, particularly in an era of human expansion into traditional animal habitats. The use of Global Navigation Satellite System (GNSS) technology has been responsible for significant advances in this field by providing the ability to obtain accurate, regular and frequent estimates of the changing distributions of many rare animal species. Employing conventional GNSS-based animal tracking methods to study drop bears is extremely difficult due to their habitat. The dense tree canopy regularly causes extended periods of complete GNSS signal loss, and sensors are often damaged during attacks on prey. This paper proposes an indirect, GNSS-based method for tracking drop bears. This involves tracking the prey rather than the predator in order to map the population of drop bears in a particular area. The method can be used to effectively estimate the number of drop bears in the study area. Analysis of the collected data provides valuable insights into the hunting behaviour of drop bears and has implications for a better understanding of the geographical distribution of other rare species, including hoop snakes and bunyips. 相似文献
19.
Summary. The mid-crustal earthquake of 1973 March 9 (mb = 5.5, h ≤ 20 km) located 60 km south-west of Sydney, Australia, provides unambiguous evidence of contemporary thrust faulting in South-eastern Australia — a region of high heat flow and Cenozoic basaltic volcanism. Aftershock locations suggest a steeply dipping fault in the depth range from 8 to 24 km with a lateral extent of about 8 km. The mechanism solution is consistent with a tectonic stress field that is dominated by east—west horizontal compression. A seismic moment of 5.7 ± 1023 ± 20 per cent dyne-cm was computed from surface-wave amplitudes. Minimum values of slip and stress drop, 2 cm and 1 bar respectively, were estimated from the moment and a fault size taken' from aftershock locations.
Refinement modelling by a controlled Monte Carlo technique was used to provide unbiased models directly from multimode group velocities. The dispersion of fundamental and higher mode surface waves recorded at the digital high-gain station at Charters Towers, Queensland, and the WWSSN station at Adelaide, South Australia, is satisfied by crust- and upper-mantle models which have neither pronounced S-wave low-velocity zones nor thick high-velocity lids within 140 km of the Earth's surface. These models have subcrustal shear velocities of 4.20–4.32 km/s which are 0.4–0.5 km/s slower than Canadian shield shear velocities (CANSD). 相似文献
Refinement modelling by a controlled Monte Carlo technique was used to provide unbiased models directly from multimode group velocities. The dispersion of fundamental and higher mode surface waves recorded at the digital high-gain station at Charters Towers, Queensland, and the WWSSN station at Adelaide, South Australia, is satisfied by crust- and upper-mantle models which have neither pronounced S-wave low-velocity zones nor thick high-velocity lids within 140 km of the Earth's surface. These models have subcrustal shear velocities of 4.20–4.32 km/s which are 0.4–0.5 km/s slower than Canadian shield shear velocities (CANSD). 相似文献
20.
山前洪积扇坡面细沟侵蚀跌坑特征的试验研究 总被引：1，自引：0，他引：1
跌坑的出现是坡面侵蚀过程中的重要现象,标志着细沟侵蚀正在发育,跌坑的贯穿标志着细沟的形成。采用土槽冲刷模型试验,选取流量、坡度、冲刷时间和坡面形态作为影响跌坑发育的因子结合正交试验调查了不同试验条件下跌坑发育特征及影响因素。结果表明：影响跌坑发育的因素主次为冲刷时间、流量、坡面形态、坡度;跌坑沿坡面连续分布,深度具有波动性,沿坡向下呈现先增大后减小的趋势,最大值一般发育在坡面中部;根据跌坑的发育特征将坡面细沟侵蚀划分为片蚀、细沟雏形、细沟发育和细沟调整四个阶段;坡面细沟侵蚀跌坑深宽积与土壤侵蚀量具有较显著线性关系。研究结果对输油管线水毁及水土流失防治具有一定的参考价值。 相似文献