Triggering and synchronization of stick slip: Waiting times and frequency-energy distribution

Triggering and synchronization are encountered in many geophysical phenomena, including geodynamics. Both these effects are generated by the action of additional forcing, which is much smaller than the main driving force. That means that triggering and synchronization are connected with nonlinear interactions of objects, in this case with initiation of instability in systems that are close to the critical state. In seismic process the main component is the tectonic stress and the additional forcing is exerted by various external impacts like tides, reservoir exploitation, big explosions, magnetic storms, etc.In the paper, the results of laboratory and field experiments on the electromagnetic (EM) initiation and synchronization of mechanical instability (slip) are presented. Slip events were recorded as acoustic emission bursts. In the first series of experiments strong EM pulses were applied to the mechanical system driven close to the critical state, namely, to the (dry) rock samples placed on an inclined supporting sample at the slope angle less than, but close to the critical slip angle. It has been found that EM impact initiates slip with probability P ≈ 0.07 at the voltage ΔV = 1.3 kV and with probability P ≈ 0.2 at ΔV ≈ 10 kV if the EM field is applied parallel to the slip surface (first mode). On the other hand, the application of EM pulse hampers the slip considerably if the EM field is directed perpendicularly to the slip surface (second mode): the slip was not observed even at the angle that was larger than the critical one.In the second series of experiments the periodic EM and mechanical forcing were applied to the standard slider-spring system. It was discovered that periodic EM force of frequency f superimposed on the constant driving force excites periodic microslips of rock samples with double frequency 2f. Combined impact of periodic and constant voltages invokes transition from double frequency synchronization to 1:1 synchronization if the direct component of voltage is larger than the periodic one. Synchronization affects not only waiting times, but also frequency-energy distribution: i. the energy of bursts emitted in synchronized mode have much less scatter than in the absence of the periodic forcing, ii. the sudden decrease of synchronizing forcing is followed by acoustic burst of much larger energy than during forcing.The elementary theory of EM triggering and synchronization is given: the effects are explained by the action of EM ponderomotive (electrostriction) forces, which modify Coulomb stress similar to the well known pore pressure model. The formalism of transition from 1:2 to 1:1 synchronization is considered.     

Love函数与地球内部的潮汐形变

把地球表面潮汐形变问题扩展到地球内部.定义了Love函数和几种潮汐形变因子,并对两个实际地球模型（1066A和PREM）进行数值计算和讨论,以了解地球内部的潮汐形变特征,专门讨论了Love函数导数以及应力固体潮张量的计算问题.本工作对了解全地球潮汐场以及潮汐触发地震等问题将有所帮助.     

1996年6月1日甘肃省天祝-古浪5.4级地震的发震构造探讨

根据震区深浅地质构造特征及现场考察结果，１９９６年６月１日天祝古浪５．４级地震极震区位于古浪断裂与武威天祝断裂的交汇部位，该部位是应力积累和释放的有利场所，本次地震即为两条断裂内次级断裂共同作用的结果     

A Review of the Study of Loess Slump

There has widely developed a specific kind of landslide-loess slump in the north of loess plateau in China, which is different from loess slide or loess fall because of the high sandy ingredient in loess. Due to the weak cement of sandy loess, the slump shows some other features than “typical” loess landslide. In distribution scopes, it often occurres in sandy loess belt, beyond that there are clay loess belt and silty clay loess belt. Due to the extreme shortage of land resources, the local people often choose the concave banks of valley as the cave dwellings sites whereas the convex banks are kept to develop agriculture. Generally, the concave banks have already been eroded much by rivers which makes the level stress of the slope release. However, the excavation for constructions reduces the level stress furthermore. Therefore, it causes the shear stress increase at the feet of the slope. The equilibrium state of the slope is destroyed, and then the failure occurres. The triggering factors of loess slump include rainfall, freeze thawing, freeze swelling, excavation, etc. In fact, these factors often comprehensively affect the slumps. In terms of their deformation and failure mechanism, scientists put forward different views, which include falling firstly and sliding secondly, sliding firstly and fall secondly, from middle parts rupture extending to top and bottom, and integrated sliding and falling. The nearest research implies that the failure mechanism be the bottom shearing failure and then the top tension failure. Referring to the failure mechanism at a micro level, few researches have been done to this kind of landslide. Future studies should investigate the triggering factors widely and deeply, summarize the characteristics of the deformation and failure, apply the numerical and physical simulation methods comprehensively, build up the geological-mechanic model, analyze and clarify the deformation and failure mechanisms quantitatively on the basis of strain and stress relationship, and study the failure mechanisms on a micro level.     

Electrical triggering of earthquakes: results of laboratory experiments at spring-block models

Recently published results of field and laboratory experiments on the seismic/acoustic response to injection of direct current(DC) pulses into the Earth crust or stressed rock samples raised a question on a possibility of electrical earthquake triggering. A physical mechanism of the considered phenomenon is not clear yet in view of the very low current density(10~(-7)–10~(-8) A/m~2) generated by the pulsed power systems at the epicenter depth(5–10 km) of local earthquakes occurred just after the current injection. The paper describes results of laboratory‘‘earthquake' triggering by DC pulses under conditions of a spring-block model simulated the seismogenic fault. It is experimentally shown that the electric triggering of the laboratory ‘‘earthquake'(sharp slip of a movable block of the spring-block system) is possible only within a range of subcritical state of the system, when the shear stress between the movable and fixed blocks obtains 0.98–0.99 of its critical value. The threshold of electric triggering action is about 20 A/m~2 that is 7–8 orders of magnitude higher than estimated electric current density for Bishkek test site(Northern Tien Shan, Kirghizia) where the seismic response to the man-made electric action was observed. In this connection, the electric triggering phenomena may be explained by contraction of electric current in the narrow conductive areas of the faults and the corresponding increase in current density or by involving the secondary triggering mechanisms like electromagnetic stimulation of conductive fluid migration into the fault area resulted in decrease in the fault strength properties.     

北部湾6．1、6．2级地震发震构造探讨

北部湾６．１、６．２级地震震源位于北部湾盆地的东部，构造上处于盆地内部ＮＥＥ向的次一级企西凸起与乌石断陷的交界地带，发震断裂主破裂是ＮＥＥ向的北部湾－珠江口外断裂，ＮＷ向的临高断裂可能也参加了活动。     

长江三峡巴东县城区三道沟滑坡成因研究

三道沟滑坡位于巴东县新城区大型古滑坡前缘临江部位。自1995年6月出现裂缝, 经过四个多月的逐步发展, 至10月29日发生滑动破坏, 体积约20万m3.变形滑动的主因是老滑坡体前缘地形较陡, 土体疏松强度低; 对岸渣角碛洪积扇窄束河道, 使南岸遭受冲蚀。诱因是降雨的影响、江水涨落的影响、新城建设人工弃土及动荷载效应的影响等, 使斜坡负担过重, 水文地质条件发生了变化, 土体力学强度进一步降低, 促使临界平衡状态的斜坡发生了滑动破坏。     

Study on the Triggering Effect of the October, 2005 Pakistan Mw7.6 Earthquake on Its Aftershocks

The influences upon aftershocks of Coulomb failure stress change (CFSC) generated by the main-shock of the October 8, 2005, Pakistan earthquake are calculated and analyzed. The following factors are included in the calculation: (1) the difference between the pore fluid pressure and the medium elastic constant in the fault plane area and those of its surrounding medium; (2) the tectonic stress direction of the seismic source area; (3) the aftershock failure mechanism of aftershocks is calculated by stacking the tectonic stress with the stress change generated by the main-shock. Our study, which includes many factors, fits fairly well with the aftershock distribution. It indicates that most of the aftershocks were triggered by the Pakistan main-shock that occurred on October 8, 2005.     

The influence of the geological and geomorphological settings on shallow landslides. An example in a temperate climate environment: the June 19, 1996 event in northwestern Tuscany (Italy)

On June 19, 1996, an extremely heavy rainstorm hit a restricted area in the Apuan Alps (northwestern Tuscany, Italy). Its max intensity concentrated over an area of about 150 km² astride the Apuan chain, where 474 mm was recorded in about 12 h (21% of the mean annual precipitation, with an intensity up to 158 mm/h). The storm caused floods and hundreds of landslides and debris flows, which produced huge damage (hundreds of millions of Euros), partially destroyed villages and killed 14 people. This paper reports the results obtained from a detailed field survey and aerial view interpretation. In the most severely involved area, 647 main landslides were investigated, mapped and related to the geologic, geomorphic and vegetational factors of the source areas. This was in order to define the influence of these factors and contribute to an evaluation of the landslide hazard in the study area. An assessment was also made of the total area and volume of material mobilised by landsliding. The study area, about 46 km² wide, includes three typically mountainous basins, characterised by narrow, deep cut valleys and steep slopes, where many rock types outcrop. Most of the landslides were shallow and linear, referable to complex, earth and debris translational slide, which quickly developed into flow (soil slip–debris flow). Usually, they involved colluvium and started in hollows underlain by metamorphic rock (metasandstone and phyllite), often dipping downslope. Therefore, bedrock lithology and impermeability appeared to be important factors in the localisation of the landslide phenomena. The investigation of the geomorphic and land use features in the source areas also frequently highlighted a rectilinear profile of the slope, a high slope gradient (31–45°) and dense chestnut wood cover. In the area, about 985,000 m² (2.1% of 46 km²) was affected by landsliding and about 700,000 m² of this area was covered by chestnut forest. The landslides removed about 7000 trees. The volume of mobilised material was about 1,360,000 m³; about 220,000 m³ remained on the slopes, while the rest poured into the streams. In addition, about 945,000 m³ was mobilised by the torrential erosion in the riverbeds.