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It is generally accepted that crustal earthquakes are caused by sudden displacement along faults, which rely on two primary conditions. One is that the fault has a high degree of synergism, so that once the stress threshold is reached, fault segments can be connected rapidly to facilitate fast slip of longer fault sections. The other is sufficient strain accumulated at some portions of the fault which can overcome resistance to slip of the highstrength portions of the fault. Investigations to such processes would help explore how to detect shortterm and impending precursors prior to earthquakes. A simulation study on instability of a straight fault is conducted in the laboratory. From curves of stress variations, the stress state of the specimen is recognized and the metainstability stage is identified. By comparison of the observational information from the press machine and physical parameters of the fields on the sample, this work reveals differences of temporalspatial evolution processes of fault stress in the stages of stress deviating from linearity and metainstability. The results show that due to in teraction between distinct portions of the fault, their independent activities turn gradually into a synergetic activity, and the degree of such synergism is an indicator for the stress state of the fault. This synergetic process of fault activity includes three stages: generation, expansion and increase amount of strain release patches, and connection between them.. The first stage begins when the stress curve deviates from linearity, different strain variations occur at every portions of the fault, resulting in isolated areas of stress release and strain accumulation. The second stage is associated with quasistatic instability of the early metainstability when isolated strain release areas of the fault increase and stable expansion proceeds. And the third stage corresponds to the late metainstability, i.e. quasidynamic instability as both the expansion of strain release areas and rise of strain level of strain accumulation areas are accelerated. The synergism is accelerated when the quasistatic expansion transforms into quasidynamic expansion, with interaction between fault segments as its mechanism. The essence of such transformation is that the expansion mechanism has changed, i.e. expansion of isolated fault segments is replaced by linkage of the interacting segments when the fault enters the critical state of a potential earthquake. Based on the experimental re sults, coupled with data on the temporalspatial evolution of earthquakes along the LaohushanMaomaoshan fault, west of the Haiyuan fault zone in northwestern China, the synergism process of this fault before the 6 June 2000 M6.2 earthquake is analyzed. GEODYNAMICS & TECTONOPHYSICS P U B L I S H E D B Y T H E I N S T I T U T E O F T H E E A R T H ' S C R U S T S I B E R I A N B R A N C H O F R U S S I A N A C A D E M Y O F S C I E N C E S T e c t o n o p h y s i c s Ma Jin et al.: Accelerated synergism along a fault… 388Аннотация: Обычно...

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The Luoshan Fault located at the northeastern margin of Tibet plateau strikes roughly N-S, and is composed of six left-stepping sections with a total length of 60 km. Much evidence suggests that the Luoshan Fault is a reverse right-lateral strike-slip fault. The largest right-lateral strike-slip displacement and the most abundant dextral offset phenomena are located along the central section. Based on the right-lateral strike-slip offsets of the oldest alluvial fan, and of a gully and on the average displacement of the same order of gullies, the minimum slip-rate has been 2.15 ± 0.2 mm/yr since Late Pleistocene. Many surface rupture phenomena, such as fault scarps with fresh free-face, ground fissures, displacements of very young gullies, imply that a recent earthquake occurred along this fault. Combining the historical catalogue and our results, we believe that the 1561 A.D. earthquake was produced by the Luoshan Fault. Three paleoearthquakes were determined by means of paleoseismic studies along the Luoshan Fault: they occurred after 8200 ± 600 years BP, between 3130 ± 240 years BP and 4150 ± ± 120 years C.BP, and before 2230 ± 170 years BP, respectively.

Oil production wells on gas lift are sometimes unstable at low gas lift rates, even though steady-state flow analysis gives most efficient production at these gas lift rates. Unstable production, often called heading, may lead to periods of reduced or even no liquid production followed by large peaks of liquid and gas. This result in average oil production less than expected, and oil and gas production less than the systems design capacity to allow for the production peaks without causing shutdowns. To solve the problem the amount of lift gas is normally increased beyond the most optimum rate. When the lift gas supply is limited, other gas lifted wells must then be shut in. This paper describes field proven sequence based automation for oil production, plus a new model-based automatic controller. Both technologies solve the problem of unstable production from gas lifted wells through manipulation of the production and/or the gas-injection chokes. The sequence-based automation and the model-based controller stabilize the production at operating points that would be unstable under standard operation. Field measurements such as wellhead pressure, annulus pressure etc. are inputs to the controllers (or are used by the controller). Examples are given of the modelbased controller working together with realistic transient flow models of gas lift wells. Testing of the model-based controller on real unstable wells is planned.