Millennium recurrence interval of morphogenic earthquakes on the Qingchuan fault,northeastern segment of the Longmen Shan Thrust Belt,China

The 2008 M _w 7.9 Wenchuan produced a ～285–300-km-long coseismic surface rupture zone, including a 60-km-long segment along the Qingchuan fault, the northeastern segment of the Longmen Shan Thrust Belt (LSTB), Sichuan Basin, central China. Field investigations, trench excavations, and radiocarbon dating results reveal that (i) the Qingchuan fault is currently active as a seismogenic fault, along which four morphogenic earthquakes including the 2008 Wenchuan earthquake occurred in the past ca. 3500 years, suggesting an average millennium recurrence interval of morphogenic earthquakes in the late Holocene; (ii) the most recent event prior to the 2008 Wenchuan earthquake took place in the period between AD 1400 and AD 1100; (iii) the penultimate paleoseismic event occurred in the period around 2000 years BP in the Han Dynasty (206 BC–AD 220); (iv) the third paleoseismic event occurred in the period between 900 and 1800 BC; and (v) at least three seismic faulting events occurred in the early Holocene. The present results are comparable with those inferred in the central and southwestern segments of the LSTB within which the Wenchuan magnitude earthquakes occurred in a millennium recurrence interval, that are in contrast with previous estimates of 2000–10,000 years for the recurrence interval of morphogenic earthquakes within the LSTB and thereby necessitating substantial modifications to existing seismic hazard models for the densely populated region at the Sichuan region.     

Determining tidal turbine farm efficiency in the Western Passage using the disc actuator theory

Tidal power potential is determined across the Western Passage in Passamaquoddy Bay using the Finite Volume Community Ocean Model (FVCOM). The tidal turbines are implemented in FVCOM using the disc actuator theory method to determine the power potential for different densities and arrangements of tidal turbines. At the most efficient setting for 10 turbines across the Western Passage, the optimal turbine drag coefficient is 2.0 and the average power output, in a 2-week period, is ～819 kW. Results suggest that for a single row of turbines, the addition of turbines decreases the efficiency of the turbine farm, but this decrease in efficiency is less than 7 %. A parallel distribution of turbines in an array diminishes the average power for turbines in the shadow of other turbines, while staggered distribution in an array increases the average power extraction for some turbines, due to the speed gains in the gaps between turbines. A simple tidal farm optimization using the OpenTidalFarm (OTF) model suggests a similar tidal farm distribution.     

2007年9—11月T213与ECMWF及日本模式中期预报性能检验

2007年9-11月冷空气频繁,气温变幅大,热带气旋活跃,给中期天气预报增加了一定的难度.为积累预报经验,提高中期预报准确率,对9-11月T213模式96小时预报产品进行了天气学检验,并与ECMWF、日本模式进行对比分析.结果表明,T213、ECMWF和日本模式对亚洲中高纬大气环流调整具有较好的中期预报性能,对转折性、灾害性天气的预报有较强指示意义.综合来看,ECMWF对天气系统和要素的预报误差最小,T213模式误差最大,日本模式温度预报性能与ECMWF相当.对0713号台风登陆前的位置和移速,ECMWF预报较为准确,T213模式对台风登陆前移速预报偏慢,对台风登陆后的位置预报偏差最小,日本模式对台风位置和强度预报较为失败.     

Effect of Salinity and Currents on Sea Level Variation Along the Indian Coast: A Numerical Study

The process of upwelling/sinking and associated sea level variations are seen as a response of coastal ocean to pure wind stress forcing. Further,precipitation and monsoonal floods, apart from the marine meteorological parameters, are expected to influence the sea level fluctuations along the coast. This study comprises determining the sea level from the various parameters together with the pure wind stress forcing, which is compared with the observed cycle. However, it is found that there is considerable difference between the computations and observations. This suggests that the sea level is dependent not just on the local forcing alone, but also on the induced background circulation as well. For example, the sea level changes along the east coast of India, particularly the northern region, are more sensitive to freshwater discharge from various rivers joining the Bay of Bengal. This is due to more frequently occurring pre- and postmonsoon cyclonic storms and the associated surges in the Bay of Bengal as compared to the Arabian Sea. Hence the salinity effects are particularly important in the coastal waters off the east coast of India during monsoon months (June-September). For the west coast of India, however, it is expected that the large-scale coastal circulation may play a role in determining sea level changes in addition to other forcings. The salinity effects are negligible along the west coast in the absence of any major river systems that join the Arabian Sea. The local advection currents caused by the offshore directed freshwater discharge from various estuaries joining the coastal bay also seemed to influence the sea level. In order to elucidate the essential dynamics involved and to study the effect of the remote forcing, a three-dimensional baroclinic, nonlinear numerical model is used with appropriate open boundary conditions. The local effect of the current has been incorporated in the west coast model by means of opening a channel at Cochin through which the rainwater is carried away to the model ocean. The low saline plume, cascading from north along the east cost of India, has been incorporated in the east coast model through a proper forcing applied at the northern boundary of the model. With the inclusion of these remote forcings in the models, the disagreement between the simulations and the observations is minimized.     

Behavior of a helical anchor under vertical repetitive loading

Abstract

In the field of ocean engineering, anchors are used for several purposes. This article studies the behavior of a helical anchor embedded in soft marine clay under vertical repetitive loading. Helical anchors are simple steel shafts to which one or more helical plates are attached at regular intervals. The tests are conducted on a model helical anchor installed in a soft marine clay bed prepared in a test tank. Repetitive loading is applied using a pneumatic loading arrangement. Different cyclic load ratios and time periods are adopted. In each test, after the application of repetitive loading, poststatic‐pullout tests are conducted to observe the effect of repetitive loading on anchor behavior. From the test results, it is found that, up to a cyclic load ratio of 55%, there is no reduction in capacity. Instead, there seems to be a marginal increase in capacity and reduction in displacement. The reasons for this behavior are explained in terms of induced changes in strength and deformation behavior of marine clay under repetitive load. However, at higher cyclic load ratios, there seems to be reduction in pullout capacity of the anchor, and the reason for this is explained in terms of strain criteria. From this investigation, it can be concluded that the deep anchor is more suitable to a marine environment than a shallow anchor.     

Uplifted crust in parts of western India

During northward movement of the Indian sub-continent, after its breakup from the Gondwanaland in the Late Cretaceous, the western part of India traversed over the Reunion plume. The Saurashtra peninsula and the Cambay Basin are two important geological regions in this part. Two and half dimensional density models, based on the crustal seismic structure, were generated to establish a relationship between these two regions. These models indicate that the crust is 32–33 km thick in the eastern Saurashtra and the northern part of the Cambay Basin. The shallower crust is in a triangular region formed by the extension of the western limb of the Proterozoic Aravalli trend in Saurashtra, its eastern limb and the Narmada fault in the south. Compared to 36–37 km thick crust to the west and 38–40 km to the east of this region the crust in the above triangular region is uplifted by 4 to 6 km. This uplift took place either after the deposition of Mesozoic sediments or was concomitant with the rise of Reunion plume prior to the extrusion of the Deccan volcanics as the region was close to the axis of the plume.