Site response studies in Agartala Urban agglomeration

A systematic investigation using digital microearthquake recorders with short period SS-1 seismometers, covering 76 sites in and around Agartala city, has been carried out for site response (SR) studies in the area. In the northern part of the area, SR varies from 1.15 to 1.85 corresponding to peak frequency 0.76 to 0.93 Hz where soil is mostly semi-consolidated and stiffer than recent Quaternary deposits (Haora River formation). In the southern part of the area, SR varies from 1.12 to 2.42 corresponding to peak frequency from 0.71 to 0.85 Hz within the Dupitila formation (early Quaternary). It is observed that estimated SR from H/V increases from edges to middle of the Haora River valley and impedance contrast fallows the similar trend. This reflects that site response by H/V is influenced by impedance contrast, whereas computed amplification from 1-D model shows opposite trend. The maximum amplification at fundamental frequency of resonance, 1.04 Hz estimated from H/V by near to BH-7, is 2.5 times greater than the impedance contrast/ratio derived from 1-D model for same location. Relationship between resonance frequency and depth was obtained by applying quarter wavelength and Bard (2000) methodologies, which shows linearity, whereas H/V shows its nonlinearity characteristic in soil across the valley part of Haora River. Shear wave velocities, and subsequently, SPT index and factor of safety (by cyclic stress approach) were estimated from geotechnical parameters. Vs30 and site response data were used in this study for getting a first hand information about soil stiffness condition in the area. The estimation of SPT index and factor of safety could be a useful tool for delineating liquefied and none liquefied zones at various depth levels, especially where water table exists at a very shallow level. The expected liquefiable zone was observed at depths varying from 6–25 m beneath the soil bearing zones where percentage of fines is estimated to be more than 35% for the area. This knowledge about subsurface soil characteristics will be useful for the civil engineers/city planners, which can be taken into account at the time of constructing earthquake-resistant structures in the area.     

Forest cover: setting targets for the future

The International Year of Forests, declared by the UN, is a good occasion to discuss approaches to reducing forest degradation in developing countries. The articles collected in Thematic Forest Series form a diversity of ideas which is essential for setting the levels below which the countries' reduced emissions could be measured and credited. This editorial calls attention to the use of Land-Use/Land-Cover Change models.     

Lava tube shatter rings and their correlation with lava flux increases at Kīlauea Volcano,Hawai‘i

Shatter rings are circular to elliptical volcanic features, typically tens of meters in diameter, which form over active lava tubes. They are typified by an upraised rim of blocky rubble and a central depression. Prior to this study, shatter rings had not been observed forming, and, thus, were interpreted in many ways. This paper describes the process of formation for shatter rings observed at Kīlauea Volcano during November 2005–July 2006. During this period, tilt data, time-lapse images, and field observations showed that episodic tilt changes at the nearby Pu‘u ‘Ō‘ō cone, the shallow magmatic source reservoir, were directly related to fluctuations in the level of lava in the active lava tube, with periods of deflation at Pu‘u ‘Ō‘ō correlating with increases in the level of the lava stream surface. Increases in lava level are interpreted as increases in lava flux, and were coincident with lava breakouts from shatter rings constructed over the lava tube. The repetitive behavior of the lava flux changes, inferred from the nearly continuous tilt oscillations, suggests that shatter rings form from the repeated rise and fall of a portion of a lava tube roof. The locations of shatter rings along the active lava tube suggest that they form where there is an abrupt decrease in flow velocity through the tube, e.g., large increase in tube width, abrupt decrease in tube slope, and (or) sudden change in tube direction. To conserve volume, this necessitates an abrupt increase in lava stream depth and causes over-pressurization of the tube. More than a hundred shatter rings have been identified on volcanoes on Hawai‘i and Maui, and dozens have been reported from basaltic lava fields in Iceland, Australia, Italy, Samoa, and the mainland United States. A quick study of other basaltic lava fields worldwide, using freely available satellite imagery, suggests that they might be even more common than previously thought. If so, this confirms that episodic fluctuation in lava effusion rate is a relatively common process at basaltic volcanoes, and that the presence of shatter rings in prehistoric lava flow fields can be used as evidence that such fluctuations have occurred.     

Geometric consistency for regression model estimation and testing in climatology and meteorology

Abstract

The effects of outliers on linear regression are examined. The sensitivity of classical least‐squares (LS) procedures to outliers is shown to be associated with the geometric inconsistency between the data space and the analysis space. This is illustrated for both estimation and inference. A geometrically consistent procedure based on the Euclidean distance is proposed. This procedure involves the least absolute deviation (LAD) regression and a new permutation test for matched pairs (PTMP). Comparisons made with LS techniques demonstrate that the proposed procedure is more resistant to the existence of outliers in the data set and leads to more intuitive results. Applications and illustrations using meteorological and climatological data are also discussed.