Sediments of Lake Vens (SW European Alps, France) record large-magnitude earthquake events

We studied sediment cores from Lake Vens (2,327 m asl), in the Tinée Valley of the SW Alps, to test the paleoseismic archive potential of the lake sediments in this particularly earthquake-sensitive area. The historical earthquake catalogue shows that moderate to strong earthquakes, with intensities of IX–X, have impacted the Southern Alps during the last millennium. Sedimentological (X-ray images, grain size distribution) and geochemical (major elements and organic matter) analyses show that Lake Vens sediments consist of a terrigenous, silty material (minerals and organic matter) sourced from the watershed and diatom frustules. A combination of X-ray images, grain-size distribution, major elements and magnetic properties shows the presence of six homogenite-type deposits interbedded in the sedimentary background. These sedimentological features are ascribed to sediment reworking and grain sorting caused by earthquake-generated seiches. The presence of microfaults that cross-cut the sediment supports the hypothesis of seismic deposits in this system. A preliminary sediment chronology is provided by 210Pb measurement and AMS 14C ages. According to the chronology, the most recent homogenite events are attributable to damaging historic earthquakes in AD 1887 (Ligure) and 1564 (Roquebillière). Hence, the Lake Vens sediment recorded large-magnitude earthquakes in the region and permits a preliminary estimate of recurrence time for such events of ~400 years.

Influence of local geology on earthquake ground motion

As a simplified approach for estimating theoretically the influence of local subsoils upon the ground motion during an earthquake, the problem of an idealized layered system subjected to vertically incident plane body waves was studied. Both the technique of steady-state analysis and the technique of transient analysis have been used to analyze the problem.

In the steady-state analysis, a recursion formula has been derived for obtaining the response of a layered system to sinusoidally steady-state input. Several conclusions are drawn concerning the nature of the amplification spectrum of a nonviscous layered system having its layer stiffnesses increasing with depth. Numerical examples are given to demonstrate the effect of layer parameters on the amplification spectrum of a layered system.

In the transient analysis, two modified shear beam models have been established for obtaining approximately the response of a layered system to earthquake-like excitation. The method of continuous modal analysis was adopted for approximate analysis of the models, with energy dissipation in the layers, if any, taken into account. Numerical examples are given to demonstrate the accuracy of the models and the effect of a layered system in modifying the input motion.

Conditions are established, under which the theory is applicable to predict the influence of local subsoils on the ground motion during an earthquake. To demonstrate the applicability of the models to actual cases, three examples of actually recorded earthquake events are examined. It is concluded that significant modification of the incoming seismic waves, as predicted by the theory, is likely to occur in well defined soft subsoils during an earthquake, provided that certain conditions concerning the nature of the incoming seismic waves are satisfied.

Everyday preschool talk about Christchurch earthquakes

This article investigates young children’s interactions with their peers and teachers following the events of the Christchurch earthquakes in New Zealand on September 2010 and February 2011. Drawing on conversation analysis and psychological literature, we focus on one outdoor excursion to visit a broken water pipe caused by the earthquake to show how the teacher and children mutually accomplished trouble telling and storying. A particular feature of talk was the use of pivotal utterances to transition from talking about the damaged environment, to talking about reflections of actual earthquake events. This article shows how teachers initiate and prompt children’s informal and spontaneous story telling as an interactional resource for discussing traumatic events.

Seismic Hazard and Microzonation Studies using Geographic Information System - A Case Study of Bangalore city

Seismic hazard and microzonation of cities enable to characterize the potential seismic areas that need to be taken into account when designing new structures or retrofitting the existing ones. Study of seismic hazard and preparation of geotechnical microzonation maps has been attempted using Geographical Information System (GIS). GIS will provide an effective solution for integrating different layers of information thus providing a useful input for city planning and in particular input to earthquake resistant design of structures in an area. Seismic hazard is the study of expected earthquake ground motions at any point on the earth. Microzonation is the process of sub division of region in to number of zones based on the earthquake effects in the local scale. Seismic microzonation is the process of estimating response of soil layers under earthquake excitation and thus the variation of ground motion characteristic on the ground surface. For the seismic microzonation, geotechnical site characterization need to be assessed at local scale (micro level), which is further used to assess of the site response and liquefaction susceptibility of the sites. Seismotectonic atlas of the area having a radius of 350km around Bangalore has been prepared with all the seismogenic sources and historic earthquake events (a catalogue of about 1400 events since 1906). We have attempted to carryout the site characterization of Bangalore by collating conventional geotechnical boreholes data (about 900 borehole data with depth) and integrated in GIS. 3-D subsurface model of Bangalore prepared using GIS is shown in Figure 1.Further, Shear wave velocity survey based on geophysical method at about 60 locations in the city has been carried out in 220 square Kms area. Site response and local site effects have been evaluated using 1-dimensional ground response analysis. Spatial variability of soil overburden depths, ground surface Peak Ground Acceleration’s(PGA), spectral acceleration for different frequencies, liquefaction susceptibility have been mapped in the 220 sq km area using GIS.ArcInfo software has been used for this purpose. These maps can be used for the city planning and risk & vulnerability studies. Figure 2 shows a map of peak ground acceleration at rock level for Bangalore city. Microtremor experiments were jointly carried out with NGRI scientists at about 55 locations in the city and the predominant frequency of the overburden soil columns were evaluated.

A study on seismicity and seismic hazard for Karnataka State

This paper presents a detailed study on the seismic pattern of the state of Karnataka and also quantifies the seismic hazard for the entire state. In the present work, historical and instrumental seismicity data for Karnataka (within 300 km from Karnataka political boundary) were compiled and hazard analysis was done based on this data. Geographically, Karnataka forms a part of peninsular India which is tectonically identified as an intraplate region of Indian plate. Due to the convergent movement of the Indian plate with the Eurasian plate, movements are occurring along major intraplate faults resulting in seismic activity of the region and hence the hazard assessment of this region is very important. Apart from referring to seismotectonic atlas for identifying faults and fractures, major lineaments in the study area were also mapped using satellite data. The earthquake events reported by various national and international agencies were collected until 2009. Declustering of earthquake events was done to remove foreshocks and aftershocks. Seismic hazard analysis was done for the state of Karnataka using both deterministic and probabilistic approaches incorporating logic tree methodology. The peak ground acceleration (PGA) at rock level was evaluated for the entire state considering a grid size of 0.05A degrees x 0.05A degrees. The attenuation relations proposed for stable continental shield region were used in evaluating the seismic hazard with appropriate weightage factors. Response spectra at rock level for important Tier II cities and Bangalore were evaluated. The contour maps showing the spatial variation of PGA values at bedrock are presented in this work.

Constraining large earthquakes along the Andaman trench using deepwater turbidites: prospects and challenges

The 2004 Sumatra-Andaman earthquake was unprecedented in terms of its magnitude (M-w 9.2), rupture length along the plate boundary (1300 km) and size of the resultant tsunami. Since 2004, efforts are being made to improve the understanding of the seismic hazard in the Sumatra-Andaman subduction zone in terms of recurrence patterns of major earthquakes and tsunamis. It is reasonable to assume that previous earthquake events in the Myanmar Andaman segment must be preserved in the geological record in the form of seismo-turbidite sequences. Here we present the prospects of conducting deep ocean palaeoseismicity investigations in order to refine the quantification of the recurrence pattern of large subduction-zone earthquakes along the Andaman-Myanmar arc. Our participation in the Sagar Kanya cruise SK-273 (in June 2010) was to test the efficacy of such a survey. The primary mission of the cruise, along a short length (300 km) of the Sumatra Andaman subduction front was to collect bathymetric data of the ocean floor trenchward of the Andaman Islands. The agenda of our piggyback survey was to fix potential coring sites that might preserve seismo-turbidite deposits. In this article we present the possibilities and challenges of such an exercise and our first-hand experience of such a preliminary survey. This account will help future researchers with similar scientific objectives who would want to survey the deep ocean archives of this region for evidence of extreme events like major earthquakes.

Recent Seismicity in India and Adjoining Regions

In the present work, historical and instrumental seismicity data of India and its adjoining areas (within 300km from Indian political boundary) are compiled to form the earthquake catalog for the country covering the period from 1505 to 2009. The initial catalogue consisted of about 139563 earthquake events and after declustering,the total number of events obtained was 61315. Region specific earthquake magnitude scaling relations correlating different magnitude scales were achieved and a homogenous earthquake catalogue in moment magnitude (MW) scale was developed for the region. This paper also presents the results of the use of Geographic Information Systems (GIS) to prepare a digitized seismic source map of India. The latest earthquake data were superimposed on the digitized source map to get a final Seismotectonic map of India. The study area has been divided into 1225 grid points (approximately 110km×110km) and the seismicity analysis has been done to get the spatial variation of seismicity parameters ‘a’ and ‘b’ across the country. The homogenized earthquake catalogue with the event details is listed in the website http://civil.iisc.ernet.in/~sreevals/resource.htm

Ground improvement methods for liquefaction remediation

Soil liquefaction continues to be a major source of damage to buildings and infrastructure after major earthquake events. Ground improvement methods are widely used at many sites worldwide as a way of mitigating liquefaction damage. The relative success of these ground improvement methods in preventing damage after a liquefaction event and the mechanisms by which they can mitigate liquefaction continue to be areas of active research. In this paper the emphasis is on the use of dynamic centrifuge modelling as a tool to investigate the effectiveness of ground improvement methods in mitigating liquefaction risk. Three different ground improvement methods will be considered. First, the effectiveness of in situ densification as a liquefaction resistance measure will be investigated. It will be shown that the mechanism by which soil densification offers mitigation of the liquefaction risk can be studied at a fundamental level using dynamic centrifuge modelling. Second, the use of drains to relieve excess pore pressures generated during an earthquake event will be considered. It will be shown that current design methods can be further improved by incorporating the understanding obtained from dynamic centrifuge tests. Finally, the use of soil grouting to mitigate liquefaction risk will be investigated. It will be shown that by grouting the foundation soil, the settlement of a building can be reduced following earthquake loading. However, the grouting depth must extend the whole depth of the liquefiable layer to achieve this reduction in settlements.

Measuring shaft friction during earthquakes

The failure of piled foundations has been observed in many earthquake events. The manner in which a pile is able to support its applied superstructure loading during an earthquake is not yet fully understood, particularly with respect to the shaft friction capacity. In this paper, new pile group is presented which has been instrumented to measure the shaft friction distribution along the length of a pile. In addition, this pile group is able to measure the pore pressures directly beneath the pile tips. The pile group was tested in dynamic centrifuge experiments and showed differing shaft friction behaviour in dense and loose soil layers as well as strong dilation beneath the pile tips at the start of earthquake loading. A reduction in shaft friction was observed after the earthquake due to soil down-drag. © 2010 Taylor & Francis Group, London.

Study on the forecast effects of PI method to the north and southwest China

In this paper, the codes of Pattern Informatics (PI) method put forward by Rundle et al. have been worked out according to their algorithm published, and the retrospective forecast of PI method to North China (28.0 degrees-42.0 degrees N, 108.0 degrees-125.0 degrees E) and to Southwest China (22.0 degrees-28.3 degrees N, 98.0 degrees-106.0 degrees E) has been tested. The results show that the hit rates in different regions show a great difference. In Southwest China, 32 earthquakes with M(L)5.0 or larger have occurred during the predicted time period 2000-2007, and 26 out of the 32 earthquakes occurred in or near the hot spots. In North China, the total number of M(L)5.0 or larger was 12 during the predicted time period 2000-2007, and only 3 out of the 12 earthquakes occurred in or near the hot spots. From our results, we hold that if the PI method could be applied to all kinds of regions, the parameters associated with time points and time windows should be chosen carefully to obtain the higher hit rate. We also found that the aftershocks in a strong earthquake sequence affect the PI results obviously. Copyright (c) 2009 John Wiley & Sons, Ltd.

Stable oxygen isotopes of calcite veins in ODP Site 146-892

In situ secondary ionization mass spectrometry (SIMS) analyses of oxygen isotopes in authigenic calcite veins were obtained from an active thrust fault system drilled at Ocean Drilling Program (ODP) Site 892 (44°40.4'N, 125°07.1'W) along the Cascadia subduction margin. The average d18OPDB value of all samples is -9.9 per mil and the values are the lowest of any measured in active accretionary prisms. Ranges in individual veins can be as much as 19.6 per mil. There is an isotopic stratigraphy related to the structural stratigraphy. Mean isotope values in the hanging wall, thrust, and footwall are -14.4 per mil, -9.5 per mil, and -5.2 per mil, respectively. Several veins and crosscutting vein sequences show a general trend from lower to higher d18O values over time. Isotopic and textural data indicate several veins formed by a crack-seal mechanism and growth into open fractures. The best explanation for the strong 18O depletions is periodic rapid flow from 2-3 km deeper in the prism. Relatively narrow isotopic ranges for most veins suggest that fluids were derived from a similar source depth for each episode of fluid pulse and calcite crystallization. Structural and mass balance considerations are consistent with a record preserved in the veins of ten to hundreds of thousands of years. The fluid pulses may relate to periodic large earthquake events such as those recognized in the paleoseismicity records from the Cascadia margin.