Seismic microzonation of a region with complex surficial geology based on different site classification approaches

A seismic microzonation study was conducted to refine the seismic hazard model for the city of Saguenay, Canada. The Quaternary geology underlying Saguenay shows complex glacial and post-glacial stratigraphy with a number of buried valleys filled with fluvioglacial and glaciomarine sediments. High impedance contrast between rock formations and surficial sediments is prone to seismic amplification. To evaluate their applicability, advantages and limitations in capturing the geological specificity of the study area, four site classification methods were applied: the current National Building Code of Canada (NBCC) and Eurocode 8, both mainly based on the average shear-wave velocity for the surficial sediments (VS,avg) and for the top 30 m (VS,30); a method based on the fundamental site period (T0); and a hybrid method based on the combination of VS,30, T0 and VS,avg. The study specifically aimed to evaluate the importance of the site classification parameters on the resulting microzonation maps. VS,30 is capable to present the geological and geotechnical site conditions, however, the results may be further improved by considering Vs,avg in shallow and T0 in thick layers of soil sediments as secondary parameters. The T0 method gives also satisfactory results with T0 showing a better correlation to Vs,30 than to Vs,avg. The versatile hybrid method may be challenging to apply in certain cases with its nine different site categories and parameters.

Development Prototype of Indonesia Seismic Microzonation Information System (Inasmis)

Indonesia has a high level of seismic vulnerability because located in the junction area of four large plates, namely the Eurasian plate, Indo-Australian plate, Pacific plate, and Philippine Plate, and also the number of active faults that stretch throughout its regions. One of the efforts to mitigate earthquake disasters is to conduct seismic hazard microzonation efforts. The Research and Development Center of BMKG has carried out ongoing research in the field of seismic microzonation to obtain information on the level of seismic hazard in some regions based on microtremor parameter values, including in the Sukabumi (2009), Bantul (2010), Padang (2011), Cilacap (2012), Kulonprogo (2014), Tasikmalaya (2017), Garut (2018) and Pangandaran (2019). This study tries to prototype the seismic microzonation information system in a web-based spatial information system called InaSMIS (Indonesia Seismic Microzonation Information System). InaSIMS contains information about the analysis of microtremor survey results, including the value of the dominant period (T0), Ground Shear Strain (GSS), and USGS Vs30, which states the level of seismic hazard in an area. InaSMIS is still being developed in the research stage and is not yet an operational service for BMKG. In the future, InaSMIS expected to be a source of public information to determine seismic hazards in Indonesian regions.

MzS Tools: GIS Methods and Tools for Seismic Microzonation Mapping

MzSTools is a plugin for QGIS developed by the National Research Council (CNR) as part of the activities concerning the coordination of seismic microzonation studies in Italy. It train from the need to create a practical and easy-to-use tool to carry out seismic microzonation (SM) studies by producing standards compliant geographic database and maps, thus making them accurate, homogeneous and uniform for all municipalities in Italy. A geodatabase based on SQLite/SpatiaLite Relational Database Management System (RDBMS). It has been designed to collect and store data related to elements such as: geognostic surveys; bedrocks and cover terrains; superficial and buried geomorphological elements; tectonic-structural elements; elements of geological instability such as landslide zones, liquefaction zones and zones affected by active and capable faults; homogeneous microzones in seismic perspective, microzones characterized by a seismic amplification factor. The QGIS plugin provides tools such as data entry forms designed with Qt Designer; a QGIS project template with layers, symbol libraries and graphic styles; layouts for the SM Maps. MzSTools assembles in a single software environment a set of useful tools for those who work in. The plugin is open source, whose code hosted on the GitHub platform, and is published via the official QGIS plugins repository (https://plugins.qgis.org/plugins/MzSTools/).

Developing effective subsoil reference model for seismic microzonation studies.

A general methodological approach is here discussed to integrate geological and geophysical information in seismic microzonation studies. In particular, the methodology aims at maximizing the exploitation of low-cost data for extensive preliminary assessment of ground motion amplification phenomena induced by the local seismostratigraphical configuration. Three main steps are delineated: a) the combination of geological/geomorphological analyses to develop an Engineering-Geological Model of the study area; b) targeted geophysical prospecting to provide an Engineering-Geological/Geophysical Model; c) evaluating effectiveness of Engineering-Geological/Geophysical Model by estimating expected ground motion amplification phenomena by the use of suitable computational tools. The workflow is illustrated by a case-study based on a set of villages in the Umbro-Marchean Apennine (Central Italy) damaged during the Seismic sequence occurred in Central Italy during 2016–2017.

The Possible use of Equivalent Homogeneous Subsoil Models for 1D Seismic Response Analyses in Seismic Microzonation Studies

The possibility is here explored to use an ‘equivalent’ homogeneous configuration to simulate 1D seismic response of heterogeneous engineering-geological bodies when relatively weak seismic impedance contrasts (150 m/s) only exist above the seismic bedrock. This equivalent configuration is obtained by considering an equivalent Vs value the harmonic average of the actual Vs values and a linear combination of G/G 0 and D curves relative to the lithotechnical components present in the actual configuration. To evaluate feasibility of this approach, a wide set of numerical simulations was carried out by randomly generating subsoil layering including sequences of alternating thin layers of geotechnical units ( e.g., sands and clays) each characterized by a characteristic nonlinear curve. Outcomes of these simulations are compared with those provided by considering a single homogeneous layer characterized by equivalent nonlinear curves obtained as a weighted average of the original curves. By comparing the heterogeneous and the homogeneous columns seismic response in terms of amplification factors and fundamental period, the results confirm the possibility to model a 1D column characterized by a generic lithostratigraphic succession with an equivalent one without introducing significative errors that, at least for the studied cases, do not exceed the 6%. This conclusion is substantially confirmed by extending the comparison to a real case, i.e. the 113 m-thick heterogeneous soil profile at Mirandola site (Norther Italy), presented in the last part.

Estudio comparativo de análisis y diseño de estructuras aporticadas de hormigón armado, aplicando los espectros de la microzonificación sísmica del cantón Portoviejo y los espectros de la NEC-15

La ciudad de Portoviejo cuenta desde el 2017 con espectros de diseño específicos resultados del estudio de microzonificación sísmica del cantón. Esta investigación se origina por el escaso uso que se da a los espectros de diseño indicados en el estudio de microzonificación por parte de calculistas estructurales, quienes continúan usando los espectros que se indican en la Norma Ecuatoriana de la Construcción del 2015 para analizar y diseñar las estructuras, tanto de hormigón armado como de acero estructural. El objetivo de esta investigación es medir las variaciones de desplazamientos laterales y esfuerzos globales en estructuras aporticadas de hormigón armado mediante un análisis sísmico, empleando tanto los espectros de la norma ecuatoriana como los espectros de la microzonificación sísmica de Portoviejo. Se analizaron seis estructuras, tres de ellas regulares en planta y en elevación, las cuales eran de tres, cinco y ocho niveles de altura y otras tres estructuras irregulares de tres y dos niveles. Las estructuras fueron modeladas en Etabs, y se usó el análisis sísmico modal espectral en que se variaba el espectro de diseño. En cuanto a los espectros de la microzonificación se usaron los indicados para las microzonas M4 y M5 mientras que los espectros de la norma considerados corresponden a la zona sísmica VI en suelos tipo D y E consistentes con los tipos de suelo de las microzonas M4 y M5. Adicionalmente se evaluó la variación del área de aceros de refuerzo en elementos principales. De este esta investigación se puede concluir que: i) en las estructuras de dos, tres y cinco niveles, los desplazamientos se amplificaron al emplear los espectros de la microzonificación sísmica, la microzona 4 entrega el mayor valor. En la estructura de ocho niveles los espectros de la norma dan los mayores desplazamientos. ii) en la estructura de ocho niveles los mayores esfuerzos de corte y momento se presentaron al emplear los espectros de la norma ecuatoriana de la construcción. iii) al diseñar las estructuras los resultados no presentaron una variación significativa, se obtuvo una mínima diferencia en cuanto a los aceros de refuerzos longitudinales. iv) en la ciudad de Portoviejo, las estructuras construidas con frecuencia no superan los tres niveles y al ser diseñadas con los espectros de la norma se estaría subestimando la aceleración sísmica de diseño. Se recomienda el uso de los espectros de la microzonificación, que son resultado de un trabajo específico para las condiciones de la ciudad. Palabras claves: Microzonificación Sísmica, Espectros de Diseño, Análisis Sísmico, Diseño Sísmico. Abstract— The Portoviejo city has since 2017 with specific design spectrum results of the study of seismic microzoning in the canton. This research originates from the limited use given to the design spectrum indicated in the microzonation study by structural engineers, who continue to use the spectrum indicated in the 2015 Ecuadorian Construction Standard to analyze and design the structures, both reinforced concrete and structural steel. The objective of this investigation is to measure the variations of lateral displacements and global efforts in structures provided with reinforced concrete to be analyzed seismically using both the spectrum of the ecuadorian norms and the spectrum of the seismic microzoning of Portoviejo. Six structures were analyzed, three of them regular in plan and elevation, which were three, five and eight levels high and three other irregular structures of three and two levels. The structures were modeled in Etabs, and the spectral modal seismic analysis was used, varying the design spectrum. Regarding the microzonation spectrum, those indicated for microzones M4 and M5 were used, while the spectrum of the standard considered correspond to seismic zone VI in soils type D and E consistent with the soil types of microzones M4 and M5. Additionally, the variation of the area of reinforcement steels in main elements was evaluated. From this research it can be concluded that: i) in the structures of two, three and five levels, the displacements were amplified by using the spectrum of the seismic microzonation, the microzone 4 generated the highest value. In the eight-level structure the spectrum of the norm give the greatest displacements. ii) in the eight-level structure the greatest cutting and momentum efforts were presented when using the spectrum of the Ecuadorian construction standard. iii) when designing the structures, the results did not show a significant variation, a minimum difference was obtained regarding the longitudinal reinforcement steels. iv) in the city of Portoviejo, the structures built frequently do not exceed three levels and being designed with the spectrum of the standard would be underestimating the seismic acceleration of design. The use of microzonation spectra is recommended, which are the result of specific work for city conditions. Keywords: Seismic microzonation, Design spectrum, Seismic Analysis, Seismic Design.

The antiresonance phenomena in seismic microzonation

The interference of incident and reflected seismic waves in the ground massif near the day surface is the cause of resonant effects. In the practice of seismic microzonation (SMZ), a resonant increase in the total seismic intensity is of particular importance. At the same time, the presence of inverse layers in the ground layers interference leads to a decrease in the intensity of the total seismic impact, what is naturally named antiresonance. The article considers the conditions for the occurrence of antiresonance and evaluates the limits of its effectiveness. The natural and man-made causes of antiresonance and the possibility of using it to reduce the intensity of seismic impacts are particularly noted.