The Baranja earthquakes of 1922 and 1924

Influence of the lateral restoring force of isolation system to the seismic performance of isolated buildings in low-to-moderate seismicity regions

현행 약진지역의 내진설계기준은 주로 강진지역에서의 연구결과에 근거하고 있다. 하지만, 약진지역의 경우 지진하중보다는 중력하중이나 풍하중에 의해 구조설계가 지배되므로 구조물의 초과강도가 강진지역의 경우보다 증가하게 된다. 따라서 약진지역에 적합한 내진설계기준을 마련하기 위해서는 강진지역에 적용되는 반응수정계수를 약진지역에 그대로 적용할 수 있는지에 대한 검증이 필요하다. 본 연구에서는 건축구조물에 대한 소성해석을 통해 그 연성도와 초과강도를 산정하고 이에 근거하여 현행 반응수정계수의 적절성 여부를 검토하였다. 강진, 중진, 약진지역 등에서의 초과강도와 연성요구도를 비교하기 위하여 UBC-97에 근거하여 설계된 예제구조물을 선정하여 해석을 수행하였다. 해석결과에 의하면 약진지역의 초과강도가 강진지역보다 크기 때문에 동일한 반응수정계수에 대한 약진지역의 연성요구도는 강진지역에서보다 적게 된다. 따라서 동일한 반응수정계수를 이용하여 설계된 약진지역 구조물의 경우 접합부에서의 소성회전각 요구량을 강진지역의 경우에 비하여 상대적으로 저감시킬 수 있을 것이다. Seismic design codes are mainly based on the research results for the inelastic response of structures in high seismicity regions. Since wind loads and gravity loads may govern the design in low seismicity regions in many cases, structures subjected to design seismic loads will have larger overstrength compared to those of high seismicity regions. Therefore, it is necessary to verify if the response modification factor based on high seismicity would be adequate for the design of structures in low seismicity regions. In this study, the adequacy of the response modification factor was verified based on the ductility and overstrength of building structures estimated from the result of nonlinear static analysis. Framed structures are designed for the seismic zones 1, 2A, 4 in UBC-97 representing the low, moderated and high seismicity regions and the overstrength factors and ductility demands of the example structures are investigated. When the same response modification factor was used in the design, inelastic response of structures in low seismicity regions turned out to be much smaller than that in high seismicity regions because of the larger overstrength of structures in low seismicity regions. Demands of plastic rotation in connections and ductility in members were much lower in the low seismicity regions compared to those of the high seismicity regions when the structures are designed with the same response modification factor.

Reinforced concrete structural slurry walls have been used in the United States since the early 1960s. The typical practice, and one that makes the economics of slurry walls particularly attractive, is to design the walls to act as both temporary excavation support and permanent basement walls. They often serve as multi-story basements and below grade parking for buildings, for tunnels, subway stations, and other buried structures. One of the early applications was for a foundation for a subway station in San Francisco, but for the most part they have been used more extensively in regions of low seismicity. The purpose of this report is to investigate the requirements for extension of this practice to more common use in regions of high seismicity. Structural slurry walls are concrete walls constructed below the ground surface. In slurry wall construction, a trench is excavated using a rectangular clamshell bucket or other specialized equipment. During excavation, the trench is held open by introduction of a bentonite or polymer slurry. Steel reinforcement, if required, is lowered into the slurry-filled trench, and concrete is subsequently deposited by tremie, displacing the slurry. The length of trench open at any one time is limited to a typical maximum of about 20 to 24 feet by excavation stability and concrete placement volume considerations. Each individual concrete placement is referred to as a “panel,” and vertical construction joints separate the panels. Temporary “end-stops” are used as formwork to control the geometry of the panel joints, and horizontal reinforcement is discontinuous at the joints. Structural slurry panels range from 1.5 to 5.0 feet thick, 7 to 24 feet long, and up to 300 feet deep. In the United States, panels that are 2.0 to 3.5 feet thick and depths of 40 to 150 feet are commonplace. Structural basement walls support earth pressures acting laterally against the wall, dead and live loads acting vertically, and in-plane shear and flexure from wind and earthquake loads. The design of permanent slurry walls in regions of low or moderate seismicity is often limited to providing the strength necessary to resist out-of-plane soil pressures and vertical dead and live loads from the superstructure and basement framing. Although these walls also transfer in-plane lateral forces from the superstructure into the soils, the walls are often not specifically designed for these in-plane forces because their inherent strength is usually much greater than the forces being transferred. If resistance to in-plane forces acting on a wall required an increase in vertical reinforcement at the ends of a wall segment, an increase in the cap beam strength, or an increase in the horizontal reinforcement for shear strength, the overall design and construction approach would not vary significantly from current practice. Structural slurry walls have been used to a limited extent for buildings designed for high seismic risk, but there is reluctance on the part of design engineers to use them more often because of concern for how to design these walls to resist in-plane lateral forces, lack of code provisions for reinforcement detailing, and damage that may occur at panel joints. For buildings designed for high seismic risk, such as those assigned to Seismic Design Categories (SDC) D, E, and F as defined in Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10), in-plane shear and flexural actions may likely require modifications of a structural slurry wall only designed for out-of-plane soil pressures and vertical live and dead loads. Design would need to address in-plane lateral forces acting on structural slurry walls and the interaction of the in-plane actions with the out-of-plane and vertical actions. These issues are discussed in this report, and approaches to design for high seismic risk are presented.

Seismic design codes are developed mainly based on the observation of the behavior of structures in the high seismicity regions where structures may experience significant amount of inelastic deformations and major earthquakes may result in structural damages in a vast area. Therefore, seismic loads are reduced in current design codes for building structures using response modification factors which depend on the ductility capacity and overstrength of a structural system. However, structures in low seismicity regions, subjected to a minor earthquake, will behave almost elastically because of the larger overstrength of structures in low seismicity regions such as Korea. Structures in low seismicity regions may have longer periods since they are designed to smaller seismic loads and main target of design will be minor or moderate earthquakes occurring nearby. Ground accelerations recorded at stations near the epicenter may have somewhat different response spectra from those of distant station records. Therefore, it is necessary to verify if the seismic design methods based on high seismicity would he applicable to low seismicity regions. In this study, the adequacy of design spectra, period estimation and response modification factors are discussed for the seismic design in low seismicity regions. The response modification factors are verified based on the ductility and overstrength of building structures estimated from the farce-displacement relationship. For the same response modification factor, the ductility demand in low seismicity regions may be smaller than that of high seismicity regions because the overstrength of structures may be larger in low seismicity regions. The ductility demands in example structures designed to UBC97 for high, moderate and low seismicity regions were compared. Demands of plastic rotation in connections were much lower in low seismicity regions compared to those of high seismicity regions when the structures are designed with the same response modification factor. Therefore, in low seismicity regions, it would be not required to use connection details with large ductility capacity even for structures designed with a large response modification factor.

Damaging earthquakes in Australia and other regions characterised by low seismicity are considered low probability but high consequence events. Uncertainties in modelling earthquake occurrence rates and ground motions for damaging earthquakes in these regions pose unique challenges to forecasting seismic hazard, including the use of this information as a reliable benchmark to improve seismic safety within our communities. Key challenges for assessing seismic hazards in these regions are explored, including: the completeness and continuity of earthquake catalogues; the identification and characterisation of neotectonic faults; the difficulties in characterising earthquake ground motions; the uncertainties in earthquake source modelling, and; the use of modern earthquake hazard information to support the development of future building provisions. Geoscience Australia recently released its 2018 National Seismic Hazard Assessment (NSHA18). Results from the NSHA18 indicate significantly lower seismic hazard across almost all Australian localities at the 1/500 annual exceedance probability level relative to the factors adopted for the current Australian Standard AS1170.4–2007 (R2018). These new hazard estimates have challenged notions of seismic hazard in Australia in terms of the recurrence of damaging ground motions. This raises the question of whether current practices in probabilistic seismic hazard analysis (PSHA) deliver the outcomes required to protect communities and infrastructure assets in low-seismicity regions, such as Australia. This manuscript explores a range of measures that could be undertaken to update and modernise the Australian earthquake loading standard, in the context of these modern seismic hazard estimates, including the use of alternate ground-motion exceedance probabilities for assigning seismic demands for ordinary-use structures. The estimation of seismic hazard at any location is an uncertain science, particularly in low-seismicity regions. However, as our knowledge of the physical characteristics of earthquakes improve, our estimates of the hazard will converge more closely to the actual – but unknowable – (time independent) hazard. Understanding the uncertainties in the estimation of seismic hazard is also of key importance, and new software and approaches allow hazard modellers to better understand and quantify this uncertainty. It is therefore prudent to regularly update the estimates of the seismic demands in our building codes using the best available evidence-based methods and models.

Most of metropolitan France and conterminous Western Europe is currently located within the Eurasia intraplate domain, far from major plate boundaries (the Atlantic ridge and Nubia – Eurasia convergence zone). As in other intraplate regions, present-day deformation and seismicity rates are very slow, resulting in limited data and strong uncertainties on the ongoing seismotectonics and seismic hazards. In the last two decades, new geological, seismological and geodetic data and research have brought to light unexpected deformation patterns in metropolitan France, such as orogen-normal extensionca.0.5 mm yr−1in the Pyrenees and Western Alps that cannot be associated with their mountain-building history. Elsewhere, present-day deformation and seismicity data provide a partial picture that points to mostly extensive to strike-slip deformation regimes (except in the Western Alps foreland). A review of the numerous studies and observations shows that plate tectonics (plate motion, mantle convection) are not the sole, nor likely the primary driver of present-day deformation and seismicity and that additional processes must be considered, such as topography potential energy, erosion or glacial isostatic adjustment since the last glaciation. The exact role of each process probably varies from one region to another and remains to be characterized. In addition, structural inheritance (crust or mantle weakening from past tectonic events) can play a strong role in deformation localization and amplification up to factors of 5–20, which could explain some of the spatial variability in seismicity. On the basis of this review, we identify three research directions that should be developed to better characterize the seismicity, deformation rates and related processes in metropolitan France: macroseismic and historical seismicity, especially regarding moment magnitude estimations; geodetic deformation, including in regions of low seismicity where the ratio of seismic to aseismic deformation remains a key unknown; an integrated and consistent seismotectonic framework comprising numerical models, geological, seismological and geodetic data. The latter has the potential for significant improvements in the characterization of seismicity and seismic hazard in metropolitan France but also Western Europe.

<p>Methods to determine seismic hazard in any region vary depending on the regional seismicity, but can be roughly grouped into two main groups: one based on probabilistic methods that use data about known seismicity in the region, and another, which is based on data related to the faulting processes and determination of seismically active faults. Both groups of methods are relatively good for seismically active regions. However, in regions of low seismic activity and slow deformations, there is neither enough data for proper probabilistic determination of seismic hazard, nor enough data about deformation that can indicate possibly active faults. Because of that, all sets of data have to be combined in order to gather necessary information needed to determine seismic hazard for a given area.</p><p>One of such regions of low seismicity and very slow deformation is the region of Carpatho-Balkan orogen, situated in Eastern Serbia. This orogen represents the western part of the Carpatho-Balkan orogenic chain, extending in the north to the Romanian Southern Carpathians and in its southeastern part to the Balkan massif in Bulgaria. In its central part, in Eastern Serbia, Carpatho-Balkanides are made up of a system of east-vergent nappes, that have been formed in Early Cretaceous and were multiply activated during their geological history. This activity led to the formation of faults that are favorably oriented in respect to the main thrust system. It is suspected that some of these fault systems are also active in recent times.</p><p>Relatively complex geological structure and existence of a large number of rock discontinuities, as well as relatively long time during which these geological units have been exposed on the surface, led to intensive karst process and formation of both surface and underground karst forms. Therefore, investigations of faults and deformations on the field surface are very difficult, but investigations of neotectonically active faults inside the karst caves can give a lot more information.</p><p>In this abstract, we present evidence about the youngest and recently active faults in the region of interest, based on data from karst caves. Age of activity of faults mapped inside the caves was determined based on indicators of faults cutting speleothems, forming fault breccias that incorporate cave sediments (broken speleothems), and based on speleogenetic considerations. Samples for radiometric dating have been collected, that will help to quantify fault activity rate.</p><p>Preliminary results show that the research area is characterized by strike-slip tectonics, most likely resulting from far-field stress generated by the collision of the Adriatic microplate, the Moesian indenter and the tectonic units in-between. Such stress field is shown to be highly heterogeneous even in this relatively small research area, so local areas of transtension and transpression have also been very important in controlling the recent fault kinematics in this part of the Carpatho-Balkanides. These preliminary conclusions are also of high importance for seismic hazard characterization.</p>

SUMMARY Seismic hazard estimations are compared using two approaches based on two different seismicity models: one which models earthquake recurrence by applying the truncated GutenbergRichter law and a second one which smoothes the epicentre location of past events according to the fractal distribution of earthquakes in space (Woo 1996). The first method requires the definition of homogeneous source zones and the determination of maximum possible magnitudes whereas the second method requires the definition of a smoothing function. Our results show that the two approaches lead to similar hazard estimates in low seismicity regions. In regions of increased seismic activity, on the other hand, the smoothing approach yields systematically lower estimates than the zoning method. This epicentre-smoothing approach can thus be considered as a lower bound estimator for seismic hazard and can help in decision making in moderate seismicity regions where source zone definition and estimation of maximum possible magnitudes can lead to a wide variety of estimates due to lack of knowledge. The two approaches lead, however, to very different earthquake scenarios. Disaggregation studies at a representative number of sites show that if the distributions of contributions according to source‐site distance are comparable between the two approaches, the distributions of contributions according to magnitude differ, reflecting the very different seismicity models used. The epicentre-smoothing method leads to scenarios with predominantly intermediate magnitudes events (5 ≤ M ≤ 5.5) while the zoning method leads to scenarios with magnitudes that increase with the return period from the minimum to the maximum magnitudes considered. These trends demonstrate that the seismicity model used plays a fundamental role in the determination of the controlling scenarios and ways to discriminate between the most appropriate models remains an important issue.

The seismic hazard assessment of a site that lies in the low seismic region affected by the future existence of a large dam has been given less attention in many studies. Moreover, this condition is not addressed directly in the current seismic codes. This paper explains the importance of such information in mitigating the seismic hazard properly. Ulu Padas Area in Northern Borneo is used as an example for a case study of a site classified as a low seismic region. It is located close to the border of Malaysia, Brunei Darussalam, and Indonesia and may have a large dam in the future as the region lies in hilly geography with river flow. This study conducts probabilistic and deterministic seismic hazard analyses, and reservoir-triggered seismicity of a site affected by the future existence of a large dam. The result shows that the spectrum acceleration of the maximum design earthquake for the investigated site in the Ulu Padas Area in Northern Borneo is taken from the reservoir-triggered seismicity earthquake at short periods and from the current condition at longer periods.

Seismic hazard assessment and design spectra for the Kozani-Grevena region (Greece) after the earthquake of May 13, 1995

West Africa is considered a region of low seismicity. However, the monitoring of earthquake activity by local seismic arrays began very early (as early as 1914) in West Africa but seismic catalogs are very incomplete. In 1991, Bertil studied the seismicity of West Africa based on networks of seismic stations in Ivory Coast and neighboring countries. The reference work of Ambraseys and Adams as well as the recent earthquakes given by the international data centres on the seismicity of West Africa were also used for the computations of earthquake hazard parameters. Different earthquake event data have been compiled and homogenised to moment magnitude (Mw). The obtained catalog covers a period of over four centuries (1615-2021) and contains large historical events and recent complete observations. The complete catalog part has been subdivided into four complete subcatalogs with each a level of completeness. The minimum magnitude and the maximum observed magnitude are equal to 2.89 and 6.8 respectively for the whole catalog. The seismic code software developed by Kijko was used to calculate the earthquake hazard parameters. The results give a b value of 0.83 ± 0.08 for the whole period and preliminary seismic hazards curves are also plotted for return periods 25, 50 and 100 years. This is a good and practical example showing that this procedure can be used for seismic hazard assessment in West Africa.

Investigations of risk targeted peak ground acceleration (PGA) maps for European countries typically adopt a single generic fragility curve definition. The aim of this study is to investigate the use of typology‐specific fragility curves in the derivation of risk‐targeted PGA maps in low seismicity regions. This study differs from previous works in that it derives expressions for the relationship between design PGA and the median collapse capacity for several different structural typologies designed according to European standards, namely, reinforced concrete (RC) moment‐resisting frames (MRFs), reinforced concrete wall and dual systems, and steel MRFs. The expressions were determined from the regression analysis of a database of fragility curve parameters collected from the literature. The influence of the different typology‐specific fragility functions on the derivation of risk‐targeted seismic maps in regions of low seismicity, using Germany as a case‐study, is discussed. The key findings of this study are as follows: the fragility curves derived using the database possess significant inherent lateral capacity; the consideration of this inherent lateral capacity implies the reduction of the regions where seismic design is compulsory; and that fragility curves adapted from fragility analyses from structures in the US may not be representative of European structures. Recommendations for the direction of future research include focusing primarily on improving the definition of the typology‐specific fragility curves; developing fragility curves for modern IMs such as average spectral acceleration; and investigating the use of typology‐specific curves in regions of high seismicity.

Off Sanriku region is a well known plate subduction zone, which lies on the west side of the Japan Trench. High seismic activities have been observed, but there are also low seismic activity regions. We conducted an OBS-controlled source seismic experiment across the seismic-aseismic boundary in 1996 to examine the relationship between seismic activity variation and crustal structure. A traveltime analysis using refraction and reflection waves was applied to the observed data to determine the 2D crustal structure. An amplitude analysis of reflection waves from the subducting plate boundary revealed a good correlation between acoustic-impedance (seismic reflectivity) and seismic activity; that is, high-impedance region is a low seismic region and vice versa. We propose the hypothesis that high-impedance is caused by fluid or hydrate rocks around the plate boundary, and this hypothesis explains the crustal velocity structure and the variations of observed reflection waves and seismic activities.

On 1999 September 21, the Mw 7.6 Chi‐Chi earthquake ruptured a segment of the Chelungpu Fault, a frontal thrust fault of the Western Foothills of Taiwan. The stress perturbation induced by the rupture triggered a transient deformation across the island, which was well recorded by a wide network of continuously operating GPS stations. The analysis of more than ten years of these data reveals a heterogeneous pattern of postseismic displacements, with relaxation times varying by a factor of more than ten, and large cumulative displacements at great distances, in particular along the Longitudinal Valley in eastern Taiwan, where relaxation times are also longer. We show that while afterslip is the dominant relaxation process in the epicentral area, viscoelastic relaxation is needed to explain the pattern and time evolution of displacements at the larger scale. We model the spatiotemporal behavior of the transient deformation as the result of afterslip on the décollement that extends downdip of the Chelungpu thrust, and viscoelastic flow in the lower crust and in the mid‐crust below the Central Range. We construct a model of deformation driven by coseismic stress change where afterslip and viscoelastic flow are fully coupled. The model is compatible with the shorter relaxation times observed in the near field, which are due to continued fault slip, and the longer characteristic relaxation times and the reversed polarity of vertical displacements observed east of the Central Range. Our preferred model shows a viscosity of 0.5–1 × 1019Pa s at lower‐crustal depths and 5 × 1017Pa s in the mid‐crust below the Central Range, between 10 and 30 km depth. The low‐viscosity zone at mid‐crustal depth below the Central Range coincides with a region of low seismicity where rapid advection of heat due to surface erosion coupled with underplating maintain high temperatures, estimated to be between 300°C and 600°C from the modeling of thermo‐chronology and surface heat flow data.

Seismic risk in regions of low seismicity is evaluated using the UK as an example. This involves quantifying the seismic hazard and also the vulnerability functions for a range of typical structures. Predictions are made as to the effects of a range of specific earthquakes occurring under two existing areas of the UK. By integrating the hazard and vulnerability seismic risk has been calculated in terms of cost and fatalities and compared to those arising from other types of hazard. Recommendations are made as to when, and in what form, seismic considerations may be necessary.