Cyclic testing and analysis of a full-scale cast-in-place reinforced concrete wall-slab-wall structure

The phenomena of induced seismicity have recently become of increasing interest for public/private authorities and stakeholders due to the impact that some of these low-intensity earthquakes might have on the built environment of regions that were not historically prone to hazard from natural seismicity. This has led to the deployment of experimental campaigns aimed at investigating and assessing the seismic response of existing structures in such now-seismically active regions, which were typically built without any particular seismic design and/or detailing criteria. A pseudostatic cyclic test on a full-scale one-storey two-bay cast-in-place reinforced concrete (RC) wall-slab-wall structure, representative of a building typology that is a very common form of housing in the Netherlands, was thus carried out. The test specimen was designed according to Dutch building practice and tried to combine several common characteristics as well as customary detailing of multi-unit cast-in-place RC terraced houses, which, together with precast ones, constitute the vast majority of the Groningen RC building stock. The present paper describes the main features of the building mock-up and discusses the foremost results obtained by the testing, which was performed in a bi-directional fashion and was therefore meant to address several open questions regarding the seismic behaviour/performance of this specific structural typology in both longitudinal/weak and transverse/strong directions. Hysteretic response curves of the specimen and key parts of it are provided, along with a discussion of the building damage evolution during the phases of testing, showing how rocking of walls around their base was the dominating response mechanism for both directions of loading. Issues of strength degradation and energy dissipation were also addressed, illustrating the main trends observed with the changing of imposed drift level as well as with the number of applied cycles for a given drift amplitude. Finally, a simple yet reliable fibre-based finite element model was developed, with a view to allow the readily assessment of the seismic response of structures of this type; feasibility and limitations of such numerical model were evaluated through comparison with the obtained test data.     

Loading protocols for European regions of low to moderate seismicity

Existing loading protocols for quasi-static cyclic testing of structures are based on recordings from regions of high seismicity. For regions of low to moderate seismicity they overestimate imposed cumulative damage demands. Since structural capacities are a function of demand, existing loading protocols applied to specimens representative of structures in low to moderate seismicity regions might underestimate structural strength and deformation capacity. To overcome this problem, this paper deals with the development of cyclic loading protocols for European regions of low to moderate seismicity. Cumulative damage demands imposed by a set of 60 ground motion records are evaluated for a wide variety of SDOF systems that reflect the fundamental properties of a large portion of the existing building stock. The ground motions are representative of the seismic hazard level corresponding to a 2 % probability of exceedance in 50 years in a European moderate seismicity region. To meet the calculated cumulative damage demands, loading protocols for different structural types and vibration periods are developed. For comparison, cumulative seismic demands are also calculated for existing protocols and a set of records that was used in a previous study on loading protocols for regions of high seismicity. The median cumulative demands for regions of low to moderate seismicity are significantly less than those of existing protocols and records of high seismicity regions. For regions of low to moderate seismicity the new protocols might therefore result in larger strength and deformation capacities and hence in more cost-effective structural configurations or less expensive retrofit measures.     

中国大陆地震低活动异常及其在中期预测中的效能研究

在对中国大陆地震活动的分析研究中发现, 在某些大地震发生前的一、 两年中, 中国大陆往往出现地震活动显著平静的低活动异常。 本文应用最近100 a(1918—2017年)间5级以上地震资料, 分析地震低活动异常与未来一、 两年中国大陆地震活动的关系。 从M≥5.0地震的年频度、 年度最大地震震级、 6级以上地震的平静等方面建立了地震低活动异常的判据指标, 并从概率增益指数β、 预测效能比α、 地震预测R值评分等方面对地震低活动异常与未来大震关系作了多参数检验。 初步研究结果显示, 地震低活动异常与未来一、 两年中国大陆地震活动有一定的关系, 主要表现为其与8级左右(M≥7.8)巨大地震具有一定的相关性, 但其关联度较低, 概率增益指数β与预测效能比α为3左右, R值评分为0.2左右。 对于M＜7.5的地震, 地震低活动异常未显示, 其概率增益β, R值评分接近于0, 似呈现为近于随机关系。 M7.5是开始显示地震低活动异常与其有一定相关性的转折点。     

水口水库地震对南平地区的影响

水口水库建成蓄水后诱发了频繁的地震活动，延续时间已达8年之久，,学在继续活动。近年来，南平地区的邵武，政和等地也相继有中小地震活动，为了摸清南平地区地震活动与水口水库诱发地震是否关联，本文对该地区及水口水库诱发地震前后的地震活动性及时空特征，区域构造背景及水库诱发地震的活动趋势进行综合分析研究。     

Field experiment in Soultz-sous-Forêts, 1993: Changes of the pattern of induced seismicity

The data of the known field experiment on water injection in the borehole were analyzed. Parameters of self-similarity of seismicity were estimated in comparison with the changes of water pressure. Changes of seismicity parameters that indicate the redistribution of the failure from lower scales to upper are revealed. The total number of earthquakes per series of the water initiation found to be depended exponentially on the water pressure and seismic activity maximum is delayed gradually relative to beginning of initiation. The growth of induced seismicity zone in time differs from diffusion model for water flow in the porous medium. Analysis carried out from laboratory data indicates that diffusion growth of the failure area may be realized in the dry specimen, without fluid. It could be assumed that both kinetic processes — water and the failure diffusion — can be significant for the development of seismicity induced by the water injection.     

Seismic rehabilitation and analysis of Chaohe earth dam

Stability of earth dams during earthquakes has been a major concern for geotechnical engineers in seismic active regions. Liquefaction induced slope failure occurred at the upstream slope of a major earth dam in the suburb of Beijing, China, during the 1976 Tangshan Earthquake. The gravelly soil with loose initial condition liquefied under relatively small ground vibration. In recent years, a major seismic rehabilitation project was carried out on a similar earth dam nearby using dumped quarry stone. Seismic stability analysis was carried out using model test, finite element simulation, and pseudo-static slope stability program after taking into account the influence of excess pore pressure.     

大岗山水库地震监测系统建设及监测能力概估算和验证

本文阐述了大岗山水电站水库诱发地震监测系统建设和监测能力概况,并利用运行后的监测地震目录,分析了近年来库区附近地震活动的时空分布特征。     

利用并行计算方法实现地震活动速率变化参数Z值的空间扫描处理及其计算效能评价

近30多年的研究表明,地震平静可能是中长期的地震前兆现象,而地震平静的内在特征体现了地震活动速率的变化. Z 检验法是检验地震活动速率变化应用最为广泛的方法之一,其度量值即 Z 值. 由于 Z 值空间扫描中采用等地震数的空间扫描窗口,每个空间扫描窗口的大小通常都是变化的,这样带来了大计算量的问题,因此,本文在自行组建的4节点PC机群上实现了 Z 值空间扫描的并行化处理. 初步测试结果说明,并行化处理中获取了很高的加速比和并行计算效率,可以将此种方法应用于其他地震活动性参数的空间扫描处理.     

Elastic models of seismic failure in the Valley of Mexico

Restricted parts of the Valley of Mexico have been plagued by swarms of small earthquakes during recent years. The cause of these events is puzzling, but they may relate to massive artificial changes in the hydrology of the lake-bed which underlies most of Mexico City. If the valley responds elastically, plane strain finite element calculations can be used to relate geologic and seismic structure to observed seismicity and overall stability of the valley. These calculations show that unstable zones and areas of local seismicity coincide if the triggering mechanism of the seismicity is related to variations of pore pressure and water load.In order to make such a connection it is necessary to ascribe the generating mechanism of the seismicity to the density and elastic modulus anomalies associated with the valley structure, and to suggest that the phenomenon that triggers this seismicity is related to changes in water content of the near surface formations.     

湖南东江水库诱发地震

东江水库自蓄水前约半年起就开始进行地震监测，发震后通过密集台网强化观测、震源机制解及地震地质等多项工作，综合研究该库诱发地震发震原因，认为由于库水渗透引起的多种局部性应力调整导致水库诱发地震活动．其发展趋势将表现为逐渐衰减，今后不致于发生破坏性地震．     

Characteristics of the low energy seismicity of central Apulia (southern Italy) and hazard implications

The central part of the Apulia region, in southern Italy, has been generally considered practically free from significant level of seismicity, but historical documentation, geological indicators and recent instrumental observations suggest that the activity of local minor tectonic structures could have been masked (and partly also induced) by that of major seismogenic structures located in the neighbouring regions. A revision of the central Apulia seismicity characteristics was conducted considering its space and time distribution, energy release rate and focal mechanisms, in view of possible hazard implications. To better constrain the seismicity rates inferable from the set of available historical data, special attention was paid to the declustering of a catalogue of low energy events (magnitude < 3.5) instrumentally detected in about 20 years: a new declustering procedure, useful for cases like to the one at hand, was purposely devised taking into account the peculiarity of local seismicity characteristics and the limitations of the available database. The results obtained by combining instrumental and historical data show that this area is affected by a rather sporadic seismicity, likely associated to a general tensional regime and possibly stimulated by the interaction with Apenninic and northern Apulia seismogenic activity. Even though less energetic, the local seismicity contributes to increase the moderately damaging shaking probability due to the activity of seismic sources located in the near areas, so to justify the adoption of at least a minimum level of caution in relation to the local definition of seismic protection measures.     

历史资料短缺地区估计地震活动性参数方法初探

在地震危险性分析中，历史地震资料是确定地震活动性参数的关键。鉴于我国西部地震资料记载年限短，地震活动水平高的情况，本文提出一种对中国西部地区如何使用有限资料，确定未来地震活动水平和合理估计地震活动性参数的方法。该方法是引入相对应变这一物理量，通过分析各地震带中强以上地震在时间上的相对应变释放速率，达到尽可能合理估计地震活动性参数的目的。     

Comparison of seismic effects during deep tunnel excavation with different methods

The rapid release of strain energy is an important phenomenon leading to seismic events or rock failures during the excavation of deep rock.Through theoretical analysis of strain energy adjustment during blasting and mechanical excavation,and the interpretation of measured seismicity in the Jin-Ping Ⅱ Hydropower Station in China,this paper describes the characteristics of energy partition and induced seismicity corresponding to different energy release rates.The theoretical analysis indicates that part of the strain energy will be drastically released accompanied by violent crushing and fragmentation of rock under blast load,and this process will result in seismic events in addition to blasting vibration.The intensity of the seismicity induced by transient strain energy release highly depends on the unloading rate of in-situ stress.For mechanical excavation,the strain energy,which is mainly dissipated in the deformation of surrounding rock,releases smoothly,and almost no seismic events are produced in this gradual process.Field test reveals that the seismic energy transformed from the rock strain energy under high stress condition is roughly equal to that coming from explosive energy,and the two kinds of vibrations superimpose together to form the total blasting excavation-induced seismicity.In addition,the most intense seismicity is induced by the cut blasting delay; this delay contributes 50% of the total seismic energy released in a blast event.For mechanical excavation,the seismic energy of induced vibration(mainly the low intensity acoustic emission events or mechanical loading impacts),which accounts only for 1.5‰ of that caused by in-situ stress transient releasing,can be ignored in assessing the dynamic response of surrounding rock.     

Microseismicity around rupture area of the 1944 Tonankai earthquake from ocean bottom seismograph observations

Along the Nankai trough, southwestern Japan, the Philippine Sea plate (PSP) is subducting beneath the Eurasian plate, and large interplate earthquakes have occurred repeatedly with a recurrence interval of about 100-200 years. The most recent large thrust event in the eastern Nankai trough off Kii Peninsula was the 1944 Tonankai earthquake. In this region, current seismicity is very low and hypocenters are not determined accurately by the land seismic network. We conducted microseismicity observations around the rupture area of the 1944 Tonankai earthquake using ocean bottom seismographs (OBSs). Hypocenters were determined using a 2-D seismic velocity structure model based on an airgun-OBS seismic survey. Results obtained show that the seismicity was relatively active near the trough axis. These earthquakes may relate to deformation of the subducting Philippine Sea plate. On the other hand, microseismicity in the rupture area of the 1944 Tonankai earthquake was very low. This low-level seismic activity in the co-seismic rupture area of the 1944 Tonankai earthquake likely relates to a single large asperity off Kii Peninsula.     

Prognostic anomalies of induced seismicity in the region of the Koyna-Warna water reservoirs, West India

In 2000, the region of the Koyna-Warna water reservoirs in West India was hit by two strong earthquakes, which occurred six months apart and had magnitudes M > 5. The Koyna-Warna seismic zone is a typical region of induced seismicity with a pronounced correlation between seismicity and water level variations in the reservoirs. This indicates that the stress level in the region is close to critical; thus, insignificant variations in stress caused by the variations in the water level may trigger a strong earthquake. In order to study the preparatory processes in the sources of the induced earthquakes, in this paper we analyze the seismic catalogue for the Koyna-Warna region before a pair of strong earthquakes of 2000. The induced seismicity is found to exhibit prognostic variations, which are typical of preparation of tectonic earthquakes and indicative of the formation of metastable source zones of future earthquakes. Based on the obtained results, we suggest that initiation of failure in these metastable zones within the region of induced seismicity could have been caused by the external impacts associated with water level variations in the reservoirs and by the internal processes of avalanche unstable crack propagation.     

Machine learning-based automatic construction of earthquake catalog for reservoir areas in multiple river basins of Guizhou province,China

Large reservoirs have the risk of reservoir induced seismicity. Accurately detecting and locating microseismic events are crucial when studying reservoir earthquakes. Automatic earthquake monitoring in reservoir areas is one of the effective measures for earthquake disaster prevention and mitigation. In this study, we first applied the automatic location workflow (named LOC-FLOW) to process 14-day continuous waveform data from several reservoir areas in different river basins of Guizhou province. Compared with the manual seismic catalog, the recall rate of seismic event detection using the workflow was 83.9%. Of the detected earthquakes, 88.9% had an onset time difference below 1 s, 81.8% has a deviation in epicenter location within 5 km, and 77.8% had a focal depth difference of less than 5 km, indicating that the workflow has good generalization capacity in reservoir areas. We further applied the workflow to retrospectively process continuous waveform data recorded from 2020 to the first half of 2021 in reservoir areas in multiple river basins of western Guizhou province and identified five times the number of seismic events obtained through manual processing. Compared with manual processing of seismic catalog, the completeness magnitude had decreased from 1.3 to 0.8, and a b-value of 1.25 was calculated for seismicity in western Guizhou province, consistent with the b-values obtained for the reservoir area in previous studies. Our results show that seismicity levels were relatively low around large reservoirs that were impounded over 15 years ago, and there is no significant correlation between the seismicity in these areas and reservoir impoundment. Seismicity patterns were notably different around two large reservoirs that were only impounded about 12 years ago, which may be explained by differences in reservoir storage capacity, the geologic and tectonic settings, hydrogeological characteristics, and active fault the reservoir areas. Prominent seismicity persisted around two large reservoirs that have been impounded for less than 10 years. These events were clustered and had relatively shallow focal depths. The impoundment of the Jiayan Reservoir had not officially begun during this study period, but earthquake location results suggested a high seismicity level in this reservoir area. Therefore, any seismicity in this reservoir area after the official impoundment deserves special attention.     

Statistics of seismic events at the Groningen field

Depletion of gas fields, even in a tectonically inactive area can induce earthquakes. This is the case for the Groningen gas field, located in the north of the Netherlands. Increased seismic activity raised public concern which led to the government trying to understand the cause of the earthquakes and optimize production such as to minimize the risk of induced seismicity. The main question is how production is correlated with induced seismicity. In this paper we deal with the statistics of seismic events using Bayesian model comparison and a Bayesian change point model. We have developed a method to assess seismic event rate, its changes and tendencies. These statistical analyses are in agreement with each other and find a constant event rate up to 2003, an increasing event rate from 2003 to 2014 and a preference for a decreasing event rate from early 2014 to now. Seasonality in the production and the number of events is present. The seasonality indicates a delay ranging between 2 and 8 months between seismicity and production changes. The question of interest is whether the production reduction since January 2014 has had an effect on the seismicity occurring in the Groningen field. The number of events in the Groningen field in the area affected by the production change has been reduced significantly. We present evidence that changes in seismicity are closely related to changes in production.     

Seismic source identification and characterisation for probabilistic seismic hazard analyses conducted in the Buller–NW Nelson Region, South Island, New Zealand

A seismic source model is presented for use in probabilistic seismic hazard analyses to be conducted for sites within the Buller–NW Nelson region of New Zealand. The application of common probabilistic seismic hazard analysis (PSHA) methodology for sites in this region has been complicated by the long-held suspicion that the observed rates of seismic activity are high and not representative of long-term earthquake activity. However, recent analyses of geological, seismicity and geodetic data indicate that the extent of this anomaly may have been overestimated and that current rates of seismic activity within this region are likely to continue into the foreseeable future. Probable bounds for the most appropriate long-term rates of seismic activity are estimated after considering all available sources of constraint. These include geodetic analyses, plate-motion modelling, finite element modelling, structural geological considerations, paleoseismic information, tree-ring analyses, precarious rock information, observed seismicity and fundamental mechanics. A suite of fault sources is identified, and the observed seismicity is partitioned between these sources and a background source using Bayesian inference, and then analysed to obtain a magnitude–frequency distribution for each seismic source. The annual moment release rate for the region, resulting from the identified and characterised sources, is shown to be consistent with available constraints. Consequently, it is demonstrated that the observed seismicity in the Buller–NW Nelson region can be used to model future earthquake occurrence within the region and that standard PSHA may therefore be implemented within the region.     

鄂尔多斯周缘地震带地震活动的分期和相关分析

利用最优分割法对鄂尔多斯周缘地震带(区)的地震活动进行了分期,并作了相关分析。结果表明,1000年以来汾渭地震带北段可划为4个活跃期,每个活跃期的中心间距约300年,最大地震为7级,天水—陕南地震带(区)近一、二百年的地震活动可分为持续约10年左右的几个地震簇。所有分期中,海原地震带能量释放最大,且最近仍很活跃。汾渭地震带和海原—银川地震带活跃期存在相关现象,且其南段往往先于北段活跃,目前活动水平较低。该带1900年以来地震还有与河套地震带地震相呼应的现象和沿构造带南北迁移的规律。根据分期和相关结果推测,鄂尔多斯周缘各地震带都存在中强地震背景。其中海原—银川地震带近期发生5—6级或更大地震的可能性较大,天水—陕南地震带近期也有发生中强地震的可能。汾渭地震带虽然存在孕育6级以上大震的背景,但近年还不会发生强震。