Damage to residential buildings in Hveragerði during the 2008 Ölfus Earthquake: simulated and surveyed results

This study analyses the performance of residential buildings in the town of Hveragerði in South Iceland during the 29 May 2008 Mw 6.3 Ölfus Earthquake. The earthquake occurred very close to the town, approximately 3–4 km from it. Ground shaking caused by the earthquake was recorded by a dense strong-motion array in the town. The array provided high-quality three-component ground acceleration data which is used to quantify a hazard scenario. In addition, surveys conducted in the town in the aftermath of the earthquake have provided information on macroseismic intensity at various locations in the town. Detailed information regarding the building stock in the town is collected, and their seismic vulnerability models are created by using building damage data obtained from the June 2000 South Iceland earthquakes. Damage to buildings are then simulated by using the scenario hazard and vulnerability models. Damage estimates were also obtained by conducting a survey. Simulated damage based on the scenario macroseismic intensity is found to be similar to damage estimated from survey data. The buildings performed very well during the earthquake—damage suffered was only 5 % of the insured value on the average. Correlation between actual damage and recorded ground-motion parameters is found to be statistically insignificant. No significant correlation of damage was observed, even with macroseismic intensity. Whereas significant correlation was observed between peak ground velocity and macroseismic intensity, neither of them appear to be good indicators of damage to buildings in the study area. This lack of correlation is partly due to good seismic capacity of buildings and partly due to the ordinal nature of macroseismic intensity scale. Consistent with experience from many past earthquakes, the survey results indicate that seismic risk in South Iceland is not so much due to collapse of buildings but rather due to damage to non-structural components and building contents.     

A note on the M w 6.3 earthquake in Iceland on 29 May 2008 at 15:45 UTC

This preliminary study aims to investigate a M _w 6.3 earthquake that occurred in South Iceland on Thursday 29 May 2008 at 15:45 UTC. The epicentre was in the Olfus District between the towns of Selfoss and Hveragerdi. This study examines the data recorded and the damage observed immediately after the event. Horizontal accelerations of up to 80%g were recorded in the epicentral region and there is visual evidence that the vertical acceleration exceeded 1 g. The PGA data is compared to a ground motion estimation model developed for the South Iceland earthquakes in June 2000. In general the basic properties of this event are found to be similar to the characteristics of the South Iceland earthquakes in June 2000. The duration of strong-motion is short and the intensity attenuates rapidly with increasing distance. The earthquake action resisted by buildings in the near fault area is inspected through evaluation of elastic as well as inelastic response spectra. The vast majority of structures seemed to withstand the strong-motion fairly competently and without significant visual damage due firstly to the low-rise, predominantly reinforced concrete or timber, style of buildings. Secondly, the short duration of strong-motion contributed to the endurance of structures.     

Earthquake intensity distributions: a new view

Pioneering work by Nicolas Ambraseys and many collaborators demonstrates both the tremendous value of macroseismic data and the perils of its uncritical assessment. In numerous publications he shows that neglect of original sources and/or failure to appreciate the context of historical accounts, as well as use of unreliable indicators such as landsliding to determine intensities, commonly leads to inflated magnitude estimates for historical earthquakes. The U.S. Geological Survey “Did You Feel It?” (DYFI) system, which now collects and systematically interprets thousands of first-hand reports from felt earthquakes, provides the opportunity to explore further the biases associated with traditional intensity distributions determined from written (media or archival) accounts. I briefly summarize and further develop the results of Hough (2013), who shows that traditional intensity distributions imply more dramatic damage patterns than are documented by more spatially rich DYFI data, even when intensities are assigned according to the conservative practices established by Ambraseys’ work. I further consider the separate intensity–attenuation relations that have been developed to characterize intensities for historical and modern earthquakes in California, using traditionally assigned intensities and DYFI intensities, respectively. The results support the conclusion that traditionally assigned intensity values tend to be inflated by a fundamental bias towards reporting of dramatic rather than representative effects. I introduce an empirical correction-factor approach to correct for these biases. This allows the growing wealth of well-calibrated DYFI data to be used as calibration events in the analysis of historical earthquakes.     

Radon Changes Associated with the Earthquake Sequence in June 2000 in the South Iceland Seismic Zone

An earthquake sequence at the transform plate boundary in South Iceland, that included two magnitude 6.5 earthquakes in June 2000, was anticipated on the basis of a centuries-long seismicity pattern in the area. A program of radon monitoring in geothermal water from drill holes, initiated in 1999, rendered distinct and consistent variations in radon in association with these events. All seven sampling stations in a 50 × 30 km zone covering the epicentral area showed a consistent pattern. Four types of change could be identified: 1) Preseismic decrease of radon. Anomalously low values were measured 101–167 days before the earthquakes. 2) Preseismic increase. Spikes appear in the time series at six stations 40–144 days prior to the earthquakes. These anomalies were large and unusual if compared to a 17-years history of radon monitoring in this area. 3) Coseismic step, most likely related to the coseismic change in groundwater pressure observed over the entire area. 4) Postseismic return of the radon values to the preseismic level about three months later, also concurrent with groundwater pressure changes.     

Exploration tree approach to estimate historical earthquakes Mw and depth,test cases from the French past seismicity

The estimation of the seismological parameters of historical earthquakes is a key step when performing seismic hazard assessment in moderate seismicity regions as France. We propose an original method to assess magnitude and depth of historical earthquakes using intensity data points. A flowchart based on an exploration tree (ET) approach allows to apply a consistent methodology to all the different configurations of the earthquake macroseismic field and to explore the inherent uncertainties. The method is applied to French test case historical earthquakes, using the SisFrance (BRGM, IRSN, EDF) macroseismic database and the intensity prediction equations (IPEs) calibrated in the companion paper (Baumont et al. Bull Earthq Eng, 2017). A weighted least square scheme allowing for the joint inversion of magnitude and depth is applied to earthquakes that exhibit a decay of intensity with distance. Two cases are distinguished: (1) a “Complete ET” is applied to earthquakes located within the metropolitan territory, while (2) a “Simplified ET” is applied to both, offshore and cross border events, lacking information at short distances but disposing of reliable data at large ones. Finally, a priori-depth-based magnitude computation is applied to ancient or poorly documented events, only described by single/sporadic intensity data or few macroseismic testimonies. Specific processing of “felt” testimonies allows exploiting this complementary information for poorly described earthquakes. Uncertainties associated to magnitude and depth estimates result from both, full propagation of uncertainties related to the original macroseismic information and the epistemic uncertainty related to the IPEs selection procedure.     

Making sense of Australian earthquakes: the Ambraseys legacy

Through his students, Professor Nick Ambraseys has had a strong impact on the introduction of earthquake engineering practices in Australia, including historical earthquake studies, strong motion instrumentation and analysis, foundation studies including liquefaction, and building code formulation. In Australia the process of upgrading the third and current edition of the earthquake code and hazard map has begun. There are now about 150 digital strong motion recorders installed in cities, on major structures and at Australian National Seismograph Network sites. Three volumes of an isoseismal atlas have been published totalling more than 300 maps, mainly historical earthquakes. Significant progress has been made in paleoseismological studies across the continent, adding to the complexity of the intraplate seismicity model. With time and more installed accelerographs, the peak ground acceleration recorded in Australia has increased from .25 g in 1984 to .5 g in 1988 and 1 g in 1994, all from earthquakes smaller than magnitude 5, supporting Ambraseys contention that PGA alone is not a suitable parameter for a design ground motion.     

Earthquake response of a base-isolated bridge subjected to strong near-fault ground motion

The South Iceland Lowland is an active seismic zone. In May 2008 a magnitude 6.3 (M_w) earthquake struck the area. The 370-m-long base-isolated Oseyrar Bridge situated close to the epicentre was hit by strong ground motion. Concrete stoppers at the top of all piers were badly smashed and all four wing walls of the abutments were severely damaged. The study showed that it is possible with numerical models to back-calculate the observed damage based on recorded ground motion in the area. The recorded loads were larger than prescribed by Eurocode 8 for the site even though the magnitude and location of the earthquake was as expected. The main reason was a near-fault pulse which is not covered in the code. The pulse dominated the bridge response and is therefore important to consider. Finally, an improved design is presented that could have prevented the damage.     

Macroseismic field of the October 27, 2004 Vrancea (Romania) moderate subcrustal earthquake

On October 27, 2004, a moderate size earthquake occurred in the Vrancea seismogenic region (Romania). The Vrancea seismic zone is an area of concentrated seismicity at intermediate depths beneath the bending area of the southeastern Carpathians. The 2004 M _w?=?6 Vrancea subcrustal earthquake is the largest seismic event recorded in Romania since the 1990 earthquakes. With a maximum macroseismic intensity of VII Medvedev–Sponheuer–Kárník (MSK-64) scale, the seismic event was felt to a distance of 600 km from the epicentre. This earthquake caused no serious damage and human injuries. The main purpose of this paper is to present the macroseismic map of the earthquake based on the MSK-64 intensity scale. After the evaluation of the macroseismic effects of this earthquake, an intensity dataset has been obtained for 475 sites in the Romanian territory. Also, the maximum horizontal accelerations recorded in the area by the K2 network are compared to the intensity values.     

The August 27, 2008, <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript> = 6.3, Kultuk earthquake effects in the Nearepicenter zone: Macroseismic survey results

In this work, the macroseismic effects of the Kultuk earthquake (M _W = 6.3), which took place on August 27, 2008 in the southwestern closure of the Baikal Lake, are under consideration. The intensity of shocks in inhabited localities located in the epicentral zone reached 7–8 points on the MSK-64 scale. The earthquake was named after the local settlement of Kultuk, which was the mostly damaged area by the earthquake. The considered seismic event caused significant material damage (about 250 million rubles according to preliminary estimates). In inhabited localities of Southern Pribaikalie thousands of cases of damaged ovens and chimneys were registered. Some buildings were highly damaged and, accordingly, they are unfit for further use. The earthquake was the cause of numerous rockslides, rockfalls, and landslides on steep natural and artificial slopes. A macroseismic survey allowed us to establish the asymmetrical distribution of the intensity of shocks relative to the epicenter.     

Empirical seismic vulnerability assessment of Icelandic buildings affected by the 2000 sequence of earthquakes

In June 2000, two Mw6.5 earthquakes occurred within a 4-day interval in the largest agricultural region of Iceland causing substantial damage and no loss of life. The distance between the earthquake epicentres and the fault rupture was approximately 15 km. Nearly 5000 low-rise residential buildings were affected, some of which were located between the faults and exposed to strong ground motion from both events. The post-earthquakes damage and repair costs for every residential building in the epicentral region were assessed for insurance purposes. The database is detailed and complete for the whole region and represents one of the best quality post-earthquake vulnerability datasets used for seismic loss estimation. Nonetheless, the construction of vulnerability curves from this database is hampered by the fact that the loss values represent the cumulative damage from two sequential earthquakes in some areas, and single earthquakes in others. A novel methodology based on beta regression is proposed here in order to define the geographical limits on areas where buildings sustained cumulative damage and predict the seismic losses for future sequence of events in each area. The results show that the average building loss in areas affected by a single event is below 10% of the building replacement value, whilst this increases to an average of 25% in areas affected by the two earthquakes. The proposed methodology can be used to empirically assess the vulnerability in other areas which experienced sequence of events such as Emilia-Romagna (Italy) in 2012.     

Historical development of the British and Scandinavian earthquake archives

The study of historical earthquakes itself has an interesting history, and Nick Ambraseys figures highly in it. This historiographic tribute relates the pivotal role that he played in the development of the British and Scandinavian Earthquake Archives. Maintaining his academic distance from direct hazard consulting, he nevertheless exerted a significant influence on the scientific conduct of hazard analyses. From the collection of instrumental and macroseismic data to the assignment of magnitudes, the modus operandi which he recommended was followed in pioneering seismic hazard studies in Britain, the North Sea and Norway, during the 1980s. Because of the confidentiality of seismic hazard reports produced for the UK nuclear industry, there is little record in the open seismological literature of the contribution of Nick Ambraseys. As a civil engineer, he had an exceptional interest in the earthquake evidence that could be gleaned from historical documents. An illustrative reference is given to the discovery of fresh evidence from an Aegean earthquake that shows how even the most knowledgeable of earthquake historians can be surprised.     

Macroseismic evidence and site effects for the Lunigiana (Italy) 1995 Earthquake

On October 10, 1995, an M_L= 4.8 (ING) earthquake occurred in the region of Lunigiana (northwestern Italy). The shock was felt over a large area and produced significant damage. We performed a macroseismic survey and damage zonation and assessed a maximum intensity VII MCS in the epicentral area. The damage pattern, that we investigated in detail for some of the villages in the most heavily damaged area, emphasises the role of surface geology in amplifying the effects. Topographic effects and near-surface geology are largely responsible for broadening the damage area. Given the moderate size of the earthquake, many of the macroseismic observations, including rotations of objects and the propagation of visible waves in the ground, are suggestive of amplification phenomena.     

Intensity attenuation in the Campania region,Southern Italy

Ground motion prediction equations (GMPE) in terms of macroseismic intensity are a prerequisite for intensity-based shake maps and seismic hazard assessment and have the advantage of direct relation to earthquake damage and good data availability also for historical events. In this study, we derive GMPE for macroseismic intensity for the Campania region in southern Italy. This region is highly exposed to the seismic hazard related to the high seismicity with moderate- to large-magnitude earthquakes in the Appenninic belt. The relations are based on physical considerations and are easy to implement for the user. The uncertainties in earthquake source parameters are accounted for through a Monte Carlo approach and results are compared to those obtained through a standard regression scheme. One relation takes into account the finite dimensions of the fault plane and describes the site intensity as a function of Joyner–Boore distance. Additionally, a relation describing the intensity as a function of epicentral distance is derived for implementation in cases where the dimensions of the fault plane are unknown. The relations are based on an extensive dataset of macroseismic intensities for large earthquakes in the Campania region and are valid in the magnitude range M _w = 6.3–7.0 for shallow crustal earthquakes. Results indicate that the uncertainties in earthquake source parameters are negligible in comparison to the spread in the intensity data. The GMPE provide a good overall fit to historical earthquakes in the region and can provide the intensities for a future earthquake within 1 intensity unit.     

井下、地表地震计记录波形对比——以松原地震台为例

分析了相邻井下、地表地震计记录的背景噪声及近震、远震波形特征,以及相关的振幅谱及近震、远震接收函数波形特征。结果表明,井下地震计记录的背景噪声比地表地震计小1个数量级,其近震、远震波形记录较清晰、简洁;井下、地表地震计记录的远震振幅谱和径向接收函数具有较好的一致性,但背景噪声、近震振幅谱、近震接收函数、远震切向接收函数波形存在一定差异;由地表地震计得到的近震震级比井下地震计的大0.12,地表地震计到井下地震计之间的P波速度约为2 km/s。     

中国历史地震烈度表研究

在比较分析以往烈度表的基础上,着重增加了社会反响标志;对Ⅹ-Ⅻ度的房屋建筑物和地表现象标志进行了调整与补充,完善了作为12阶烈度表相应的《中国历史地震烈度表》.文中对烈度表的各项标志作了简要说明,并列举了国内外10次历史地震事件的评定实例.本文提出的历史地震烈度表,保持了以往烈度表的适用性与一致性.     

云南楚雄地区成组强震与单个强震的前兆差异性研究

以1995与2000年发生在云南楚雄地区武定、姚安的两次6．5级强震为例,研究了成组强震和单个强震的前兆差异性。发现在远源区、近源区及源区的前震活动、地震序列、前兆群体异常及边界动力条件方面的差异十分显著。同时指出这些差异对于识别成组强震和单个强震及后续强震预报具有重要意义。     

Validation through HVSR measurements of a method for the quick detection of site amplification effects from intensity data: an application to a seismic area in Northern Italy

An instrumental validation is attempted of an innovative approach devoted to the quick individuation, from macroseismic data, of site amplification phenomena able to significantly modify seismic hazard levels expected on the basis of average propagation effects only. According to this methodology, two evaluations of hazard are performed at each investigated locality: the former, obtained by epicentral intensity data ‘reduced’ at the site through a probabilistic attenuation function and, the latter, computed by integrating such data with seismic effects actually observed at the site during past earthquakes. The comparison, for each locality, between these two hazard estimates allow to orientate the identification of those sites where local amplifications of earthquake ground motion could be significant. In order to check such methodology, indications obtained in this way from macroseismic data are compared with the estimates of transfer functions performed through the HVSR technique applied to microtremors. Results concerning municipalities located in a seismic area of Northern Italy indicate a good agreement between macroseismic and instrumental estimates.     

Long-term seismicity in regions of present day low seismic activity: the example of western Europe

In western Europe, the knowledge of long-term seismicity is based on reliable historical seismicity and covers a time period of less than 700 years. Despite the fact that the seismic activity is considered as low in the region extending from the Lower Rhine Embayment to England, historical information collected recently suggests the occurrence of three earthquakes with magnitude around 6.0 or greater. These events are a source of information for the engineer or the scientist involved in mitigation against large earthquakes. We provide information relevant to this aspect for the Belgian earthquake of September 18, 1692. The severity of the damage described in original sources indicates that its epicentral intensity could be IX (EMS-98 scale) and that the area with intensity VII and greater than VII has at least a mean radius of 45 km. Following relationships between average macroseismic radii and magnitude for earthquakes in stable continental regions, its magnitude M_s is estimated as between 6.0 and 6.5. To extend in time our knowledge of the seismic activity, we conducted paleoseismic investigations in the Roer Graben to address the question of the possible occurrence of large earthquakes with coseismic surface ruptures. Our study along the Feldbiss fault (the western border of the graben) demonstrates its recent activity and provides numerous lines of evidence of Holocene and Late Pleistocene large earthquakes. It suggests that along the 10 km long Bree fault scarp, the return period for earthquakes with magnitude from 6.2 to 6.7 ranges from 10,000 to 20,000 years during the last 50,000 years. Considering as possible the occurrence of similar earthquakes along all the Quaternary faults in the Lower Rhine Embayment, a large earthquake could occur there each 500–1000 years. These results are important in two ways. (i) The evidence that large earthquakes occur in western Europe in the very recent past which is not only attested by historical sources, but also suggested by paleoseismic investigations in the Roer Graben. (ii) The existence of a scientific basis to better evaluate the long-term seismicity in this part of Europe (maximal magnitude and return period) in the framework of seismic hazard assessment.     

Historical intermediate-depth earthquakes in the southern Aegean Sea Benioff zone: modeling their anomalous macroseismic patterns with stochastic ground-motion simulations

We model the macroseismic damage distribution of four important intermediate-depth earthquakes of the southern Aegean Sea subduction zone, namely the destructive 1926 M?=?7.7 Rhodes and 1935 M?=?6.9 Crete earthquakes, the unique 1956 M?=?6.9 Amorgos aftershock (recently proposed to be triggered by a shallow event), and the more recent 2002 M?=?5.9 Milos earthquake, which all exhibit spatially anomalous macroseismic patterns. Macroseismic data for these events are collected from published macroseismic databases and compared with the spatial distribution of seismic motions obtained from stochastic simulation, converted to macroseismic intensity (Modified Mercalli scale, I_MM). For this conversion, we present an updated correlation between macroseismic intensities and peak measures of seismic motions (PGA and PGV) for the intermediate-depth earthquakes of the southern Aegean Sea. Input model parameters for the simulations, such as fault dimensions, stress parameters, and attenuation parameters (e.g. back-arc/along anelastic attenuation) are adopted from previous work performed in the area. Site-effects on the observed seismic motions are approximated using generic transfer functions proposed for the broader Aegean Sea area on the basis of V_S30 values from topographic slope proxies. The results are in very good agreement with the observed anomalous damage patterns, for which the largest intensities are often observed at distances >?100 km from the earthquake epicenters. We also consider two additional “prediction” but realistic intermediate-depth earthquake scenarios, and model their macroseismic distributions, to assess their expected damage impact in the broader southern Aegean area. The results suggest that intermediate-depth events, especially north of central Crete, have a prominent effect on a wide area of the outer Hellenic arc, with a very important impact on modern urban centers along northern Crete coasts (e.g. city of Heraklion), in excellent agreement with the available historical information.