Integrating macroseismic intensity distributions with a probabilistic approach: an application in Italy

The geographic distribution of earthquake effects quantified in terms of macroseismic intensities, the so-called macroseismic field, provides basic information for several applications including source characterization of pre-instrumental earthquakes and risk analysis. Macroseismic fields of past earthquakes as inferred from historical documentation may present spatial gaps, due to the incompleteness of the available information. We present a probabilistic approach aimed at integrating incomplete intensity distributions by considering the Bayesian combination of estimates provided by intensity prediction equations (IPEs) and data documented at nearby localities, accounting for the relevant uncertainties and the discrete and ordinal nature of intensity values. The performance of the proposed methodology is tested at 28 Italian localities with long and rich seismic histories and for two well-known strong earthquakes (i.e., 1980 southern Italy and 2009 central Italy events). A possible application of the approach is also illustrated relative to a 16th-century earthquake in the northern Apennines.

The experience of detailed probabilistic assessments of possible seismic effects on the territory of North Ossetia-Alania

В настоящее время во многих сейсмоактивных районах Российской Федерации (включая и территорию РСО-Алания) наблюдается определенный дефицит точных и детальных данных о возможных на этих территориях сейсмических воздействиях. Помимо того, что без этих данных не могут решаться многие академические задачи инженерной и общей сейсмологии, они также необходимы для обеспечения должного качества антисейсмического проектирования и прогнозов сейсмического риска, принятия эффективных управленческих решений и других мер защиты населения, объектов инфраструктуры и окружающей среды от возможных здесь сильных землетрясений. Поэтому настоящее исследование, направленное на уменьшение дефицита таких данных в сейсмогеологических условиях РСО-Алания представляется актуальным. Цель исследования. Совершенствованиеметодики и получение на этой основе более точных и детальных вероятностных оценок используемых в инженерно- сейсмологической практике параметров колебаний грунта при землетрясениях, представляющих опасность для территории РСО-Алания. Методы исследования. Работа выполнена с использованием оригинальной вероятностной технологии оценки сейсмической опасности, включающей в себя разработку на основе анализа большого объема экспериментальных данных по инструментальной регистрации и макросейсмическому описанию землетрясений Кавказа и других сейсмоактивных районов мира региональных моделей сильных движений грунта и сейсмичности с последующими расчетами (на основе формулы полной вероятности) результирующих характеристик возможных сейсмических воздействий на исследуемой территории. Результаты.Впервые для изучаемого района с единых методологических позиций получены уточненные вероятностные оценки широкого перечня параметров (макросейсмических интенсивностей, амплитуд, периодов, длительностей, спектров и акселерограмм) колебаний грунта при возможных на данной территории землетрясениях со средними периодами повторяемости 500, 1000, 2500 и 5000 лет, которые в ряде случаев существенно отличаются от действующих нормативов. Например, показано, что на изучаемой территории прогнозируемые амплитуды и спектры ускорений грунта в пределах одной и той же зоны балльности могут существенно (до 100%) различаться в зависимости от периодов повторяемости ожидаемых землетрясений. Вероятные спектры реакции ускорений колебаний грунта, в зависимости от частот спектральных составляющих также могут существенно (до 100%) отличаться от нормативных спектров At present, in many seismically active regions of the Russian Federation (including the territory of North Ossetia-Alania), there is a certain deficit of accurate and detailed data on possible seismic effects on these territories. In addition to the fact that without these data many academic problems of engineering and general seismology cannot be solved, they are also necessary to ensure the proper quality of earthquake-proof design and seismic risk predictions. These data is also necessary to make effective management decisions and other measures to protect the population, infrastructure and the environment from possible strong earthquakes. Therefore, the present study aimed at reducing the deficit of such data in the seismogeological conditions of North Ossetia-Alania is relevant. Aim. Improvement of the methodology and obtaining, on this basis, more accurate and detailed probabilistic estimates of the ground vibration parameters used in engineering and seismological practice during earthquakes, which are dangerous for the territory of the Republic of North Ossetia-Alania. Methods. The work was carried out using an original probabilistic technology for seismic hazard assessment, which includes the development, based on the analysis of a large amount of experimental data on instrumental registration and macroseismic description of earthquakes in the Caucasus and other seismically active regions of the world, regional models of strong ground motion and seismicity with subsequent calculations (based on the formula for complete probabilities) of the resulting characteristics of possible seismic effects on the studied territory. Results. For the first time for the studied territory, from a unified methodological standpoint, refined probabilistic estimates of a wide range of parameters (macroseismic intensities, amplitudes, periods, durations, spectra and accelerograms) of ground vibrations were obtained for possible earthquakes in a given territory with average recurrence periods of 500, 1000, 2500 and 5000 years, which in some cases differ significantly from the current standards. For example, it has been shown that in the considered territory the predicted amplitudes and spectra of ground accelerations within the same magnitude zone can differ significantly (up to 100%) depending on the recurrence periods of expected earthquakes. The probable response spectra of ground vibration accelerations, depending on the frequencies of the spectral components, can also significantly (up to 100%) differ from the standard spectra

The Influence of the Rate of Increase of Ground Vibration Accelerations During Earthquakes on the Value of the Observed Macroseismic Effect

Aims: It is known that along with the traditionally considered amplitudes and durations of ground vibrations, the rate of increase in the intensity of ground vibrations in time can also affect the level of macroseismic effects caused by earthquakes. According to the previously obtained correlations, the differences between the observed macroseismic effects during earthquakes with slow and fast increases in the amplitude level of oscillations can reach one point of the macroseismic scale. The purpose of these study is to obtain, on the basis of a significantly (almost 9 times) larger than before, the volume of initial data (in combination with a more effective method of analysis) new and more accurate quantitative estimates of the studied dependences, as well as their possible interpretation. Background: The work continues the research began in 1985-1989. A representative statistical material was used, including 1250 accelerograms of earthquakes that occurred in different regions of the world, with magnitudes M = 2.5-7.7, distances of 5-230 km and independent estimates of macroseismic intensities I = 3-10 points by the MSK or MMI. Objective: Correlations between the absolute and relative rates of increase of ground vibration accelerations during earthquakes with different magnitudes and distances, on the one hand, and macroseismic effects caused by these vibrations, on the other, are considered. Methods: The study was carried out in the form of a direct statistical comparison of the parameters describing the form of ground vibrations during earthquakes with the characteristics of variations in macroseismic effects caused by these vibrations. A sample was formed and analyzed, including 1250 accelerograms of sensible and strong earthquakes recorded in various regions of the world and having independent estimates of the macroseismic intensity of shaking at instrumental registration sites. Results: It is shown that the macroseismic intensity of shaking can depend on the relative rate of increase of acceleration amplitudes in the general wavetrain of ground vibrations. An increase in the macroseismic intensity of shaking was observed with an increase in the relative rate of increase of the amplitudes and, conversely, it decreases with a slowdown in the rate of increase of the acceleration intensity. Similar constructions, made according to the data of the Time-Frequency Signal Analysis (TFSA) of 50 accelerograms of earthquakes with M = 3.3-6.2, a distance of 7-139 km and a macroseismic intensity of 4-7 MMI points, showed the same dependence, but clearer and with large coefficients of regression and correlation. The difference between earthquakes with “fast” and “slow” accelerations in the intensity I can reach one MSK point. Conclusion: The results of this study indicate that the rate of increase in the acceleration of ground vibrations during earthquakes can in a certain way affect the macroseismic effects. Earthquakes with slowly increasing amplitudes of ground vibration accelerations form average less macroseismic effects than those with rapidly growing accelerations. Variations in the shaking intensity, at the same time, are quite significant and can be compared with variations associated with differences in soil-geomorphological conditions, focal mechanisms, general seismotectonic conditions and other factors that are traditionally taken into account in detailed assessments of seismic hazard. Therefore, this factor should also be taken into account when conducting such studies.

Integrating macroseismic intensity distributions by a probabilistic approach: an application in Italy

The geographic distribution of earthquake effects quantified in terms of macroseismic intensities, the so-called macroseismic field, provides basic information for several scopes including source characterization of pre-instrumental earthquakes and risk analysis. Macroseismic fields of past earthquakes as inferred from historical documentation may present spatial gaps, due to the incompleteness of the available information. We present a probabilistic approach aimed at integrating incomplete intensity distributions by considering the Bayesian combination of estimates provided by Intensity Prediction Equations (IPEs) and data documented at nearby localities, accounting for the relevant uncertainties and the discrete and ordinal nature of intensity data. The performance of the proposed methodology is tested at 28 Italian localities with long and rich seismic histories, and for two well-known strong earthquakes (i.e., 1980 Southern Italy and 2009 Central Italy events). A possible application of the approach is also illustrated relative to a sixteenth century earthquake in Northern Apennines.

Inhomogeneity of Macroseismic Intensities in Italy and Consequences for Macroseismic Magnitude Estimation

We show that macroseismic intensities assessed in Italy in the last decade are not homogeneous with those of the previous periods. This is partly related to the recent adoption of the European Macroseismic Scale (EMS) in place of the Mercalli–Cancani–Sieberg (MCS) scale used up to about one decade ago. The underestimation of EMS with respect to MCS is about a half of a degree on average and, even more significant, if the MCS intensities are estimated according to the approach developed for the quick evaluations of damage by macroseismic seismologists of the Italian Department of Civil Protection. We also show the inhomogeneity over time of the average differences between instrumental and macroseismic magnitudes computed from intensity data, indicating an average overestimation of magnitudes of about 0.3 units for the instrumental ones before year 1960 and of about 0.2 units for the macroseismic ones after such date. This is consistent with previous studies that hypothesized the incorrect calibration of mechanical recording seismometers operating in Italy and in the surrounding countries before the introduction of the standard electromagnetic seismometers from the beginning of 1960s. For such reasons, the magnitudes of preinstrumental earthquakes in the Catalogo Parametrico dei Terremoti Italiani seismic catalog, used for the most recent seismic hazard assessment in Italy, might be overestimated, on average, by about 0.1–0.2 magnitude units.

The new Italian seismic hazard model (MPS19)

We describe the main structure and outcomes of the new probabilistic seismic hazard model for Italy, MPS19 [Modello di Pericolosità Sismica, 2019]. Besides to outline the probabilistic framework adopted, the multitude of new data that have been made available after the preparation of the previous MPS04, and the set of earthquake rate and ground motion models used, we give particular emphasis to the main novelties of the modeling and the MPS19 outcomes. Specifically, we (i) introduce a novel approach to estimate and to visualize the epistemic uncertainty over the whole country; (ii) assign weights to each model components (earthquake rate and ground motion models) according to a quantitative testing phase and structured experts’ elicitation sessions; (iii) test (retrospectively) the MPS19 outcomes with the horizontal peak ground acceleration observed in the last decades, and the macroseismic intensities of the last centuries; (iv) introduce a pioneering approach to build MPS19_cluster, which accounts for the effect of earthquakes that have been removed by declustering. Finally, to make the interpretation of MPS19 outcomes easier for a wide range of possible stakeholders, we represent the final result also in terms of probability to exceed 0.15 g in 50 years.

Source and Attenuation Properties of the 2012 Moe, Southeastern Australia, Earthquake Sequence

The 19 June 2012 local magnitude ML 5.4 (Mw 5.1) Moe earthquake, which occurred within the Australian stable continental region, was the largest seismic event for the state of Victoria, for more than 30 yr. Seismic networks in the southeast Australian region yielded many high-quality recordings of the moderate-magnitude earthquake mainshock and its largest aftershock (ML 4.4 and Mw 4.3) at a range of distances from the epicenter. The source and attenuation characteristics of the earthquake sequence are analyzed. Almost 15,000 felt reports were received following the mainshock, and its ground motions tripped a number of coal-fired power generators in the region amounting to the loss of, approximately, 1955 MW of generation capacity. The attenuation of macroseismic intensities is shown to mimic the attenuation shape of eastern North America (ENA) models but requires an interevent bias to reduce predicted intensities. Furthermore, instrumental ground-motion recordings are compared to ground-motion models (GMMs) considered applicable for the southeastern Australian (SEA) region. Some GMMs developed for ENA and SEA provide reasonable estimates of the recorded ground motions of spectral acceleration within epicentral distances of, approximately, 100 km. The mean Next Generation Attenuation-East GMM, recently developed for stable ENA, performs relatively poorly for the 2012 Moe earthquake sequence, particularly, for short-period accelerations. These observations will help inform future seismic hazard assessments for eastern Australia.

Seismic Hazard and Risk Due to Induced Earthquakes at a Shale Gas Site

ABSTRACT Hydraulic fracturing of the first shale gas well at Preston New Road (PNR), Blackpool, United Kingdom, in late 2018, marked the end of a 7 yr United Kingdom-wide moratorium on fracking. Despite a strict traffic-light system being in place, seismic events up to ML 2.9 were induced. The ML 2.9 event was accompanied by reports of damage and was assigned European Macroseismic Scale 1998 (EMS-98) intensity VI by the British Geological Survey. The moratorium was subsequently reinstated in late 2019. The study here presents a pseudo-probabilistic seismic risk analysis and is applied to the larger of the induced events at PNR, in addition to hypothetical larger events. Initially, site characterization analysis is undertaken using direct and indirect methods. These analyses show low-velocity deposits dominate the region (VS30‾=227 m/s). We test existing ground-motion prediction equations using spatially dependent VS30 to determine applicability to the recorded waveform data and produce a referenced empirical model. Predicting median and 84th percentile peak ground velocity fields, we subsequently determine macroseismic intensities. Epicentral intensities of IV, IV–V, and VI–VII are predicted for the observed ML 2.9, and hypothetical ML 3.5 and 4.5 scenarios, respectively. A probabilistic analysis of damage is performed for 3500 ground-motion realizations (2.1≤ML≤4.5) using the OpenQuake-engine, with nonlinear dynamic analysis undertaken to define building fragility. Based on these analyses, the onset of cosmetic damage (DS1) in terms of median risk is observed for the ML 2.9 event. Mean modeled occurrences of DS1 and DS2 (minor structural damage), 75 and 10 instances, respectively, are consistent with reported damage (DS1:97, DS2:50). Significant occurrences (median≥30 buildings) of DS2, DS3, and DS4 (minor to major structural damage) are likely for ML 3.5, 4.0, and 4.5 events, respectively. However, by comparing reported damage with modeled damage due to the ML 2.9 event and considering the fact that low macroseismic intensities (EMS-98 <4) are often not reported by the public, we conclude that the previously assigned intensity of VI is too high, with V being more appropriate.

Seismological Data Acquisition and Analysis within the Scope of Citizen Science

From 2017 till 2020 a low cost seismic sensor network was built in the southern Vienna Basin, Lower Austria, as a part of ongoing educational and citizen science projects. The purpose of the project is to inform society about the seismic activity in this area and to include authorities and interested citizens into data acquisition and exploitation. Near real time (NRT) seismic data are made accessible online. Seismic events are detected and archived automatically. The visualization of these events online facilitates instantaneously estimates of the extent of the shaking area and potential damage. Peak ground velocities (PGV) are related to macroseismic intensities (EMS-98) derived from reports about ground motion felt in the vicinity of the network stations. Observed amplitudes and travel times are modeled by simple, but effective relations. Traditional and innovative localization methods based on travel times and amplitudes are applied and analyzed with respect to data quality and localization accuracy. All results are accessible online and the computer code is open and applicable, e.g. for educational purposes.