Macroseismic Questionnaires Research Articles

The Hydrological Impact of Historical and Modern Earthquakes in Central-Southern Italy: Data Analysis and Seismotectonic Insights

Abstract We present the results of research conducted on the hydrological effects induced by historical and modern earthquakes in the central-southern Apennines of Italy. We investigated thirty-nine seismic events of magnitude between M 4.1 and 7.1 that occurred between 1688 and 2020. We collected 526 observations of coseismic and postseismic hydrological effects such as increase or decrease in streamflow, variations in the water levels in wells, formation and/or disappearance of springs and changes in their discharge, and changes in the chemical and physical characteristics of waters. More than half of the observations were new data unpublished to date in the scientific literature. We find that the ease of finding data of interest is strongly influenced by the historical period in which the seismic event occurred. We summarize the methodology of data retrieval and its classification and present examples and maps of coseismic hydrological changes associated with individual seismic events. Most of our novel data derive from seismic postcards, macroseismic questionnaires, and hydrographic annals. However, a nonnegligible set of data comes from a careful rereading of sources—both historical and modern—that were at first considered fruitless. The majority of data concerns an excess flow in streams and springs, and physical–chemical variations such as turbidity, an increase in temperature, and gas emission. We also find that the maximum distance to which seismically induced streamflow responses have been reported can be related to the earthquake magnitude and use this distance to derive an alternative magnitude for some of the strongest earthquakes of our dataset. Finally, we show a potential application of this type of research because the distribution of coseismic hydrological changes can provide constraints in discriminating between the causative faults of strong historical seismic events for which the instrumental data are scarce or not available.

Testing the Environmental Seismic Intensity Scale on Data Derived from the Earthquakes of 1626, 1759, 1819, and 1904 in Fennoscandia, Northern Europe

Earthquake environmental effects (EEEs) were compiled for the earthquakes of 1626, 1759, 1819, and 1904 in the Fennoscandian Peninsula, northern Europe. The principal source of information was the contemporary newspaper press. Macroseismic questionnaires collected in 1759 and 1904 were also consulted. We prepared maps showing newly discovered EEEs together with previously known EEEs and analyzed their spatial distribution. We assigned intensities based on the 2007 Environmental Seismic Intensity (ESI) scale to 27 selected localities and compared them to intensities assigned based on the 1998 European Macroseismic Scale. While the overall agreement between the scales is good, intensities may remain uncertain due to the sparsity of written documentation. The collected data sets are most probably incomplete but still show that EEEs are not unprecedented cases in the target region. The findings include landslides and rockfalls as well as cascade effects with a risk potential and widespread water movements up to long distances. The winter earthquake of 1759 cracked ice over a large area. This investigation demonstrates that the ESI scale also has practical importance for regions with infrequent EEEs.

Re-evaluation of Seismic Intensities and Relocation of 1969 Saint Vincent Cape Seismic Sequence: A Comparison with the 1755 Lisbon Earthquake

Seismic intensity for the February 28, 1969 (Mw = 7.8) earthquake have been re-evaluated using original documents in local archives, such as, contemporary newspapers, council minutes, monographic studies, among other sources for Spain, Portugal and Morocco and answers to macroseismic questionnaires for Morocco. This information is used to plot a new intensity map for the whole region affected by the earthquake: Portugal, Spain and Morocco. The intensity values vary from VIII to IX in the E-W coast of Algarve, southern Portugal, to II–III. Furthermore, we have relocated the hypocentres for main shock and 24 aftershocks using a new 3D crustal velocity model for the Gulf of Cadiz region and a non-linear probabilistic location methodology, most of them previously lacking a depth estimate. The new locations show an E-W distribution of epicenters, with focus located in the uppermost mantle, most of them with depths between 30 and 50 km. No earthquakes have been located at depths shallower than 30 km. A comparison between peak ground accelerations (PGAs) estimated from the observed intensities for the 1969 and the Lisbon 1755 earthquakes, and synthetic PGA values, generated assuming two different scenarios (using the 1969 and 2009 earthquakes) for the 1755 Lisbon event, shows that the observed damage produced by the 1755 earthquake may be better explained assuming a reverse dip-slip mechanism oriented in NE-SW direction, similar to that of the 2009 earthquake, rather than assuming focal mechanism similar to that of the 1969 earthquake.

Regional macroseismic field and intensity residuals of the August 24, 2016, Mw=6.0 central Italy earthquake

A macroseismic investigation of the August 24, 2016, Mw=6.0 Central Italy earthquake, was carried out through an online web survey. Data were collected through a macroseismic questionnaire available at the website www.haisentitoilterremoto.it, managed by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). Over 12000 questionnaires were compiled soon after the seismic occurrence, coming from over 2600 municipalities. A statistical analysis was applied to the data collected in order to investigate the spatial distribution of intensity of the earthquake. The macroseismic intensity field (I) was described by identifying three main components: an isotropic component (II), a regional anisotropic component (IA) and a local random variations parameter (). The anisotropic component highlighted specific and well-defined geographical areas of amplification and attenuation. In general, the area between the Adriatic coast and Apennines Chain was characterized by an amplification of intensity, while the West side of the Apennines showed attenuation, in agreement with the domains found by other works focused on the analysis of instrumental data. Moreover, the regional macroseismic field showed similarities with instrumental PGA data. The results of our analysis confirm the reliability of web questionnaire data.

Macroseismic intensity investigation of the November 2014, M=5.7, Vrancea (Romania) crustal earthquake

On November 22, 2014 at 21:14:17 local hour (19:14:17 GMT) a M<sub>L</sub>=5.7 crustal earthquake occurred in the area of Marasesti city of Vrancea county (Romania) - the epicenter was located at north latitude 45.87° and east longitude 27.16°, with a focal depth of 39 km. This earthquake is the main shock of a sequence that started with this and lasted until the end of January. During the sequence, characterized by the absence of foreshocks, a number of 75 earthquakes were recorded in 72 hours, the largest of which occurred in the same day with the main shock, at 22:30 (M<sub>L</sub>= 3.1). The crustal seismicity of Vrancea seismogenic region is characterized by moderate earthquakes with magnitudes that have not exceeded M<sub>W</sub> 5.9, this value being assigned to an earthquake that occurred in historical times on March 1, 1894 (Romplus catalogue). Immediately after the 2014 earthquake occurrence, the National Institute for Earth Physics (NIEP) sent macroseismic questionnaires in all affected areas, in order to define the macroseismic field of ground shaking. According to macroseismic questionnaires survey, the intensity of epicentral area reached VI MSK, and the seismic event was felt in all the extra-Carpathian area. This earthquake caused general panic and minor to moderate damage to the buildings in the epicentral area and the northeast part of country. The main purpose of this paper is to present the macroseismic map of the earthquake based on the MSK-64 intensity scale.

An ordered probit model for seismic intensity data

Seismic intensity, measured through the Mercalli–Cancani–Sieberg (MCS) scale, provides an assessment of ground shaking level deduced from building damages, any natural environment changes and from any observed effects or feelings. Generally, moving away from the earthquake epicentre, the effects are lower but intensities may vary in space, as there could be areas that amplify or reduce the shaking depending on the earthquake source geometry, geological features and local factors. Currently, the Istituto Nazionale di Geofisica e Vulcanologia analyzes, for each seismic event, intensity data collected through the online macroseismic questionnaire available at the web-page www.haisentitoilterremoto.it . Questionnaire responses are aggregated at the municipality level and analyzed to obtain an intensity defined on an ordinal categorical scale. The main aim of this work is to model macroseismic attenuation and obtain an intensity prediction equation which describes the decay of macroseismic intensity as a function of the magnitude and distance from the hypocentre. To do this we employ an ordered probit model, assuming that the intensity response variable is related through the link probit function to some predictors. Differently from what it is commonly done in the macroseismic literature, this approach takes properly into account the qualitative and ordinal nature of the macroseismic intensity as defined on the MCS scale. Using Markov chain Monte Carlo methods, we estimate the posterior probability of the intensity at each site. Moreover, by comparing observed and estimated intensities we are able to detect anomalous areas in terms of residuals. This kind of information can be useful for a better assessment of seismic risk and for promoting effective policies to reduce major damages.

The Importance of Smartphones as Public Earthquake-Information Tools and Tools for the Rapid Engagement with Eyewitnesses: A Case Study of the 2015 Nepal Earthquake Sequence

This study investigates how people affected by the 2015 Ghorka earthquake sequence in Nepal have adopted and used the LastQuake smartphone application and associated Twitter robot, which offer rapid earthquake information focusing on felt and damaging earthquakes. It is shown that mobile devices played a key role in Nepal by providing access to earthquake information and that the LastQuake smartphone application was rapidly identified and adopted through viral spread after the mainshock. Thumbnail-based questionnaires, which have replaced online macroseismic questionnaires on the smartphone application and on the website for mobile devices, ease language barriers and have proven to be very efficient for collecting testimonies within a few tens of minutes of a felt-earthquake occurrence. Offering rapid information on felt and damaging earthquakes is an effective strategy to engage with eyewitnesses and optimize the collection of eyewitnesses’ observations. The two-way and real-time communication channels that smartphones offer raises eyewitnesses’ expectations in terms of real-time information but also offer an opportunity to improve situation awareness and provide rapid and direct guidance to individuals immediately after shaking to contribute to risk reduction.

Automatic Clustering of Macroseismic Intensity Data Points from Internet Questionnaires: Efficiency of the Partitioning around Medoids (PAM)

Online Material: Tables of earthquakes and clustering results, maps of questionnaire results, ananimation showing the evolution of the Barcelonnette event questionnaire clustering with time, and figures showing clustering comparisons (zipped archive). In the early history of scientific seismology, the systematic collection of macroseismic information was performed through standardized paper questionnaires. The Internet is changing the way earthquakes are monitored by citizens (Wald et al. , 1999; Bossu et al. , 2011, 2014). With the phenomenal growth of the Internet, many Internet‐based macroseismic survey systems have been implemented in the last decade in many parts of the world (see Wald et al. , 2011, for a survey of these systems). These kinds of systems include automatic procedures for the assignments of intensity values. The two main processing stages in assigning intensity consist of grouping data by place then determining the intensity degree that best fits the descriptions. Grouping macroseismic observations is an elaborate process: groups should be large enough to allow a general assignment of the intensity (here, “general” means that the intensity value is not assigned to an individual questionnaire) but small enough to fairly represent local geographical conditions. In well‐recognized organizations dealing with Internet macroseismic questionnaires, some ad hoc partitioning techniques are currently being used. Among these techniques, the most commonly used in macroseismology makes regular tessellation with grid squares, perhaps because it is the easiest to create and deal with. Results from grid partitioning are expected to be influenced by grid geometries. Grid geometries are probably not significantly biasing results when raw data are already artificially discretized. Artificial distributions may be expected when city center point coordinates or postal ZIP codes are used to …

Macroseismic Intensity Assessment Method for Web Questionnaires

Online Material: Tables of questions and answers of the macroseismic questionnaire, score matrices, and a figure showing a macroseismic field example. Macroseismic investigation with data collected through web‐based questionnaires is today routinely applied by most important seismological institutions, such as the U.S. Geological Survey (http://earthquake.usgs.gov/earthquakes/dyfi/; last accessed December 2014), British Geological Survey (http://www.earthquakes.bgs.ac.uk/questionnaire/EqQuestIntro.html; last accessed December 2014), European‐Mediterranean Seismological Centre (http://www.emsc-csem.org/Earthquake/Contribute/choose\_earthquake.php?lang=en; last accessed December 2014), Schweizerische Erdbebendienst (http://www.seismo.ethz.ch/eq/detected/eq\_form/index_EN; last accessed December 2014), Bureau Central Sismologique Francais (http://www.seisme.prd.fr/english.php; last accessed December 2014), and the New Zealand GeoNet project (http://www.geonet.org.nz/quakes/; last accessed December 2014). The wide diffusion of Internet and the citizen collaboration (crowdsourcing) allow documentation of information on seismic effects and production of a macroseismic field with low costs and almost in real time. Transformation from qualitative information (as given by questionnaires) to numerical quantification is a crucial issue. In the traditional evaluation of intensity, experts used to work through a complex comparison of effects basically driven by personal experience. The major problem with this approach concerns the difficulty in verifing and reproducing the evaluation process due to the lack of a detailed explanation of the employed workflow and to the large variability of possible cases. On the other hand, an automatic method for the estimation of macroseismic intensities needs to be completely well defined and specified in order to be reproducible and verifiable. For these reasons, this paper presents a comprehensive explanation of our intensity assessment method. A useful automatic method for intensity assessment should be computationally fast and strictly follow the macroseismic scales. To meet these requirements in 2010, we proposed a method that firstly quantified the effects using additive scores associated with each answer of the questionnaire item and then determined an intensity estimate for each questionnaire (Sbarra et al. , 2010). After …

Macroseismic intensity data of the 22 April 2013 Tenk (Hungary) earthquake

On 22 April 2013 an earthquake of magnitude 4.8 occurred near the village of Tenk (Hungary), which was the strongest Hungarian earthquake in the past 28 years. This event was detected by a number of seismological stations, thus it is well documented. Nevertheless, it is still possible to get further data through macroseismic surveys, which cannot be obtained using seismological instruments, but are very useful for the understanding of seismic properties of the affected area. The studied earthquake was felt in approximately third of the territory of Hungary. The number of incoming macroseismic questionnaires was over eight hundred and damage descriptions for the epicentral area reached almost one thousand. Intensity evaluation was carried out following the European Macroseismic Scale guidelines. Intensities were assigned to 211 places, including 23 districts of Budapest. The earthquake caused non-structural building damages, the epicentral intensity was estimated as VI on the EMS-98 scale and was assigned to three villages: Tenk, Átány and Erdőtelek. The event was widely felt west to the epicentre, but much less observed in the east direction. The asymmetry of the intensity distribution raises questions and requires further investigation.

How Observer Conditions Impact Earthquake Perception

Online Material: Figures of earthquake magnitude distribution; list of events; questionnaires and analysis; felt percentages analysis, Chi‐squared tests. Intensity scales define the criteria used to determine different levels of shaking in relation to environmental effects. Objective evaluations of low‐intensity degrees based on transient effects may be difficult. In particular, estimations for the number of people feeling an earthquake are critical and are qualitatively described by words such as “few,” “many,” and “most” for determining various intensity levels. In general, such qualitative amounts are converted into specific percentages for each macroseismic scale. Additionally, estimations of macroseismic intensity are influenced by variables that are mentioned in macroseismic scale degree descriptions. For example, the Mercalli–Cancani–Sieberg (MCS; Sieberg, 1930) and the modified Mercalli intensity scales (Wood and Neumann, 1931) describe intensity II as a vibration felt only by a few people, extremely susceptible, almost always on the upper floors of buildings. Another example is the European Macroseismic Scale (EMS) (Grunthal, 1998), which describes the intensity V as “felt indoors by most, outdoors by few. Many sleeping people awake.” In this work, we focus on two variables referred to as (1) people’s physical situation (what were you doing?), here categorized as “sleeping,” “at rest,” or “in motion,” and (2) the observer’s location, here categorized as “higher floors,” “lower floors,” and “outdoors.” Both variables have a partial influence on intensity assessments because they condition vibration perception. However, it is important to use an experimental method to study the weights of these variables in the quantification of felt effects. Musson (2005a) also recognized the influence of such conditions on the number of people feeling an earthquake, stating that the proportion of people in different conditions “are generally difficult to quantify in any case.” Today, we have a large quantity of data, gathered through an online macroseismic questionnaire, associated with …

Re-evaluation of the macroseismic effects produced by the March 4, 1977, strong Vrancea earthquake in Romanian territory

<p>In this paper, the macroseismic effects of the subcrustal earthquake in Vrancea (Romania) that occurred on March 4, 1977, have been re-evaluated. This was the second strongest seismic event that occurred in this area during the twentieth century, following the event that happened on November 10, 1940. It is thus of importance for our understanding of the seismicity of the Vrancea zone. The earthquake was felt over a large area, which included the territories of the neighboring states, and it produced major damage. Due to its effects, macroseismic studies were developed by Romanian researchers soon after its occurrence, with foreign scientists also involved, such as Medvedev, the founder of the Medvedev-Sponheuer-Karnik (MSK) seismic intensity scale. The original macroseismic questionnaires were re-examined, to take into account the recommendations for intensity assessments according to the MSK-64 macroseismic scale used in Romania. After the re-evaluation of the macroseismic field of this earthquake, the intensity dataset was obtained for 1,620 sites in Romanian territory. The re-evaluation was necessary as it has confirmed that the previous macroseismic map was underestimated. On this new map, only the intensity data points are plotted, without tracing the isoseismals.</p>

Application of Environmental Seismic Intensity scale (ESI 2007) to Krn Mountains 1998 <i>M</i><sub>w</sub> = 5.6 earthquake (NW Slovenia) with emphasis on rockfalls

Abstract. The 12 April 1998 Mw = 5.6 Krn Mountains earthquake with a maximum intensity of VII–VIII on the EMS-98 scale caused extensive environmental effects in the Julian Alps. The application of intensity scales based mainly on damage to buildings was limited in the epicentral area, because it is a high mountain area and thus very sparsely populated. On the other hand, the effects on the natural environment were prominent and widespread. These facts and the introduction of a new Environmental Seismic Intensity scale (ESI 2007) motivated a research aimed to evaluate the applicability of ESI 2007 to this event. All environmental effects were described, classified and evaluated by a field survey, analysis of aerial images and analysis of macroseismic questionnaires. These effects include rockfalls, landslides, secondary ground cracks and hydrogeological effects. It was realized that only rockfalls (78 were registered) are widespread enough to be used for intensity assessment, together with the total size of affected area, which is around 180 km2. Rockfalls were classified into five categories according to their volume. The volumes of the two largest rockfalls were quantitatively assessed by comparison of Digital Elevation Models to be 15 × 106 m3 and 3 × 106 m3. Distribution of very large, large and medium size rockfalls has clearly defined an elliptical zone, elongated parallel to the strike of the seismogenic fault, for which the intensity VII–VIII was assessed. This isoseismal line was compared to the tentative EMS-98 isoseism derived from damage-related macroseismic data. The VII–VIII EMS-98 isoseism was defined by four points alone, but a similar elongated shape was obtained. This isoseism is larger than the corresponding ESI 2007 isoseism, but its size is strongly controlled by a single intensity point lying quite far from others, at the location where local amplification is likely. The ESI 2007 scale has proved to be an effective tool for intensity assessment in sparsely populated mountain regions not only for very strong, but for moderate earthquakes as well. This study has shown that the quantitative definition of rockfall size and frequency, which is diagnostic for each intensity, is not very precise in ESI 2007, but this is understandable since the rockfall size is related not only to the level of shaking, but also depends highly on the vulnerability of rocky slopes.

Influence of Observation Floor and Building Height on Macroseismic Intensity

Research Article| March 01, 2012 Influence of Observation Floor and Building Height on Macroseismic Intensity Paola Sbarra; Paola Sbarra Istituto Nazionale di Geofisica e Vulcanologia Via di Vigna Murata 605, 00143 Rome, Italypaola.sbarra@ingv.it (P. S.) 1Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Search for other works by this author on: GSW Google Scholar Patrizia Tosi; Patrizia Tosi Istituto Nazionale di Geofisica e Vulcanologia Via di Vigna Murata 605, 00143 Rome, Italypaola.sbarra@ingv.it (P. S.) 1Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Search for other works by this author on: GSW Google Scholar Valerio De Rubeis; Valerio De Rubeis Istituto Nazionale di Geofisica e Vulcanologia Via di Vigna Murata 605, 00143 Rome, Italypaola.sbarra@ingv.it (P. S.) 1Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Search for other works by this author on: GSW Google Scholar Antonio Rovelli Antonio Rovelli Istituto Nazionale di Geofisica e Vulcanologia Via di Vigna Murata 605, 00143 Rome, Italypaola.sbarra@ingv.it (P. S.) 1Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2012) 83 (2): 261–266. https://doi.org/10.1785/gssrl.83.2.261 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paola Sbarra, Patrizia Tosi, Valerio De Rubeis, Antonio Rovelli; Influence of Observation Floor and Building Height on Macroseismic Intensity. Seismological Research Letters 2012;; 83 (2): 261–266. doi: https://doi.org/10.1785/gssrl.83.2.261 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Online material: Distribution of data as a function of MCS intensity and distance; additional information about the intensity residuals and the applied statistical tests. It is well known that perception of transitory effects is quite dependent on an observer’s location. The perception of an earthquake depends on whether the observer is located on a lower or upper floor within a building. Inside a building, all other things being equal, there are some specific factors that increase the perception of macroseismic effects. Macroseismic scales propose only a qualitative approximate description of the varying effects felt at lower or upper floors.... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Macroseismic effects highlight site response in Rome and its geological signature

A detailed analysis of the earthquake effects on the urban area of Rome has been conducted for the L’Aquila sequence, which occurred in April 2009, by using an online macroseismic questionnaire. Intensity residuals calculated using the mainshock and four aftershocks are analyzed in light of a very accurate and original geological reconstruction of the subsoil of Rome based on a large amount of wells. The aim of this work is to highlight ground motion amplification areas and to find a correlation with the geological settings at a subregional scale, putting in evidence the extreme complexity of the phenomenon and the difficulty of making a simplified model. Correlations between amplification areas and both near-surface and deep geology were found. Moreover, the detailed scale of investigation has permitted us to find a correlation between seismic amplification in recent alluvial settings and subsiding zones, and between heard seismic sound and Tiber alluvial sediments.

Web-based macroseismic survey in Italy: method validation and results

A new method of macroseismic surveys, based on voluntary collaboration through the Internet, has been running at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) since July 2007. The macroseismic questionnaire is addressed to a single non-specialist; reported effects are statistically analysed to extrapolate a probabilistic estimate of Mercalli Cancani Sieberg and European Macroseismic Scale intensities for that observer. Maps of macroseismic intensity are displayed online in almost real time and are continuously updated when new data are made available. For densely inhabited zones, we have received reports of felt effects for even very small events (M = 2). Six earthquakes are presented here, showing the ability of the method to give fast and interesting results. The effects reported in questionnaires coming from three towns are carefully analysed and assigned intensities are compared with those derived from traditional macroseismic surveys, showing the reliability of our web-based method.

State-of-the-art of historical earthquake research in Fennoscandia and the Baltic Republics

We review historical earthquake research in Northern Europe. 'Historical' is defined as being identical with seismic events occurring in the pre-instrumental and early instrumental periods between 1073 and the mid-1960s. The first seismographs in this region were installed in Uppsala, Sweden and Bergen, Norway in 1904-1905, but these mechanical pendulum instruments were broad band and amplification factors were modest at around 500. Until the 1960s few modern short period electromagnetic seismographs were deployed. Scientific earthquake studies in this region began during the first decades of the 1800s, while the systematic use of macroseismic questionnaires commenced at the end of that century. Basic research efforts have vigorously been pursued from the 1970s onwards because of the mandatory seismic risk studies for commissioning nuclear power plants in Sweden, Finland, NW Russia, Kola and installations of huge oil platforms in the North Sea. The most comprehensive earthquake database currently available for Northern Europe is the FENCAT catalogue covering about six centuries and representing the accumulation of work conducted by many scientists during the last 200 years. This catalogue is given in parametric form, while original macroseismic observations and intensity maps for the largest earthquakes can be found in various national publications, often in local languages. No database giving intensity data points exists in computerized form for the region. The FENCAT catalogue still contains some spurious events of various kinds but more serious are some recent claims that some of the presumed largest historical earthquakes have been assigned too large magnitude values, which would have implications for earthquake hazard levels implemented in national building codes. We discuss future cooperative measures such as establishing macroseismic data archives as a means for promoting further research on historical earthquakes in Northern Europe.

Web-based macroseismic survey: fast information exchange and elaboration of seismic intensity effects in Italy

A new method of macroseismic survey, based on voluntary collaboration through the internet, has been running at Istituto Nazionale di Geofisica e Vulcanologia (INGV), Italy, since June 2007. The macroseismic questionnaire is addressed to a single nonspecialist person and reported effects are statistically analysed to extrapolate the Mercalli-Cancani-Sieberg (MCS) scale and the European Macroseismic Scale (EMS) intensity referred to by that observer. Maps of macroseismic intensity are displayed online in almost real time and are continuously updated. The aim of the questionnaire is to evaluate seismic effects as felt by the compiler. The final result is a definition of a particular degree of intensity with an evaluation of an associated uncertainty. Results of medium to low magnitude earthquakes are presented here, showing the ability of the method to give fast and interesting results. Effects reported in questionnaires coming from towns are analysed in-depth and assigned intensities are compared with those derived from traditional macroseismic surveys, showing the reliability of the web-based method.

Revising the 1759 Kattegat earthquake questionnaires using synthetic wavefield analysis

In aseismic regions the lack of large earthquakes in modern times makes the study of historical earthquakes essential as such events often serve as the yardstick for estimating seismic hazards. Since the largest earthquakes with few exceptions took place hundreds of years ago we may claim that their recurrence times are very large or alternatively their magnitudes are biased upwards. Scandinavia is no exception in this regard and one interesting event in particular is the Kattegat earthquake of 22 December 1759. It is unique in the sense that a macroseismic questionnaire was circulated in Zealand within days of its occurrence. The puzzling feature here is that intensity ranges from VII to I across Zealand but then given I=III for Hamburg and other North German cities. We have chosen two avenues in reassessing the magnitude of the Kattegat earthquake, namely (1) 2D finite-difference (FD) waveform modelling to evaluate possible amplitude amplifications due to sub-surface basin structures and (2) re-assessing the intensity reports of the 1759 Kattegat earthquake in light of the new European Macroseismic Scale (EMS-98). Our 2D FD results show that the presence of the Danish and the North German basins led to shear wave amplitude amplification by factors of 10 and 5, respectively. This is equivalent to a significant increase or overestimation of the area of perception (tied to I=III). The re-assessment of the macroseismic observations in light of the above results and the EMS-98 downgrade the Kattegat earthquake MS magnitude from 5.6 to 5.1.

Spatial patterns of earthquake sounds and seismic source geometry

Earthquake sounds are heard during or immediately before an event up to several kilometres around the epicentral area. Sound data as heard by people have been collected by way of macroseismic questionnaires, showing that sound audibility does not follow isotropic patterns. The explanation of this effect is found in the behaviour of the particular radiation pattern of the P‐waves for each earthquake, in connection with its focal mechanism. This result opens the possibility to get more information about the earthquake source parameters in the future and past events, making use of non‐instrumental observations.