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Improving the Mediterranean seismicity picture thanks to international collaborations
Physics and Chemistry of the Earth 63 (2013) 3–11
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Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce
Improving the Mediterranean seismicity picture thanks to international collaborations S. Godey a,⇑, R. Bossu a,b, J. Guilbert b a b
Euro-Mediterranean Seismological Centre, c/o CEA, DAM, DIF, F-91297 Arpajon Cedex, France CEA, DAM, DIF, F-91297 Arpajon Cedex, France
a r t i c l e
i n f o
Article history: Available online 9 May 2013 Keywords: Bulletin Seismicity Mediterranean
a b s t r a c t The seismicity of the Mediterranean, compiled by the Euro-Mediterranean Seismological Centre (EMSC), is presented for the period 1998–2010. The data set contains earthquakes recorded by more than 3000 stations operated by 78 networks. The Euro-Med Bulletin (EMB) provides parametric information for 273,000 tectonic events. The location resolution is characterised by a constant improvement over the years made possible by gathering steadily increasing data contribution. Dedicated collaboration with the network operators through regular discussion and interaction allow proper review of input and output information. In addition to natural events, seismic networks report data related to nontectonic activity. Discrimination is a crucial step in the production of seismicity catalogues to provide accurate information used for seismic hazard assessment. Over 13 years, 62,000 artificial events are characterised, mostly located in the vicinity of the largest mine and quarry areas of the region. While a large amount of man-related seismic activity is properly described, we show that through statistical analysis, discrimination in the EMB can still be improved. This is especially useful to scan past data, when routine discrimination was usually not performed by local networks. Seismicity catalogues are also useful for integrated access data portal to diverse and distributed seismological products as multi-disciplinary approaches of seismology is necessary. It leads us to propose and develop an e-Bulletin where the scientists could access all type of seismological and engineering products (from strong motion or broad band waveforms to macroseismic analysis) by simply connecting to a single event dedicated web page. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction International seismological bulletins are an essential source of information for geophysical analysis, such as seismic hazard assessment or tomography. To determine an accurate image of the seismicity, it is crucial to rely on proper location methods but also on comprehensive data collection from a variety of institutes of regional or national scale. In this paper, we present how the EMSC succeeds to produce a reference bulletin for the EuroMediterranean region based on the merging of parametric data (Godey et al., 2006). These data are supplied to the ISC (International Seismological Centre) and provide significant information for their worldwide bulletin. Thanks to close collaborations with the local networks and extensive data description, such as event type information or amplitude and period measurements, we obtain higher constrained seismicity information, displayed for example through the azimuthal gap improvement. ⇑ Corresponding author. Tel.: +33 01 69 26 54 86; fax: +33 01 69 26 71 47. E-mail address: [email protected] (S. Godey). 1474-7065/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pce.2013.04.012
The ability to produce an accurate seismicity catalogue also requires proper identification of anthropogenic events. We present here the information available in the EMB and propose statistical tools to further discriminate nontectonic events polluting the picture of the seismicity of the region. Finally we suggest how the Euro-Med Bulletin could become an entry point to additional seismological data available for the scientific community. 2. Methodology The Euro-Med Bulletin has for goal to compute and distribute parametric event information for a large European region, from Iceland to Oman, using homogeneous procedures (Godey et al., 2006). It relies thoroughly on the data contributions from the various countries of the region. By dedicating its activities at regional scale, the EMSC has established close collaborations and interactions with local networks allowing a comprehensive data collection and the integration of new travel time observations not contributing previously, especially from Northern Africa (Amaru, 2007).
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Available for the period 1998–2010, the EMB has become a reference bulletin for the scientific community and a support for seismic hazard assessment. It provides homogeneously computed seismicity information for the region. By merging all data available, the seismicity imaged within the national networks is preserved while off-shore and bordering events benefit from crucial cross boundaries data collection. The fusion provides the integration of the various data available into a consistent and accurate event location. This process requires computing time and an unavoidable manual expertise to ensure geophysical coherence of the results (Godey et al., 2006). The event locations are computed using the ak135 global velocity model (Kennett et al., 1995). It is the only bulletin that includes all magnitude events, though depending on the data contributors’ distribution policies. The EMB is based on revised parametric data from 3154 seismic stations (this value includes any station which has contributed at least once) operated by 78 networks of 53 countries (the list of contributors is available at http://www.emsc-csem.org/Bulletin/ contrib.php), several weeks or months after earthquake occurrences. The information collected are manually validated by the operators and therefore display higher accuracy compared to automatic detections, making them suitable for scientific research purposes. The EMB includes three classes of event locations depending on the received data contributions (Godey et al., 2009). When several networks provide data for a same event, the data are associated and a new location is computed. If an event is recorded by a single network (e.g. no phase association is possible), no EMSC location is computed as the local network location cannot be improved. If the contributing network is authoritative in the region of occurrence (namely the national or regional networks), the event is kept as reported. If the contributing network is not authoritative (very distant stations from the epicentre) or if less than 4 stations contributed, the event is considered as deprecated and is discarded from the EMB. For each event, the EMB includes all the hypocentral information provided by the local networks along with arrival times. The parametric data collected by the EMSC and the EMB output are available in GSE2.0 format. The repository of raw data provided by the local networks is available online, along with the merged EMB information (http://www.emsc-csem.org/Bulletin/). They can also be retrieved by automatic data request (autodrm). Identified nontectonic seismicity is discarded from the EMSC online search tools. All the data available in the EMB are disseminated to the International Seismological Center (ISC) to be integrated in their global bulletin of magnitude larger than 3.5 (Engdahl and Bondar, 2011), providing a useful external evaluation.
3. Collaborative effort Established in 2002 by 11 seismological institutions in Europe during the EC-project EPSI, the EMB plays a federative role in the seismological community of the Euro-Med region. Focusing on a specific geographic area, close and personal discussions with the local institutes have been set up. Regularly, we review the contribution status for each current or possible partner. Several countries have distributed for the first time their data to the international community through the EMSC, in particular in the Middle East and Northern Africa (among which Egypt, Yemen, Tunisia or Azerbaijan). The EMSC has developed several procedures to interact actively with the local networks in order to ensure accurate and comprehensive data collection. Networks often reprocess their data several months after the first publication of their provisional bulletins. The EMSC collects both provisional and finalised data
and tools to avoid their duplications are applied. Before the computation of a set of EMB data, a review is performed by the contributors to verify and update their information based on monthly catalogues and contributing station details sent by email. This procedure has proven to be useful and allows the identification of local data format misinterpretation, the update of event classification which was not available in the provisional data (cf paragraph 6) and the verification of station information (location, ownership). One third of the data contributors regularly provide valuable updates. The EMSC works in coordination with international data centres like the ISC and the World Data Center for Seismology (NEIC). The international registry (IR) of seismograph stations is maintained by the ISC and the NEIC to assign and archive stations information. Through its collaborations with local networks, the EMSC collects and requests information from a significant amount of stations. Over 13 years, about 800 stations were registered or updated to the IR, the EMSC relaying the information from the data contributors. Furthermore, the three international centres are defining a new nomenclature for event type description (cf paragraph 6). The EMSC also collaborates with international organisations like IASPEI (International Association of Seismology and Physics of the Earth) which provides guidelines for good practices in seismology (Bormann, 2012). The velocity model ak135 and the GSE2.0 output format used by the EMSC are recommendations from IASPEI, along with the seismic phases coding conventions. The recent station and network coding standard was proposed by the international data centres in association with the FDSN (International Federation of Digital Seismograph Networks) and its implementation is in discussion. This new standard should fulfil the description needs of the constantly increasing number of stations. The seismicity catalogue developed by the EMSC has become a crucial entry point to link seismological data and products gathered and managed by the seismological community. The EMB, in addition to worldwide real time data from the EMSC is the base of the seismic data portal implemented under the NERIES and NERA projects (www.seismicportal.eu). Through a unique event identification source, distributed and various seismological products (e.g. broadband waveforms, accelerometric data, shakemaps) can be accessed.
4. The Euro-Med Bulletin 1998–2010 In Fig. 1, the seismicity of the Mediterranean region as available in the EMB is presented in terms of magnitude and depth for the period 1998–2010. The study area is defined by: 22–52° latitude, 15° to 63° longitude. A total of 273,000 tectonic events are described with 7360 displaying a magnitude larger than 4. The whole region presents high seismic rates spread from the West along the Gibraltar Strait to the East with the Zagros Arc. Clusters of seismicity are observed following the Hellenic Arc, along the Northern and Eastern Anatolian faults and in the Zagros Mountains. Lower seismicity rate is also observed along the Absheron Sill across the Caspian Sea and in the Tyrrhenian Sea. The Gibraltar Strait and the Northern Coast of Morocco and Algeria also define a segment of seismicity. 14 events of M > 6.5 occurred in the region over the 13 years analysed, mostly in Iran and Turkey. Deep events, generally connected to lithospheric subduction are concentrated in three specific regions in Greece, down to 300 km along the Hellenic Arc; in Italy down to 500 km following the subduction of the Calabrian Arc into the Tyrrhenian Sea and in Romania, depicting the Vrancea intra-continental slab down to 170 km. Since 1998, the number of events with magnitude larger than 3 included in the EMB has steadily increased, following the growth of contributing stations and networks, leading to the sampling of a
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Fig. 1. Seismicity of the Mediterranean region from the Euro-Med Bulletin of 1998–2010. Tectonic events with magnitude greater than 3 are plotted. Top: magnitude distribution. Bottom: depth distribution.
wider seismicity region (Fig. 2). The large value observed in 2008 is related to several large earthquakes in Greece (6 events of magnitude larger than 6 in January, February, June and July 2008) and the subsequent high seismicity. 5. Bulletin completeness 5.1. Resolution overview This paragraph provides figures reflecting the importance of data exchange to accurately constrain the seismicity of the Mediterranean region, in particular for off-shore events.
With increasing event magnitude, the number of recording stations increases. In the EMB, data association from different operators is systematically available for events of magnitude larger than 5 (Fig. 3). However, data merging is also possible for low magnitude seismicity as 45% of the events with magnitude between 2 and 3 being recorded by more than one network. Since the beginning of the EMB production, the EMSC has received data from an increasing number of institutes. 20 additional networks have joined the data exchange since 1998, summing to 78 contributors from 53 countries. However operational duty depends on various factors, including national political changes, and some countries are not able to provide continuously seismic
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Fig. 2. Evolution of the number of events with magnitude larger than 3 in the EuroMed Bulletin since 1998 (histogram) in comparison with the number of contributing stations.
Fig. 4. Evolution with time of the proportion of events with azimuthal gap lower than 90° for magnitude above 3 (diamond), 4 (square) and 5 (triangle).
Fig. 5. Evolution of the average number of stations used to locate events in the EMB for different magnitudes. The high value in 2007 is driven by the high number of contributing stations from North America for three Azores island earthquakes.
Fig. 3. Proportion of associated events available in the Euro-Bulletin as a function of magnitude.
information. It has particularly affected the Mediterranean Basin in the last few years. To ensure the bulletin quality and completeness, important contributions are and will be included in the EMB after its production. Despite this fluctuating contribution, the number of stations entering the EMSC collection regularly increases thanks to the new contributors but also thanks to various new installations (Fig. 2). Over 13 years, the number of contributing stations in the EMB has more than doubled. Consequently, the constraint on the seismicity has steadily increased since 1998 as reflected by the number of phases used in the hypocentre determination of events of magnitude larger than 4 (Figs. 4 and 5) with an average value jumping from 64 in 1998 to 134 in 2010. In parallel the average azimuthal gap of 80% of the events went from 190° in 1998 to 160° in 2010 (Godey et al., 2009).
5.2. Importance of data merging The association of parametric data from a maximum number of seismic operators is necessary to produce the most comprehensive archive of earthquake information. The station geographic distribution obtained allows to sample more accurately the seismicity. Events occurring off shore or in border regions are better
constrained when adding international data to the local network information. It impacts significantly the event azimuthal coverage and hypocentral location. Fig. 6 provides the example of data merging effect in the Calabrian Arc region, where the remaining segment of oceanic crust subducts into the Tyrrhenian Sea. 4620 events are recorded in the focused region over 13 years. The local network information display an average gap of 115°. The azimuthal coverage decreases away from the shore due to the absence of stations to the North West. The seismicity depth distribution shows a diffuse signature down to 300 km and few events at 500 km (top panel of Fig. 6). The EMB for the same events display an improved azimuthal coverage with an average gap of 98°. This is particularly significant for off shore events about 50 km away from the shore with a gap improvement of more than 90°. Moreover, the events depth distribution unravelled in the EMB is significantly different. The dipping signature presents a narrower width at 300 km with a less dispersive seismicity. Two plunging angles are characterised from almost vertical down to 300 km, it reaches about 45° down to 500 km. It corresponds to the deflection of the Ionian lithosphere subduction underneath the Calabrian arc as imaged in tomographic studies (Faccenna et al., 2004, Giacomuzzi et al., 2011). This example shows how data merging can impact the azimuthal coverage for off-shore events in a well monitored region. However monitoring systems and data availability vary among countries in particular in the countries surrounding the Mediterra-
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Fig. 6. Influence of data merging on the seismicity azimuthal resolution in South Italy. Top: hypocentral location and azimuthal gap of the 4620 events analysed in the study as reported by the Italian network from 1998 to 2010. Centre: evolution of the azimuthal gap between the Italian network solutions to the EMB solutions. Bottom: hypocentral location and azimuthal gap obtained in the EMB.
nean Sea. Seismic bulletin are generally available earlier from the Northern Side of the Mediterranean Sea where real time operating systems are implemented, while Northern African and Middle East networks usually provide data on a yearly base. The Euro-Med Bulletin is particularly dedicated to collect data from countries which
do not have rapid data exchange system. Those data are of great importance for unravelling the Mediterranean Basin tectonic regimes. The integration of data from a single network can provide significant information for a specific region and help constraining the seismicity. This is particularly the case in the Eastern Mediter-
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Fig. 7. Impact on events azimuthal gap of adding 2010 data from the Libyan network. Top: preliminary result without using Libyan data. Bottom: final EMB result when using Libyan data. Black triangles correspond to the 14 Libyan stations added for that period.
ranean side. Fig. 7 displays an example around the Hellenic Arc where the azimuthal gap is significantly improved by adding the 2010 data from the Libyan network for 174 events. In Crete, the gap is divided by two for several events in the Southern side of the Hellenic Arc. The 14 stations installed in Libya, for which data were received in 2010, are one of the only sources of information available from the south-western direction. Improvements in the Strait of Sicily are also clearly observed, in Tunisia and Algeria. In general, the gap has improved of more than 50° for 80 events.
6. Nontectonic seismicity Discrimination is a key issue to compute accurate seismicity catalogue useful for seismic hazard assessment. This is particularly important when integrating all magnitude events as anthropogenic activity mostly generates low magnitude seismicity. The EMSC collaborates with the local networks to improve the identification of quarry and mine blasts for the region using different means. Firstly, event type information is collected and archived in our database. A monthly verification is performed before producing the bulletin, which allows us to integrate
the latest event type classification available from the seismic institutes. After the production, the catalogue is available to all data providers for further analysis and discrimination. This is particularly important if a network discards anthropogenic events from its contribution to the EMSC while the neighbouring networks record and provide data without performing event type identification. As an example, the EMB is verified against the LDG (Laboratoire de Détection et de Géophysique, France) anthropogenic events list and leads to re-classification of 700 events over 13 years, mostly in France and Northern Spain but also off-shore marine explosions in the Mediterranean Sea, English Channel and the Bay of Biscay. The GSE2.0 event type definition is used in the EMB. Information collected from the contributors in various format are parsed into this single format. However, the description of each event type and the classification procedure vary tremendously among operators. These inconsistencies lead the EMSC, ISC and NEIC to define a new international nomenclature submitted to IASPEI. Its goal is to provide guidelines to commonly describe event type information by using a hierarchy that clarifies and reflects more accurately the analyst knowledge. This includes in particular the introduction of a general anthropogenic event type.
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Fig. 8. Location density map of anthropogenic events as classified in the Euro-Med Bulletin (1998–2010). Contour areas show the number of events within a 0.25° region.
In this paragraph, we focus on the EMB data available from 2007 to 2010, period for which all magnitude events are integrated and where discrimination is most crucial. In the Mediterranean region, there are 44,315 events labelled as related to nontectonic or induced activity over 4 years (about 30 per day). The whole distribution (Fig. 8) is dominated by sources in Spain, particularly in the North, in relation with very active mines. Most of the sources are located in Catalonia, Navarra and in the North West of Castile y Leon. The mining areas of Lubin and Upper Silesia in Poland are also depicted as very high and well limited patterns of events. Other mine regions are well defined in Germany, Switzerland, Croatia, Czech Republic, Austria, Turkey and Ukraine (Fig. 8). A more spread pattern is also observed at the border between Lebanon, Israel and Jordan. The majority of the anthropogenic events are identified as mine explosions (68% of all nontectonic events) and induced events (30%) while rock bursts are a minor contribution (Fig. 9). Being related to human activity, mine and quarry blasts are generally performed during working hours. The time distribution of events in the EMB including nontectonic events (Fig. 10) shows a clear and typical peak from 10am to 4 pm (Gulia, 2010). Meanwhile, when discarding nontectonic events (Fig. 10), a regular time distribution is observed throughout the day. On both histograms, a typical increase of the number of events is observed during night hours, when the signal to noise ratio is higher due to minimum anthropogenic noise, and detection level is lower (Rydelek and
Fig. 10. Histograms of the number of events per hour in the Mediterranean region as described in the Euro-Med Bulletin including (white) and discarding (gray) nontectonic events. All magnitude events (1998–2010) are taken into account and local time is used.
Sacks, 1989). However, a residual increase of about 500 events per hour is observed around noon, showing that discrimination is still incomplete. Thanks to the local network information, contamination of the EMB by human activity is strongly limited, though indication of residual nontectonic events is identified. Additional statistical analysis of the catalogue can provide useful discrimination tools which are presented in the next paragraph. 7. Perspectives 7.1. A posteriori discrimination
Fig. 9. Distribution of anthropogenic event type in the Euro-Med Bulletin (values computed over 4 years 2007–2010) where all magnitude events are incorporated.
Over the years, identification of artificial events has improved, based on spectral analysis, cross-correlation or on expert knowledge and quarry operators listing. Several studies have proven to be efficient to detect artificial seismicity in national bulletins (Gulia, 2010) but also at local level in north western Turkey (Horasan et al., 2009), the Vrancea region (Telesca et al., 2012) or in Sicily (Ursino et al., 2001). As first approach, the event temporal distribution contains useful information which can be used to improve identification of nontectonic events. Often, quarry blasts are not only performed during daytime but at very specific hour.
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Fig. 11. Time–space analysis of natural (black) and anthropogenic (red) events. Top: in the vicinity of the Luzenac talc quarry (Southern France) from 2007 to 2010. Central point: 42.8° North, 1.8° East. Search radius: 20 km. 33 events out of 112 are classified as tectonic. Bottom: in the vicinity of the Panaguyrichté copper mine (Bulgaria) from 2007 to 2010. Central point: 42.5° North, 24.1° East. Search radius: 20 km. 190 events out of 480 are classified as tectonic.
Here we propose to apply an a posteriori tool to seismicity catalogues based on today’s knowledge. We show hereafter how past information can be further discriminate. In Southern France, the Luzenac quarry operators perform blasts at 17:50 (local time). The events distribution 20 km around the Luzenac quarry is shown in Fig. 11-Top and includes 112 events over 4 years. The preferred blasting time is clearly seen, along with a seasonal trend for summer operation. However, a third of the events are still labelled as being of natural origin while their origin time is precisely the preferred blasting time. We can also see that since 2009, the majority of the blasts are well identified, while in the two previous years, most events are labelled as natural. The inspection of known quarry areas allows the identification of numerous misclassified events to be discarded from seismicity catalogues. By collecting the known quarries location from the network operators, an epistemic analysis based on occurrence time could be performed to search for nontectonic events which were not properly identified. The geographic distribution of known artificial events is also a starting point for further analysis of the discrimination performances. For example, a cluster of nontectonic events is observed in central Bulgaria (Fig. 8). By searching on satellite images, this cluster can be associated with a copper mine, located near the city of Panaguyrichté. Plotting the event occurrence time as a function of date and distance to the centre of the mining area, shows that almost all events occur at 14:30 and 15:00 (local time). However, 190 events over 4 years are classified as nontectonic (40% of the total number of events) in particular for the first semester of 2007 and 2008 (Fig. 11-Bottom).
These two examples show how the current event type information provided by the contributing networks, though incomplete, can be used to back process the whole EMB and improve discrimination. It is therefore important to encourage the distribution of event type information and the collection of known quarries location to allow scanning of the EMB using event temporal distribution. With an improved discrimination between man-made and tectonic events, the seismicity imaged in the EMB will be more accurate for further active tectonic interpretation and will avoid spreading erroneous results into seismic hazard assessment. 7.2. E-Bulletin Technical developments now allow to exchange and link distant information to a unique entry point thanks to dedicated communication protocols. Users are not only interested in parametric data but also wish to access various seismological products related to a particular event. The EMB by focusing on a specific region and dedicated to the research community could become the place to reach several types of information which would be gathered in an event portfolio. Thanks to a single identifier developed within the NERIES project, each seismic event is assigned a unique identification number (UNID) shared by several Institutes. Currently all data from the EMSC real time system can be linked to the EMB data through the UNID, including photos, macroseismic questionnaires and special reports. Within the Seismic Portal (www.seismicportal.eu), seismic waveforms hosted at ODC (Orfeus Data Center)
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are accessible thanks to the UNID. The SED (Swiss Seismological Service) at ETH and gempa GmbH developed an accelerometric waveform parametrization module embedded in SeisComp3 (Hanka et al., 2010) for real time strong motion data exchange based on EMSC earthquake location. Further independent data can be linked under the same identifier such as moment tensors of the EMMA database (Pondrelli et al., 2011), shakemaps, etc. 8. Conclusion It has been shown that through dedicated interaction with local networks, an enhanced seismicity bulletin can be achieved. The seismic monitoring of the Mediterranean region has improved over the 13 years for which the Euro-Med Bulletin is available, leading to a steadily increasing event azimuthal coverage. Nontectonic events should be properly qualified to be used for seismic hazard assessment. We have demonstrated that event discrimination has become more reliable thanks to cross border reviews. We describe how artificial event identification can be completed by a posteriori analysis using event occurrence time and quarry operations knowledge from the operating institutes. While the EMSC is constantly working on improving the parametric information available in its bulletin, we propose to develop an e-Bulletin for each event within which users will access all seismological products and scientific documentation related to it. Acknowledgment This work could not be done with the support of the EMSC members and all the EMB data contributors. The EMB has partially benefited from funding provided by the NERA Project (SA1) in the Seventh Framework Programme [FP7/2007–2013] under Grant agreement n° 262330. We thank the ISC for their help in data integration coordination. And last but not least, we thank the LDG for their daily collaboration and crucial support.
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References Amaru, M., 2007. Global travel time tomography with 3-D reference models, Geol. Traiectina. PhD thesis, Utrecht Univ. 1_174. Bormann, P., 2012. New Manual of Seismological Observatory Practice (NMSOP-2), IASPEI, GFZ German Research Centre for Geosciences, Potsdam, doi: 10.2312/ GFZ.NMSOP-2. Faccenna, C., Piromallo, C., Crespo-Blanc, A., Jolivet, L., Rossetti, F., 2004. Lateral slab deformation and the origin of the western Mediterranean arcs. Tectonics 23. http://dx.doi.org/10.1029/2002TC001488. Engdahl, R., Bondar, I., 2011. Seismological Networks, Encyclopedia of Solid Earth Geophysics, Springer Reference, Gupta, Harsh, K., (Ed.), XXXVIII, p. 1328. Giacomuzzi, G., Chiarabba, C., De Gori, P., 2011. Linking the Alps and Apennines subduction systems: new constraints revealed by high-resolution teleseismic tomography. Earth Planet. Sci. Lett. 301 (3–4), 531–543. Godey, S., Mazet-Roux, G., Bossu, R., Merrer, S., Guilbert, J., 2009. Ten years of seismicity in the Euro-Mediterranean region: Panorama of the EMSC bulletin 1998–2007, Cahiers du Centre Européen de Géodynamique et de Séismologie, pp. 1–14. Godey, S., Bossu, R., Guilbert, J., Mazet-Roux, G., 2006. The Euro-Mediterranean Bulletin: a comprehensive seismological bulletin at regional scale. Seismol. Res. Lett. 77/4, 460–474. Gulia, L., 2010. Detection of quarry and mine blast contamination in European regional catalogues. Nat. Hazards 53, 229–249. http://dx.doi.org/10.1007/ s11069-009-9426-8. Hanka, W., Saul, J., Weber, B., Becker, J., Harjadi, P., 2010. Real-time earthquake monitoring for tsunami warning in the Indian Ocean and beyond. Nat. Hazards Earth Syst. Sci. 10 (12), 2611–2622. Horasan, G., Guney, A.B., Kusmezer, A., Bekler, F., Ogutcu, Z., Musaoglu, N., 2009. Contamination of seismicity catalogs by quarry blasts: an example from Istanbul and its vicinity, northwestern Turkey. J. Asian Earth Sci. 34 (1), 90–99. Kennett, B.L.N., Engdahl, E.R., Buland, R., 1995. Constraints on seismic velocities in the Earth from travel times. Geophys. J. Int. 122, 108–124. Pondrelli, S., Salimbeni, S., Morelli, A., Ekström, G., Postpischl, L., Vannucci, G., Boschi, E., 2011. European-Mediterranean regional centroid moment tensor catalog: solutions for years 2005–2008. Phys. Earth Planet. In. 185 (3–4), 74–81, doi:10.1016/j.pepi.2011.01.007. Rydelek, P.A., Sacks, I.S., 1989. Testing the completeness of earthquake catalogs and the hypothesis of self-similarity. Nature 337, 251–253. Telesca, L., Alcaz, V., Sandu, I., 2012. Analysis the 1978–2008 crustal and sub-crustal earthquake catalog of Vrancea region. Nat. Hazards Earth Syst. Sci. 12, 1321– 1325. http://dx.doi.org/10.5194/nhess-12-1321-2012. Ursino, A., Langer, H., Scarfì, L., Di Grazia, G., Gresta, S., 2001. Discrimination of quarry blasts from tectonic microearthquakes in the Hyblean Plateau (Southeastern Sicily). Ann. Geophys. 44 (4), 703–722.
Contents lists available at SciVerse ScienceDirect
Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce
Improving the Mediterranean seismicity picture thanks to international collaborations S. Godey a,⇑, R. Bossu a,b, J. Guilbert b a b
Euro-Mediterranean Seismological Centre, c/o CEA, DAM, DIF, F-91297 Arpajon Cedex, France CEA, DAM, DIF, F-91297 Arpajon Cedex, France
a r t i c l e
i n f o
Article history: Available online 9 May 2013 Keywords: Bulletin Seismicity Mediterranean
a b s t r a c t The seismicity of the Mediterranean, compiled by the Euro-Mediterranean Seismological Centre (EMSC), is presented for the period 1998–2010. The data set contains earthquakes recorded by more than 3000 stations operated by 78 networks. The Euro-Med Bulletin (EMB) provides parametric information for 273,000 tectonic events. The location resolution is characterised by a constant improvement over the years made possible by gathering steadily increasing data contribution. Dedicated collaboration with the network operators through regular discussion and interaction allow proper review of input and output information. In addition to natural events, seismic networks report data related to nontectonic activity. Discrimination is a crucial step in the production of seismicity catalogues to provide accurate information used for seismic hazard assessment. Over 13 years, 62,000 artificial events are characterised, mostly located in the vicinity of the largest mine and quarry areas of the region. While a large amount of man-related seismic activity is properly described, we show that through statistical analysis, discrimination in the EMB can still be improved. This is especially useful to scan past data, when routine discrimination was usually not performed by local networks. Seismicity catalogues are also useful for integrated access data portal to diverse and distributed seismological products as multi-disciplinary approaches of seismology is necessary. It leads us to propose and develop an e-Bulletin where the scientists could access all type of seismological and engineering products (from strong motion or broad band waveforms to macroseismic analysis) by simply connecting to a single event dedicated web page. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction International seismological bulletins are an essential source of information for geophysical analysis, such as seismic hazard assessment or tomography. To determine an accurate image of the seismicity, it is crucial to rely on proper location methods but also on comprehensive data collection from a variety of institutes of regional or national scale. In this paper, we present how the EMSC succeeds to produce a reference bulletin for the EuroMediterranean region based on the merging of parametric data (Godey et al., 2006). These data are supplied to the ISC (International Seismological Centre) and provide significant information for their worldwide bulletin. Thanks to close collaborations with the local networks and extensive data description, such as event type information or amplitude and period measurements, we obtain higher constrained seismicity information, displayed for example through the azimuthal gap improvement. ⇑ Corresponding author. Tel.: +33 01 69 26 54 86; fax: +33 01 69 26 71 47. E-mail address: [email protected] (S. Godey). 1474-7065/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pce.2013.04.012
The ability to produce an accurate seismicity catalogue also requires proper identification of anthropogenic events. We present here the information available in the EMB and propose statistical tools to further discriminate nontectonic events polluting the picture of the seismicity of the region. Finally we suggest how the Euro-Med Bulletin could become an entry point to additional seismological data available for the scientific community. 2. Methodology The Euro-Med Bulletin has for goal to compute and distribute parametric event information for a large European region, from Iceland to Oman, using homogeneous procedures (Godey et al., 2006). It relies thoroughly on the data contributions from the various countries of the region. By dedicating its activities at regional scale, the EMSC has established close collaborations and interactions with local networks allowing a comprehensive data collection and the integration of new travel time observations not contributing previously, especially from Northern Africa (Amaru, 2007).
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Available for the period 1998–2010, the EMB has become a reference bulletin for the scientific community and a support for seismic hazard assessment. It provides homogeneously computed seismicity information for the region. By merging all data available, the seismicity imaged within the national networks is preserved while off-shore and bordering events benefit from crucial cross boundaries data collection. The fusion provides the integration of the various data available into a consistent and accurate event location. This process requires computing time and an unavoidable manual expertise to ensure geophysical coherence of the results (Godey et al., 2006). The event locations are computed using the ak135 global velocity model (Kennett et al., 1995). It is the only bulletin that includes all magnitude events, though depending on the data contributors’ distribution policies. The EMB is based on revised parametric data from 3154 seismic stations (this value includes any station which has contributed at least once) operated by 78 networks of 53 countries (the list of contributors is available at http://www.emsc-csem.org/Bulletin/ contrib.php), several weeks or months after earthquake occurrences. The information collected are manually validated by the operators and therefore display higher accuracy compared to automatic detections, making them suitable for scientific research purposes. The EMB includes three classes of event locations depending on the received data contributions (Godey et al., 2009). When several networks provide data for a same event, the data are associated and a new location is computed. If an event is recorded by a single network (e.g. no phase association is possible), no EMSC location is computed as the local network location cannot be improved. If the contributing network is authoritative in the region of occurrence (namely the national or regional networks), the event is kept as reported. If the contributing network is not authoritative (very distant stations from the epicentre) or if less than 4 stations contributed, the event is considered as deprecated and is discarded from the EMB. For each event, the EMB includes all the hypocentral information provided by the local networks along with arrival times. The parametric data collected by the EMSC and the EMB output are available in GSE2.0 format. The repository of raw data provided by the local networks is available online, along with the merged EMB information (http://www.emsc-csem.org/Bulletin/). They can also be retrieved by automatic data request (autodrm). Identified nontectonic seismicity is discarded from the EMSC online search tools. All the data available in the EMB are disseminated to the International Seismological Center (ISC) to be integrated in their global bulletin of magnitude larger than 3.5 (Engdahl and Bondar, 2011), providing a useful external evaluation.
3. Collaborative effort Established in 2002 by 11 seismological institutions in Europe during the EC-project EPSI, the EMB plays a federative role in the seismological community of the Euro-Med region. Focusing on a specific geographic area, close and personal discussions with the local institutes have been set up. Regularly, we review the contribution status for each current or possible partner. Several countries have distributed for the first time their data to the international community through the EMSC, in particular in the Middle East and Northern Africa (among which Egypt, Yemen, Tunisia or Azerbaijan). The EMSC has developed several procedures to interact actively with the local networks in order to ensure accurate and comprehensive data collection. Networks often reprocess their data several months after the first publication of their provisional bulletins. The EMSC collects both provisional and finalised data
and tools to avoid their duplications are applied. Before the computation of a set of EMB data, a review is performed by the contributors to verify and update their information based on monthly catalogues and contributing station details sent by email. This procedure has proven to be useful and allows the identification of local data format misinterpretation, the update of event classification which was not available in the provisional data (cf paragraph 6) and the verification of station information (location, ownership). One third of the data contributors regularly provide valuable updates. The EMSC works in coordination with international data centres like the ISC and the World Data Center for Seismology (NEIC). The international registry (IR) of seismograph stations is maintained by the ISC and the NEIC to assign and archive stations information. Through its collaborations with local networks, the EMSC collects and requests information from a significant amount of stations. Over 13 years, about 800 stations were registered or updated to the IR, the EMSC relaying the information from the data contributors. Furthermore, the three international centres are defining a new nomenclature for event type description (cf paragraph 6). The EMSC also collaborates with international organisations like IASPEI (International Association of Seismology and Physics of the Earth) which provides guidelines for good practices in seismology (Bormann, 2012). The velocity model ak135 and the GSE2.0 output format used by the EMSC are recommendations from IASPEI, along with the seismic phases coding conventions. The recent station and network coding standard was proposed by the international data centres in association with the FDSN (International Federation of Digital Seismograph Networks) and its implementation is in discussion. This new standard should fulfil the description needs of the constantly increasing number of stations. The seismicity catalogue developed by the EMSC has become a crucial entry point to link seismological data and products gathered and managed by the seismological community. The EMB, in addition to worldwide real time data from the EMSC is the base of the seismic data portal implemented under the NERIES and NERA projects (www.seismicportal.eu). Through a unique event identification source, distributed and various seismological products (e.g. broadband waveforms, accelerometric data, shakemaps) can be accessed.
4. The Euro-Med Bulletin 1998–2010 In Fig. 1, the seismicity of the Mediterranean region as available in the EMB is presented in terms of magnitude and depth for the period 1998–2010. The study area is defined by: 22–52° latitude, 15° to 63° longitude. A total of 273,000 tectonic events are described with 7360 displaying a magnitude larger than 4. The whole region presents high seismic rates spread from the West along the Gibraltar Strait to the East with the Zagros Arc. Clusters of seismicity are observed following the Hellenic Arc, along the Northern and Eastern Anatolian faults and in the Zagros Mountains. Lower seismicity rate is also observed along the Absheron Sill across the Caspian Sea and in the Tyrrhenian Sea. The Gibraltar Strait and the Northern Coast of Morocco and Algeria also define a segment of seismicity. 14 events of M > 6.5 occurred in the region over the 13 years analysed, mostly in Iran and Turkey. Deep events, generally connected to lithospheric subduction are concentrated in three specific regions in Greece, down to 300 km along the Hellenic Arc; in Italy down to 500 km following the subduction of the Calabrian Arc into the Tyrrhenian Sea and in Romania, depicting the Vrancea intra-continental slab down to 170 km. Since 1998, the number of events with magnitude larger than 3 included in the EMB has steadily increased, following the growth of contributing stations and networks, leading to the sampling of a
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Fig. 1. Seismicity of the Mediterranean region from the Euro-Med Bulletin of 1998–2010. Tectonic events with magnitude greater than 3 are plotted. Top: magnitude distribution. Bottom: depth distribution.
wider seismicity region (Fig. 2). The large value observed in 2008 is related to several large earthquakes in Greece (6 events of magnitude larger than 6 in January, February, June and July 2008) and the subsequent high seismicity. 5. Bulletin completeness 5.1. Resolution overview This paragraph provides figures reflecting the importance of data exchange to accurately constrain the seismicity of the Mediterranean region, in particular for off-shore events.
With increasing event magnitude, the number of recording stations increases. In the EMB, data association from different operators is systematically available for events of magnitude larger than 5 (Fig. 3). However, data merging is also possible for low magnitude seismicity as 45% of the events with magnitude between 2 and 3 being recorded by more than one network. Since the beginning of the EMB production, the EMSC has received data from an increasing number of institutes. 20 additional networks have joined the data exchange since 1998, summing to 78 contributors from 53 countries. However operational duty depends on various factors, including national political changes, and some countries are not able to provide continuously seismic
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Fig. 2. Evolution of the number of events with magnitude larger than 3 in the EuroMed Bulletin since 1998 (histogram) in comparison with the number of contributing stations.
Fig. 4. Evolution with time of the proportion of events with azimuthal gap lower than 90° for magnitude above 3 (diamond), 4 (square) and 5 (triangle).
Fig. 5. Evolution of the average number of stations used to locate events in the EMB for different magnitudes. The high value in 2007 is driven by the high number of contributing stations from North America for three Azores island earthquakes.
Fig. 3. Proportion of associated events available in the Euro-Bulletin as a function of magnitude.
information. It has particularly affected the Mediterranean Basin in the last few years. To ensure the bulletin quality and completeness, important contributions are and will be included in the EMB after its production. Despite this fluctuating contribution, the number of stations entering the EMSC collection regularly increases thanks to the new contributors but also thanks to various new installations (Fig. 2). Over 13 years, the number of contributing stations in the EMB has more than doubled. Consequently, the constraint on the seismicity has steadily increased since 1998 as reflected by the number of phases used in the hypocentre determination of events of magnitude larger than 4 (Figs. 4 and 5) with an average value jumping from 64 in 1998 to 134 in 2010. In parallel the average azimuthal gap of 80% of the events went from 190° in 1998 to 160° in 2010 (Godey et al., 2009).
5.2. Importance of data merging The association of parametric data from a maximum number of seismic operators is necessary to produce the most comprehensive archive of earthquake information. The station geographic distribution obtained allows to sample more accurately the seismicity. Events occurring off shore or in border regions are better
constrained when adding international data to the local network information. It impacts significantly the event azimuthal coverage and hypocentral location. Fig. 6 provides the example of data merging effect in the Calabrian Arc region, where the remaining segment of oceanic crust subducts into the Tyrrhenian Sea. 4620 events are recorded in the focused region over 13 years. The local network information display an average gap of 115°. The azimuthal coverage decreases away from the shore due to the absence of stations to the North West. The seismicity depth distribution shows a diffuse signature down to 300 km and few events at 500 km (top panel of Fig. 6). The EMB for the same events display an improved azimuthal coverage with an average gap of 98°. This is particularly significant for off shore events about 50 km away from the shore with a gap improvement of more than 90°. Moreover, the events depth distribution unravelled in the EMB is significantly different. The dipping signature presents a narrower width at 300 km with a less dispersive seismicity. Two plunging angles are characterised from almost vertical down to 300 km, it reaches about 45° down to 500 km. It corresponds to the deflection of the Ionian lithosphere subduction underneath the Calabrian arc as imaged in tomographic studies (Faccenna et al., 2004, Giacomuzzi et al., 2011). This example shows how data merging can impact the azimuthal coverage for off-shore events in a well monitored region. However monitoring systems and data availability vary among countries in particular in the countries surrounding the Mediterra-
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Fig. 6. Influence of data merging on the seismicity azimuthal resolution in South Italy. Top: hypocentral location and azimuthal gap of the 4620 events analysed in the study as reported by the Italian network from 1998 to 2010. Centre: evolution of the azimuthal gap between the Italian network solutions to the EMB solutions. Bottom: hypocentral location and azimuthal gap obtained in the EMB.
nean Sea. Seismic bulletin are generally available earlier from the Northern Side of the Mediterranean Sea where real time operating systems are implemented, while Northern African and Middle East networks usually provide data on a yearly base. The Euro-Med Bulletin is particularly dedicated to collect data from countries which
do not have rapid data exchange system. Those data are of great importance for unravelling the Mediterranean Basin tectonic regimes. The integration of data from a single network can provide significant information for a specific region and help constraining the seismicity. This is particularly the case in the Eastern Mediter-
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Fig. 7. Impact on events azimuthal gap of adding 2010 data from the Libyan network. Top: preliminary result without using Libyan data. Bottom: final EMB result when using Libyan data. Black triangles correspond to the 14 Libyan stations added for that period.
ranean side. Fig. 7 displays an example around the Hellenic Arc where the azimuthal gap is significantly improved by adding the 2010 data from the Libyan network for 174 events. In Crete, the gap is divided by two for several events in the Southern side of the Hellenic Arc. The 14 stations installed in Libya, for which data were received in 2010, are one of the only sources of information available from the south-western direction. Improvements in the Strait of Sicily are also clearly observed, in Tunisia and Algeria. In general, the gap has improved of more than 50° for 80 events.
6. Nontectonic seismicity Discrimination is a key issue to compute accurate seismicity catalogue useful for seismic hazard assessment. This is particularly important when integrating all magnitude events as anthropogenic activity mostly generates low magnitude seismicity. The EMSC collaborates with the local networks to improve the identification of quarry and mine blasts for the region using different means. Firstly, event type information is collected and archived in our database. A monthly verification is performed before producing the bulletin, which allows us to integrate
the latest event type classification available from the seismic institutes. After the production, the catalogue is available to all data providers for further analysis and discrimination. This is particularly important if a network discards anthropogenic events from its contribution to the EMSC while the neighbouring networks record and provide data without performing event type identification. As an example, the EMB is verified against the LDG (Laboratoire de Détection et de Géophysique, France) anthropogenic events list and leads to re-classification of 700 events over 13 years, mostly in France and Northern Spain but also off-shore marine explosions in the Mediterranean Sea, English Channel and the Bay of Biscay. The GSE2.0 event type definition is used in the EMB. Information collected from the contributors in various format are parsed into this single format. However, the description of each event type and the classification procedure vary tremendously among operators. These inconsistencies lead the EMSC, ISC and NEIC to define a new international nomenclature submitted to IASPEI. Its goal is to provide guidelines to commonly describe event type information by using a hierarchy that clarifies and reflects more accurately the analyst knowledge. This includes in particular the introduction of a general anthropogenic event type.
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Fig. 8. Location density map of anthropogenic events as classified in the Euro-Med Bulletin (1998–2010). Contour areas show the number of events within a 0.25° region.
In this paragraph, we focus on the EMB data available from 2007 to 2010, period for which all magnitude events are integrated and where discrimination is most crucial. In the Mediterranean region, there are 44,315 events labelled as related to nontectonic or induced activity over 4 years (about 30 per day). The whole distribution (Fig. 8) is dominated by sources in Spain, particularly in the North, in relation with very active mines. Most of the sources are located in Catalonia, Navarra and in the North West of Castile y Leon. The mining areas of Lubin and Upper Silesia in Poland are also depicted as very high and well limited patterns of events. Other mine regions are well defined in Germany, Switzerland, Croatia, Czech Republic, Austria, Turkey and Ukraine (Fig. 8). A more spread pattern is also observed at the border between Lebanon, Israel and Jordan. The majority of the anthropogenic events are identified as mine explosions (68% of all nontectonic events) and induced events (30%) while rock bursts are a minor contribution (Fig. 9). Being related to human activity, mine and quarry blasts are generally performed during working hours. The time distribution of events in the EMB including nontectonic events (Fig. 10) shows a clear and typical peak from 10am to 4 pm (Gulia, 2010). Meanwhile, when discarding nontectonic events (Fig. 10), a regular time distribution is observed throughout the day. On both histograms, a typical increase of the number of events is observed during night hours, when the signal to noise ratio is higher due to minimum anthropogenic noise, and detection level is lower (Rydelek and
Fig. 10. Histograms of the number of events per hour in the Mediterranean region as described in the Euro-Med Bulletin including (white) and discarding (gray) nontectonic events. All magnitude events (1998–2010) are taken into account and local time is used.
Sacks, 1989). However, a residual increase of about 500 events per hour is observed around noon, showing that discrimination is still incomplete. Thanks to the local network information, contamination of the EMB by human activity is strongly limited, though indication of residual nontectonic events is identified. Additional statistical analysis of the catalogue can provide useful discrimination tools which are presented in the next paragraph. 7. Perspectives 7.1. A posteriori discrimination
Fig. 9. Distribution of anthropogenic event type in the Euro-Med Bulletin (values computed over 4 years 2007–2010) where all magnitude events are incorporated.
Over the years, identification of artificial events has improved, based on spectral analysis, cross-correlation or on expert knowledge and quarry operators listing. Several studies have proven to be efficient to detect artificial seismicity in national bulletins (Gulia, 2010) but also at local level in north western Turkey (Horasan et al., 2009), the Vrancea region (Telesca et al., 2012) or in Sicily (Ursino et al., 2001). As first approach, the event temporal distribution contains useful information which can be used to improve identification of nontectonic events. Often, quarry blasts are not only performed during daytime but at very specific hour.
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Fig. 11. Time–space analysis of natural (black) and anthropogenic (red) events. Top: in the vicinity of the Luzenac talc quarry (Southern France) from 2007 to 2010. Central point: 42.8° North, 1.8° East. Search radius: 20 km. 33 events out of 112 are classified as tectonic. Bottom: in the vicinity of the Panaguyrichté copper mine (Bulgaria) from 2007 to 2010. Central point: 42.5° North, 24.1° East. Search radius: 20 km. 190 events out of 480 are classified as tectonic.
Here we propose to apply an a posteriori tool to seismicity catalogues based on today’s knowledge. We show hereafter how past information can be further discriminate. In Southern France, the Luzenac quarry operators perform blasts at 17:50 (local time). The events distribution 20 km around the Luzenac quarry is shown in Fig. 11-Top and includes 112 events over 4 years. The preferred blasting time is clearly seen, along with a seasonal trend for summer operation. However, a third of the events are still labelled as being of natural origin while their origin time is precisely the preferred blasting time. We can also see that since 2009, the majority of the blasts are well identified, while in the two previous years, most events are labelled as natural. The inspection of known quarry areas allows the identification of numerous misclassified events to be discarded from seismicity catalogues. By collecting the known quarries location from the network operators, an epistemic analysis based on occurrence time could be performed to search for nontectonic events which were not properly identified. The geographic distribution of known artificial events is also a starting point for further analysis of the discrimination performances. For example, a cluster of nontectonic events is observed in central Bulgaria (Fig. 8). By searching on satellite images, this cluster can be associated with a copper mine, located near the city of Panaguyrichté. Plotting the event occurrence time as a function of date and distance to the centre of the mining area, shows that almost all events occur at 14:30 and 15:00 (local time). However, 190 events over 4 years are classified as nontectonic (40% of the total number of events) in particular for the first semester of 2007 and 2008 (Fig. 11-Bottom).
These two examples show how the current event type information provided by the contributing networks, though incomplete, can be used to back process the whole EMB and improve discrimination. It is therefore important to encourage the distribution of event type information and the collection of known quarries location to allow scanning of the EMB using event temporal distribution. With an improved discrimination between man-made and tectonic events, the seismicity imaged in the EMB will be more accurate for further active tectonic interpretation and will avoid spreading erroneous results into seismic hazard assessment. 7.2. E-Bulletin Technical developments now allow to exchange and link distant information to a unique entry point thanks to dedicated communication protocols. Users are not only interested in parametric data but also wish to access various seismological products related to a particular event. The EMB by focusing on a specific region and dedicated to the research community could become the place to reach several types of information which would be gathered in an event portfolio. Thanks to a single identifier developed within the NERIES project, each seismic event is assigned a unique identification number (UNID) shared by several Institutes. Currently all data from the EMSC real time system can be linked to the EMB data through the UNID, including photos, macroseismic questionnaires and special reports. Within the Seismic Portal (www.seismicportal.eu), seismic waveforms hosted at ODC (Orfeus Data Center)
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are accessible thanks to the UNID. The SED (Swiss Seismological Service) at ETH and gempa GmbH developed an accelerometric waveform parametrization module embedded in SeisComp3 (Hanka et al., 2010) for real time strong motion data exchange based on EMSC earthquake location. Further independent data can be linked under the same identifier such as moment tensors of the EMMA database (Pondrelli et al., 2011), shakemaps, etc. 8. Conclusion It has been shown that through dedicated interaction with local networks, an enhanced seismicity bulletin can be achieved. The seismic monitoring of the Mediterranean region has improved over the 13 years for which the Euro-Med Bulletin is available, leading to a steadily increasing event azimuthal coverage. Nontectonic events should be properly qualified to be used for seismic hazard assessment. We have demonstrated that event discrimination has become more reliable thanks to cross border reviews. We describe how artificial event identification can be completed by a posteriori analysis using event occurrence time and quarry operations knowledge from the operating institutes. While the EMSC is constantly working on improving the parametric information available in its bulletin, we propose to develop an e-Bulletin for each event within which users will access all seismological products and scientific documentation related to it. Acknowledgment This work could not be done with the support of the EMSC members and all the EMB data contributors. The EMB has partially benefited from funding provided by the NERA Project (SA1) in the Seventh Framework Programme [FP7/2007–2013] under Grant agreement n° 262330. We thank the ISC for their help in data integration coordination. And last but not least, we thank the LDG for their daily collaboration and crucial support.
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