Abstract
The Carpathian Mountainsarc is the most seismically active area in Central Europe. Analysis of the seismicity of entire Carpathian arc requires data from each of the particular catalogues which have to be properly and uniformly entered, standardized and merged. For our study we first had to prepare a database of seismic events (ML ≥ 1.6) compiled from the data of earthquakes taken from individual national seismic networks as well as data from international seismic centers. However, a careful review of these catalogues has uncovered significant inconsistencies, particularly discrepancies in the description of the location, magnitude and completeness of seismic events. To address these inconsistencies, a newly created compound earthquake catalogue was compiled from the aforementioned seismic catalogues and included events that occurred in the Carpathian Mountains arc area between 1976 and 2017. This work is intended to point out some of the problems associated with collecting data from various seismic catalogues as well as the need for their very careful verification, in order to create a uniform set of seismic data across a large area spanning numerous countries. The results suggest that compiling a uniform and dependable earthquake catalogue is crucial for reliable seismic studies.
1 Introduction
Earthquake catalogues represent the most important data sets for studying different aspects of seismicity within a given area. The homogeneity of the data is considered to be a principal requirement of a catalogue. Many scientists developed unified earthquakes databases, for example homogeneous earthquake catalogues are essential in comparing geodetic and seismic estimates of crustal strain rates [1, 2] and in implementing various probabilistic seismic hazard approaches [3, 4, 5]. Also uniform databases covering individual countries [6, 7] as well as extensive areas [8]. Unfortunately, most earthquake catalogues are non-homogeneous because of a variety of factors that depend on both random and systematic errors introduced during the acquisition process and database construction procedure [9]. Homogeneous earthquake catalogues with high quality data covering large territories and long historical time spans are lacking for many parts of the Earth [10]. For the planned future study the elaboration of one unified database of seismic events containing information about strong earthquakes occurred in the Carpathian Mountainsarc was essential for us. The general purpose of our research is to check if seismic activity of the Central and Southern Europe flows the occurrence of strong seismic shocks in the Upper Silesian Coal Basin (USCB) in Poland. Former research of seismicity in the USCB showed that it has a bimodal character. Tremors occurring in this area group into two energetic modes – those caused directly by the underground coal exploitation (low-energy mode) and regional ones (high-energy mode), the genesis of which are not yet fully explained. The second type of seismicity is probably induced by the combination of two factors: the mining and tectonic one. These high-energy shocks occurred in areas of tectonic zones and frequently are felt in the surface. The cause of the strongest tremors can be the cumulation of mining and tectonic stresses acting in the same parts of the rock mass. Confirmation or denial of this hypothesis could be important for explaining the genesis of strong seismic events in the USCB. We focus on a detailed analysis of seismic catalogues connected with the Carpathian arc as the area having the possible impact on the occurrence of strong events in the USCB. For our further research we needed an uniform seismic database containing basic information on earthquakes occurring there. Because of the data available in the seismic catalogue from the USCB we needed to compile the database containing events occurred between 1976-2017 which strength was expressed by local magnitude (ML), similarly like in the USCB catalogue.
Until now, there has been not any study that focused on the analysis of seismicity of the whole Carpathian arc, although some papers have discussed the seismicity of particular regions. Analyses of the entire Carpathian area would require data from all available catalogues of the individual national seismic networks and international seismological centers, all of which would need to be completed, standardized, and merged in order to obtain a homogeneous earthquake data. This article is intended to show the importance of proper data collecting from various seismic networks, as well as the need for very careful verification of partial seismic catalogues, in order to create a single set of data regarding the seismic phenomena across a large area.
2 Seismotectonics of the research area
The Carpathian Mountains are commonly known as the most seismically active area in Central Europe. The arc has an inhomogeneous distribution of earthquakes and is therefore a very interesting region for seismologists. Localization of the shocks hypocentre is very important in terms of tectonics, because of most of the earthquakes are located in a restricted area in the bending zone between the Eastern and Southern Carpathians where at least three units are in contact: the East European plate, Intra-Alpine and Moesian sub-plates [11].
As it is well known the Carpathians were formed in the Alpine–Carpathian–Pannonian (ALCAPA) region during the Alpine orogenesis. This area has very complicated tectonics. According to [12] the Carpathians arc recorded a complex tectonic history, involving extrusion of microplates, ocean closure, subduction, slab rollback, slab detachment and asthenospheric upwelling. The recent ALCAPA region consist of the Carpathian orogen and Pannonian back-arc basin [13]. Because the tectonic evolution of this region is still a matter of discussion we can distinguish two evolutionary theories. First of them explained the evolution of the ALCAPA region in terms of gravitational collapse of the continental lithosphere. Some authors confirmed this theory [14, 15]. According to the second hypotesis, the subduction of the oceanic lithosphere underneath the Carpathians is a key process during the tectonic evolution of the ALCAPA [13]. Because the processes occurring in the Vrancea zone – still active seismic zone located in the bend region in Romania are seen as the latest stage of the subduction underneath the Carpathian Mountains [16, 17] the second thesis is more commonly accepted. This interpretation is well supported by former volcanic activity of the region [13]. There is also a third theory of the tectonic evolution of this region – lithospheric delamination due to continental underthrusting and orogenic thickening [14, 18]. This model could be generated through closure of an intracontinental basin and lithospheric thickening and does not require the former presence of an ocean-floored basin, or the subduction of oceanic lithosphere [14].
3 Database
The construction of mentioned database was based on individual seismic data from the following national and international seismic networks: seismic data portal Czech-Geo operated by the Czech Geological Survey [19], Kövesligethy Radó Seismological Observatory of the Hungarian Academy of Sciences [20], National Institute for Earth Physics (NIEP) in Romania [21], Seismological Survey of Serbia [22], IRIS Data Management Center [23], the European– Mediterranean Seismological Centre (EMSC) [24] and on-line bulletins from the International Seismological Centre (ISC) [25].
We included earthquakes within the Carpathian arc from Romanian and Hungarian catalogues. Data about tremors that occurred between 1976-2017 in northern part of the arc were taken from EMSC, ISC and IRIS. Only events with a known location and strength corresponding to Mw or ML ≥ 1,6 (depending on the magnitude type given in the catalogue) were entered into our final database. Several suspected erroneous entries with incomplete data (e.g date, hour, latitude, longitude, magnitude and depth) were not included in the final database.
4 Inconsistencies in the catalogues
Careful review of the particular catalogues has showed their significant inconsistencies, especially discrepancies in the location, magnitude, and completeness of events which had to be pointed out.
4.1 Different time window
The first step of the analysis was to align time windows for all included catalogues. The period we considered starts from 1st January 1976 and end in 31st December 2016 .We have accepted this time window due to data from the USCB which are available for this period. Firstly, the data were sorted and filtered to avoid including shocks repeated in analyzed catalogues as different shocks in the our database. All shocks that had the same date, hour, latitude, longitude, magnitude and depth were treated as one record. Secondly, some of the catalogues contained time gaps, for example the Romanian catalogue started in 1976 while the Hungarian one began in 1995. To make the final database complete in the time domain we filled up the gaps by the data taken from EMSC, ISC and IRIS.
4.2 Inhomogeneous earthquake distribution of the research area
The fact that epicentre distribution is not homogeneous throughout the entire area, tremors group mostly in the Southern part of the arc (Figure 1), consequently affects the density of the seismic network within the different parts of the Carpathians arc. It may be important for the completeness of seismic catalogues regarding weak events.
Epicentres map of shocks (ML ≥ 1.6) throughout the Carpathians arc during 1976-2017
4.3 Different magnitude scales of used catalogues
Common known, the magnitude is one of the most important parameters of earthquakes and the first quantitative measure of earthquake strength.
The main problem that we had to solve in creating a complete and uniform seismic database for the Carpathian Mountains arc was the various types of magnitude given in the particular catalogues, especially mb, ML, and Mw magnitudes (Table 1).
The number of events in the Carpathians characterized by different types of magnitude.
| Magnitude type | Number of earthquakes |
|---|---|
| mb | 18 |
| ML | 363 |
| Mw | 17 541 |
| Mw/ ML | 5 316* |
| all shocks | 23 238 |
* in the Romanian catalogue, 5 316 tremors recorded since 2014 were catalogued as both Mw and ML.
5 Methods
In our merged database of seismic events from the entire Carpathian region, different magnitudes of earthquakes were unified into local magnitude for the whole study area.
The biggest problem with the unification of catalogue was caused by the Romanian data, where magnitude Mw were given mostly. However, since 2014 the ML value of earthquake sizes has been given together with the magnitude Mw (Table 2). For this reason we were able to compare the values of Mw and ML from the Romanian catalogue for 5 316 tremors occurring between 2014 and 2016 (Table 1) and subsequently to determine the regression function of Mw versus ML. After careful analysis, it turned out that this relationship is bimodal and has two branches (Figure 2). One branch refers to shocks occurring shallowly, up to a depth of 65 km, while the other refers to tremors located deeper. We noticed that for shocks occurring below a depth of 65 km (Figure 3) the linear regression function Mw vs. ML is done as:
Regression function Mw versus ML in the Romanian catalogue
The examples of events characterized by different types of magnitude – ML vs. MW above ML >.0 in the Romanian catalogue between 2014-2017.
| Date | Time | ML | MW | ML −MW |
|---|---|---|---|---|
| 24.08.2014 | 07:12:49.66 | 4.6 | 4.2 | 0.4 |
| 03.04.2014 | 12:38:56.95 | 4.7 | 4.3 | 0.4 |
| 10.09.2014 | 19:45:57.79 | 4.8 | 4.3 | 0.5 |
| 24.01.2015 | 07:55:47.31 | 4.7 | 4.3 | 0.4 |
| 16.03.2015 | 15:49:49.14 | 4.7 | 4.3 | 0.4 |
| 29.03.2015 | 00:44:58.44 | 4.8 | 4.3 | 0.5 |
| 23.01.2014 | 06:15:04.66 | 4.9 | 4.4 | 0.5 |
| 29.03.2014 | 19:18:05.07 | 5 | 4.6 | 0.4 |
| 23.09.2016 | 23:11:20.06 | 5.3 | 4.9 | 0.4 |
| 27.12.2016 | 23:20:55.96 | 5.3 | 4.9 | 0.4 |
| 22.11.2014 | 19:14:17.11 | 5.7 | 5.4 | 0.3 |
Regression function Mw versus ML in the Romanian catalogue for shocks occurring below 65 km of depth
For shocks whose hypocentre depth is less than 65 km the linear regression formula is:
In addition, regardless of the depth range, the local magnitude of strongest shocks (MW above 4,5) presented in the Romanian catalogue can be calculated from the formula:
These formulas (1)-(3) were used to unify the magnitudes of all the shocks from the Romanian catalogue by converting them into ML value (Figure 3, Figure 4, Figure 5).
Regression function Mw versus ML in the Romanian catalogue for shocks whose depth is less than 65 km
Regression function Mw versus ML in the Romanian catalogue of shocks for which MW > 4.5
As one can see, the hypocentre depth range is a very important parameter to distinguish different types of magnitudes contained in the Romanian seismic catalogue. This data agrees with Kanamori’s [26] suggestion that Mw can be determined for both shallow and deep earthquakes whereas ML depends on depths of the hypocentre and the ‘size’ of earthquake.
Another considered problem was the different types of magnitudes listed in the Hungarian catalogue as well as the catalogues for the northern part of the Carpathians. In both catalogues the local magnitude ML was given primarily but the magnitudes of 7 shocks (out of 209) described in the Hungarian catalogue and 11 (out of 172) depicted in the catalogue for the northern part of the Carpathians were of the mb type.
According to Kanamori [26] mb is almost equal to ML for shallow seismic events. Taking into consideration the small amount of shocks with magnitude mb in relation to the number of shocks with magnitude ML and also a small depth range (less than 33 km of depth) of seismic events, we could attribute the mb value to the ML.
6 Results
Our database, which was compiled basing on the data from the aforementioned seismic catalogues, includes 23 238 events on ML ≥ 1.6 from period 1976 to 2017 that occurred in the studied area. Analysis of the database revealed an apparent incompleteness of smaller earthquakes with magnitudes less than 2 (Figure 6). For this reason, we decided to construct the Carpathian arc database for shocks with magnitudes ML ≥ 2.0 to obtain the relatively complete data set.
Histogram presenting the number of tremors for consecutive magnitude intervals: for the whole Carpathian Mountain arc area. N– number of events, M– magnitude intervals
7 Discussion and conclusions
As it is already well known, the main purpose of earthquake catalogues is to provide users with simple parameters for initial interpretations of the data. Any and every earthquake catalogue should endeavour to homogenize the given parameters, especially the magnitude, and any additional strength measure [27].
Unifying the seismic data of a huge area like the Carpathian Mountain arc (190 000 km2) required solving a few problems, among others:
collecting data from various available partial catalogues operated by several agencies,
capturing and rejecting the same shocks that are repeated in different catalogues,
sorting and filtrating the data to make correction of incorrectly described events,
The biggest problem was unification of magnitudes, because in different catalogues the various types of magnitude are given.
Grünthal and Wahlström [27] emphasize that ML is by far the dominant magnitude in most of the used catalogues despite a heterogeneity between different local ML scales, unknown to its extent. As Kanamori [26] wrote, a mix use of different magnitude scales (especially those determined at different periods) often causes confusion and should be avoided as much as possible. For these reasons, we have decided to unify all magnitudes to ML, because they concern for a relatively homogeneous area which is the Carpathian Mountains arc. As a result, our methodology could reasonably be applied to the seismic data of all such homogenous mountain ranges.
The database we receive will be available soon to the seismological community.
References
[1] Mazzotti et al., Seismic hazard in western Canada from GPS strain rates versus earthquake catalog. Journal of Geophysical Research, 2011, Vol. 116, issue B12, https://doi.org/10.1029/2011JB00821310.1029/2011JB008213Search in Google Scholar
[2] Chousianitis K., et al., Strain and rotation rate patterns of mainland Greece from continuous GPS data and comparison between seismic and geodetic moment release. Journal of Geophysical Research, 2015, Vol. 120, Issue 5. p. 3909-3931. https://doi.org/10.1002/2014JB01176210.1002/2014JB011762Search in Google Scholar
[3] Kijko A., Sellevoll M.A, Estimation of earthquake hazard parameters from incomplete data files. Part I. Utilization of extreme and complete catalogs with different threshold magnitudes. Bulletin of the Seismological Society of America, 1989, 79(3), p. 645-654.10.1785/BSSA0790030645Search in Google Scholar
[4] Kijko A., Graham G., Parametric-historic Procedure for Probabilistic Seismic Hazard Analysis Part I: Estimation of Maximum Regional Magnitude mmax. Pure and Applied Geophysics, 1998, Vol. 152, Issue 3, p. 413–442. https://doi.org/10.1007/s00024005016110.1007/s000240050161Search in Google Scholar
[5] Burton P.W, Makropoulos K.C., Seismic risk of circum-Pacific earthquakes: II. Extreme values using Gumbel’s third distribution and the relationship with strain energy release. Pure and Applied Geophysics, 1985, Vol. 123, Issue 6, pp 849–869. https://doi.org/10.1007/BF0087697410.1007/BF00876974Search in Google Scholar
[6] Leptokaropoulos K.M., et al., A Homogeneous Earthquake Catalog for Western Turkey and Magnitude of Completeness Determination. Bulletin of the Seismological Society of America, 2013, 103 (5), p. 2739-2751. https://doi.org/10.1785/012012017410.1785/0120120174Search in Google Scholar
[7] Makropoulos K. C., Drakopoulos J. K., Latousakis J. B., A revised and extended earthquake catalogue for Greece since 1900. Geophysical Journal International, 1989, Vol. 98, Issue 2, p. 391–394. https://doi.org/10.1111/j.1365-246X.1989.tb03360.x10.1111/j.1365-246X.1989.tb03360.xSearch in Google Scholar
[8] Grünthal G., Wahlström R., The European-Mediterranean Earthquake Catalogue (EMEC) for the last millennium. Journal of Seismology, 2012, Vol. 16, Issue 3, p. 535–570. https://doi.org/10.1007/s10950-012-9302-y10.1007/s10950-012-9302-ySearch in Google Scholar
[9] Leptokaropoulos K. M., et al., A Homogeneous Earthquake Catalog for Western Turkey and Magnitude of Completeness Determination. Bulletin of the Seismological Society of America, 2013, 103, No. 5, 2739–2751. DOI: 10.1785/012012017410.1785/0120120174Search in Google Scholar
[10] Grünthal G., Wahlström R., Stromeyer D., The unified catalogue of earthquakes in central, northern, and northwestern Europe (CENEC) – updated and expanded to the last millennium. Journal of Seismology, 2009, 13, 517–541. DOI 10.1007/s10950-008-9144-910.1007/s10950-008-9144-9Search in Google Scholar
[11] Marmureanu G., Cioflanc C. O., Intensity seismic hazard map of Romania by probabilistic and (neo)deterministic approaches, linear and non linear analyses. Romanian Reports in Physics, 2011, 63, No. 1, p. 226–239.Search in Google Scholar
[12] Bielik, M. et al., The Western Carpathians — interaction of Hercynian and Alpine processes. Tectonophysics, 2004, 393 (1-4), p. 63–86.10.1016/j.tecto.2004.07.044Search in Google Scholar
[13] Tašárová A. et al., The lithospheric structure of the Western Carpathian – Pannonian Basin region based on the CELEBRATION 2000 seismic experiment and gravity modelling. Tectonophysics, 2009, 475, p. 454–469.10.1016/j.tecto.2009.06.003Search in Google Scholar
[14] Knapp, J.H., Knapp, C.C., Raileanu, V., Matenco, L., Mocanu, V., Dinu, C., Crustal constraints on the origin of mantle seismicity in the Vrancea Zone, Romania: the case for active continental lithospheric delamination. Tectonophysics, 2005, 410, p. 311–323.10.1016/j.tecto.2005.02.020Search in Google Scholar
[15] Gemmer, L., Houseman, G.A., Convergence and extension driver by lithospheric gravitational instability: evolution of the Alpine–Carpathian–Pannonian system. Geophys. J. Int., 2007 168, p. 1276–1290.10.1111/j.1365-246X.2006.03327.xSearch in Google Scholar
[16] Sperner, B., Ioane, D., Lillie, R.J., Slab behaviour and its surface expression: new insights from gravity modelling in the SE-Carpathians. Tectonophysics, 2004, 382, p. 51–84.10.1016/j.tecto.2003.12.008Search in Google Scholar
[17] Martin, M.,Wenzel, F., High-resolution teleseismic body wave tomography beneath SE-Romania — II. Imaging of a slab detachment scenario. Geophys. J. Int., 2006, 164, p. 579–595.10.1111/j.1365-246X.2006.02884.xSearch in Google Scholar
[18] Fillerup M., Knapp J., Knapp C., Raileanu V., Mantle earthquakes in the absence of subduction? Continental delamination in the Romanian Carpathians. Lithosphere, 2010, vol. 2, no. 5, p. 333–340. Doi: 10.1130/L102.1.10.1130/L102.1Search in Google Scholar
[19] Catalogues of regional seismic events recorded by the Czech Regional Seismological Network downloaded from: http://www.czechgeo.cz/en/gfu-catalog/ and Bulletins of seismic events recorded by the Czech Regional Seismological Network downloaded from: http://www.czechgeo.cz/en/gfubulletin/Search in Google Scholar
[20] Hungarian National Seismological Bulletin. HU ISSN 1589-9756 (online) and also download from: http://www.seismology.hu/index.php/en/seismicity/recent-felt-earthquakes/28-list-of-recent-felt-earthquakes-in-hungarySearch in Google Scholar
[21] National Institute for Earth Physics (NIEP Romania) (1994): Romanian Seismic Network. International Federation of Digital Seismograph Networks. Dataset/Seismic Network. 10.7914/SN/RO download from: http://www.infp.ro/wp-content/uploads/2015/12/romplus.cat_.txtSearch in Google Scholar
[22] Seismological Survey of Serbia (2013): Catalogue of earthquakes M ≥ 3.5 Republic of Serbia. Downloaded from: http://www.seismo.gov.rs/Seizmicnost/Katalog-zemljotresa.pdfSearch in Google Scholar
[23] Catalogue downloaded using IRIS Interactive Earthquake Browser from: https://ds.iris.eduSearch in Google Scholar
[24] The European– Mediterranean Seismological Centre (EMSC) catalogue downloaded from: https://www.emsc-csem.org/Search in Google Scholar
[25] on-line bulletins from the International Seismological Centre (ISC) downloaded from: http://www.isc.ac.uk/iscbulletin/search/catalogue/Search in Google Scholar
[26] Kanamori H.,Magnitude scale and quantification of earthquakes, In: S. J. Duda and K. Aki (eds.) Quantification of Earthquakes. Tectonophysics, 1983, 93, 185-199.10.1016/0040-1951(83)90273-1Search in Google Scholar
[27] Grünthal G., Wahlström R., An Mw based earthquake catalogue for central, northern and northwestern Europe using a hierarchy of magnitude conversions. Journal of Seismology, 2003, 507–531.10.1023/B:JOSE.0000005715.87363.13Search in Google Scholar
© 2019 A. Braclawska and A. F. Idziak, published by De Gruyter
This work is licensed under the Creative Commons Attribution 4.0 Public License.
