RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES1001, doi:10.2205/2005ES000195, 2006


Analysis of Macroseismic Data

[10]  The intensity data were read from the maps in Figure 2 (a and b, named Source 2 and Source 3, respectively) and they were supplemented with data from two other sources: the archive of the Cartographic Institute of Catalonia [Susagna and Goula, 1999, named Source 1] and Fontseré [1940] (named Source 4). All these data are summarized in Table 2. The localities for which the intensity values from 2 or more data sources coincide are highlighted in bold. There are only 9 such cases from a total number of 74 localities. On one hand, this fact demonstrates the lack of detailed and accurate data, which make possible unique intensity assessments and, on the other hand, it proves that different data sources present independent researches. However, in most of the cases the intensity discrepancy does not exceed 1 degree, which could be accepted as a reasonable accuracy of assessment for the majority of reported intensities. Of course in some cases the error could be greater. The last column of the table compiles the intensity degrees considered in this paper. In agreement with European Macroseismic Scale [Granthat, 1998] we follow the convention that sign "-" means uncertainty and not the precision, i.e. I =4-5 means that we are not able to distinguish between 4 or 5 and not that the intensity is 4.5 (which would imply an accuracy of 0.5 degree). The epicenter locations from different sources do not differ significantly because all of them are within a 7 km radii circle. It also has to be mentioned that though none of the sources report observed intensities greater than 5, the Source 1 assesses an epicentral intensity 5-6.

2005ES000195-fig04
Figure 4
[11]  The map of intensity data points considered in this work is presented in Figure 4. Question marks denote doubtful cases, such as when the intensity in a locality differs significantly from the ones for cluster of nearby localities. For example, intensity 4 is assigned to locality Sant Feliu de Llobregat by Source 1, but there are other four close localities with intensity 2. Other type of doubtful case is when the reported locality is in obvious contradiction with any reasonable attenuation, like in Solsona for which the only source is Source 1. This source assigns intensity 4 to this locality, which is at about 100 km from the epicenter of an earthquake with epicentral intensity 5. The same source assigns an intensity degree 4 to Riumors (42.229o N; 3.044o E), which even falls out of the presented map boundaries, with an epicentral distance of about 180 km. Nevertheless, such strong incoherencies are rare (6 points from a set of 74 localities). This also confirms our assumption that the accuracy for most of the intensity assessments is not worse than 1 degree. It is important to emphasize that the fact of adding the information from two new sources does not change the main features observed when comparing the isoseismal maps of Figures 2 and 3: the felt area and the I i=4 area for the 1930 earthquake are larger than those for the 1949 seismic event. Moreover, this difference becomes sharper.

[12]  The isolines drawn in Figure 4 are calculated using equations (1) and (2). The epicenter location is taken from the IGN catalogue. Because all the earthquake locations from different sources are close to each other, the arbitrariness of this choice does not affect the result. Then we plot 8 isoline sets for different depth values (from 5 km to 40 km with a 5 km step). In the macroseismic field equations a mean radius of isoline is assumed. For the elliptical isolines we assumed that R mean = (ab)1/2, where a and b are the ellipse semiaxis. The ratio a/b depends on the number of isoline from the epicenter. The ratio is larger for those, which are close to the epicenter. We also vary the orientation of isolines. The isoline set shown in Figure 4 gives the best separation of plotted data (it should be remembered that the symbol "-" is interpreted as uncertainty). The set corresponds to the following parameters: H = 30 km; (a/b) V=2.5; (a/b) IV=2.25; (a/b) III=2.2.

[13]  To fit the epicentral intensity I0=5 and the isoseismal radii in Figure 4 the earthquake magnitude has to be set to MS=4.8. Taking into account the accuracy of intensity assessments in the localities together with the number of model variables used for the calculation of each set of isoseismals, one has to be cautious with the obtained magnitude value. However, we have strong reasons to conclude that a larger earthquake magnitude than that reported in the IGN catalogue is expected. To be more confident we have to verify instrumental data.


RJES

Citation: Tatevossian, R., A. Ugalde, J. Batlló, and R. Maciá (2006), Macroseismic and instrumental data comprehensive analysis: Earthquake of June 2, 1930 in Catalonia (Spain), Russ. J. Earth Sci., 8, ES1001, doi:10.2205/2005ES000195.

Copyright 2006 by the Russian Journal of Earth Sciences

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