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


Analysis of Instrumental Data

[14]  According to Hughes and Bellamy [1935] the 1930 earthquake was recorded on 13 stations at epicentral distances ranging from 0.6o to 11.2o. The fact that the earthquake was recorded up to 1200 km distance already supports a relatively large magnitude value. The reported magnitude by the International Seismological Summary (ISS) Kárnik [1969] is MS=4.6 (based on data of 2 stations) whereas in the column of remarks a magnitude M=5.0 calculated by Munuera [1963] with a local magnitude scale is given. Our assessment ( MS=4.8 ) based on macroseismic data analysis is in a very good agreement with these values. The source depth is not defined in Kárnik [1969]; it is only marked as "normal". On the other hand, the 1949 earthquake is not consigned by the ISS. Kárnk [1969] gives a magnitude (4.2) (brackets in the original) taken from Munuera [1963]. However, as it has been shown recently [López and Mu noz, 2003] the methodology used by Munuera [1963] presents some shadows really difficult to elucidate now. In an unpublished research, Sánchez-Contador [1988] calculated a local magnitude ( M L ) equation for the Mainka's seismographs and a duration magnitude ( Mt ) equation for the Vicentini seismograph of the Fabra Observatory (FBR) in Barcelona, situated approximately at one degree of distance from the epicentral zone. The application of these equations gives M L=5.0 and Mt=4.9 for the 1930 earthquake and M L=Mt=4.5 for the 1949 event. Samardjieva et al. [1998] also calculated the Mt and MS magnitudes for the Horizontal Wiechert seismograph at Toledo Observatory (TOL) and obtained 4.5 and 4.1 values, respectively, for the 1930 earthquake. The 1949 event was not recorded at that observatory.

[15]  Although all presented data sustain the assumption that the 1949 earthquake had a lower magnitude than the 1930 event, in order to shed light on these discrepancies the instrumental magnitudes for both events were re-calculated using the original seismographic recordings summarized in Table 3. All the recordings have been previously scanned, digitized and processed as described in Dineva et al. [2002]. However, some problems with the stylus inscription, time marks and not damped instruments make most of the records not valid for the seismic moment calculation. The only useful records are those from Toledo Observatory. Thus, the moment magnitude ( Mw ) has been calculated for the 1930 event. The obtained ground displacement spectra ( U ) of the digitized recordings have been modelled following Brune [1970, 1971] by fitting:

eq003.gif(3)

using the non-linear c2 criteria where U0 is the low-frequency level and fc (wc = 2pfc) the corner frequency.

[16]  Then, the seismic moment was estimated using the formulation of Keilis-Borok [1960] from the low-frequency level of the spectra of body-waves:

eq004.gif(4)

where r = 2.7 g cm -3 is the density in the source region; v the wave velocity; U0 the low-frequency level of spectrum in m cdot s; G(r) the geometrical spreading factor which depends on the distance r; Rqj is the correction for the radiation pattern, which takes an average value of 0.4 for P waves [Wyss and Brune, 1968] and 0.63 for S waves [Boore and Boatwright, 1984]; and C=2.0 is the correction for the free surface [Moskvina, 1987].

2005ES000195-fig05
Figure 5
[17]  The moment magnitude, Mw, is calculated using the empirical relation [Hanks and Kanamori, 1979]:

eq005.gif(5)

with M0 given in N cdot m. Figure 5 shows one example of the digitized records and the ground displacement spectra fits (3) for P and S waves. From the obtained U0 values and after applying (4) and (5) results give a value of Mw = 4.4 for P waves and Mw = 4.2 for S waves. Results are also larger than the value previously reported in the IGN catalogue ( MS=3.9 ). However, the relationship between both magnitude scales show that MS values are larger than Mw, for Mw less than about 6 [Utsu, 2002].

2005ES000195-fig06
Figure 6
[18]  On the other hand, although a moment magnitude calculation for the 1949 earthquake has been impossible, the original records give us information about the different size of both events. The Mainka instruments of EBR and FBR stations recorded both events with unchanged working characteristics. Figure 6 shows, in the same scale, the records on the Mainka N-S component of EBR. The Ebro observatory seismic bulletin states an epicentral distance of 56 km for the 1930 event and 32 km for the 1949 one. A rough visual comparison of maximum amplitudes supports a difference on magnitude between the two events of almost half unit.


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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