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


Introduction

[2]  The modern concept of seismic hazard assessment is based on the assumption that what was observed in the past could likely be expected in the future. Usually this principle is stated explicitly but even if not so, it is followed implicitly. It could be easily recognized by comparison of the spatial distribution of maximum intensity of shaking ( I max ) from past earthquakes with seismic hazard maps. Tatevossian et al. [2006] showed recently that the spatial distribution of I max (the observed ones supplemented by calculated values based on the epicentral intensities) over the Spanish territory is well correlated with the peak ground acceleration (PGA) contours obtained within the frames of the Global Seismic Hazard Assessment Program [Jiménez and Garcia-Fernández, 1999]. This fact makes crucially important to re-evaluate the parameters of earthquakes close to the maximum observed ones in a certain area: even a small change of them can affect considerably the final hazard assessment. In this paper comprehensive analysis of macroseismic and instrumental data for accurate re-evaluation of earthquake parameters is presented. The most promising time period for this objective is the so-called early instrumental period. Before that time period we simply do not have instrumental data, and for the time period after the 1960's, when the World Wide Standardized Seismograph Network (WWSSN) was set in operation, the instrumental observations ensure relatively high data quality, which is enough to get reliable earthquake parameter determinations.

2005ES000195-fig01
Figure 1
[3]  In this work we will address to the seismicity of southern Catalonia, Spain. Though Catalonia is not large, the spatial distribution of seismicity over its territory is rather inhomogeneous (Figure 1). The highest activity both in historical and modern times is concentrated in its northern part along the Pyrenean Range, approximately up to Barcelona. On the other hand, the part from Tarragona to Ebro valley in the south presents moderate seismic activity. According to the seismic catalogue of the Spanish National Geographic Institute (IGN) [Martinez-Solares and Mézcua, 2002; Mézcua and Martinez-Solares, 1983], (IGN Database, 2004, http://www.mfom.es/ign/), earthquakes with magnitudes greater than 4 are not known in this part of Catalonia. The spatial distribution of seismicity is also reflected in the hazard map (Figure 1). PGA values along the Pyrenees are greater than or equal to 1.25 m s-2, whereas to the north of Barcelona it exceeds 1.75 m s-2. On the other hand, the isoline of 1.0 m s-2 contours southern Catalonia.

[4]  The importance of every earthquake for hazard assessment becomes higher in areas of low and moderate seismicity. At the same time, the parameters of small earthquakes ( Mle4 ) are usually defined with greater uncertainty, especially before the operation of the WWSSN started, because only few stations could record them. In a certain sense, the level of seismic activity justifies our choice of the study region.

[5]  Stars in Figure 1 show the relatively large earthquakes occurred in southern Catalonia. Black stars correspond to seismic events with M = 3.5-4 (larger magnitudes are not known in this region). The epicentral intensity ( I0 ) of the 1845 earthquake (white star) is the highest reported ever. Its magnitude (in fact there were 3 shocks on October 1, 3 and 7) is not defined in the IGN catalogue. The epicentral intensity of the first shock is 4, whereas for next two shocks it is 6.5. These are the highest reported I0 in the area. But probably I0 is somewhat exaggerated because of superposition of macroseismic effects from the sequence of shocks.

[6]  We will focus our study on the earthquake of 1930. It clearly can be considered as a seismic event of the early instrumental period. Its magnitude in the IGN catalogue is 3.9 whereas I0 is 5 and hypocenter depth is not assigned. It has to be noted that the source depth is neither reported for 1949, nor for 1976 earthquakes with magnitudes 4.0 and 3.6; both with epicentral intensity 5. The absence of hypocenter depth determination could be interpreted as an evidence for low accurate solution for this earthquake. This is again a reason to look more carefully at the seismicity of the region, which was not well monitored instrumentally probably up to mid of 1970's. The importance of source depth for seismic hazard analysis is evident from the macroseismic field equation [Shebalin, 1971]:

eq001.gif(1)

which in the epicentral area becomes:

eq002.gif(2)

where I i is the intensity at a distance R km from the epicenter, I0 is the epicentral intensity, M is the magnitude, H is the hypocenter depth in km, and b, n, c are coefficients which are equal to 1.5, 3.5 and 3.0 respectively for earthquake sources within the Earth crust. These values of coefficients were proved in most of the world seismic active regions [Kondorskaya and Shebalin, 1982; Shebalin et al., 1974] and they suggest a strong dependence of intensity with depth. It has to be emphasized that the absence of depth for largest earthquakes is an essential drawback not only for hazard assessment but also for the understanding of seismotectonic features governing the seismicity of the region.

[7]  Further we will present the record lines for the 1930 earthquake from the parametric catalogue, then we will examine the macroseismic data referenced in the catalogue and finally we will revise the macroseismic and instrumental data to get mutually consistent interpretation.


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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