✍ Scribed by D. Benouar
No coin nor oath required. For personal study only.
This paper presents the results of an investigation of the magnitude-intensity and intensity-attenuation relationships for earthquakes in the Atlas block and Algeria using macroseismic data. This work is based on a selected sample of isoseismal maps from 32 events which were recently revised. Surface-wave magnitudes, M,, are recalculated using the Prague formula and range from 4.2 to 7.45. Because the Atlas mountains block is in a collision zone, earthquakes occur in general within a layer 15 km deep. Expressions of general form for the magnitude-intensity and intensity-attenuation correlations are adopted and are, respectively, M,, = A , + A,(Zi) + A,(Ri) + A4 log Ri + U P and
where R2 = d2 + h2, d the source distance in km, h the focal depth in km, M , the revised surface-wave magnitude, M,, the predicted surface-wave magnitude, I i the intensity at isoseismal i, I the predicted intensity, 6 the standard deviation and P is Zero for 50-percentile values and one for 84-percentile, and the coefficients A's andB's are determined by regression analysis. The results of this study show that the intensity-attenuation models are adequate to predict quite well the dieout of intensity with distance in the Atlas zone and coastal Algeria; it is also found that magnitude can be predicted accurately by calibrating isoseismal radii against revised instrumental surface-wave magnitude. Such magnitude-intensity relationships may be used to evaluate the magnitude of historical earthquakes in the region under survey, with no instrumental data, for which isoseismal radii and intensities are available.
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