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Reappraisal of surface wave magnitudes in the Eastern Mediterranean region and the Middle East

Ambraseys, N. and Douglas, J. (2000) Reappraisal of surface wave magnitudes in the Eastern Mediterranean region and the Middle East. Geophysical Journal International, 141 (2). pp. 357-373. ISSN 0956-540X

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Abstract

There have been many attempts to improve parametric catalogues for surface wave magnitudes for earthquakes of this century, and many of these attempts have been based on empirical adjustments to homogenize and complete catalogues without recourse to the instrumental data with which these magnitudes have been calculated. Using the Prague formula with station corrections and a substantial volume of amplitude and period readings of surface waves, culled from station bulletins, we calculated uniformly the magnitude of all significant earthquakes, 1519 in all, in the Eastern Mediterranean region and in the Middle East between 1900 and 1998. We also calculated station corrections and their variation with time, and examined the effect of distance adjustment of the Prague formula on M(s) estimates. We find that the current procedure of averaging station magnitudes underestimates M(s). This underestimation depends on the variance and on the number of station magnitudes available for the calculation of M(s), which can be as large as 0.3 magnitude units or more. We also find that station corrections have a rather small overall effect on event magnitude, of less than +0.1, except when the number of observing stations is small, in which case the correction may reach +0.3 magnitude units. Event magnitudes derived from station magnitudes with distance adjustment, calculated from the original Prague formula, are on average 0.1 units larger than M(s) without distance correction. We find that for shallow events, Gutenberg's estimates are, on average, larger by 0.12 units and show a significant scatter, with a standard deviation three times the mean value. We find similar differences and scatter for surface wave magnitudes estimated by other workers and agencies.