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

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

The Sarez-Pamir Earthquake and Landslide of 18 February 1911

“…The body-wave magnitude, which was calculated from long-or medium-period instruments, gave m = 7.3 ( Table 2). The surface-wave magnitude Ms* calculated from 27 Milne seismograms (Table 3) is Ms* = 7.4 ± 0.4 (see Douglas 2000 andAmbraseys 2001).…”

Comparison of frequency of occurrence of earthquakes with slip rates from long-term seismicity data: the cases of Gulf of Corinth, Sea of Marmara and Dead Sea Fault Zone

“…For the instrumental period, after 1912, the standard deviation of station magnitudes σ S remains nearly constant for the whole 100‐yr‐long period, with a mean value of ±0.25 M S units but with considerable scatter (±0.09). The standard deviation of the mean σ varies somewhat with magnitude and is on average ±0.10 in M S units (±0.05) (Ambraseys & Douglas 2000).…”

Reassessment of earthquakes, 1900-1999, in the Eastern Mediterranean and the Middle East

“…The example shown in Fig. 6, which compares uniformly recalculated magnitudes (Abe 1988, 1994; Abe & Noguchi 1983a,b; Ambraseys & Douglas 1999, 2000) with magnitudes estimated by Gutenberg & Richter (1965) and Duda (1965), demonstrates, for instance, that for M S > 7.0 an overestimation of magnitudes by 0.3–0.5 units is possible and it happens in our case: there is an overestimation sufficiently large to obscure or to eliminate the break in the regional frequency relation, to blur scaling laws and to exaggerate grossly early 20th‐century seismicity (Satyabala & Gupta 1996 for northern India).…”

“…Station magnitudes were estimated from amplitudes and periods of long waves taken from horizontal and vertical components separately and corrected for station and distance. For the period before 1935 a considerable number of events with 6 < M S < 7.2, not included in other catalogues, have been identified and their magnitudes assessed (Ambraseys & Douglas 1999, 2000).…”