ISC-GEM : Global Instrumental Earthquake Catalogue (1900–2009), III. Re-computed MS and mb, proxy MW, final magnitude composition and completeness assessment

Di Giacomo, Domenico and Bondár, István and Storchak, Dmitry A. and Engdahl, E. Robert and Bormann, Peter and Harris, James (2015) ISC-GEM : Global Instrumental Earthquake Catalogue (1900–2009), III. Re-computed MS and mb, proxy MW, final magnitude composition and completeness assessment. PHYSICS OF THE EARTH AND PLANETARY INTERIORS, 239. pp. 33-47. ISSN 0031-9201

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Abstract

Abstract This paper outlines the re-computation and compilation of the magnitudes now contained in the final ISC-GEM Reference Global Instrumental Earthquake Catalogue (1900–2009). The catalogue is available via the ISC website (www.isc.ac.uk/iscgem/) and lists in a comma separated format the location and magnitude parameters (with corresponding uncertainties) of global large earthquakes. In this work we report on the procedures adopted to obtain the final magnitude composition of the nearly 20,000 earthquakes processed. We have made every effort to use uniform procedures of magnitude determination throughout the entire period of the catalogue. The re-computation of the surface wave MS and short-period body-wave mb values benefitted from new hypocentres (Bondár et al., 2014), previously unavailable amplitude-period data digitized during this project (Di Giacomo et al., 2014), and a more reliable algorithm for magnitude estimation based on a 20% alpha-trimmed median magnitude (Bondár and Storchak, 2011). In particular, for MS until the end of 1970 we have processed an unprecedented amount of data and obtained several thousands of station magnitudes not available before. The available re-computed MS and mb provided an ideal basis for deriving new conversion relationships to moment magnitude MW. Therefore, rather than using previously published regression models, we derived new empirical relationships using both generalized orthogonal linear and exponential non-linear models to obtain MW proxies from MS and mb. The new models were tested against true values of MW, and the newly derived exponential models were then preferred to the linear ones in computing MW proxies. For the final magnitude composition of the ISC-GEM catalogue, we preferred directly measured MW values as published by the Global CMT project for the period 1976–2009 (plus intermediate-depth earthquakes between 1962 and 1975). In addition, over 1000 publications have been examined to obtain direct seismic moment M0 and, therefore, also MW estimates for 967 large earthquakes during 1900–1978 (Lee and Engdahl, 2014) by various alternative methods to the current GCMT procedure. In all other instances we computed MW proxy values by converting our re-computed MS and mb values into MW, using the newly derived non-linear regression models. The final magnitude composition is an improvement in terms of magnitude homogeneity compared to previous catalogues. The magnitude completeness is not homogeneous over the 110 years covered by the ISC-GEM catalogue. Therefore, seismicity rate estimates may be strongly affected without a careful time window selection. In particular, the ISC-GEM catalogue appears to be complete down to MW 5.6 starting from 1964, whereas for the early instrumental period the completeness varies from ∼7.5 to 6.2. Further time and resources would be necessary to homogenize the magnitude of completeness over the entire catalogue length.

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