[S01-2] Open session II

[S01-2-01] Calculating the ISC's own magnitudes

^○Elizabeth Entwistle, Domenico Di Giacomo, Dmitry Storchak (International Seismological Centre, Thatcham, United Kingdom)

The International Seismological Centre (ISC) routinely reports body and surface wave magnitudes for earthquakes recorded at teleseismic distances. Current ISC procedures for calculating magnitudes rely on amplitude-period pairs reported to the ISC by contributing agencies. Here, we develop a new method to calculate magnitudes in house at the ISC, in accordance with the IASPEI standards outlined by the IASPEI/COSOI Working Group on Magnitudes. These new ISC magnitudes are calculated using waveforms downloaded from international data centres (IRIS, EIDA, GeoNet) and our own automatic algorithms for measuring amplitude (A) and period (T). We present results for broadband body wave magnitudes (mB(BB)) calculated using the relation mB(BB) = log10((A/T)max) + Q(d,h), where Q(d,h) is the attenuation function for P-waves recorded on vertical component seismographs, established by Gutenberg and Richter (1956), at epicentral distances d and for a focal depth h, and (A/T)max = (Vmax/2pi), where Vmax is the ground velocity associated with the maximum velocity trace-amplitude in the entire P-wave train (Bormann and Saul, 2008). We use a dataset of recordings of up to 1000 earthquakes of magnitude greater than 5 in the period 2012 – 2013, that are reviewed in the ISC Bulletin. We compare our own ISC mB(BB) with MW from GCMT, mb from NEIC, and conventional ISC mb and MS. These new ISC-calculated magnitudes will be an extra dataset available to users of the ISC Bulletin, and will provide a robust measure of magnitude that is in agreement with IASPEI standards. To complement the existing amplitude and period measurements reported to the ISC by contributing agencies, ISC-picked amplitude and period measurements and their associated amplitude-phase arrival time will also be reported and made available to users.

References:

Bormann, P., J. Saul (2008): The New IASPEI Standard Broadband Magnitude mB. Seismological Research Letters, 79, 5, p. 698-706.