On the 26 of January 2017, the version 4.0 of the ISC-GEM catalogue was publicly released. The release marked the end of the third year of the Extension Project. During this year we added to the catalogue a) earthquakes listed in the ISS with phase data and surface wave amplitude measurements that occurred between 1920 and 1934 (i.e., earthquakes not included in the catalogue’s previous versions); b) earthquakes with magnitude 5.5 and above that occurred in 2013.
To process earthquakes during 1920-1934 we digitized body wave arrival times as well as amplitudes and periods for various phases using a multitude of sources to allow relocation and magnitude re-computation (for this time period only MS) using the same approaches described in Di Giacomo et al. (2015a). Compared to the first catalogue’s release, for this version we were able to add amplitudes from printed station bulletins that were not available in the original ISC collection. The additional station bulletins were kindly provided from the personal collection of Prof. N. Ambraseys, British Geological Survey (BGS) collection, bulletins from the Geophysical Institute of the Czech Academy of Sciences in Prague, the Geophysical Survey of the Russian Academy of Science in Obninsk and the University of Strasbourg (France). Such an enriched bulletin collection allowed us to add important surface wave amplitudes from different stations and to obtain for the first time MS for many historical earthquakes and also to improve the MS for large earthquakes (MS>6.0) for earthquakes that were already listed in previous versions of the ISC-GEM catalogue.
Fig. 1 shows the ISC-GEM locations with symbols according to Agnew (2014) as obtained from the procedure described in Bondár et al. (2015). One feature that makes the ISC-GEM catalogue a unique product is that the locations (after 1903) are achieved by the same location technique and velocity model (ak135) and comes with uncertainties for each focal parameter.
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| Fig.1: map showing the earthquakes listed in the version 4.0 of the ISC-GEM catalogue (more than 26,000 earthquakes, see Fig.2). The symbols are plotted according to Agnew (2014) and colour coded according to the ISC-GEM depth. |
Fig. 2 shows the current time-magnitude distribution of the ISC-GEM Catalogue (1900-2013).
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| Fig. 2: Top: cumulative annual number of earthquakes with Mw ≥ 5.5 (red), ≥ 6.5 (blue) and ≥ 7.5 (yellow); Bottom: time-magnitude distribution color-coded in cells of 0.1 units of Mw for each year of the ISC-GEM main catalogue.. |
The earthquakes processed in 2013 exhibit the same distribution we can observe in recent years and add significant earthquakes to the catalogue. The biggest difference with previous versions occurs during 1920-1934, where, in the past versions, nearly no earthquake below Mw 6.1 was included. Both the time-frequency distribution and the annual number of events per year above 5.5 show, however, that in the 1920-1934 the ISC-GEM catalogue is not as complete as in the more recent decades. The dark shaded grey area before 1920 is the time period that will be processed in the remaining year of the Extension Project, the shaded light gray areas highlight the years extended or added. For comparison with the original version, see Fig. 20 in Di Giacomo et al. (2015b).