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We apply cross correlation between multichannel seismic waveforms as atechnique for signal detection and automatic event building at theInternational Data Centre (IDC). This technique allows detecting signals withamplitudes by at least a factor of two lower than those found in the currentversion of IDC processing. Previously, we processed with a cross correlationdetector aftershock sequences of a large earthquake with thousands ofaftershocks detected by the International Monitoring System (IMS) and amiddle-size earthquake (hundreds of aftershocks). Our study has revealed thatthe official Reviewed Event Bulletin (REB) of the IDC misses from 50% to 70%valid seismic events. Since the IDC is a major contributor to the InternationalSeismological Centre (ISC) these extra events together with the associatedarrivals are missing from the ISC bulletin which is an open data source for thebroader seismological and geophysical community.Here, we assess the ultimateresolution of the cross correlation technique with specific IDC constraints.The aftershock sequence of the October 5, 2011 mb(IDC)4.2 earthquake in theNorth Atlantic is an example of a weak sequence and includes only 38 REBevents. The number and quality of these REB events, which are used as masterevents, allow conducting a comprehensive interactive review by experiencedanalysts of all event hypotheses obtained by the cross correlation technique.In an iterative procedure starting from the main shock, all 38 REB events werefound and analysts added 26 REB events. Therefore, the cross correlationpipeline reduces the detection threshold by a factor of 2 to 3 andapproximately doubles the number of events in the REB, and thus, in the ISCbulletin for the North Atlantic.

Comprehensive recovery of a weak aftershock sequence in the North Atlantic using waveform cross correlation