  We apply cross correlation between multichannel seismic waveforms as a
technique for signal detection and automatic event building at the
International Data Centre (IDC). This technique allows detecting signals with
amplitudes by at least a factor of two lower than those found in the current
version of IDC processing. Previously, we processed with a cross correlation
detector aftershock sequences of a large earthquake with thousands of
aftershocks detected by the International Monitoring System (IMS) and a
middle-size earthquake (hundreds of aftershocks). Our study has revealed that
the 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 International
Seismological Centre (ISC) these extra events together with the associated
arrivals are missing from the ISC bulletin which is an open data source for the
broader seismological and geophysical community.Here, we assess the ultimate
resolution of the cross correlation technique with specific IDC constraints.
The aftershock sequence of the October 5, 2011 mb(IDC)4.2 earthquake in the
North Atlantic is an example of a weak sequence and includes only 38 REB
events. The number and quality of these REB events, which are used as master
events, allow conducting a comprehensive interactive review by experienced
analysts of all event hypotheses obtained by the cross correlation technique.
In an iterative procedure starting from the main shock, all 38 REB events were
found and analysts added 26 REB events. Therefore, the cross correlation
pipeline reduces the detection threshold by a factor of 2 to 3 and
approximately doubles the number of events in the REB, and thus, in the ISC
bulletin for the North Atlantic.
