The data from the Bulletin of the International Seismological Centre (ISC) have always been and still remain in demand for a wide range of studies in Geosciences. The unique features of the Bulletin include long-term coverage (1904-present), the most comprehensive set of included seismic data from the majority of permanent seismic networks at any given time in the history of instrumental recording (currently ~ 150) and homogeneity of the data and their representation. In order to preserve this homogeneity, the ISC has followed its own standard seismic event processing procedures that have not substantially changed until the early 2000s. Several considerable and necessary advancements in the ISC data collection and seismic event location procedures have created a need to rebuild the data for preceding years in line with the new procedures. Thus was set up a project to rebuild the ISC Bulletin for the period from the beginning of the ISC data till the end of data year 2010. The project is known as the Rebuild of the ISC Bulletin. From data month of January 2011, the ISC data have already been processed with the fully tested and established new procedures and do not require an alteration. It was inconceivable even to think about such a project for many tens of years, but great advances in computer power and increased support by the ISC MemberInstitutions and Sponsors have given us a chance to perform this project. Having obtained a lot of experience on the way, we believe that within a few years the entire period of the ISC data will be reprocessed and extended for the entire period of instrumental seismological recordings from 1904 till present. The purpose of this article is to describe the work on reprocessing the ISC Bulletin data under the Rebuild project. We also announce the release of the rebuilt ISC Bulletin for the period 1964-1979 with all seismic events reprocessed and relocated in line with the modern ISC procedures, ~ 68,000 new events, 255 new stations, ~ 815,000 new seismic phases, more robust and reliable mb and M S magnitude evaluations and the addition of ~ 2700 new M S magnitudes.
S U M M A R YIn 1999, an unusual earthquake swarm at the 85 • E/85 • N volcanic centre on the ultraslow spreading Gakkel Ridge, Arctic Ocean, was detected teleseismically. The swarm lasted over 9 months and counted 252 events with m b ≥ 3.1. It represents the strongest and largest ever recorded mid-ocean ridge earthquake swarm, and it occurred at a site where spreading rates are only about 10 mm yr -1 . We relocated the earthquake swarm comparing the performance of three different localization algorithms: (1) the absolute least-squares routine HYPOSAT, (2) the absolute probabilistic routine NonLinLoc and (3) the relative least-squares routine Mlocate. The epicentres as calculated by each algorithm mostly did not agree within their error ellipses. Thus, the choice of location algorithm proved more critical than, for example, the choice of a local velocity model. We compiled a set of well-localized events which closely agree in at least two routines, mostly Mlocate and NonLinLoc.We conclude that the earthquake swarm of 1999 was related to a spreading episode and shows a complex interplay of tectonic and magmatic events. Our geological interpretation revealed three phases in swarm activity: In the first phase from January 17 up to February 1 fracturing of the crust took place, either as a result of or enabling magmatic intrusion. Seismicity in the second phase from February 2 to April 6 expanded along-and across axis. It showed signs of magmatic interaction, but a clear dyke migration pattern is absent. At the beginning of the third phase, a distinct break in the event rate suggested a change in the physical process, either an adjustment of the stress field to the new regime or a transition to an effusive stage.
SUMMARY The ultraslow spreading Southwest Indian Ridge (SWIR) is a prominent end‐member of the global mid‐ocean ridge system. It spreads with a full‐rate of 14–16 mm y−1 and shows several segments of various obliquities. The western SWIR consists of the Oblique and Orthogonal Supersegments lying at an epicentral distance of ∼21° to the VNA2 seismic array operated by the German Neumayer station in East Antarctica. The array monitors backazimuth, apparent velocity and signal‐to‐noise ratio of arriving waves and provides a data set of seismicity from the western SWIR over several years. Compared to the global seismological network, its detection threshold for earthquakes occurring at the western SWIR is more than 0.5 mb lower enabling a more comprehensive study of mid‐ocean ridge processes than the teleseismic earthquake catalogues. We identified a total number of 743 earthquakes occurring at the western part of the SWIR and calculated the body‐wave magnitudes (mb) from P‐wave amplitude picks on the VNA2 broad‐band sensor obtaining a magnitude range from mb 3.18 to mb 5.34. In the years of 2001, 2004, 2005 and 2008, significantly increased event rates indicated four earthquake swarms with up to 164 events lasting for several days. All swarms had strong events registered in the International Seismological Centre catalogue. The relocalization of these events confirmed that all swarms occurred in the same region on the Orthogonal Supersegment. We analysed event and moment release rate histories, b‐values and aftershock decay rates (Modified Omori Law) finding that the swarms of 2001, 2004 and 2005 have similarities in the temporal distribution of seismic moment and event numbers. The swarm of 2008 is smaller with high magnitude events at the swarm's onset which represent shear failure on normal faults. The application of the Modified Omori Law and the b‐value show that the earthquakes of the swarms do not follow the classical main shock–aftershock pattern of purely tectonic earthquake sequences. At the Orthogonal Supersegment, a continuous positive magnetic anomaly along the rift axis, a negative mantle Bouguer anomaly, basalts at the seafloor and potentially volcanic edifices indicate robust magmatic crustal accretion. The high‐resolution bathymetry shows ubiquitous rift‐parallel ridges with steep flanks towards the rift axis indicating high‐angle normal faults. The high‐magnitude earthquakes detected teleseismically during the swarms are generated there. We interpret that the swarms are caused by magmatic accretion episodes at a suggested volcanic centre of the Orthogonal Supersegment and that possible magma injection activates the steeply dipping fault planes.
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