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Wecoma Response Cruise Synopsis:
The earthquake swarms north of the Blanco Transform and on the northern Gorda Ridge segment have subsided and seismicity is at background levels. Overall, it was a very interesting progression of seismicity, beginning with an intense, 10-day intraplate swarm north of the central Blanco Transform. This was followed over the next week by magnitude 5 earthquakes (with aftershocks) at the Cascadia basin (central BTF) and along the eastern Blanco Transform. Then surprisingly during the response cruise, another intense earthquake swarm began on the northern Gorda Ridge segment adjacent to the Blanco Transform. The Gorda swarm produced >1000 detected earthquakes over the 5 day duration of the event, with earthquakes distributed from the mid-segment high to north along the rift-zone. This swarm was also of interest because of previous magmatic activity near its location. In 1996, a similar-sized earthquake swarm occurred on the southern part of the northern Gorda ridge segment that was associated with a seafloor eruption and a massive hydrothermal plume release.
Although clear water-column temperature and transisometer anomalies associated with magmatic activity were not found during real-time sampling at either swarm site, water samples are still being processed by the NOAA/PMEL Helium Isotope lab, UW (M. Lilley) and OSU (S. Giovannoni) for unique chemical and biological signatures that will provide additional information on the cause of these events. Moreover, multichannel seismic data (courtesy LDEO, Carbotte and Tolstoy) and 3.5 khz sub-bottom profile data collected during the response cruise indicate the intraplate swarm occurred in an area of recently active faults that offset abyssal sediments. More data processing is required, however, to establish if these faults are the source of the intraplate swarm.
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