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Gorda Ridge/Blanco Transform - July 2007

epicenter map
Epicenter map of the recent 2007 events. (click for full size)

For the past 10 days we have watched a week long, systematic progression of earthquakes beginning at the Northern Gorda, moving to the eastern and western Blanco Transform, and ending at the Cleft-Vance intersection at the southern Juan de Fuca Ridge.This week of earthquake activity on SOSUS stands out since we had been observing little to no earthquake activity from the JdF during  the prior three months.
The recent plate boundary sequence began on July 18 at 2014Z with a large earthquake that occurred in the central valley of the northern Gorda Ridge. The event was followed by 20 aftershocks. Although this was a relatively a small sequence of events, it is interesting to note the earthquakes were located on the intra-segment high where the 1996 eruption centered. This Gorda earthquake swarm was followed for the next 5 days by earthquakes along the entire Blanco Transform.

On July 23, a relatively small swarm of earthquakes occurred at the Cleft-Vance intersection. The largest events in the swarm were located at ~45-00' 130-19'  and produced 79 events over 11 hours. This was followed on July 26 (yesterday) by a sequence of 3 large earthquakes (magnitudes 4-4.7) with ~150 aftershocks along the western Blanco - Cleft intersection. The western Blanco events were still going on as of this posting (July 30, 2007)

The earthquake swarm activity at Vance-Cleft is interesting and uncommon. It would be worth a CTD cast, but only if a ship is in the area. If the swarm continues then investigation of the site should become a high priority, but activity at this time has already died down.

Past western Blanco earthquake sequences have caused temperature changes at the Cleft hydrothermal vent sites. However, the earthquakes we used to establish these temperature change relationships were an order of magnitude larger than this recent event. Therefore although there may have been affects at the Cleft vents, they were likely not large and it would not be worth diverting a ship.

Although the observation of a progression of earthquakes (strain pulse) along the plate boundary is interesting from an active tectonics standpoint, there is very likely no magmatic activity associated with this phenomena.


Last Updated: 07/30/07

Address inquiries to:
Robert Dziak - Seismologist -
Matt Fowler - Analyst -