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July
18- 28 2007: Gorda-Blanco-Southern Juan de Fuca earthquake sequences
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. |
