White circles show approximate locations of seismic events.
Error bars show the earthquake location uncertainty at the
95% confidence interval .Click
map for full-size.
relatively small, but intense earthquake swarm occurred on
the eastern side of West Valley along the northern Juan de
Fuca Ridge. The swarm occurred on 21 July at 06:36Z, lasted
for ~4 hours, and produced 87 detectable earthquakes. We were
only able to locate 20 of these events, however, because of
the limited number of SOSUS hydrophones currently available.
None of the earthquakes appear to have been detected by land-based
seismic networks along the western U.S. or Canada.
swarms along West and Middle Valleys are not common, but because
of its short duration and relatively few earthquakes (by comparison
the 1998 Axial eruption produced >8200 earthquakes in 11
days) a dedicated response cruise does not seem warranted.
It may, however, be worthwhile to investigate possible water-column
signatures from a ship of opportunity.
of the end of 21 July 2003, we detected 10 more earthquakes
and were able to locate one more event originating from the
previous swarm site.