The Hubbard cluster is named after Hubbard Glacier, north of Yakutat,
Alaska. Nearly all events lie under an icesheet. The cluster is based on
the 7.0 Mw earthquake on December 6, 2025, and it includes 42
aftershocks over the next 4 days. The cluster spans the U.S.-Canada
border. The region contains extreme variations in elevation, from near
sealevel to 3 km above sealevel, and this seems to make difficult the
specification of a crustal velocity model that is broadly satisfactory.
More than the usual number of events prefer extremely shallow focal
depths and such events are especially common among the aftershocks of
the December 6 earthquake. The arrival time dataset has few observations
from very close range so the shallow depths are driven by negative
residuals at distances greater than 50 km, likely at least partially due
to an inadequate crustal model. Most aftershocks of the December 6
mainshock prefer a shallow depth, less than ~5 km. Despite some
uncertainty in focal depths, the location calibration is very robust,
due to good azimuthal coverage by seismic stations. The arrival time
data for the December 6 mainshock are not consistent with a single,
simple rupture process. The initial rupture is reported by most stations
within about 600 km but at greater distances, especially teleseismic,
many reported first arrival times are delayed by 2-5 seconds from that
initial source, apparently representing one or more later pulses of
energy release. To avoid location bias, only data within 6.0° epicentral
distance were used for this event. At the northwestern end of the
aftershock sequence the epicenters spread orthogonal to the main trend
over ~15 km. All events have depth control from near-source or
local-distance readings and some events have teleseismic depth phase
data in general agreement with those depths.
