Category: Uncategorized

The Acapulco cluster is named for the city of Acapulco on the coast of
Guerrero State, Mexico. This series replaces the acapulco5 cluster
previously published in GCCEL. The cluster includes a 7.0 Mw earthquake
on September 8, 2021 and four earthquakes with magniude 6.0 or greater
(6.8 Mw on 4/25/1989, 6.0 Mw on 4/13/2007, 6.2 Mw on 8/21/2013 and 6.5
Mw on 1/2/2026). Earthquakes located more than ~15 km off-shore or
deeper than ~60 km were not included, to avoid location bias. The
distribution of seismograph stations for direct calibration is quite
good. All events have depth control, mainly from near-source and
local-distance readings.

Further work on the Hubbard Glacier cluster involved adding 10 additional aftershocks and resolving an issue with many events wanting to go to zero depth or negative depth.

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 53
aftershocks through January 1, 2026. The cluster spans the U.S.-Canada
border. The local distance arrival time data are best fit with a
relatively high velocity at shallow depths (6.2 and 3.55 km/s) and a
single-layer crustal model with these velocities fits the data well.
Most aftershocks of the December 6 mainshock prefer a shallow depth,
with a peak of ~5 km. The location calibration is very robust, due to
good azimuthal coverage. The arrival time data for the December 6
mainshock are not consistent with a single, simple rupture process. The
initial rupture is reported consistently 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
to estimate the relative location 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.

The Sapporo cluster is named for the city of Sapporo on the island of
Hokkaido, Japan. The cluster covers the southeastern part of the island,
including events to a depth of ~80 km. The cluster lies in the “Hokkaido
Corner” at the intersection of the Japanese and Kurile trench systems.
It includes three major events, the 6.4 Mw Hikada earthquake on January
20, 1970, the 6.7 Mb Urakawa-oki earthquake on March 21, 1982 and the
6.6 Mw Eastern Iburi earthquake on September 5, 2018. The distribution
of seismograph stations is very good and the location calibration is
very robust. All events have depth control from near-source and
local-distance arrival time data and many events also have teleseismic
depth phases that are usually in close agreement. The distribution of
depths is bi-modal with a peaks at 32 and 60 km.

The arrival time dataset of the 2018 Eastern Iburi earthquake is quite
complex and the normal procedure of using data at all distances to
estimate the relative location resulted in significant location bias;
the mainshock epicenter was shifted several tens of km away from its
aftershock cloud. From a detailed analysis of the aftershock sequence,
Katsumata et al. concluded
that the main rupture consisted of reverse faulting on two fault segments
separated by a short step-over on which the rupture initiated with
strike-slip motion and magnitude ~5. There is an unspecified time lag
(likely several seconds) between the initial rupture and the two
subsequent larger subevents. The arrival time data at short epicentral
distances consistently appears to be from this triggering subevent, but
at greater distances the arrival times display significant scatter,
apparently as the arrivals from one or the other of the two larger
reverse faulting events dominate. In the calibrated relocation all
arrival times beyond 8.0° for this event were skipped, so the calibrated
epicenter represents the triggering subevent near the middle of the
overall aftershock pattern.

The version of the Hubbard Glacier cluster uploaded on December 14 (hubbard4.12) contained a biased location the M7.0 mainshock on December 6, 2025. That has been corrected in hubbard5.7.

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.

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 mainshock
epicenter, with a preferred depth of 8 km, consistently lies near the
southeastern end of the SE-NW trending aftershock pattern, but always
5-10 km south of that trend, suggesting some geometrical complexity in
the rupture. 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.

The Ushkan cluster is named for the Ushkan Islands in Lake Baykal,
Russia. Many of the included earthquakes are small and recorded only to
regional distances, but there is a modest amount of teleseismic data
from the larger events (up to 5.3 Mw). The distribution of seismograph
stations provides good azimuthal coverage at local distances for a
robust location calibration. All events have depth control, mainly from
near-source and local-distance readings.

The Davsha cluster is named for the town of Davsha on the eastern shore
of Lake Baykal in eastern Russia. The earthquakes are mainly on the
eastern shore of the northern third of the lake. The number of seismic
stations in the area is not large but they are well situated to provide
robust locations and the ISC contains a large dataset of the seismicity,
upwards of 1000 events whose locations could be calibrated. The cluster
is composed of two classes of events selected from that large dataset:
1) those with the closest station coverage for depth control and good
azimuthal coverage, and 2) those with the best teleseismic datasets. All
events have depth control. Many of the larger events have depth
constraint from teleseismic depth phases that agrees well with the depth
inferred from near-source and local-distance readings.

The Novy cluster is named for the town of Novy Uoyan in the Republic of
Buryatia, near the northern end of Lake Baykal in the Russian far east.
The area is quite seismically active, at magnitudes as high as 5.9, and
fairly well instrumented, so location calibration is very robust. The
depth distribution of earthquakes tends toward bimodality, with about
half the events quite shallow, from 0-10 km, and another peak around 16
km, with some events near 30 km depth. The location analysis is
ill-suited to resolving very shallow depths. All events have depth
control, mostly from near-source and local-distance observations, or in
some cases from teleseismic depth phases, especially for earlier events
that had few or no local observations. Most events are observed at
teleseismic distances, but a few events that were observed only to
regional distances are retained for their contribution to the
statistical power of the location calibration.

The Chara cluster is named for the town of Chara in the northern art of
Zabaykalsy Krai in eastern Russia. It includes several earthquakes in
the magnitude 5 range, the largest being a 5.8 mb event on August 21,
1994. All events were observed to at least 4.0° epicentral distance and
there is abundant teleseismic data. All events have depth control from
near-source and local-distance readings and some have teleseismic depth
phase constraint as well. The number of seismograph stations close
enough to be suitable for direct calibration is small but they are well
distributed and the location calibration is very robust.

The Tofino cluster is named for the village of Tofino on the west coast
of Vancouver Island, Canada. As with many calibrated clusters along
coastlines, azimuthal coverage is a major problem, ameliorated somewhat
in this case by the availability of observations for some events from
ocean bottom seismometers. About half the events in the cluster are
observed only to regional distances but they are retained to provide a
sufficient number of local-distance readings to adequately constrain the
crustal velocity structure and depth constraint. Similarly, about half
the events are beneath the Moho (30 km) of the crustal model, to a
maximum depth of 45-50 km. Even though there are only two events with
magnitude 5.0 or greater (the largest is 5.7) the cluster does have
reasonable coverage at teleseismic distances. All events have depth
control from near-source or local-distance observations, and a few also
have teleseismic depth phases for corroboration.