Category: Uncategorized

The Oran cluster is named for the city of Oran on the Mediterranean
coast of Algeria. It includes a few events across the border in coastal
Morocco. The cluster contains four events with magnitude > 5, the
largest being the 5.5 Mw event on June 6, 2008. Most events were
observed to teleseismic distances but some smaller events that were
recorded only to near-regional distances were retained to improve the
azimuthal coverage and statistical power of the location calibration.
All events have depth control, from near-source or local-distance
readings, or in a several cases, a waveform modeling study or
teleseismic depth phases.

The Afar cluster is named for the Afar region of Ethiopia, Eritrea and
Djibouti. The cluster occupies the northern part of what has been termed
the Danakil or Arrata microplate. Much of the seismicity has occurred in
swarms associated with volcanic activity. The cluster includes five
events with magnitude greater than 6.0, in swarms that occurred in April
1969 and August 1989. All but five events in the cluster were recorded
at teleseismic distances. All events have depth control from near-source
and local-distance readings or (more frequently than usual) teleseismic
depth phases. Location calibration is based mainly on a seismic swarm
that was well-instrumented with a temporary network from November 2007
to February 2009, and included events large enough to be recorded at
teleseismic distances, which established the necessary connectivity with
events that lack local observations.

The Sumba cluster is named for the island of Sumba in the Lesser Sunda
Islands of eastern Indonesia. The cluster is restricted to events with
constrained focal depths of 40 km or less since 2008. Earlier events
lack station coverage to adequately link them to the more recent
earthquakes. The largest earthquake has magnitude 5.8 Mw, on February
21, 2022. With one exception (depth constrained by teleseismic depth
phases) all events have depth constraint from near-source or
local-distance readings. Many small events that were recorded only to
near-regional distances are retained to improve the statistical power of
the location calibration.

The Lucia cluster is named for the island of St. Lucia in the Windward
Islands. It includes events near Barbados and St. Vincent and the
Grenadines. Station coverage is fairly good since 2008. Many events are
recorded only locally but there are useful amounts of data at far
regional and teleseismic distances. The largest event is 5.7 mb on
August 12, 1987. All events have depth control, mostly from near-source
and local-distance readings, but several events are constrained in depth
by teleseismic depth phases.

The Sparta cluster is named for the city of Sparta in northwestern North
Carolina. It covers a rather broad area and includes events in eastern
Tennessee, western Virginia and southernmost West Virginia. The cluster
is based on a 5.1 Mw earthquake on August 9, 2020 very close to Sparta.
The Sparta earthquake is remarkable for its shallow depth (with various
estimates in the 2-4 km range) and the observation of surface faulting.
It has a depth of 4 km in the calibrated relocation, based on arrivals
at local distance, but a shallower depth cannot be ruled out. The
cluster as a whole is at quite shallow depths, with only a single event
as deep as 10 km. It is quite likely that many of the events are mining
blasts. Most events have magnitudes in the 2-3 range. The distribution
of seismic stations is very good and the location calibration is very
strong, with an uncertainty of ~500 m for the hypocentroid at the 90%
confidence level. However the calibrated epicenter, like most other
location-based estimates that were reported, is a few km south of the
surface faulting. This could be a consequence of lateral heterogeneity
in the region. Seismograph stations to the southeast are on the coastal
plain, while the rest are in the Appalachian Mountains.

The Chardon cluster is named for the city of Chardon, Ohio. The cluster
covers the northeastern corner of Ohio along the southern bank of Lake
Erie, and includes some events in western Pennsylvania. The largest
event is a 5.3 mb event on January 31, 1986; no other event is larger
than 3.9 mb. Most events are recorded only to regional distances. All
events have depth control from near-source or local-distance readings.

The Greenback cluster is named for town of Greenback in eastern
Tennessee. The seismicity is of low magnitude and scattered over most of
eastern Tennessee, with some events over the border in western North
Carolina and northern Georgia. The largest events are a 4.3 mb event on
December 12, 2018 and a 4.1 Mw event on May 10, 2025. The seismic
network in this area expanded greatly around 2012, but earlier events in
the region are not well connected to those recorded in the past 15
years. Most events are recorded only to regional distances. All events
have depth control from near-source and local-distance readings.

The Willits cluster is named for the town of Willits in northern
California. The cluster is composed of small events, mostly recorded
only to regional distances. The largest event has magnitude 4.4 Mw on
June 22, 2024. Densification of the seismic network in this region
results in extremely good coverage since ~2012, and the events since
that time were relocated with free depth, and then held fixed as older
events were added, with depths constrained by near-source and local-
distance readings.

The Ferron cluster is named for the town of Ferron in central Utah. The
cluster is based on a pair of moderate magnitude events: 5.9 mb on
August 14, 1988 (known as the San Rafael Swell event) and 5.0 mb on
January 30, 1989 (Southern Wasatch Plateau event). The region is
moderately active seismically with small events associated with the
western margin of the Colorado Plateau. The cluster retains a large
number of those smaller events to improved statical power for the
location calibration. Regional distance observations are abundant and
the cluster carries a modest number of teleseismic observations.
Azimuthal coverage is excellent. Except for one event (the Southern
Wasatch Plateau event on January 30, 1989) whose depth is constrained by
teleseismic depth phases, all events have depth control from near-source
or local-distance readings.

This version of the Mentone cluster replaces an earlier result
(mentone5) posted in GCCEL. The Mentone cluster is named after the town
of Mentone in west Texas, U.S.A. The cluster was originally motivated by
the occurrence of a 5.0 Mw earthquake on March 26, 2020 and the sequence
includes several other events near magnitude 5: 5.3 Mw on November 16,
2022, 5.2 Mw event on November 8, 2023, 4.9 Mw on February 15, 2025 and
5.2 Mw on May 4, 2025. The region is a major producer of gas and oil and
the bulk of the seismicity is likely induced by excessive waste water
injection, as in Oklahoma and elsewhere. The earliest events in the
sequence are not included, due to lesser quality datasets. The
distribution of seismograph stations in the source region is now quite
dense; this version of the cluster was limited to events since January
2021. Azimuthal coverage and near-source data are exceptionally good for
nearly all events. The entire cluster has been relocated with free
depth.