Category: Uncategorized

The Nias cluster is named for island of Nias off the southwest coast of
North Sumatra, Indonesia. The selection of events went to ~150 km depth.
The cluster includes the 8.6 MW earthquake on March 28, 2005. It also
includes two events with magnitude >7, a 7.1 MS event on February 4,
1971 and a 7.8 MW event on April 6, 2010, and 20 earthquakes with
magnitudes between 6.0 and 6.9. Arrival time data from station SNSI on
Simeulue Island, GSI on Nias Island and PBSI on Tanahbala Island provide
unusually good azimuthal coverage for a subduction zone cluster. All
events have depth control from near-source and local-distance readings
and many events also have depth control from teleseismic depth phases.

The Umluj cluster is named for the city of Umluj on the Red Sea coast of
Saudi Arabia. The cluster is dominated by small events on shore but
includes a half dozen offshore events in the Red Sea. Most events are
small and observed only to local or near-regional distances but they are
retained in order to provide statistical power for calibrated location
of five events that were recorded teleseismically. The largest
earthquake is a 5.1 Mw event on May 19, 2009. All events have depth
control from near-source and local-distance readings.

The Oriente cluster is named for the Oriente Fault along the southern
coast of Cuba. It is larger in area than usual for GCCEL (~500 km
across) in order to include enough station coverage for stable location
calibration. The cluster includes the 7.7 Mw earthquake on January 28,
2020 and three M6 events: on May 25, 1992 (6.8 Mw), February 4, 2007
(6.2 Mw) and November 10, 2024 (6.8 Mw). In order to stabilize the
relocation many small events were retained. About 1/3 of the earthquakes
were recorded to teleseismic distances. All events have depth constraint
from near-source and local distance readings, as well as teleseismic
depth phases for some of the larger events.

The Sumbing cluster is named for Mount Sumbing, a strato-volcano in
central Java, Indonesia. The cluster includes shallow earthquakes on
land and also earthquakes in the subduction zone, up to ~100 km offshore
and to a depth of ~160 km. It includes three M6 earthquakes: on June 9,
1992 (6.1 Mw), May 26, 2006 (6.3 Mw) and January 25, 2014 (6.3 Mw). Most
events are recorded to teleseismic distances but a few smaller,
locally-recorded earthquakes are retained for improved statistics for
direct calibration. All events have depth constraint from near-source
and local distance readings, as well as teleseismic depth phases in many
instances. The distribution of stations for the events on land is very
good and the location calibration is robust, but like most subduction
zone clusters, offshore events tend to be less stable.

The Paria cluster is named for the Gulf of Paria between Venezuela and
Trinidad and Tobago. There are many relatively deep events immediately
to the north, associated with the Lesser Antilles subduction zone, but
when they are included in the cluster the lateral heterogeneity in
velocity structure causes location bias. The cluster is comprised of
shallow events (less than 35 km). There are two M6 events in the
cluster, on April 11, 1983 (6.1 Mw) and on June 11, 1986 (6.3 mw). The
distribution of seismic stations for direct calibration is quite good.
All events have depth control from near-source and local distance
stations and, in many cases, good teleseismic depth phase datasets. Most
events are recorded to teleseismic distances; the pattern of teleseismic
P residuals features a remarkable discontinuity near 65°.

The Cumana cluster is named for the city of Cumana, capital of the
Venezuelen state of Sucre. This cluster, along with the Valencia cluster
replaces the Caracas cluster in GCCEL. The cluster includes a 6.1 MS
earthquake on June 12, 1974 and a 6.9 Mw earthquake on July 9, 1997.
With most events distributed along the coast and few seismograph
stations to the north, the azimuthal coverage for location calibration
is a bit weak, but the relocations were stable. All events have depth
control from near-source or local distance readings, or teleseismic
depth phases.

The Valencia cluster is named for the city of Valencia, capital of
Carabobo State in Venezuela. A cluster named Valencia in the same region
was published earlier in GCCEL but this version is a fresh start, based
on the large, damaging earthquake(s) of June 24, 2026. There has been
controversy as to whether it should be treated as two separate events or
one very complex event. It is treated as two events here. The initial
event of magnitude 7.2 Mw was followed by a second large pulse of energy
(7.5 Mw) after about 30 seconds. Because of interference from the coda
of the initial event, assembling a consistent arrival time dataset for
the second event was challenging. The arrival times reported by the ISC
were supplemented by a carefully curated set of arrival times by NEIC
analysts that helped resolve the second mainshock hypocenter, which
locates ~150 km east of the first one, just offshore of Caracas. The
cluster also includes a 6.6 MS earthquake on July 30, 1967 and a 6.4 Mw
earthquake on September 12, 2009. The June 24, 2026 earthquakes produced
surprisingly few aftershocks. 17 aftershocks over the following ~month
are included in the cluster but most of them do not have depth control
and are held at a default depth of 12 km. All other events in the
cluster have depth control from near-source and local-distance readings
and teleseismic depth phases in the case of the 1967 earthquake. The
distribution of local seismograph stations is quite good and the cluster
has a robust location calibration.

The Ambon cluster is named for the island of Ambon in the Maluku Islands
of Indonesia. The cluster consists of earthquakes with depths to a
little over 50 km. It contains three M6 earthquakes, a 6.0 Mw event on
December 30, 1996, a 6.5 event on March 19, 2001, and a 6.5 Mw event
(the so-called Ambon earthquake) on September 25, 2019. All earthquakes
are recorded to teleseismic distances. All events have depth control from
near-source and local-distance arrivals or teleseismic depth phases. The
number of local distance seismograph stations is small but they are well
distributed and provide a robust location calibration.

The Kochi cluster is named for the city of Kochi on the island of
Shikoku, Japan. The cluster includes most of Shikoku. It includes
earthquakes to a depth of ~50 km. The only M6 event is a 6.4 mb
earthquake on March 24, 2001. About half the events are recorded only to
near-regional distances (~5.0°) but they were retained because the
relocation suffered from instability without them. All events have depth
control from near-source and local distance readings, and many have
consistent estimates of focal depth from teleseismic depth phases. The
local seismic network is very dense and direct calibration is very
robust.

The Caucasia cluster is named for the town of Caucasia in northern
Colombia. It covers a relatively large area, which was necessary to
obtain adequate station coverage. Many of the events are subcrustal in
depth, as deep as 190 km. There are especially large earthquakes
(maximum magnitude is 5.7) but all events are observed at teleseismic
distances. All events have depth control, from teleseismic depth phases
in many cases, but also from near-source and local-distance
observations. The location calibration is robust overall but some of the
deeper events are susceptible to instability.