Category: Uncategorized

The Salina cluster is named for the town of Salina Cruz on the coast of
Oaxaca, Mexico. About half the cluster is in the neighboring state of
Chiapas or offshore of it, including the 8.2 Mw Tehuantepec earthquake
of September 8, 2017. The cluster also includes three M6 events, on July
6, 2007 (6.0 Mw), December 17, 2015 (6.6 Mw) and September 23, 2017 (6.0
Mw) and a large number of earthquakes in the magnitude 5 range. Because
of the geometry of the coastline and location of the subduction zone,
most of the epicenters on land are subcrustal. Depths range from
approximately 10 to 160 km. All events have depth control from
near-source and local-distance readings, and most also have depth
constraint from teleseismic depth phases that is in close agreement. The
distribution of seismograph stations in the area is such that a
relatively large area needs to be included in order to ensure adequate
azimuthal coverage for location calibration.

The Sadyr cluster is named for the town of Sadyr in eastern Kazakhstan,
close to the border with China. Some earthquakes, including the largest
one (6.4 mb), on December 21, 1958, are in China. Many small events that
are only recorded to near-regional distances are included to improve the
statistics for location calibration. The station coverage at local
distances is quite good since ~2003. All events except the large 1958
event (fixed to a depth of 15 km) have depth control, from a combination
of near-source and local-distance readings and teleseismic depth phases.
A few events locate to very shallow depths, usually based on readings at
least 50 km away.

The Imphal cluster is named for the city of Imphal in the state of Manipur,
bordering Myanmar in eastern India. The cluster contains four events
with magnitude 6: July 1, 1957 (6.6 Ms), May 6, 1984 (6.0 Mw), December
30, 1984 (6.0 Mw) and January 3, 2016 (6.7 Mw). Most events are between
25 and 75 km deep. Depth constraint for many events comes from
teleseismic depth phases, often in combination with near-source and
local-distance readings. The distribution of seismograph stations for
location calibration is not dense but it provides good azimuthal
coverage.

The Charyn cluster is named for the Charyn Canyon National Park in the
Almaty region of eastern Kazakhstan. The cluster includes a set of
events close to the canyon, but also includes some earthquakes (which
are generally larger) to the east, along the Kazakhstan-China border.
The cluster includes two magnitude 6+ events, 6.0 Mw on December 1, 2003
and 6.2 Mw on January 28, 2013. Some events are small and recorded only
to near-regional distances, but they were retained to improve the
location calibration. Earthquakes prior to about 2003 are poorly
connected to the more recent events that have good local coverage, and
therefore could not be included in the cluster. All events have depth
control from near-source and local-distance observations and some have
teleseismic depth phases that are reasonable agreement. This cluster
exhibits greater than usual variability in travel time residuals, which
especially complicates depth estimation. This is likely a consequence of
especially severe crustal structure variations.

The Larino cluster is named for town of Larino in the the province of
Campobasso, Italy. The largest earthquake in the cluster is a 5.7 Mw
event on November 1, 2002; there are several other events with magnitude
5.0 or greater. All events are observed to at 8.0° epicentral distance
and most are observed teleseismically. The station distribution is quite
dense since 2002 and the location calibration is very robust. All events
have depth control from near-source and local-distance readings, and a
few have teleseismic depth phases that are consistent with those depths.

The Caleta cluster is named after the town of Caleta de Campos on the
coast of Michoacan, Mexico. It is unusally rich in large events,
including the 8.0 Mw earthquake on September 19, 1985. In addition there
are two other events with magnitude greater than 7.0 and six with
magnitude between 6.0 and 6.9. All the larger earthquakes are associated
with the subduction of the northernmost portion of the Cocos plate
(perhaps involving the Rivera plate as well) beneath the North American
plate. The cluster includes events down to ~100 km in the subduction
zone and a patch of generally very shallow earthquakes further inland
near the town of Los Reyes, an area that is considered somewhat
susceptible to volcanic activity. Many of these events want to locate at
zero depth, although the topography is 1.2-1.5 km in this area. All
events in the cluster have depth control, either from near-source and
local distance readings, or from teleseismic depth phases (sometimes
both). The location calibration is very robust, due to the dense local
network installed in the last several decades. Many smaller events that
were recorded only to near regional distances (minimum 3.0° epicentral
distance) were retained to improve the statistics of the relocation.

The Limnos cluster is named for the island of Limnos in the Thracian Sea
(northern Aegean). Most of the seismicity is associated with the North
Aegean Trough, the site of the two largest events, on August 6, 1983
(6.6 Mw) and May 24, 2014 (6.9 Mw). The distribution of seismic stations
on the nearby islands is quite good and the location calibration is very
robust. All events have depth control from near-source and
local-distance readings and some have constraint from teleseismic depth
phases that is in good agreement.

The Bandung cluster is named for the city of Bandung on the island of
Java, Indonesia. Earthquakes prior to about 2010 could not be included
in the relocation because of a lack of local network observations. Some
constraint on location and depth for these earlier events could be
obtained from regional and teleseismic observations but not enough for
stable calibrated relocation. The cluster consists mainly of shallow
earthquakes (to ~20 km) on the island, but the cluster includes about a
dozen events offshore to the south, which are generally associated with
the subduction zone at depths of 30-50 km. Most off-shore events proved
to be unstable in the relocation analysis. Location calibration is
completely controlled by the on-shore events, many of which are small
and recorded only locally. The largest on-shore event, on November 21,
2022, has magnitude 5.6 mb. All events have depth control from
near-source and local distance arrivals, and some also have consistent
depth estimates from teleseismic depth phases.

The Salton Trough cluster now contains many more events and covers a wider area.

The Salton cluster is named for the Salton Trough in southeastern
California, a transitional region between sea-floor spreading in the
Gulf of California and the San Andreas transform fault system on land.
The largest event is 5.5 Mw for the July 7, 2010 earthquake. Only events
recorded to at least 10° epicentral distance were retained. Station
coverage for direct calibration is superb since about 2009 but events
prior to about 2007 could not be kept in the cluster because of poor
connectivity with the modern dense network in the area. Because of the
dense network coverage. Free-depth relocation was used to determine the
focal depth for most events (which were held fixed in the final runs),
but a few focal depths were adjusted manually to better fit the
near-source arrival time data. The cluster covers both sides of the
transform fault zone, as well as events in the fault zone itself, and
crustal heterogeneity is very evident in the arrival time dataset.
Because of the immense number of close-in data (~6000 readings for the
hypocentroid) the calibrated epicenters have quite small confidence
ellipses but it is likely that the true uncertainties are greater due to
the strong crustal heterogeneity. All events have depth control from
near-source and local-distance readings, and a few have depth estimates
from teleseismic depth phases which are in good agreement.

The El Mayor cluster now contains many more events and covers a wider area.

The elmayor cluster is named for a mountain named Sierra El Mayor, 60 km
south of Mexicali, Mexico on the Mexico-California border. The cluster
crosses the U.S.-Mexico border, about 30 km into California. The cluster
includes the 7.2 Mw El Mayor – Cucapah earthquake on April 4, 2010, but
no other events are larger than 5.9. All events are observed to at least
10° epicentral distance. The region is heavily instrumented and location
calibration is robust enough for depth control to be done with a free
depth relocation for most events. A few events were unstable in a free
depth relocation and are set manually for the final relocation.