Author: Eric Bergman

The Prince William Sound cluster is named for the sound on the eastern
side of the Kenai Peninsula of southern Alaska, but it extends well
north of the sound, through the Chugach Mountains and south into the
Gulf of Alaska. The epicenter of the 9.2 Mw “Good Friday” earthquake on
March 28 (March 27 local time), 1964 is on the northern shore of the
sound. The cluster includes the mainshock and most of the significant
aftershocks of the eastern part of the 1964 sequence, east of ~148°W.
The full aftershock zone extends westward to ~154°W. Until the mid-1980s
there were very few seismograph stations in the area; the nearest
station to the 1964 mainshock was station COL at a distance of more than
300 km. Since the 1990s the density of stations has increased
dramatically. Selection of earthquakes for the cluster reflected the
time-dependence in station density. For all dates, events were only
selected if they had a minimum of 30 arrival time observations in the
ISC Bulletin, but for events after 1989 a minimum magnitude of 3.5 was
also imposed to keep the number of events within a manageable limit.
Events were only retained if some form of depth constraint was
available. For events prior to late 1966 this was almost exclusively
from teleseismic depth phases. For subsequent events the preferred
source of depth constraint for most events is near-source and
local-distance readings but teleseismic depth phases are available for
many of the more recent events and they were analyzed for validation.
Approximately 1/3 of the events were observed only to regional
distances; they were retained partly to provide stronger statistical
power for the location calibration, and also because of their
significance for seismotectonic investigations. In addition to
containing more earthquakes (596) than any GCCEL cluster to date, the
Prince William Sound cluster is one of the largest in geographical
terms, approximately 180 x 280 km. Moreover it extends over the
transition from inland Alaska, through the Prince William Sound, to the
continental shelf. The use of a single crustal model for the entire
region is certainly optimistic and it is likely that epicenters near the
perimeter are subject to greater uncertainty than the formal error
estimate.

The Piritu cluster is named for a town in the state of Falcón, in
northwestern Venezuela (there are several other towns named Piritu in
other states). It contains one magnitude 6.0 Ms event, the Boca del
Tocuyo earthquake on April 30, 1989, and several events with magnitudes
5.0-5.9. Station distribution for direct calibration is good, but it was
necessary to retain many smaller events that are observed only to
near-regional distances to obtain good statistical power. All events
have depth control, mainly from near-source and local-distance readings,
but earlier, larger events are often constrained by teleseismic depth
phases.

A new calibrated cluster in the eastern part of the Republic of Tuva, in southern Siberia, has been uploaded.

A new version of the Shushtar cluster (shushtar10.28) in the central Zagros Mountains of Iran replaces the former shushtar8.211 cluster. This version contains almost twice as many events (378 vs 200), including a recent 5.4 Mw event on December 5, 2024.

The former Fin-Tiab calibrated cluster (fintiab7) in southern Iran has been replaced by a new version (fintiab11) that has many more events (505 vs. 117).

New calibrated cluster posted to GCCEL:

The Dezhgan cluster is named for the village of Dezhgan in Hormozgan
Province in southern Iran. The cluster includes seismicity at Qeshm
Island in the Strait of Hormuz. The cluster contains many earthquakes
with magnitudes in the range 5.0-5.9, and three larger events, 6.0 mb on
September 10, 2008 (at Qeshm Island) and 6.0 and 6.3 mb on July 1, 2022.
In order to obtain adequate azimuthal coverage for direct calibration it
was necessary to extend the distance limit used for estimating the
hypocentroid to 1.2°, to pick up readings from seismograph stations to
the south, across the Strait of Hormuz, in the UAE. In the distance
range 1.0-1.2° Pn and Sn arrivals are often found following close to the
arrival times of direct crustal phases Pg and Sg. This required careful
inspection of residuals to avoid biasing depth estimates. The data set
includes arrival times from temporary network stations that were
operated on Qeshm Island by Yamini Fard and colleagues from the IIEES,
as well as observations of S-P reported from strong motion stations very
near epicenters on Qeshm Island by Reza Ghods from IASBS. These data
help constrain focal depths and also helped guard against location bias
from unmodeled lateral heterogeneity, which is likely rather strong in
this area.

Latest upload for GCCEL:

The Kurikoma cluster is named for Mt. Kurikoma, a stratovolcano in
northern Honshu, Japan. Much of the seismicity in the region is thought
to be related to magmatic activity. The cluster includes 5 earthquakes
of magnitude 6.0-7.0, and most of the smaller events appear to have
occurred as aftershocks or swarms related to these larger events.
Station coverage for location calibration and depth control is
excellent. All events have depth control from near-source or
local-distance observations. All events are recorded at teleseismic
distances.

Since the last update on new clusters in August I’ve posted eight new ones, in China, Iran, Tajikistan, Russia, and Uzbekistan.

Version 12 of mloc is now being used for new calibrated clusters. Version 11 was meant to be the release of this greatly updated code but it ended up being mainly used for development and testing. The changes in v12 vastly improve memory management and execution time, making it possible to analyze much larger clusters than before. Clusters of 500 events or more are practical on a Mac Mini with M2 chip and only 8 GB of RAM. This version of mloc will be posted soon, when I’ve finished updating the User’s Manual, but if you’re anxious to get started with it, get in touch with me directly.

Since I last posted about new calibrated clusters for GCCEL in April, I have uploaded 15 new ones, bringing the total number of calibrated clusters to 359. The regions I’ve been working in include the western U.S., Japan and China. You can see them at the “MLOC data for GCCEL” page .