The Mandalay cluster is named for the city of Mandalay in central
Myanmar. The cluster was motivated by the destructive Mw 7.7 earthquake
on March 28, 2025, but it had to be handled in a fashion different from
most calibrated clusters, due to the scarcity of seismograph stations in
the region. By considering earthquakes in a much larger area than usual
(~700 km in latitude, ~400 km in longitude), it was possible to obtain
enough arrival time data at near-source and local distances to carry out
a direct calibration. A larger area than usual would have been required
in any case if the cluster was intended to represent the aftershock
sequence of the March 28, 2025 earthquake, as the event is estimated to
have ruptured a much longer, as much as 500 km, section of the Sagaing
Fault than expected for an earthquake of this magnitude. The cluster
includes a previous large event (7.5 Ms on September 12, 1946) on the
Sagaing Fault, to the north of the latest sequence, and several events
with magnitude >6 in the years between the 1946 and 2025 events. Because
of the great areal extent of the cluster and unmodeled variations in
crustal structure that perturb the travel times of local phases, the
location calibration is less precise than usual. This is reflected in
larger confidence ellipses than typical. Depth control for this cluster
is weaker than usual as well, since the depths of many events could only
be constrained by small numbers of reported teleseismic depth phases.
Eleven events, mostly prior to 1964 or aftershocks of the March 28
event, were set at a default value of 20 km, based on the mean of focal
depths for events with depth constraint. Because of the importance of
the seismicity in this region for earthquake hazard and seismotectonic
applications and the poor prospects for being able to ever achieve
higher accuracy in location calibration here it was decided to make
these results available in GCCEL. They should be utilized with due
appreciation for the larger uncertainties, as compared to most
calibrated earthquake clusters in GCCEL.
