The Thera cluster is named for the volcanic island Thera (also known as
Santorini), the southernmost member of the Cyclades island group in the
Aegean Sea. The cluster is dominated by an earthquake sequence that
began in early February, 2025, 20-40 km northeast of Thera. The sequence
has swarmlike characteristics but it is, as yet, unclear if the activity
is volcanic or tectonic in nature. The cluster includes the 7.7 Mw
tsunamigenic earthquake of July 9, 1956 and 3 of its aftershocks. The
mainshock is located ~10 km southeast of the southern end of the ongoing
2025 swarm, midway between the southern end of the Santorini-Amorgos
Fault and the Anafi-Astypalaea Fault. Some of the earlier members of the
cluster are grouped near an apparent submarine volcanic feature midway
between the Thera island and the 2025 sequence. The 2025 activity
includes several events with magnitude 5+, up to M5.3. Most events in
the cluster are observed to teleseismic distances, but some smaller
events prior to the 2025 sequence are retained because they provide
important azimuthal control for the location calibration. All events
except the 1956 events and a single small event in 1968, which are held
at 10 km focal depth, have depth control from near-source or
local-distance readings.
