The swarm of earthquakes that began on August 24, 2022, beneath Trident stratovolcano, Alaska, U.S. continues. The last eruption of this volcano took place in 1974 (VEI 3).
In addition, episodes of weak seismic tremor and low frequency earthquakes have been detected since August 28.
“Together, these observations mean that Trident is exhibiting signs of elevated unrest above known background level. Therefore, we are raising the Aviation Color Code to YELLOW and the Volcano Alert level to ADVISORY,” the Alaska Volcano Observatory (AVO) reports.1
During the current swarm, earthquake depths were initially mostly deep at around 25 km (16 miles) below sea level but became progressively shallower to around 5 km (3 miles) by August 28.
Since then, earthquakes have mostly occurred 3 to 6 km (about 2 to 4 miles) below sea level, although some deeper events have been detected.
Earthquake magnitudes have ranged from 0.7 to 1.9. At the peak of the swarm, dozens of earthquakes occurred daily beneath the volcano, but earthquake rates have since decreased to just a few per day. No other signs of unrest have been detected in monitoring data.
The increase in seismic activity is likely caused by movement of magma or magmatic fluids, AVO said.
Increases in seismic activity have been detected previously at Trident and other similar volcanoes, with no subsequent eruptions.