M5.5 Earthquake near Adak, Alaska — 21 September 2025
A moderate earthquake of magnitude 5.5 was recorded near Adak, Alaska on , with an epicentre 118 km west-southwest of Adak, at a focal depth of 35.0 km.
Time (UTC): 2025-09-21 09:30:30
Time (your timezone): —
Magnitude: 5.5 (moderate)
Focal depth: 35.0 km (shallow)
Epicentre: 51.3590, -178.1340
Location: 118 km WSW of Adak, Alaska
USGS link to NOAA tsunami information: Available
Review status: Reviewed
Source: USGS event page
How strong was this earthquake?
Moderate earthquakes can damage poorly built or unreinforced structures near the epicentre, while well-engineered buildings typically escape serious harm. Shaking is often strong enough to wake sleeping residents over a wide area.
A magnitude 5.5 earthquake radiates roughly 2,682 tons of TNT equivalent in seismic energy.
The USGS assigns this earthquake a significance score of 465, which is below the USGS threshold of more than 600 for its significant-earthquake list. The ranking combines magnitude with available Did You Feel It responses and PAGER alert level; it is not a direct measurement of damage.
Where was the epicentre?
The epicentre was located at 51.3590, -178.1340 — 118 km west-southwest of Adak. The closest other recorded event was a magnitude 2.8 earthquake 13 km north of here on 28 July 2026.
Why the 35 km depth matters
At 35 km this was a shallow earthquake. Shallow events release their energy close to the surface, so for a given magnitude they produce noticeably stronger shaking — and more damage — than deeper ones. Roughly three quarters of all earthquakes worldwide occur at shallow depths.
This catalogue does not ingest eyewitness responses, so it cannot confirm where this earthquake was felt. Based on magnitude alone, an event this size can be felt widely and may cause minor local damage. Its shallow focus makes stronger surface shaking more likely than for a deeper event of the same magnitude.
What does the tsunami information flag mean?
The USGS event record links to NOAA tsunami information because this was a large event in an oceanic region. That link flag does not mean a tsunami occurred or that a warning is active. Check tsunami.gov and instructions from your local authority for the current status of your coastline.
How earthquakes generate tsunamis, and what the alert levels mean →
Aftershocks and local seismic history
3 further earthquakes have been recorded within about 150 km of the epicentre in the seven days after this event, the largest of them magnitude 0.6.
88 earthquakes of magnitude 4 or above have been logged within roughly 200 km of this epicentre in our catalogue. The strongest was a magnitude 6.2 event near Adak on 21 March 2025. This M5.5 event released about 11 times less energy than that event.
How long aftershock sequences last, and how large they get →
Where this event sits in the record
This earthquake is part of the September 2025 worldwide record. Browse the full monthly archive to compare periods.
Nearest recorded earthquakes to this epicentre
- M2.8 earthquake near Adak, Alaska — (13 km N)
- M2.6 earthquake near Adak, Alaska — (14 km N)
- M2.4 earthquake near Adak, Alaska — (19 km N)
- M2.1 earthquake near Adak, Alaska — (26 km SE)
- M1.9 earthquake near Adak, Alaska — (27 km NW)
- M2.6 earthquake near Adak, Alaska — (33 km SSE)
- M1.9 earthquake near Adak, Alaska — (33 km E)
- M1.9 earthquake near Adak, Alaska — (53 km ESE)
- M1.2 earthquake near Adak, Alaska — (55 km NW)
- M0.7 earthquake near Adak, Alaska — (56 km N)
Other magnitude 5 earthquakes worldwide
- M5.6 earthquake near Port McNeill, Canada —
- M5.5 earthquake near Mutsu, Japan —
- M5.6 earthquake near San Fernando, Peru —
- M5.4 earthquake near Abepura, Indonesia —
- M5.3 earthquake near Kokopo, Papua New Guinea —
- M5.6 earthquake near Pacific-Antarctic Ridge —
- M5.4 earthquake near Rapu-Rapu, Philippines —
- M5.4 earthquake near Tsunagi, Japan —
Questions about this earthquake
How strong is a magnitude 5.5 earthquake?
Moderate earthquakes can damage poorly built or unreinforced structures near the epicentre, while well-engineered buildings typically escape serious harm. Shaking is often strong enough to wake sleeping residents over a wide area. A magnitude 5.5 earthquake radiates roughly 2,682 tons of TNT equivalent in seismic energy.
How deep was the earthquake near Adak?
At 35 km this was a shallow earthquake. Shallow events release their energy close to the surface, so for a given magnitude they produce noticeably stronger shaking — and more damage — than deeper ones. Roughly three quarters of all earthquakes worldwide occur at shallow depths.
Was the M5.5 earthquake near Adak felt?
This catalogue does not ingest eyewitness responses, so it cannot confirm where this earthquake was felt. Based on magnitude alone, an event this size can be felt widely and may cause minor local damage. Its shallow focus makes stronger surface shaking more likely than for a deeper event of the same magnitude.
What does the USGS tsunami field mean for this event?
The USGS event record links to NOAA tsunami information because this was a large event in an oceanic region. That link flag does not mean a tsunami occurred or that a warning is active. Check tsunami.gov and instructions from your local authority for the current status of your coastline.
Have there been aftershocks?
3 further earthquakes have been recorded within about 150 km of the epicentre in the seven days after this event, the largest of them magnitude 0.6.
How often do earthquakes happen near Adak?
88 earthquakes of magnitude 4 or above have been logged within roughly 200 km of this epicentre in our catalogue. The strongest was a magnitude 6.2 event near Adak on 21 March 2025. This M5.5 event released about 11 times less energy than that event.