USGS Volcano Notice - DOI-USGS-NMI-2025-11-05T19:31:14+00:00

NMI/USGS Volcanic Activity Notice

Volcano: Ahyi Seamount (VNUM #284141)

Current Volcano Alert Level: ADVISORY
Previous Volcano Alert Level: UNASSIGNED

Current Aviation Color Code: YELLOW
Previous Aviation Color Code: UNASSIGNED

Issued: Thursday, November 6, 2025, 6:42 AM ChST
Source: U.S. Geological Survey
Notice Number: 2025/N67
Location: N 20 deg 25 min E 145 deg 1 min
Elevation: -259 ft (-79 m)
Area: Northern Mariana Islands

Volcanic Activity Summary:

Signs of unrest at Ahyi Seamount have been observed routinely in satellite data over the past few weeks. Plumes of discolored water were observed drifting up to 1 mile (1.6 km) away from the volcano in satellite images on October 20, 21, 26, 28 and November 5. Retrospective analysis of older satellite data shows weak plume activity becoming more consistent beginning September 21, following sporadic plume observations from August to early September. No significant activity has been detected from the direction of Ahyi Seamount in hydroacoustic array data at Wake Island, although submarine plumes have been observed at the volcano in the past without clear hydroacoustic signals.

The plumes of discolored water suggest ongoing submarine unrest at Ahyi Seamount. The Aviation Color Code and Volcano Alert Level are being raised to YELLOW and ADVISORY. We will continue to monitor satellite and distal hydroacoustic data for additional evidence for, or escalation of, volcanic activity.

Eruptive activity at Ahyi Seamount may present a hazard to mariners in the water above and near the volcano. The summit has shallowed over the course of previous eruptions to 164 ft (50 m) below sea level, but the lack of local real-time data means we are unable to forecast or warn of impending eruptions. We also cannot be certain if plumes of discolored water represent vigorous hydrothermal activity or active eruptions, and it is possible that a large explosion from the seamount could breach the surface due to the decreased water depth above the vent.

Recent Observations:
[Volcanic cloud height] No volcanic cloud produced
[Other volcanic cloud information] Discolored water observed above the seamount in satellite images.

Hazard Analysis:
[Other hazards] Shallow submarine eruptions may present hazards to mariners in the water above the vent.

Remarks:

Ahyi seamount is a large conical submarine volcano that rises to within 164 ft (50 m) of the sea surface about 11 miles (18 km) southeast of Farallon de Pajaros (Uracas) Island in the northern Marianas, about 370 miles (600 km) north of the island of Saipan. In the past, discolored water has been observed over the submarine volcano. In 1979, the crew of a fishing boat felt shocks over the summit area and then observed upwelling of sulfur-bearing water. On April 24-25, 2001 an explosive submarine eruption was detected seismically from a seismic station on Rangiroa Atoll, Tuamotu Archipelago. The event was well constrained (+/- 9 miles or 15 km) at a location near the southern base of Ahyi.

Contacts:

CNMI Homeland Security and Emergency Management
https://opd.gov.mp/library/agency/homeland-security-and-emergency-management.html

USGS Northern Mariana Duty Scientist (907) 786-7497
http://volcano.wr.usgs.gov/cnmistatus.php

Satellite information, Washington VAAC
http://www.ssd.noaa.gov/VAAC/washington.html

Next Notice:

A new VAN will be issued if conditions change significantly or alert levels are modified.

(1) VOLCANO OBSERVATORY NOTICE FOR AVIATION (VONA)
(2) Issued:	(20251105/2042Z)
(3) Volcano:	Ahyi Seamount (VNUM #284141)
(4) Current Color Code:	YELLOW
(5) Previous Color Code:	UNASSIGNED
(6) Source:	U.S. Geological Survey
(7) Notice Number:	2025/N67
(8) Volcano Location:	N 20 deg 25 min E 145 deg 1 min
(9) Area:	Northern Mariana Islands
(10) Summit Elevation:	-259 ft (-79 m)
(11) Volcanic Activity Summary:	Signs of unrest at Ahyi Seamount have been observed routinely in satellite data over the past few weeks. Plumes of discolored water were observed drifting up to 1 mile (1.6 km) away from the volcano in satellite images on October 20, 21, 26, 28 and November 5. Retrospective analysis of older satellite data shows weak plume activity becoming more consistent beginning September 21, following sporadic plume observations from August to early September. No significant activity has been detected from the direction of Ahyi Seamount in hydroacoustic array data at Wake Island, although submarine plumes have been observed at the volcano in the past without clear hydroacoustic signals. The plumes of discolored water suggest ongoing submarine unrest at Ahyi Seamount. The Aviation Color Code and Volcano Alert Level are being raised to YELLOW and ADVISORY. We will continue to monitor satellite and distal hydroacoustic data for additional evidence for, or escalation of, volcanic activity. Eruptive activity at Ahyi Seamount may present a hazard to mariners in the water above and near the volcano. The summit has shallowed over the course of previous eruptions to 164 ft (50 m) below sea level, but the lack of local real-time data means we are unable to forecast or warn of impending eruptions. We also cannot be certain if plumes of discolored water represent vigorous hydrothermal activity or active eruptions, and it is possible that a large explosion from the seamount could breach the surface due to the decreased water depth above the vent.
(12) Volcanic cloud height:	No volcanic cloud produced
(13) Other volcanic cloud information:	Discolored water observed above the seamount in satellite images.
(14) Remarks:	Ahyi seamount is a large conical submarine volcano that rises to within 164 ft (50 m) of the sea surface about 11 miles (18 km) southeast of Farallon de Pajaros (Uracas) Island in the northern Marianas, about 370 miles (600 km) north of the island of Saipan. In the past, discolored water has been observed over the submarine volcano. In 1979, the crew of a fishing boat felt shocks over the summit area and then observed upwelling of sulfur-bearing water. On April 24-25, 2001 an explosive submarine eruption was detected seismically from a seismic station on Rangiroa Atoll, Tuamotu Archipelago. The event was well constrained (+/- 9 miles or 15 km) at a location near the southern base of Ahyi.
(15) Contacts:	CNMI Homeland Security and Emergency Management https://opd.gov.mp/library/agency/homeland-security-and-emergency-management.html USGS Northern Mariana Duty Scientist (907) 786-7497 http://volcano.wr.usgs.gov/cnmistatus.php Satellite information, Washington VAAC http://www.ssd.noaa.gov/VAAC/washington.html
(16) Next Notice:	A new VAN will be issued if conditions change significantly or alert levels are modified.