An intense episode of apparent submarine volcanism was recorded 21-22 September by an 11-channel hydrophone array on Wake Island, an ocean bottom seismometer off Japan's Boso Peninsula, and the Polynesian Seismic Network's Pomariorio station, on Rangiroa atoll. Strong activity began at about 1100 and peaked between 1230 and 1520, with several hundred distinct events detected before the episode ended abruptly at about 0755 the next morning. During the activity, continuous noise levels in the SOFAR channel remained at least 20 dB above normal ambient values, at frequencies of 5-30 Hz. T-Phase arrivals at the three sites were used to determine a rough location for the activity at 21.9°N, 145.9°E, with a typical potential error of about ± 100 km [but see 14:12]. However, the calculated position is displaced >200 km E of the northern Marianas volcanic arc, falling on the W edge of the trench.

Activity was last reported from the area on 2 September 1985, when a 3-km zone of discolored water near the 1969 eruption site... was observed from an aircraft. Between 2 August and 5 September 1985, the Polynesian Seismic Network's Rangiroa station recorded 109 T-Phase events, with characteristics typical of submarine eruptions, originating from the NW Pacific. However, no other seismic stations were known to have recorded the activity and a precise location was impossible.

Reference. McCreery, C., Oliveria, F., and Walker, D., 1989, Submarine volcano: EOS, v. 70, p. 1466.

[Report reviewed and attributed to Ahyi in 2023.]

Geological Summary. Ahyi seamount is a large conical submarine volcano that rises to within 75 m of the ocean surface ~18 km SE of the island of Farallon de Pajaros in the northern Marianas. Water discoloration has been observed there, and in 1979 the crew of a fishing boat felt shocks over the summit area, followed by upwelling of sulfur-bearing water. On 24-25 April 2001 an explosive eruption was detected seismically by a station on Rangiroa Atoll, Tuamotu Archipelago. The event was well constrained (+/- 15 km) at a location near the southern base of Ahyi. An eruption in April-May 2014 was detected by NOAA divers, hydroacoustic sensors, and seismic stations.

Information Contacts: D. Walker, Univ of Hawaii; J. Talandier, LDG, Tahiti; Y. Sawada, JMA.