HVO reported that an earthquake swarm in the Manu'a Islands of American Samoa continued to be felt by residents of Ta'u Island and Ofu-Olosega islands during 24-30 August. Three advanced seismometers were installed beginning the second week of August, one on Ofu Island and two on Ta'u, and by 26 August the data indicated that the source of the activity was related to Ta'u Island. Around 30 earthquakes per hour were recorded each day. At 2033 on 23 August an earthquake widely felt by residents of the Manu'a islands and Tutuila Island was characterized as producing light-to-moderate shaking (a maximum Intensity of V on the Modified Mercalli Intensity Scale); the estimated M 3-3.5 event was the largest recorded since the new instruments were installed. The largest earthquakes recorded during 25-26 August had estimated magnitudes between 3 and 4 and were felt by residents of Manu'a. An estimated M 2.8 event was strongly felt at 0932 on 27 August. Numerous events were felt during 27-28 August, but none were reported during 28-30 August. Booming noises had been reported and investigated for weeks; scientists confirmed that they were related to the earthquakes, noting that the sound waves generated by some earthquakes could produce sounds audible to humans. No signs of ground cracking, landslides, rockfalls, or other activity that could have caused the sounds were seen. The Volcano Alert Level remained at Advisory and the Aviation Color Code remained at Yellow.

Geological Summary. The 10-km-wide Ta'u Island, located at the E end of the Samoan islands, is ringed by sea cliffs. It is the emergent portion of the large Lata shield volcano. A major flank collapse event around 22 ka resulted in the steep scarps on the southern side of the island. Two smaller shields were constructed along rift zones at the NW and NE tips of the island. The NW corner of the island has a tuff-cone complex that ejected large dunite xenoliths and coral blocks. Numerous Holocene post-caldera cones occur at the summit and on the flanks.

Source: US Geological Survey Hawaiian Volcano Observatory (HVO)