USGS Volcano Notice - DOI-USGS-HVO-2022-08-18T16:25:23-07:00

Summary

The earthquake swarm in the Manuʻa islands of American Samoa continues without significant changes. Approximately 20 earthquakes per hour are being recorded by the two microseismometers installed in the Manuʻa islands; most events are too small to be felt. Estimated magnitudes of the largest earthquakes are between magnitude 2 and 3. Seismic data indicate that earthquakes are related to magma moving beneath the Manuʻa Islands, likely closer to Taʻū island than Ofu-Olosega. Satellite data from the past 24 hours showed fairly clear views and no signs of volcanic activity. Additional microseismometers are being installed today, and a permanent seismic monitoring station is being delivered this week.

Observations and Reports 

• Residents of the Manuʻa group of islands in American Samoa continue to feel earthquakes. The first felt earthquake report was on July 26th.    

• Reports from both Taʻū and Ofu-Olosega islands suggest that the earthquakes vary in intensity, but are generally short, sharp jolts.

Upcoming Change in Communication

Volcanoes in the Manuʻa islands have not previously been monitored and were classified as unassigned in the US Geological Survey’s Volcano Alert System. https://www.usgs.gov/programs/VHP/volcanic-alert-levels-characterize-conditions-us-volcanoes

Current seismic monitoring shows that the level of earthquake activity is above NORMAL (Green) and that the appropriate level should be ADVISORY (Yellow). On Friday, August 19, the USGS will change Taʻū and Ofu-Olosega volcanoes to ADVISORY (Yellow) status if conditions remain the same. This change in Alert Level will be communicated in a Volcano Activity Notice (VAN) that acknowledges the new level of activity. Beginning Saturday, August 20, Daily Updates will be issued for Taʻū and Ofu-Olosega instead of Information Statements. To subscribe to receive VANs and Updates, see STAY INFORMED at:  https://www.usgs.gov/volcanoes/ta-u-island 

Analysis 

Not all earthquake swarms on volcanoes result in eruptions. Current low-level earthquake activity may continue and vary in intensity for days to months or longer without an eruption. It is also possible that the swarm is an early precursor to an eventual eruption. At this time, we cannot determine which of these possibilities is more likely. A large explosive eruption similar to the Tonga eruption earlier this year is extremely unlikely.  

Response

Experts at the Pago Pago National Weather Service Office (NWS), U.S. Geological Survey (USGS) Volcano Hazards Program, NOAA Pacific Tsunami Warning Center, NOAA-IOC International Tsunami Information Center, and USGS National Earthquake Information Center are working together with the local authorities in American Samoa to understand the source of these earthquakes. These agencies are working together to install monitoring stations so earthquake locations and magnitudes can be more precisely determined. 

USGS Hawaiian Volcano Observatory (HVO) staff have been in Pago Pago since August 11 and are consulting with local authorities. Additional HVO personnel and permanent seismometers for earthquake detection are expected to arrive in American Samoa later this week and will be installed as soon as installation sites are identified. 

Monitoring

American Samoa’s volcanoes are monitored for earthquakes by a microseismometer in Fitiʻuta village on Taʻū island, a microseismometer in Olosega village on Ofu-Olosega island, and a more distant global seismic station in Apia, Samoa. Due to limited earthquake monitoring equipment, the exact location, depth, and magnitude depth of these earthquakes are currently unknown.  

Remote sensing satellite data is being used to monitor American Samoa for events potentially related to volcanic activity, such as thermal anomalies, volcanic plumes, or volcanic gases. This monitoring is being done by USGS Alaskan Volcano Observatory scientists and the Wellington Volcano Ash Advisory Center (VAAC).  

Residents can assist these monitoring efforts by noting and reporting accurate times that they feel earthquake shaking, hear unusual noises, or see changes that might be volcanically related to either the Pago Pago National Weather Service Office (https://www.weather.gov/ppg/wsopagooffice) or the American Samoa EOC in Pago Pago (684-699-3800).   

Hazards

Currently, the primary hazard of concern is earthquake shaking. For information on how to prepare for an earthquake, see: https://www.usgs.gov/faqs/what-can-i-do-be-prepared-earthquake. 

If you feel shaking and are not near the coast, immediately drop, cover, and hold on until the shaking stops. 

If you are at the coast, heed the natural tsunami warning signs. If you feel a strong or long-duration earthquake, see a sudden rise or fall of the ocean, hear a loud roar from the ocean, or see a large aerial plume from an eruption, a tsunami may follow, and you should immediately move to higher ground.  

It is unclear if this unrest will escalate to a volcanic eruption. An eruption could pose significant hazards to residents of American Samoa; these hazards include volcanic gases, low-level localized explosions of lava, lava flows, earthquake shaking, and tsunami. Information about these hazards, which are similar to those in Hawaii, can be found at this HVO website: https://www.usgs.gov/observatories/hvo/hazards.