Summary

• Residents of the Manuʻa group of islands in American Samoa continue to feel earthquakes. Reports from National Park of American Samoa staff and Taʻū residents suggest that the activity began on July 26th. Since August 10th, earthquakes have also been reported by residents of Ofu and Olosega islands.
• Reports suggest that the earthquakes vary in intensity, but are generally short, sharp jolts. The earthquakes are more likely to be felt by people indoors at rest and along the coast, where buildings sit on sediment that amplifies shaking. These factors are probably responsible for the variability in reporting.
• Based on the reports, these earthquakes are probably related to either Taʻū or Vailuluʻu volcanoes.
• Scientists are investigating earthquakes and unconfirmed reports of other activity. Several residents of Taʻū reported loud booming noises on Wednesday night, August 10; no other noises have been reported since then.
• Scientists plan to install additional instruments to monitor earthquakes and other activity in the coming week.

Analysis

The earthquake activity reported to date suggests a local volcanic source.

Due to limited earthquake monitoring equipment, the exact location of these earthquakes is currently unknown.

Not all earthquake swarms result in eruptions. Current low-level earthquake activity may continue and vary in intensity for days to months 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.

Volcanoes in American Samoa are similar to those in Hawaii. If activity escalates to an eruption, it will most likely include slow-moving lava flows or low-level explosions of lava that are localized to a small area. An eruption like Hunga Tonga–Hunga Ha'apai in Tonga earlier this year is extremely unlikely as it is a different type of volcano. Volcanoes in Tonga erupt much more explosively than ones in American Samoa and Hawaii.  

Response

Experts at the Pago Pago National Weather Service Office, 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 American Samoa EOC to understand the source of these earthquakes better.   The Samoa Meteorological Service is also reporting increased seismicity south or east of Tutuila Island.

Dr. Natalia Deligne of the US Geological Survey Hawaiian Volcano Observatory (HVO) arrived on August 11 in Pago Pago and is consulting with local authorities on the situation. Additional HVO personnel and earthquake detection instruments are expected to arrive in American Samoa next week.   Dr. Charles McCreery, Director of the Pacific Tsunami Warning Center, will arrive next Monday to advise tsunami concerns.

Currently, American Samoa’s volcanoes are monitored remotely by satellites and a distant seismic (earthquake detection) station in Apia, Samoa. These instruments might detect significant explosive activity in American Samoa, but the lack of ground-based monitoring stations at the volcanoes does not allow for advanced warning of new activity. With the existing real-time earthquake-monitoring network in American Samoa, the earthquakes’ locations and magnitudes cannot be precisely determined. HVO scientists plan to install additional earthquake monitoring instruments in the coming weeks.

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

HVO will issue another information statement on Monday, August 15. Additional messages may be issued as needed.

Background

Taʻū is a shield volcano with rift zones to the northeast and northwest; the last eruption of Taʻū occurred in 1866 as a submarine cone that formed between Taʻū and Ofu-Olosega islands.

Vailuluʻu is a submarine seamount whose summit is about 1970 feet (600 m) below sea level.  The last eruption of Vailuluʻu was in 2003, during which a cone formed within the summit caldera.

Hazards

It is unclear if this unrest will escalate to a volcanic eruption. Volcanic hazards associated with eruptions in American Samoa could include volcanic gases, low-level explosions of lava localized to a small area, 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.

A submarine volcanic eruption or landslide can generate a tsunami. The Pacific Tsunami Warning Center will issue a warning if they detect earthquake activity that is likely to cause a tsunami. However, volcanic eruptions do not usually generate large enough earthquakes to warrant a tsunami warning. If there is a tsunami from a nearby volcanic eruption, residents of the Manuʻa islands and elsewhere in American Samoa are more likely to experience natural warning signs before receiving an official tsunami warning.

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.

Here is information on what you can do to protect yourself and your family if you see a tsunami or receive a warning: https://www.weather.gov/safety/tsunami-during.

Additional information will be provided as it becomes available.

More information

• Subscribe to USGS Volcano Notification Service: https://volcanoes.usgs.gov/vns2/
• National Park of American Samoa: https://www.nps.gov/npsa/index.htm
• Protecting yourself and your family from earthquakes: https://www.usgs.gov/node/277816 
• Smithsonian Institution Global Volcanism Program webpage on Taʻū Island: https://volcano.si.edu/volcano.cfm?vn=244001
• Smithsonian Institution Global Volcanism Program webpage on Vailuluʻu: https://volcano.si.edu/volcano.cfm?vn=244000

Tsunami information

• Subscribe to U.S. Tsunami Warning Center: https://www.tsunami.gov/?page=productRetrieval
• National Weather Service Pago Pago Office: https://www.weather.gov/ppg/wsopagooffice 
• Pacific Tsunami Warning Center: https://tsunami.gov/
• International Tsunami Information Center and American Samoa Tsunami Awareness Information: http://www.tsunamiwave.org
• Protecting yourself and your family from tsunami: https://www.weather.gov/safety/tsunami

How you can help scientists

Contact askHVO@usgsg.gov if you have information or observations to share, such as times and types of earthquake shaking, steaming ground, discolored or steaming water, unusual changes to the ground surface, ocean, or new large areas of dead vegetation or wildlife. These are most useful if you include time and location of the observation plus a photo or video with scale, if possible, that shows what you see. A person, boat, house, or tree can provide scale. Only take a photo or video if it is safe to do so.

Stay informed

The U.S. Geological Survey (USGS) Hawaiian Volcano Observatory will be sharing information about unrest in American Samoa via the USGS Volcano Notification Service (VNS) in addition to the existing website.. This free service sends notices via email about volcanic activity in the US.

Subscribe to the VNS at https://volcanoes.usgs.gov/vns2/. For emails about American Samoa unrest, select Ofu-Olosega, Ta’u Island, and Tutuila Island from the list of available volcanoes. Alternatively, select “Hawaiian Volcano Observatory - Add All Volcanoes” from the list of available volcano observatories to receive notices about volcanoes in Hawaii and American Samoa.

For more information about the different types of VNS notifications, please see:  https://volcanoes.usgs.gov/vhp/notifications.html

If you have questions, please contact: askHVO@usgs.gov.