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Exact cause of earthquakes around Manu’a remains unexplained

USGS and NOAA scientists are looking at the phenomena

Pago Pago, AMERICAN SAMOA — To date it is unclear where the earthquakes are coming from around the Manu’a Islands.

For the last week, residents of the Manuʻa group of islands in American Samoa continue to feel earthquakes.

The US Geological Survey Hawaiian Volcano Observatory (HVO) in its latest report says that Taʻu residents suggest that the activity began on July 26.

 “Since August 10, earthquakes have also been reported by residents of Ofu and Olosega islands,” according to the report.

These 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.”

On Friday some residents in Manu’a called on the American Samoa Government to evacuate them, and on Saturday a chartered flight took Emergency Operations Center personnel to Manu’a.

The plane returned to Tutuila with some of Manu’a’s residents on Saturday,

The Manu’atele also arrived in Manu’a Sunday morning, where a meeting was scheduled to take place in the afternoon to discuss evacuation. However questions about shelters, and supplies for people evacuated from Manu’a remain unanswered.

Repeated efforts to get comments from the director of Homeland Security, Samana Semo Ve’ave’a were unsuccessful including visits to the office Saturday afternoon and calls to his cell phone.

Dr Natalia Deligne of the US Geological Survey Hawaiian Volcano Observatory (HVO), arrived in Pago Pago on August 11 and is consulting with territorial authorities.

During a recorded program of the National Weather Service Office, Elinor Lutu-McMoore said Tutuila people are not experiencing anything and the feeling is “hopeless” — not knowing what is happening in Manu’a.

Also the source of the explosives sounds heard by the people of Manu’a is unclear.

Deligne said the USGS is working on getting monitoring equipment to the islands this week that will provide more information on from where the earthquakes are originating, “but they have been going on for several weeks now, … please keep the reports coming in, they are very helpful for us.”

Adding that if these earthquakes are related to a volcanic unrest as they called it in Tonga, there are differences in American Samoa.

 “Tonga volcanoes and volcanoes here are different.

 “The volcanoes here are much more like Hawaii with the basaltic lava, whereas in Tonga it’s what we call subduction zone volcano and it can be very explosive,” said Deligne.

 “And so the type of volcanic eruptions that we could expect in the two locations (Tonga and American Samoa) are very different.

 “The worst case scenario here is not what we saw in Tonga, it would be an eruption, but the worst case scenario here would be an eruption that is either on the island or offshore — just offshore — but it would be much much smaller.

 “That does not mean that it's not scary — a volcanic eruption is generally scary but it wouldn't be as huge as what we saw in Tonga earlier this year,” said the USGS official in the territory.

Experts at the Pago Pago National Weather Service Office, USGS Volcano Hazards Program, NOAA Pacific Tsunami Warning Center and NOAA-IOC International Tsunami Information Center and USGS National Earthquake Information Center said they are working together with the American Samoa government to understand the source of these earthquakes better.

According to the USGS 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.”

The USGS also noted that 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.

“The Samoa Meteorological Service is also reporting increased seismicity south or east of Tutuila Island.”

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.

As a result, “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.”


Taʻu 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 Taʻu 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.

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.