Volcano Watch: U.S. Geological Survey scientists return to American Samoa a year after seismic unrest

It’s been one year since Taʻū volcano in American Samoa started shaking residents of the Manuʻa Islands of Ofu-Olosega and Ta‘ū Islands.

Fortunately, there have been no additional earthquakes since the volcanic unrest ended last October. This month, U.S. Geological Survey staff will be returning to American Samoa to do outreach, strengthen relationships with partners, and maintain the monitoring network.

The first felt earthquake was reported on July 26, 2022. Over the next six weeks, until early September 2022, hundreds of earthquakes were felt throughout the Manuʻa Islands; one earthquake was felt as far away as Tutuila. The events were highly unusual for American Samoa, so much so that the islands did not have any volcano monitoring equipment when the unrest began.

In response to the unrest, the U.S. Geological Survey’s Hawaiian Volcano Observatory rapidly deployed staff and monitoring equipment in August and September 2022. The observatory established a monitoring network across American Samoa, and kept the local community, federal and local partners informed on the dynamic situation. In December 2022, a few months after the unrest ended, a small team returned to service the newly established monitoring network.

In August 2023, two observatory scientists will return to American Samoa for two weeks. We are looking forward to being back during less shaky (literally) conditions.

Staff will conduct outreach with our partners at the National Oceanic and Atmospheric Administration’s National Weather Service office in Pago Pago. Outreach events are planned for the Manuʻa Islands on Aug. 7 and 8, with further community meetings, media interviews and other events on Tutuila over the following week.

We will also maintain and service the volcano and earthquake monitoring network, with assistance from the National Weather Service Pago Pago Office. The network also provides important data for earthquake and tsunami monitoring across the southwest Pacific region. 

On Taʻū Island, we will replace the currently offline TAU broadband seismometer, located on northwest of Taʻū Island and damaged during a lightning storm on May 5, 2023. We will also improve the telemetry — how data is transmitted so that it can be analyzed from afar — for the broadband seismometer on eastern Tutuila.

Considering there was no monitoring network in place at the start of the 2022 unrest, we do not know how many earthquakes occurred during the entire 2022 earthquake swarm. A hydrophone (similar to an underwater microphone) on Wake Island, about 2,800 miles northwest of Taʻū, detected activity that provides a useful overview of how the earthquake swarm evolved. It began in late July, peaked in mid-August, and was mostly over by early to mid-September. It definitively ended in early October (see the attached figure).

By Aug. 20, 2022, the rapidly deployed monitoring network had enough stations for the USGS to locate earthquakes of magnitude (M) 2.5 or greater. Between Aug. 20 and Sept. 11, 2022, 301 earthquakes that were M2.5 or greater occurred north of Taʻū Island.

As the seismic unrest eased back into background activity, between Sept. 11 and Oct. 6, 2022, an additional eight earthquakes occurred. Most of these final earthquakes were not felt by local residents.

The 2022 earthquake swarm in American Samoa is a reminder that these islands are formed by volcanoes that have the potential to erupt someday. The network of instruments in American Samoa now allows the USGS to monitor the volcanoes, which have remained quiet since the 2022 unrest ended.

To receive information about the status of volcanoes in American Samoa, all of which are currently at NORMAL/GREEN, please subscribe at https://volcanoes.usgs.gov/vns2/.  More information on volcanoes in American Samoa is also available on the Hawaiian Volcano Observatory website at https://www.usgs.gov/observatories/hvo/volcanoes-american-samoa.

Editor’s Note: Volcano Watch is a weekly article and activity update written by scientists and affiliates of the U.S. Geological Survey Hawaiian Volcano Observatory. Today’s article is by observatory geologist Natalia Deligne.