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Frequently Asked Questions about earthquakes in American Samoa

By Hawaiian Volcano Observatory
Volcanoes in American Samoa

Earthquakes are being felt in eastern American Samoa.

What is happening in American Samoa? 

Earthquakes are being felt in eastern American Samoa. The earliest report of a felt earthquake is from Ta‘ū Island on July 26, 2022. By August 6, the earthquakes were reported felt by residents of all the Manu‘a Islands (Ofu-Olosega and Ta‘ū). On August 23 one of these earthquakes was large enough to be felt on Tutuila Island, which is about 60 miles (100 km) west of the Manuʻa Islands. 

Color photograph of people installing field instrument
Sources/Usage: Public Domain.
A broadband seismometer being installed on Ta‘ū island near Fiti‘uta on August 23, 2022, by the USGS Hawaiian Volcano Observatory, NOAA National Weather Service, and with help from the American Samoan government and local residents of Ta‘ū island. This is one of two broadband seismometers that have been placed on Ta‘ū island to help detect earthquakes that started to be felt by residents starting in late July. USGS photo by D. Downs.

The exact number of earthquakes is unknown. There are six small and four large seismometers recording the earthquakes. At the height of activity in late August, approximately 20–60 earthquakes per hour were occurring. This type of activity is called an earthquake swarm because many earthquakes of about the same size repeatedly occur in generally the same area. Preliminary data analysis suggests a wide area of possible earthquake locations extending from deep beneath the northern shore of Ta‘ū Island to shallower depths about 20 km (13 mi) to the north. 

Most earthquakes are too small to be felt, but at the height of the swarm in late August around 30 earthquakes per day were large enough to be felt. The largest earthquakes are estimated to be between magnitude 2 and 3.5. A widely felt earthquake at 8:33 p.m. Samoa Standard Time on August 23 is one of the largest earthquakes recorded since seismometers were installed. Its magnitude was likely 3–3.5 or larger. The earthquakes have been occurring in a fixed location according analysis of the travel-time of seismic waves.

Since late August, the number and size of earthquakes has been decreasing, but there are still a few earthquake per hour occurring. 

Why are earthquakes happening in American Samoa?  

Earthquake swarms happen for many reasons – they can occur as the earth’s crust settles in response to tectonic plate movement, when the earth’s crust adjusts to having a significant weight (a volcano) on top of it, or when magma or other fluids move and fracture rock beneath the ground surface. Because the earthquakes are not moving, we can say that the mechanism causing them isn’t moving through the earth. However, we are not completely certain why these events are occurring.

Where are the earthquakes located in the American Samoa earthquake swarm? 

The current best earthquake location estimate is that they are occurring approximately 3-9 miles (5-15 kilometers) off the northern shore of Ta‘ū Island at approximately 6-9 miles (10-15 kilometers) below the surface. Water depths are more than half a mile (about 1 kilometer) at 3 miles (5 kilometers) to the north and 2 miles (3 kilometers) at 9 miles (5 kilometers) to the north. Seismologists have determined that earthquakes are occurring in a fixed position within the earth. So, although there is difficulty in resolving the exact location within the earth, the earthquakes have not been moving from that spot. 

Why is it difficult to locate the earthquakes occurring in American Samoa? 

The precise locations of the earthquakes occurring as part of the 2022 August-September swarm are hard to resolve. For ideal earthquake monitoring at volcanoes, an array of a minimum of three seismometers would surround the earthquake location to better "see" the earthquake source from different angles. This array design allows seismologists to better resolve the latitude, longitude, and depth of earthquakes that are occurring around the volcano. Because of the geometry of the islands in American Samoa, seismometers have been installed in a relatively straight east-west line. This limits the ability to resolve the depth and north-south locations of the earthquakes, whereas the east-west locations are well resolved. 

Why are there volcanoes in American Samoa, and how often do they erupt? 

Color photograph of volcano slopes
Sources/Usage: Public Domain.
A view of the summit region of Ta‘ū island, in the Manu‘a island group of American Samoa. Ta‘ū island is the exposed top of a volcano that formed as a result of volcanic activity at the Samoa hotspot. Most of the volcano is under the ocean surface. The exposed part of this volcano that forms Ta‘ū island is covered in dense jungle vegetation though scoria cones are evident on its slopes. USGS photo by Drew Downs.

The islands that form American Samoa are the exposed tops of volcanoes, most of which are hidden beneath the ocean surface. Volcanoes in American Samoa form as the Pacific Plate moves northwest over the Samoan hotspot. A hotspot is an area where magma (molten rock beneath the ground surface) rises from the Earth’s mantle to form volcanoes. Currently, the center of the Samoan hotspot is thought to be close to the Vailulu‘u seamount, located about 25 miles (45 kilometers) east of Ta‘ū Island. 

Vailulu‘u has erupted several times in the past 50 years, but volcanoes that form the islands of Ofu-Olosega and Ta‘ū have only one documented eruption over the past 200 years. This eruption was in 1866,  between Ofu-Olosega and Ta‘ū Islands. Though this eruption happened approximately 2 miles (3 km) southeast of Olosega and stopped before the cone emerged above the ocean surface, it did impact residents. Earthquakes were felt before, and during the eruption, volcanic gases released during the eruption caused air (vog) and ocean pollution (inferred from dead fish), and tephra (small volcanic particles) was blown downwind. 

What is the USGS doing to understand this activity better? 

The U.S. Geological Survey Hawaiian Volcano Observatory (USGS HVO) is responsible for monitoring volcanoes in American Samoa. Before August 2022, there was no volcano-monitoring equipment on the ground in American Samoa. Once USGS HVO found out about the earthquakes, staff were sent to American Samoa to meet with local authorities and residents and install monitoring equipment.  

Color photograph of earthquake monitoring device
Sources/Usage: Public Domain.
A Raspberry Shake microseismometer was installed on Ta‘ū island, American Samoa, on August 19, 2022, by the USGS Hawaiian Volcano Observatory and NOAA National Weather Service team. This is one of several microseismometers that have been placed within the Manu‘a islands to help detect earthquakes in the region that started to be felt in late July. USGS photo by D. Downs.

USGS HVO staff arrived in American Samoa on August 11, and the first micro-seismometer (small earthquake-detecting device) was installed on Ta‘ū Island on August 13 with the help of the National Oceanic And Atmospheric Administration (NOAA) National Weather Service Pago Pago office. By August 24, six microseismometers and three broadband seismometers (the standard for volcano monitoring) were installed across American Samoa. A fourth broadband seismometer was installed on September 14. Data collected by seismometers will help us understand where the earthquakes are occurring and what processes might be causing them, and most importantly, to track changes in the location, rate, and size of earthquakes. Changes in any of those characteristics are essential to detect as they may indicate a change in hazards.  

In early September, two Global Positioning System (GPS) receivers were installed, which will improve the Manuʻa Islands’ monitoring network. The GPS equipment will detect very slight movements (less than an inch!) of the ground and help show what is happening beneath the ground to provide more accurate hazard assessments. It will take about a month to be able to interpret data from these GPS stations.   

Satellite remote sensing is another tool the USGS has been using since the middle of August. Satellites can detect heat, volcanic gas, and volcanic ash associated with early phases of volcanic activity. USGS scientists are examining multiple satellite images of the Manu’a Islands every day to look for signs of anything unusual.  

Color map of islands and nearby bathymetry
Sources/Usage: Public Domain.
This reference map depicts the topography and bathymetry of the volcanoes in the Manu‘a Islands, American Samoa, where seismic unrest has been ongoing for several weeks. The highest elevation in the area is Lata Mountain on Ta‘ū Island, standing 3,179 feet (969 meters) above sea level. The bathymetry data—courtesy of the National Oceanic and Atmospheric Administration (NOAA), Maxar/DigitalGlobe, and the University of Hawai‘i School of Ocean & Earth Science & Technology (SOEST)—depicts submarine topography in high-resolution down to 1,148 feet (350 meters) below sea level, and at coarser resolution to greater depths. A shallow submarine ridge extends to the northwest from Ta‘ū Island towards Olosega Island; it was along this ridge that a submarine eruption occurred in 1866. There are now 3 broadband seismometers (pink circles) and 4 microseismometers (pink squares) monitoring seismic activity in the Manu‘a Islands.

How long do earthquake swarms last? 

It can vary wildly and depends upon the tectonic and volcanic settings. Swarms that include earthquakes that don’t move within the earth can be very short-lived with seismicity lasting only a few hours to days—this happens often at large volcanic systems like Long Valley Caldera and Yellowstone. There was an episode like that in the Azores volcanic islands off the coast of Portugal) earlier in 2022. But in other situations, earthquake swarms can last for months or years. A swarm on the eastern border of Northern Nevada began in July 2014 and kept going at a declining rate into early 2017; called the “Antelope Swarm,” there is no consensus on a cause for the earthquakes, but the area is capable of producing volcanic eruptions. Before the eruption in Iceland last year, there were at least months of seismicity and ground deformation, and the most recent eruption was similarly preceded by months of earthquakes and ground movement. Some volcanoes have "personalities" and so have a tendency towards one style versus another, but we know relatively little about the volcanoes of American Samoa, and they don't erupt above the water or in shallow water that often (most recently in 1866). Experience with these volcanoes is therefore limited. 

What could happen, and what hazards might impact American Samoa Islands? 

One possibility is that the current earthquake activity continues, with variations in the number and size of earthquakes, for days, weeks, months, or years before eventually stopping. This could include an increase in the number and size of earthquakes if magma moves within the volcano as an intrusion (a term for when magma moves within a volcano but stops before erupting at the surface). 

Currently, there is no indication that the number or size of earthquakes is increasing. If there were to be an eruption, we anticipate that the number and size of earthquakes would increase. The earthquakes would likely also move closer to the surface. USGS scientists are monitoring the activity and will issue an alert if the earthquake activity begins to change. 

Color photograph of ocean and island in distance
Sources/Usage: Public Domain.
An afternoon view of the tuff cone near Faleāsao village located in northwest Ta‘ū island, with the islands of Ofu and Olosega visible in the distance. This tuff cone, which is thousands of years old, formed when hot magma interacted with either shallow groundwater or sea water. There are several tuff cones located within the northwest corner of Ta‘ū island. USGS photo by D. Downs.

There is a chance these earthquakes are caused by volcanic activity, but an eruption like Hunga Tonga–Hunga Ha'apai in Tonga earlier this year is extremely unlikely. This is because hot spot shield volcanoes like Ta‘ū Island have less explosive eruptions than a volcano like Hunga Tonga- Hunga Ha'apai. Most earthquake swarms on volcanoes do not result in an eruption. However, earthquake swarms can precede volcanic activity, such as an intrusion (magma moving within a volcano but not reaching the surface) or an eruption (magma moves within a volcano and reaches the surface). 

If the earthquake swarm leads to a volcanic eruption, the type of eruption and its hazards will largely depend on the eruption location. Eruptions do not always come from the top of the volcano, and there is evidence that past eruptions have happened in many different places on Ta‘ū Volcano. Most of Ta‘ū Volcano is underwater. 

If an eruption occurs in deep ocean water (hundreds of yards deep), it will be mostly confined by the sea water. People may see discolored water, and they may be bursts of steam or volcanic ash from the water. If an eruption happens on land in an area with little ground water, it will produce low-level explosions of lava particles confined to an area near the eruption site. Such on-land “dry” eruptions can also feed long lava flows that move downhill. The worst-case scenario would be for an eruption to occur near the coast. This could produce large explosions of rock, volcanic ash, and steam. Hazards from a coastal eruption would be wide-reaching—hot, ground-hugging surges of steam and ash might accompany the high-velocity ejection of heavy rocks. These eruptions can produce high plumes of volcanic ash that travel far distances downwind. Secondary hazards from a coastal eruption include local tsunamis. 

Any possible eruption could last days to years. The Ofu-Olosega submarine eruption in 1866 lasted a few months.  

To learn about the types of eruptions possible in American Samoa and the associated hazards, see this webpage: https://www.usgs.gov/observatories/hvo/volcanic-eruptions-and-hazards-american-samoa

What should I do to protect myself and my family now and if there is an eruption? 

Become aware of the volcanic hazards that might affect the areas where you live, work, or visit. Know who the trusted sources of information are. Make an emergency kit. Plan what to do during an emergency, such as an evacuation, by identifying responsibilities for each household member to work together as a team. 

To learn about the types of eruptions possible in American Samoa and the associated hazards, see this webpage: https://www.usgs.gov/observatories/hvo/volcanic-eruptions-and-hazards-american-samoa

Trusted sources of information on unrest in American Samoa:  

Information on how to build an emergency kit:  

Information on how to build an emergency plan:  

How can I stay informed about this activity?  

Color photograph of island from space
Sources/Usage: Public Domain.
 NASA Astronaut image of Ta'u Island (Manu'a Islands, American Samoa) in the South Pacific Ocean.

HVO is issuing daily notices about the activity in American Samoa via the USGS Volcano Notification Service (VNS). This free service sends notices via email about volcanic activity in the US. These notices are also posted on the Taʻū Island Volcano Updates webpage.  

To receive Volcano Activity Notices (VANs), subscribe to the VNS at https://volcanoes.usgs.gov/vns2/. For emails about American Samoa unrest, select Ofu-Olosega, Ta’ū 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   

Other resources:  

How can I report observations about this activity? 

Residents can assist these monitoring efforts by noting and reporting accurate times that they feel earthquake shaking or notice other changes (smells like rotten eggs, dead fish, areas with dead vegetation, ground cracks, and generally unusual activity) that might be related to volcanic activity 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).   

Is it possible that residents will be evacuated? 

Evacuation plans and decisions are the purview the responsibility of the American Samoa Government. The lead agency is of the American Samoa Department of Homeland Security. Please see https://www.americansamoa.gov/dhs and https://www.facebook.com/AmericanSamoaDHS