Volcanic Eruptions and Hazards in American Samoa

It is unclear if the current earthquake unrest in American Samoa is related to volcanic activity and, if so, whether it will escalate to a volcanic eruption. Currently, the primary hazard of concern is earthquake shaking, although no damaging earthquakes have occurred as part of this swarm. If activity escalates to an eruption, it could pose significant hazards to residents of American Samoa.  

Not all earthquake swarms result in a volcanic eruption. When they do occur, volcanic eruptions can range in size and vigor. Hazards associated with potential volcanic activity in American Samoa depend on the type of activity. Potential types of activity include earthquake activity, eruptions on land, eruptions involving water (which could occur near or offshore), and submarine eruptions. Hazards associated with these potential types of volcanic activity include ground shaking and associated hazards, local tsunami, volcanic gases, low-level localized explosions of lava, lava flows, lava ocean entry hazards, and hydrovolcanic and phreatomagmatic activity. These different types of activity and associated hazards are described below.  

Earthquakes

Earthquakes can be caused by a number of subsurface processes, such as ground movement along faults, geothermal activity, or movement of magma beneath a volcano. The word swarm is used to identify when a sequence of smaller earthquakes is occurring over time in a localized area. Most earthquakes in a swarm are small and cannot be felt by people, but magnitude 3 and larger earthquakes can be more widely felt. Shaking can be locally intensified by water-saturated soils that amplify earthquake ground motions, and can result in other hazards such as ground movements and local tsunami.

Ground shaking, ground cracking, landslides, local tsunami 

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Strong earthquakes endanger people and property by shaking structures and by causing ground cracks and ground settling. On steep slopes, intense shaking may cause such soils to fail, resulting in landslides and mudflows. Large offshore earthquakes can displace large volumes of water to form tsunamis, a series of sea waves that can be far more damaging than any of the direct seismic hazards. Strong earthquakes have the potential to also damage infrastructure such as buildings, roads and bridges, and water, sewer, and utility lines. Earthquakes can occur at any time and the exact location and timing of earthquakes cannot be predicted. 

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.  

Eruption on land

During an eruption on land, volcanic gas is emitted that is hazardous to breathe and can create volcanic air pollution (vog) that can affect broad areas downwind. Eruptions on land can form lava flows, spatter ramparts, and scoria cones. Lava flows destroy everything in their path and generate additional hazards when they enter the ocean. Small-scale explosive activity, such as lava fountains, shreds and blasts molten rock into the air, forming lava fragments called tephra that solidify and fall back to the ground. 

Volcano Gas Emissions  

Volcanic gases—primarily water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂)—are continuously released during the eruptions. As SO₂ is released, it reacts in the atmosphere with oxygen, sunlight, moisture, and other gases and particles, and within hours to days, converts to fine particles. The particles scatter sunlight and cause the visible haze that can be observed downwind, and is known as volcanic air pollution or vog (volcanic smog). Vog creates the potential for airborne health hazards, damages agricultural crops and other plants, and affects livestock operations. 

Lava Flows 

Lava flows are streams of molten rock that typically erupt from a shield volcano's summit or along rift zones on its flanks. A lava flow moves down the flank of a volcano, towards the ocean, under the influence of gravity, similar to how water flows downhill through gullies, channels, and valleys. Lava flows generally advance slowly and can be easily avoided by people. However, lava flows can destroy everything in their paths—rainforests, buildings, roads, utility and communication systems, and whole communities. Lava flows can be smooth and ropey (pāhoehoe) or rubbly and rough (‘a‘ā).