Pago Pago, AMERICAN SAMOA — The US Geological Survey has confirmed that in the past 24 hours, no felt earthquakes were reported by people in the Manuʻa Islands, this is despite earthquake swarm related to Taʻu Island are continuing.

“The frequency of earthquakes in the past 24 hours has leveled out, but remains low following the pattern of decreasing seismicity over the past week. The lower number of earthquakes does not mean the earthquake swarm will end,” according to the latest statement issued by the USGS.

The report also mentioned the magnitude 7.6 earthquake in Papua New Guinea was detected across the USGS seismic network in the Manuʻa Islands. 

“The FAGA broadband seismometer is transmitting data again as of yesterday.  Installation of the two GPS stations on Taʻū Island is complete. 

“The satellite data from the previous 24 hours observed no volcanic activity.”

According to the USGS, Volcanoes in the Manuʻa Islands are monitored with a limited real-time seismic network consisting of three sophisticated seismometers (broadband seismometers) — two on Taʻu Island and one on Ofu Island — and six microseismometers on Tutuila, Taʻu, and Ofu-Olosega Islands. Satellite remote sensing is another tool being used, which may detect heat, volcanic gas, and volcanic ash associated with early phases of volcanic activity.

Meanwhile, the American Samoa Government says that in case of imminent danger or an active disaster, the local statute authorizes the Governor to direct and compel the evacuation of all or part of a the population form a stricken or threatened area, prescribe routes and modes of transportation for evacuation, and provide for emergency housing.

This is outlined in an executive order issued in accordance with the powers and authority granted to the Governor by Article IV, Sections 6 and 7 of the Revised Constitution of American Samoa, and the American Samoa Code Annotated, Sections 4.0111 (b) and 26.0105.

“Executive Order 008-2022 declared that a state of emergency was in effect related to the earthquakes that were felt in Manu'a for thirty days.”

Members of the Public are reminded that anyone who feels a strong or long earthquake, observes a sudden drop or rise of sea level, or hears a loud roar from the ocean should immediately go to higher ground.

“Anyone who feels an earthquake should report it to the EOC and/or the National Weather Service Office in Pago Pago once they are in a safe place.

“It shall take effect as of immediately and will remain in effect for thirty days.”

According to the USGS report, the exact number of earthquakes is unknown, but data from newly-installed seismometers (earthquake-recording devices with the first one installed on August 13) indicate that approximately 20–60 earthquakes per hour are 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 Ta’u Island to shallower depths about 10 km (6 mi) to the north. 

“Most earthquakes are too small to be felt, but around 30 earthquakes per day are 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 and will be refined later when the American Samoa seismic network has been calibrated. 

“The rate or magnitude of earthquakes is not increasing, and they have been occurring in a fixed location since monitoring instruments were installed in mid-August.”

The data from newly installed seismometers will help us to understand what is causing the earthquakes.

BACKGROUND

“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.”

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.”

The precise location 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.”