The U.S. Geological Survey considers the several earthquakes that have occurred following the 8.3 magnitude quake of Sept. 29 as a “aftershock sequence”, expected following a large event.

Since last month’s major quake, which generated a tsunami killing more than 100 people in American Samoa, Samoa and Tonga, there have been seven or eight more quakes in the Samoa Island region, according to the latest information on the USGS Web site.

Locals and off-island Samoans have noted these latest earthquakes, including the one that occurred Monday— the tremor was felt island-wide— and many have been wondering why this is happening.

Responding to Samoa News inquires, Gavin Hayes, a research scientist with the USGS National Earthquake Information Center in Golden, Colo. says the “aftershock sequence near Samoa...are an expected sequence of earthquakes that occur following all large events, and reflect a redistribution of stresses following the Sept. 29th mainshock.”

“The locations of these events are of interest, in that they seem to be predominantly occurring to the west of the mainshock, within the subduction zone regime rather than outboard (to the east of) the subduction zone where the mainshock occurred,” he explained via e-mail. “This may reflect some triggering of earthquakes within the subduction zone from the large mainshock.”

(Subduction zones involve an oceanic plate sliding beneath either a continental plate or another oceanic plate. Subduction zones are often noted for their high rates of volcanism, earthquakes, and mountain building. This is because subduction processes result in melt of the mantle that produces a volcanic arc as relatively lighter rock is forcibly submerged.)

Besides the Samoa region, there have also been a series of earthquakes around the world— such as the Sept. 30 Sumatra earthquake, and the Oct. 7 and Oct. 8 Vanuatu and Santa Cruz Islands earthquakes. Hayes provides an additional explanation.

“First, the number of large events that have occurred recently is not abnormal when compared to the number of large earthquakes (7-plus magnitude) we expect to occur in a given year (approximately 15-20), based on scaling relations,” he explained.

“Secondly, while it seems unusual that they have occurred in such a short period of time, probability calculations show that such a clustering of large earthquakes should occur approximately once every 30 years, and so again this is not incredibly unusual,” Hayes said.

He also said when seismologists discuss ‘triggering’, two different types of triggering are known:

• ‘Static Stress Transfer’, which is thought to only influence earthquakes within 1 or 2 fault lengths of the triggering earthquake— in the case of the Samoa earthquake this equates to 200 kilometers.

• ‘Dynamic triggering’, relates to distant earthquakes being caused by the influence of the passage of seismic energy from the triggering earthquake through the fault zone of the triggered earthquake.

“While this has been observed in the past, where small earthquakes have been caused by the passage of energy from large earthquakes, to my knowledge it has not been demonstrated to cause very large events such as those we are talking about here,” Hayes explained.

“There is also a significant time delay between the passage of seismic energy from the Samoa earthquakes and when the following earthquakes in Sumatra and Vanuatu occurred,” he noted.

It’s “possible that this passage of energy from the Samoa earthquake ‘nudged’ the faults on which subsequent earthquakes occurred (that would have occurred soon anyway)...and this is a hypothesis that seismologists investigating these earthquakes are currently exploring,” he said.

Meanwhile, Dr. Guy Gelfenbaum, an Oceanographer with USGS based in Menlo Park, Calif., will make a presentation on Thursday at the ASCC Lecture Hall about the findings of an international team documenting the affects of the Sept. 29 tsunami in American Samoa, says USGS spokesperson Bruce Jaffe in an e-mail to Samoa News.

Title of the presentation is, “The Samoa Tsunami 2009: Preliminary Field Data on Tsunami Inundation in American Samoa.”

Abstracts of the presentation state the tsunami caused considerable damage and a number of fatalities on American Samoa.

Scientists from the USGS, as well as others from the US, Japan, and elsewhere have been on the island to measure tsunami wave heights, inundation distances, and collect other data that will be used to help understand the impact of the tsunami, according to abstracts provided by USGS to Samoa News.

Scientists measured tsunami runups— the elevation at the location of the furthest inland inundation— of about 38 feet and inundation distances of about 800 feet.

According to the USGS, Gelfenbaum will show the types of data collected and describe how the data is used to improve the understanding of tsunami hazards in American Samoa and elsewhere around the world.

Gelfenbaum has been with the USGS for over 20 years studying coastal hazards on US coasts in Florida, Oregon, and Washington. He has been investigating tsunami hazards for 12 years and has been to Papua New Guinea (1998), Peru (2001), and Sumatra (2004) after tsunamis impacted those countries, according to the federal agency.

Gelfenbaum also models tsunami inundation and sediment transport to help improve tsunami hazards assessments.